blast resistance steel tips astaneh may2010

_________________________________________________________________________________________________________________ Notes on Blast Resistance of Steel and Composite Building Structures , Copyright © 2010 by Abolhassan Astaneh-Asl. All rights reserved.

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May 2010

Notes on Blast Resistance of Steel and

Composite Building Structures

By

Abolhassan Astaneh-Asl, Ph.D., P.E. Professor

University of California, Berkeley

____________________________________________________________________________

(A copy of this report can be downloaded for personal use from www.steeltips.org)


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Notes on Blast Resistance of Steel and Composite Building Structures By Abolhassan Astaneh-Asl, Ph.D., P.E., Professor of Structural Engineering, University of California, Berkeley

Abstract— This Steel Technical Information and Product Services (Steel TIPS) report provides information on blast resistance and protection of steel and composite building structures against an intentional or accidental explosion occurring close to the building. Chapter 1 gives a background on the issues of blast-resistance and blast protection and introduces the reader to the current status of this important and rapidly developing field. Chapter 2 discusses the blast effects on humans and structures and provides a summary of blast loads used in blast resistant design. Chapter 3 provides information on material properties of steel and concrete under a high strain rate of blast. Chapter 4 focuses on the discussion of performance of specific steel and composite members and connections with emphasis on the primary members such as beams , columns , and floors as well as the connections directly subjected to blast pressures. The non-structural elements are not part of this report. In addition, Chapter 4 provides some suggestions on how to improve the detailing of steel and composite members to attain better performance under blast loads. There is a bibliography listing for further reading.

First Posting on www.steeltips.org: May 2010. ________________________________________________________________________ Abolhassan Astaneh-Asl, Ph.D., P.E. Professor of Structural Engineering, University of California, Berkeley and Consultant on Structural Engineering, Earthquake Engineering and Protection of Structures against Impact and Blast. E-mail: [email protected] Web sites: http://www.astaneh.net and http://www.ce.berkeley.edu/~astaneh _________________________________________________________________________

Disclaimer: The information presented in this publication has been prepared in accordance with recognized engineering principles and is for general information only. While it is believed to be accurate, this information should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability, and applicability by a licensed professional engineer, designer, or architect. The publication of the material contained herein is not intended as a representation or warranty, on the part of the Structural Steel Educational Council or of any other person named herein, that this information is suitable for any general or particular use or of freedom from infringement of any patent or patents. Anyone making use of this information assumes all liability arising from such use. Caution must be exercised when relying upon specifications and codes developed by others and incorporated by reference herein since such material may be modified or amended from time to time subsequent to the printing of this document. The Structural Steel Educational Council or the author bears no responsibility for such material other than to refer to it and incorporate it by reference at the time of the initial publication of this document.

COPYRIGHT © 2010 Abolhassan Astaneh-Asl. All rights reserved. Except as permitted under the United States Copyright Act of 1976 and all subsequent amendments, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without written permission of the author All drawings and photos are by and Copyright 2010 Abolhassan Astaneh-Asl unless otherwise noted. The graphics and artwork from this document are not to be used in any document, printed or electronic, by anyone for any purpose without written permission of the creator and copyright holder Abolhassan Astaneh-Asl.


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DEDICATION

This report is dedicated to the

(www.skyscrapersafety.org)


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ACKNOWLEDGMENTS

The publication of this report was made possible in part by the support of the Structural Steel Educational Council (SSEC). Funding was provided by the California Field Iron Workers Administrative Trust (CFIWAT), a union trust fund. The author wishes to thank all SSEC members as well as the CFIWAT for their support.

The comments and input received from Fred Boettler, Rich J. Denio, Pat Hassett , John Konechne, Brett Manning, Kevin Moore, and all other members of the Structural Steel Educational Council were very valuable and sincerely appreciated.

The author and his graduate and undergraduate research students at the University of California at Berkeley participated in conducting the research projects reported in this document. In particular, the excellent work of former research collaborators and students Emma Goodson, Dr. Marcus Rutner, Dr. Jin Son, Mark Wan and Prof. Qiuhong Zhao in conducting the analytical studies of structural components subjected to blast loads is acknowledged and appreciated.

Special thanks are due to Casey Heydari of the MSC Software Corporation for his support and technical input in our studies of structures subjected to blast and impact loads. The analyses of structures subjected to blast loading reported here would not be possible without using the powerful structural analysis software series Patran, Dytran and Nastran, all developed by the MSC Software Corporation (www.MSCsoftware.com). Generous donation of the software by the MSC Software Corporation to our projects at the University of California, Berkeley is sincerely appreciated.

Dr. David McCallen of the Lawrence Livermore National Laboratory provided many valuable information for this report through his research collaboration with the author.

The project on 3-story buildings was sponsored by the Advanced Technology Institute and was done for the U.S. Army. The support as well as valuable technical input and guidance received from Dr. Beatty, Dr. Adler, Mr. Plumer, Dr. Jack Hayes, Jon Tirpak, and Professor Harry Paxton are acknowledged and appreciated.

The author had no involvement with the development, production, marketing or sale of the Sideplate (www.sideplate.com) connections and has no financial interest on the product itself or the company that holds the patent on this product. The mention of this product here is only based on the author’s belief, formed by a review of test results available at this time , on the good performance of this product under structural loads especially seismic and blast effects.

The opinions expressed in this document are solely those of the author and do not necessarily reflect the views of the University of California, Berkeley, where he is a professor of structural engineering, or the Structural Steel Educational Council and other agencies and individuals whose names appear in this report.


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NOTES ON BLAST RESISTANCE OF STEEL AND COMPOSITE BUILDING

STRUCTURES By: ABOLHASSAN ASTANEH-ASL, P.E., Ph.D. Professor University of California, Berkeley

TABLE OF CONTENTS Subject Page

ABSTRACT …………………………………………………..………………………….………….………… 1

DISCLAIMER ………… …………………………………………………………………….…….………….. 1

DEDICATION ………………………………………………….…………………………………….………... 2

ACKNOWLEDGMENTS ……………………………………………..….…………………………………... 3

TABLE OF CONTENTS ………………………………………………………..………………...….……… 4

NOTATIONS …………………………………………………………………………………..……………… 6

CHAPTER 1. INTRODUCTION TO BLAST-RESISTANT DESIGN ……………….……….….……….

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1.1 INTRODUCTION……………………………………………….…………..…….…….. 8 1.2 SCOPE OF THE REPORT………………………..………………..……….….….………. 10

1.3 MAIN SOURCES OF INFORMATION ON BLAST RESISTANT DESIGN…..........…. 11 1.4 BLAST EFFECTS LEADING TO PROGRESSIVE COLLAPSE ……………………… 12 1.5 PREVENTING PROGRESSIVE COLLAPSE BY REDUCING BLAST DAMAGE TO THE STRUCTURE…………………………………………………….….………...

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1.6 HARDENING OF STEEL STRUCTURES TO MINIMIZE DAMAGE ……….............… 14

CHAPTER 2. BLAST EFFECTS ON HUMANS AND STRUCTURES…………………….……......…… 15 2.1. BLAST EFFECTS ON HUMANS …….…………………………………….………..…. 15 2.2. STAND-OFF DISTANCE AND BARRIERS…………………………………….…...... 16 2.3 SAFE AREAS WITHIN THE BUILDINGS ………………………..………………..… 18 2.4. BLAST EFFECTS ON STRUCTURES …….………………………………....……..... 18 2.5. SHOCK WAVE CREATED BY BLAST IN THE AIR…………………..…………….. 25 2.6. BLAST IMPULSE……………………………………………………..……..……..…... 29


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CHAPTER 3. MATERIAL PROPERTIES UNDER HIGH STRAIN RA TES OF BLAST………….… 30 3.1 BLAST EFFECTS ON MATERIAL PROPERTIES ………..…………………….……. 30 3.2 MECHANICAL PROPERTIES OF STEEL UNDER BLAST LOAD …….……………… 31 3.3 MECHANICAL PROPERTIES OF CONCRETE UNDER BLAST LOAD………………. 34 3.3.a. Increase in Compressive Strength of Concrete under High Strain Rate……… 35 3.3.b. Increase in Tensile Strength of Concrete under High Strain Rate……………. 37

CHAPTER 4. BLAST PROTECTION OF STEEL AND COMPOSITE STRUCTURES …………..…. 40

4.1 INTRODUCTION………………………………………………….…………………..….. 40 4.2 BEHAVIOR AND BLAST DESIGN OF STEEL COMPONENTS …….………….……. 41 4.2.a. Steel and Composite Columns Subjected to Blast Effects …….……….…..…. 41 4.2.b. Blast Damage to the Columns of the World Trade Center Tower Caused

by the 1993 Car Bomb Attack……………………………………..…..…..…. 42

4.2.c. Blast Effects on Open Shape Steel Columns…………….…………………… 45 4.2.d. Blast Test of Steel Open Shape Member …………………………..………… 46 4.2.e. Analytical Studies of Open Shape Columns Subjected to Blast Effects...……. 48 4.2.f. Preventing Flange Folding in Open Steel Cross Sections (Wide Flanges)…….. 49 4.3 FAILURE MODES OF COLUMNS SUBJECTED TO BLAST………………………...… 51 4.3.a. Notes on Overall Buckling of Steel Columns under Blast Loads……...……… 52 4.3.b. Notes on Local Buckling of Steel Columns under Blast Loads……….………. 52 4.3.c. Notes on Distortion of Column Sections under Blast Loads……….…………. 54 4.3.d. Notes on Shear Failure of Steel Columns under Blast Loads…………….…… 54 4.3.e. Notes on Fracture of Tension Flange of Columns under Blast Loads………… 55 4.3.f. Notes on Columns Splices under Blast Loads…………………………………. 55 4.3.g. Notes on Failure of Column Base Plate/Anchor Bolts Assembly…….……….. 56 4.3.h. Failure of Foundations…………………………………………………….…… 59 4.4. NOTES ON COMPOSITE COLUMNS SUBJECTED TO BLAST EFFECTS……….….60 4.5. NOTES ON DESIGN OF STEEL AND COMPOSITE BEAMS UNDER

BLAST LOADS…………………………………………………………………………. 60

4.5.a. Steel Beams under Blast Loads………………………………………………... 61 4.5.b. Composite Beams and Shear Studs……………………………………………. 61 4.6. NOTES ON DESIGN OF STEEL AND COMPOSITE CONNECTIONS UNDER BLAST LOADS………………………………………………………………..…

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4.6.a. Notes on Welds under Blast Effects……………………………………………. 62 4.6.b. Notes on Bolts and Rivets under Blast Effects…………………………………. 63 4.6.c. Steel Moment Connections under Blast Effects…………………………..…… 63 4.7. EXTERIOR WALLS AND FACADES ………………………………………………… 67 4.8. COMPOSITE SHEAR WALLS SUBJECTED TO BLAST LOADS…………..………… 67 4.9. USE OF COMPOSITE SHEAR WALLS IN BLAST HARDENING OF BUILDINGS…. 70 4.9.a. Introduction to Composite Shear Walls………………………………………… 71 4.9.b. Analysis of a 3-Story Case Study Building Subjected to Blast………………… 73 4.9.c. Analysis Results of a 3-Story Case Study Building Subjected to Blast……..…… 76 4.9.d. Summary of Blast Studies of a 3-story Building………………………………. 78 4.10. COMPARISON OF CONSTRUCTION OF R/C AND PROPOSED BLAST

RESISTANT COMPOSITE SHEAR WALLS 79

REFERENCES AND BIBLIOGRAPHY …………………………………………….………………………. 81 ABOUT THE AUTHOR …………………………………………………………….………..……………..

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LIST OF STEEL TIPS REPORTS ………………………………………………………….……………....

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Notations _________________________________________________________________________

b = Width of element C = (1) A coefficient in local buckling

(2) Viscosity factor Crα = Peak reflected pressure coefficient at angle of incidence α d = Diameter of reduced section of an upset bolt or threaded rod D = Diameter of unthreaded part in an upset bolt or threaded rod DIF = Dynamic Increase Factor E = Modulus of elasticity of steel Ec = Modulus of elasticity of concrete Eh = Strain hardening modulus for steel Eh /E = Ratio of strain hardening modulus to elastic modulus Fu = Static tensile strength of steel under normal (slow) strain rate Fud = Dynamic tensile strength of steel under high strain rate of blast Fy = Static yield stress of steel under normal (slow) strain rate Fyd = Dynamic yield stress of steel under high strain rate of blast

cf = Static ultimate compressive strength of concrete at 28 days (same as f’ c)

f’ c = Static ultimate compressive strength of concrete at 28 days

cdf = Dynamic ultimate compressive strength of concrete

fco = 10 kPa (1450 psi)

tf = Static ultimate tensile strength of concrete

tdf = Dynamic ultimate tensile strength of concrete

g = Acceleration of the gravity (32.2 ft/sec2) ir = Unit positive normal reflected impulse (psi-ms) is = Unit positive incident impulse (psi-ms) K = (1) Effective length factor in column buckling

(2) Diameter of threaded part in an upset bolt or threaded rod L = Length of a member Lw = Wave length of positive pressure phase (ft) M = Applied bending moment Mn = Nominal moment capacity of the column Mx = Applied bending moment about x-x axis My = Applied bending moment about y-y axis (M-P) = Static bending moment –axial load interaction curve (M-P) dyn = Dynamic bending moment –axial load interaction curve (M-P)splice-

brittle = Bending moment –axial load interaction curve for column splice based on governing brittle failure mode

(M-P)splice-

ductile = Bending moment –axial load interaction curve for column splice based on governing ductile failure mode

po = Ambient atmospheric pressure pr = Average peak reflected pressure


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prα = Peak value of positive reflected pressure pso = Value of incident pressure psi = Pounds per square inches P = Applied axial load Pn = Nominal buckling capacity of column Pr = Peak value of positive normal reflected pressure Pro = Peak value of positive reflected pressure Pro(-) = Peak value of negative reflected pressure Pso = Peak value of positive incident pressure Pso(-) = Peak value of negative incident pressure (partial vacum) q = Strain hardening parameter RG = Ground distance from the center of a blast to a surface r = radius of gyration of a cross section t = (1) Time

(2) Thickness of element tA = Time of arrival of blast wave (ms) to = Duration of positive phase of blast pressure (ms) Ta = Arrival time for the air pressure wave To = Duration of initial positive pressure To(-) = Duration of negative pressure Ts = The time that peak incident pressure decays to the ambient air pressure Ts(-) = The time that incident pressure is negative (partial vacum). U = Shock front velocity (ft/ms) V = Applied shear force to the wall Vmax = Maximum shear capacity of the wall W = Weight of the design charge (explosive) in equivalent TNT weight ZG = Scaled ground distance for blast =RG / W 1/3 α = (1) A parameter used to establish properties of concrete = 1 / (5 + 9 f’ c / fco)

(2) Angle of incidence of the pressure front (degrees) β = A parameter for concrete high strain behavior δ = a parameter for concrete tensile strength under high strain rate

∆ = Lateral displacement (drift) εy = Static yield strain of steel = Fy / E

εu = Static ultimate strain of steel = % of elongation

εyd = Dynamic yield strain of steel under high stain rate

εud = Dynamic ultimate strain of steel under high strain rate

εcy = “Nominal” yield strain of concrete in compression

εcu = Ultimate strain of concrete in compression

εty = Yield strain of concrete in tension

ε tu = Ultimate strain of concrete in tension ε& = Strain rate

sε& = Static strain rate (= 10-3 in/in/sec)

φw = Reduction factor for weld

µ = A parameter for concrete high strain behavior


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1. Introduction to Blast- Resistant Design 1.1. Introduction

Since the 1980’s terrorists have used car bombs to attack buildings throughout the world causing tragic consequences, loss of lives and injuries to thousands of people. Figure 1.1 shows the three important measures taken to protect buildings against car bomb attacks. The first and most effective measure is to gather intelligence on terrorist activities in an effort to find out about those activities in advance and prevent their occurrences. The second step is to provide physical barriers and standoff distances around the buildings such that car bombs cannot be detonated close to buildings. The third line of defense is to harden the building. The hardening, which is done through blast-resistant design, should be done such that if the first and second steps fail and the car bomb explodes close to the building casualties are prevented and injuries to people and damage to the building are minimal with no progressive collapse. Past experience with car bomb attacks on building indicate that if progressive collapse occurs it can cause very high number of casualties and injuries. The third measure is the responsibility of the engineer designing the blast resistance of the building and this report is prepared to be of some help to that end.

Structural engineers and other design professionals are often asked what can be done to protect structures against attacks such as the 2001 airplane attacks on the World Trade Center and the Pentagon buildings or the 1995 car bomb attack on the Murrah Federal Building in Oklahoma City. Given today’s technological advances and the cost associated with implementing various protective technologies, a rational approach in protecting buildings is:

Figure 1.1. Three Steps in Preventing Terrorist Car Bomb Damage to Buildings

(Graphics courtesy of Dr. David McCallen of LLNL)


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(a) To minimize local damage and;

(b) To prevent progressive and catastrophic collapse

A terrorist attack applying ground-based explosions, such as car bombs, or using a flying object, such as airplanes or rockets, can result in:

1. Serious but very localized damage to a few columns and beams in the vicinity of the impacted zone, and;

2. Progressive collapse initiated by the local failures which spreads in a domino effect resulting in the collapse of large portions or even the entire structure.

In the case of critical buildings, such as nuclear power plants and military command and control centers, even a local damage to a small portion of the building can have catastrophic consequences. Therefore, for those structures even a local damage due to terrorist or other attacks using explosives is not tolerated. Progressive collapse of the U.S. buildings either abroad or at home have resulted in thousands of deaths in military barracks in Beirut in the 1980’s, in embassy buildings in Africa and in Murrah Federal building in the 1990’s, in the World Trade Center and Pentagon in 2001 and various civilian buildings throughout the world. In other less sensitive buildings, such as office or residential buildings, assuming a low probability of a terrorist attack, currently the protection measures are not so unanimously accepted.

The author proposes the following criteria for protection of buildings against terrorist attacks:

1. For the critical buildings, the local damage should be minimized and kept at non-consequential and easily repairable level and any type of collapse, partial or full, should be prevented.

2. For the ordinary buildings, assuming a lower probability of terrorist attacks, some local damage and minor personal injuries may be unavoidable under the existing measures. However, a partial or full progressive collapse of all buildings should be prevented since such collapses may result in the loss of lives of many people (168 deaths in Murrah Building in Oklahoma City, 1995) (2,973 in the World Trade Center and Pentagon, 2001).

3. For both critical and ordinary buildings, the possibility of flying debris, such as glass shards and pieces of concrete or masonry, entering the building and causing injuries should be minimized or eliminated altogether.

Regarding the military facilities, the “Unified Facilities Criteria” published by the Department of Defense (UFC/DoD, 2002), states that the DoD “…has significant commitment to seek effective ways to minimize the likelihood of mass casualties from terrorist attacks against DoD personnel in the buildings in which they work and live”. The document also emphasizes high level of protection and prevention of progressive collapse for critical buildings with large number of occupants.


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1.2. Scope of the Report Building damages caused by a blast are divided into local failure (direct blast effects)

during explosion, and progressive partial or full collapse (consequential effects) under gravity load effects (Lee, Kim and Kim, 2009). It is important that in a blast resistant design both of these phases be considered.

A well-designed structure to resist blast effects is the one that:

a. During the blast phase and under highly dynamic blast pressures sustains a relatively minor, predictable and repairable local damage contained locally with no initiation of a progressive collapse.

b. During the blast, the flying debris resulting from the fracture of structural or more importantly non-structural elements, such as facade and window glass, do not separate from the building turning into flying debris penetrating the occupied space and injuring or killing the occupants or those outside the building.

c. After the blast phase ends the damaged structure still has the capacity to resist the gravity load and does not collapse in a progressive manner.

This report focuses on Item (a) of the above with regard to steel and composite structures and hopes to provide some useful information to structural engineers designing blast resistant steel and composite (steel and concrete) structures. The subject of protection of non-structural elements against blast, item (b) above and the progressive collapse prevention, item (c) above are beyond the scope of the report and the reader is referred to the literature and references listed in the Bibliography. On progressive collapse prevention of steel braced frames, the reader might find the information in another Steel TIPS by the author (Astaneh-Asl, 2007) and its Bibliography and references somewhat useful

Peter J. Rhodes (1974) in his classic paper on blast protection of buildings divided the building types into three categories of (1) Wholly Compressive, (2) Intermediate, and, (3) Tensile. He defined Wholly Compressive buildings as “The main body of the structure is of masonry or brickwork, all designed to use only compressive stresses” (Rhodes, 1974). In Intermediate buildings: “Some tensile material is provided by cast iron, or by the early use of steel plate in combination with other materials such as timber” (Rhodes, 1974). The report here does not discuss those two types of buildings. Our focus here is on the third type that Rhodes (1974) has called “Tensile” buildings and has defined as buildings that:

“Large portions, if not the whole structure, are supported by tensile material, either rolled steel or reinforced concrete. In modern buildings, with skeletons of either rolled steel or reinforced concrete, almost every single member (including columns) is designed using tensile stresses. Another property or virtue that comes with the use of steel, either as rolled sections or as reinforcement in concrete, is that the skeletal structure can be coherent. To a greater or lesser extent, the whole frame can be monolithic and it has been seen that this is probably the most important factor in


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allowing the structure to withstand bomb damage and still be repairable (Rhodes, 1974).

As mentioned earlier the author's focus in the remainder of this report is on the blast resistance and blast protection of steel and composite structures. I have put an emphasis on what Peter J. Rhodes in the above excerpt has called the virtue of steel which makes the structure, even a structure made of concrete, which is an inherently brittle material, coherent and renders the whole frame monolithic which he felt probably was the most important factor in resisting the blast and making the blast damage repairable. Today, what Rhodes called “coherent and monolithic frames”, probably are interpreted as ductile frames with a well-defined and continuous load path which include redundant elements.

Therefore, emphasis here is further narrowed down to provide notes on how to design new steel and composite structures or retrofit the existing ones economically, to:

a. make the structures ductile

b. have a well defined and continuous load path

c. limit the damage to members and connections to minor, predictable and easily repairable ones

d. ensure that the local damage sustained during the blast does not trigger a partial or full progressive collapse of the structure during the blast or afterwards under the gravity load effects

1.3. Main Sources of Information on Blast Resistant Design

In recent years, the U.S. Federal government has increasingly focused on protecting buildings and their occupants against terrorist attacks and has funded research and development projects to address the problem. These efforts are aimed at the development of protective measures to prevent casualties and serious injuries and to reduce the damage in the event of terrorist car bomb attack on a building. Major efforts are made by the federal government agencies and professional organizations such as the General Services Administration, the Department of Defense and the American Society of Civil Engineers to develop and release guidelines and information on blast protection of buildings.

An important document used for past several decades in blast resistant design of buildings and other facilities is the manual “TM5-1300, Structures to Resist the Effects of Accidental Explosions” (Army, 1990), developed by the Department of Defense in the late 1960’s. The initial version primarily had information on reinforced concrete. The 1990 version of the TM5-1300 (Army, 1990), in addition to concrete, included some information on steel as well but not much information on composite structures. As shown later in Chapter 4, structures with composite members are better suited to resist blast than both steel and reinforced concrete structures.

Until December 2008, when the Department of Defense released the Unified Force Criteria: “UFC 3-340-02 Manual (UFC/DoD, 2008), Structures to Resist the Effects of


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Accidental Explosions” the TM5-1300 was the main reference for almost all aspects of the blast resistant design. The TM5-1300 was used extensively by the military and civilian engineers for blast resistant design. The importance of UFC 3-340-02 Manual and its TM5-1300 predecessor is not only due to its extensive information on various aspects of the blast resistant design, but also because of the fact that the information is based on the actual blast tests. The UFC 3-340-02 Manual states that:

“The design techniques set forth in this manual are based upon the results of numerous full and small-scale structural response and explosive effects tests of various materials conducted in conjunction with the development of this manual and/or related projects.”

Upon its release, the 2008 UFC 3-340-02 Manual (UFC/DoD, 2008), which in most part is the same as the 1990 approved version of the TM5-1300 manual with only minor revisions, superseded the TM5-1300. In this report the author refers to the UFC 3-340-02 (UFC/DoD, 2008) instead of the more familiar TM5-1300 (Army, 1990). A PDF copy of the UFC 3-340-02 Manual may be downloaded from http://dod.wbdg.org/.

The UFC 3-340-02/2008 Manual provides extensive information and numerical examples on how to establish the blast loads and the response of building structures to such highly dynamic loads, the material behavior under high strain rate and how to establish the capacity of steel, reinforced concrete and other structures subjected to blast. In addition, UFC 3-340-02/2008 has extensive information on blast resistant design of non-structural elements including facades, doors, and windows.

The UFC 3-340-02 (UFC/DoD, 2008) manual has separate chapters on reinforced concrete and steel designs. However, this latest version of the manual still does not have much information on blast resistant design of steel-concrete composite structures other than some information on composite beams. Chapter 5 of the UFC Manual has valuable information on the behavior of steel as a material under high strain rates such as blast loads, design of beams and beam-columns, and cold-formed sections. Here in the report the author will refer to some of those items and add notes whenever appropriate.

1.4. Blast Effects Leading to Progressive Collapse

When a blast occurs outside a structure, the shock waves of the blast reaching the structure cause damage in structural and non-structural elements directly exposed to those shock waves. If the local damage to the structure is such that the damaged members are no longer able to carry their gravity load, those loads have to be transferred to the adjacent members to be carried to the rest of the structure, the foundations, and eventually to the ground. Otherwise the structure or a significant portion of it may progressively collapse. The progressive collapse may occur during the explosion, which usually lasts a fraction of a second or after the explosion has ended and the damaged structure is primarily under the effects of gravity and possibly wind. Therefore severe local damage and loss of a member or a critical connection can initiate a progressive collapse of large portions or the whole structure disproportionate to the local damage.


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This type of serious damage to a few local members or connection leading to disproportionate progressive collapse has occurred in the past when car bombs were placed close to the building columns. Tragic examples include the truck bombings of the 1983 U. S. Marine Corp Barracks, Beirut, Lebanon (298 fatalities) and the 1995 Murrah Federal Building, Oklahoma City (168 fatalities). Many more such tragic terrorist attacks have occurred throughout the world. A failed case of terrorist attack with the intention of causing this type of progressive collapse was the 1993 truck bomb attack on the north tower of the World Trade Center. A brief summary of this case is given later in Chapter 4.

In addition to a local damage initiating a progressive collapse, it is also possible that the structure as a whole is not capable of resisting the lateral dynamic pressure of the blast and therefore collapses. For this to happen the charge causing the blast has to be quite large and relatively far away from the structure to create lateral forces greater than the capacity of the lateral force resisting system which is generally designed to resist wind or seismic loads. This is not very common in terrorist car bomb attacks

1.5. Preventing Progressive Collapse by Reducing Bl ast Damage to the Structure

Currently it seems that doing a blast resistant design and designing structures not to progressively collapse are becoming two separate fields. There are separate standards of Unified Force Criteria developed by the Department of Defense for blast resistant design (UFC/DoD, 2008) and progressive collapse prevention (UFC/DoD, 2008). This approach might have practical reasons making it possible for many engineers to be involved in evaluating the possibility of progressive collapse or, designing progressive collapse prevention measures into their structures without getting involved in more complex blast resistant design. In most cases, the complex blast resistant design requires advanced level of understanding of the dynamics of shock and vibrations as well as the behavior of material under very high strain rates. The approach also might be appropriate for reinforced concrete structures where there is a relatively limited interaction among internal stresses. For example, in a reinforced concrete beam, the longitudinal bars carry tension, the ties carry balk of the shear and the concrete and compression rebars carry the compression resulting from bending. However, in steel structures, several elements of the cross section share carrying the internal stresses. In addition, in reinforced concrete structures the connections are not separate or relatively complex entities as they are in steel and composite structures.

In designing economical progressive collapse prevention measures, it might be prudent to spend some effort to design the exposed members and connections such that the damage to them during the blast is minor and non-consequential. By doing so, we might be able to stop the initiation of a progressive collapse at its local source. This is especially important for bare steel structures where, in many cases of low and mid-rise building and due to relatively small size of steel columns, under even a mid size car bomb blast the steel column may be lost. As shown later in Chapter 4, the bare steel wide flanges, if relatively thin, are susceptible to flange folding and web dishing due to very high blast pressures. When the flanges of a column fold or the web dishes out, the column no longer has the original radius of gyration and it buckles loosing strength to


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carry its gravity load. The column losing its strength sheds its gravity load to the neighboring column with a potential to trigger a progressive collapse.

1.6. Hardening of Steel Structures to Minimize Dama ge

Hardening of steel structure is done in many cases quite efficiently and economically, to limit the initial damage due to blast to a minimum and to a structurally non-consequential level. In most cases, the hardening is only needed on those components of structure that are directly subjected to dynamic blast effects during the blast pressure and rebound period. These in most cases are exterior beams and columns of the ground floor and a few floors above it as well as the floor deck-slab of the floors. The remaining parts of this report provide more information on the response of bare steel and composite components of steel and composite structures to blast and give some suggestions on how they are hardened economically, efficiently and in an easy-to-design and easy-to-fabricate manner.


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2. Blast Effects on Humans and Structures

2.1. Blast Effects on Humans

The most important consideration in design of buildings to resist blast is the safety and protection of the occupants inside and people outside the building. Some may comment that the engineering profession focuses too much on the structural elements and not so much on the inhabitants of those structures. However, if we design the structures according to the established rules of design for gravity, wind, seismic, snow and so on without allowing for unacceptable risk or damage, the occupants will enjoy the expected safety and protection.

A total protection of a building designed to resist the blast may not be feasible due to extreme dynamic forces of the blast. Collateral damages to humans are possible even though a structure escapes the collapse or damage itself. Injuries sustained due to high dynamic pressures, flying debris, or occupants hitting the walls and so on are all quite possible and we have seen those in the past car bomb attacks even though the structure may have remained rather intact. The UFC 3-340-02 Manual (UFC/DoD, 2008), formerly TM5-1300, has a section on Human tolerance of blast effects. It provides air pressures that can cause internal injuries or death. The UFC Manual gives five pounds per square inch (psi) air pressures as the threshold of eardrum rupture and 30-40 psi as the threshold of severe lung-hemorrhage while the threshold of lethality due to lung damage is given approximately at 100-120 psi.

The UFC Manual also lays out the effects of structural impact and structural motions (acceleration, velocity and displacement) on humans. It states that it is necessary that human tolerance to two types of shock exposure be considered: (1) Impacts causing body acceleration/deceleration, and (2) Body vibration as a result of the vibratory motion of the structure” (UFC/DoD, 2008). The UFC Manual states that the probable safe impact tolerance velocity is 10 ft/sec and the probable safe horizontal acceleration 0.5g. For more detailed information see the UFC Manual.

Very high velocity fragments and flying debris resulting from blasts may hit humans causing severe injuries and death. In blast resistant design, the designers do not have much control on the primary fragments, which are released by explosion itself or by the container, the car and equipment in a very close vicinity of the explosive device. However, it is important to prevent injuries and minimize damage due to the secondary fragments that are separated from the building by the impact of the blast and are high velocity sharp objects entering the building and injuring the occupants.


16

Spalling of the concrete cover over the reinforcement in members exposed directly to blast pressures has been a source of serious injuries. Steel structures have a great advantage over reinforced concrete structures since they have relatively less fragmented parts. In composite structures also, as we will discuss later in Chapter 4, one can prevent the concrete part of the component from separating and flying away. For example instead of using encased composite columns one can use concrete filled tube or box columns and have the concrete inside instead of outside exposed to blast.

However, the connections of steel and composite structures quite often have relatively small pieces of steel plates and angles as well as in many cases bolts and rivets (in some existing pre 1950’s steel structures). The possibility of these relatively small connection elements becoming fragments during the blast entering the building with high velocity and causing injuries to occupants need to be considered at the design stage. Tightened bolts are of some concern since they store considerable elastic energy during the pre-tensioning phase and release of such energy by blast impact can make them even more forceful. The UFC 3-340-02 Manual (UFC/DoD, 2008) provides information on the threshold of what weight of fragment with certain velocity may cause serious human injuries.

Later in Chapter 4 under Connections and Bolts some suggestions are given on how to prevent elements of steel connection, including bolts and rivets, from becoming potentially dangerous fragments causing injury or death to occupants of structures as well as those outside the building. The solutions in general are either to over-design the components of a connection, such as bolts, that are thought to fracture and become separated from the structure, such that they remain elastic during the blast and/or to protect them from flying away in case they fracture by some measure such as encasing the connection in concrete or steel jackets.

2.2. Stand-off Distance and Barriers

As was shown in Figure 1.1 (Chapter 1), one of the important steps in blast protection of buildings is to keep strong blast shock waves of a car bomb blast from reaching the building. This can be done by preventing the car bombs or other relatively large amount of explosives from being placed close to the building. As discussed later in this chapter, the blast pressure applied to the buildings is inversely related to the distance from the explosive to the building. The distance called standoff distance is recommended by the UFC 3-340-02 Manual (UFC/DoD, 2008) to be as shown in Figure 2.1. Of course in the case of many civilian buildings in urban settings the standoff distance is only the width of the sidewalk curbs. To ensure that the car containing the bomb cannot drive over the sidewalk and close to the building, car barriers, such as those shown in Figure 2.2 are placed on the sidewalk. These barriers are designed for car impact. There are more sophisticated and non-intrusive barriers to stop unauthorized cars from getting too close to a building. An example of such barriers developed and tested at the Lawrence Livermore National Laboratory (LLNL) is shown in Figure 2.3.

The low-cost yet very efficient LLNL system consisted of 20-ft. sections of 22-inch diameter steel pipes about ¼ inch thick arranged in a U-shape pattern. There was a 1-inch diameter steel cable inside the pipes with cable attached to anchor blocks on either side of the roadway. According to the Popular Mechanics news story, the low-tech steel barrier was able to


17

stop a remotely controlled pickup truck with 400 pounds of sand, simulating car bomb weight, driven into the steel barrier at 36 mph. The truck was literally destroyed while the steel barrier was undamaged and could be pushed back and positioned at original U-shape position.

Placing protective walls in front of the building is another form of reducing the effect of the blast and deflecting the waves before they reach the building. Chapter 4 of this report has notes and some suggestions on design and use of steel and composite blast protection walls.

Figure 2.1. Stand-off Distances Given by the UFC 3-340-02 Manual

(UFC/DoD, 2008, Formerly TM5-1300)

Figure 2.3. Cable-Based Car-stopper Developed and Tested by the Lawrence Livermore National Laboratory

Figure 2.2. Barriers Constructed on the Side Walk after the 1993 Terrorist Car Bomb Attack on the World Trade Center


18

2.3. Safe Areas within the Buildings

Safe areas within the buildings are those areas that have protection against physical (blast and impact), fire and chemical, and biological attacks. These areas are usually located in the interior of the building and away from the streets around the buildings. A safe area in the form of a room has strong walls to withstand physical attacks and has sealed spaces to prevent penetration of hazardous gases as well as biological agents into the space. Although reinforced concrete walls for the safe rooms can be designed to withstand physical attacks such as blast or impact, reliability of reinforced concrete walls in preventing penetration of gasses and biological agents is questionable since concrete can sustain cracks under the blast pressure and impact.

Steel and especially composite shear walls are two reliable wall systems to create safe room within the buildings against physical, chemical and biological attacks. By welding the steel plate of steel or composite shear walls to the steel columns and beams a “sealed safe room is created. More information on blast resistance of steel and composite shear walls is given later in Chapter 4.

2.4. Blast Effects on Structures

Buildings may be subjected to external or internal blasts depending on the location of the explosion being outside or inside the building. The external blasts may be aerial or ground blasts. We focus here on the ground external blasts where a blast occurs near the building at the ground level such as the case of a car bomb detonated outside a building.

A blast or an explosion is the process of a rapid energy release, which generates a blast wave (Baker et al., 1983). When a ground blast occurs outside a building, the high-pressure shock waves released from the high explosive material travel through the air and affect the exterior of the building. In addition, shock waves transferred to the ground, travel through the ground and affect the basement retaining walls as well as underground foundations. Figure 2.4 shows von Mises stresses in the ground as well as the building created by a ground explosion.

The shock waves generated at the center of the detonation and emanating out into the air can have velocities in the range of 15,000 to 19,000 miles/hr (22,000-28,000 ft/sec) and pressures of front waves in the range of 27,000-49,000 ksi). The temperature at detonation can range from 5,000oF-6500oF. Obviously, no construction material, even high strength steel can withstand such high pressures and temperatures. In addition, even a small fragment with such high velocity can cause serious damage to structural components and create high velocity secondary fragment such as flying concrete, steel, glass, or other construction material fragments. More importantly, the primary and secondary fragments flying with high velocity can enter the building and cause serious injuries or death. Although the pressures and detonations at the detonation center are extremely high, they drop relatively rapidly as the shockwaves move away from the center of blast. Therefore the first measure of protection of buildings against blast is to keep the blast center away from the building at a certain standoff distance. Glasstone and Dolan (1977) and Sartori (1983) conducted studies to assess the effects of high air pressures on humans and various construction materials. Their studies were used by Zipf and Cashdollar (2008) to form the following Table 2.1.


19

Figure 2.4 Von Mises Strains due to Shock Waves Traveling Through the Ground Reaching the Base of the Building (Analysis and graphics courtesy of Dr. David McCallen , LLNL)

.

Figure 2.5 shows pressure exerted on vertical rigid walls located at various distances from the center of a 5000 pounds TNT blast. If the center of the explosive charge is close to the building, the blast pressures are very high though they act over a relatively small area. On the other hand, if the building is far away from the center of the charge, the blast pressures will be relatively small but act on a larger area of the facade.

Table 2.1 – The Effect of Blast Overpressures on Various Structures and the Human Body (Table From: Zipf and Cashdollar, 2008)


20

Figure 2.6 shows a comparison of typical seismic, wind and blast design loads acting on a building. Both seismic and wind loads are generally global loads that act on the entire structure and mobilize it, although in the case of wind loads most design codes have provisions for the local effects of wind. For blast design loads, in general, both local and global effects are present as shown in Figure 2.6(c) with the local effect of blast being the most damaging and forceful in most car bomb attacks on the buildings.

Figure 2.7 shows the Khobar Towers, where a truck bomb attack in 1996 resulted in 19 U.S. Military personnel deaths and more than 500 injuries. The damage to the facade was extensive but since the car bomb exploded far from the building the structure did not sustain a progressive collapse. The structure of the 8-story Khobar Towers was precast reinforced concrete with bearing walls and floors.

\

Figure 2.5. Blast Pressures of a 5,000 lb TNT Blast on a Building Facade as a Function of Stand-off Distance

(Data courtesy of Dr. David McCallen, LLNL)

Figure 2.6. Approximate Representation of Seismic, Wind and Blast

Design Loads Acting on a Typical Building


21

A study by Engineering Analysis Inc. (EAI, 2003) simulated the effects of blast on Khobar

Tower. Figure 2.8 shows overpressure contours for the blast wave as they propagate from the truck bomb and move towards the Khobar Tower 131, which was the building closest to the explosion. As can be seen in Figures 2.8(b) and (c), because of the relatively long distance from the center of the blast to the building, the high overpressures of 200 and 100 psi never reached the building facade. Figure 2.8(d) indicates that the 50 psi pressure contour affected the left half of the tower over about half of the height from the ground. It is interesting to note that the 10 psi contour line affected the entire area of the building facade. As indicated earlier if the standoff distance is relatively large, as was the case here, the local over-pressure effect is small but the entire surface of the building can be subjected to blast pressures similar to a case of wind pressure. However, even in this case, with a stand-off distance of about 105 feet, Figure 2.7 (bottom-left) still the overpressure on the building facade was between 10 to 50 psi (1440 to 7200 psf). One can compare this number to wind pressures that are about 20 to 40 psf. The blast pressures are more than 600 times larger than the wind pressures!

Figure 2.7. Khobar Tower Before ( top-left) and After 1996 Truck Bomb Attack


22

Figures 2.9 shows other cases of the car bomb attacks on buildings where a partial or total progressive collapse of the structures ensued.

Figure 2.8. Three Dimensional Over-pressure Contours as the Blast Wave Moves Towards the Khobar Towers and Hits Them

(Analysis and graphics from a study done by Engineering Analysis Inc. (EAI, 2003))


23

Figure 2.9. Important Cases of Past Car Bomb Attacks on Buildings, in Most Cases Resulting in Partial or Complete Progressive Collapse.


24

Figure 2.10 shows the two major cases of car bomb attacks in the United States. The 1995 attack on Murrah Federal Building in Oklahoma City, a reinforced concrete building, resulted in partial progressive collapse of the building. In the case of the 1993 car bomb attack on the World Trade Center, although the damage to the floors was extensive, no progressive collapse ensued. More information on this case is given in Chapter 4.

To protect a building and especially its occupants against blast effects, structural engineers and designers of building protection systems need to consider all of the above effects. In particular, the effects of blast pressure and the impact of projectiles from either the explosive device or the flying debris separated from the building need to be addressed. The effect of ground shaking due to blast can also be important if the blast occurs near the building especially if the building has basements and retaining walls located near the blast.

Figure 2.10. Important Cases of Past Car Bomb Attacks on Buildings in the United States


25

2.5. Shock Wave Created by Blast in the Air

When condensed high explosive charges such as TNT detonate, the ensuing detonation waves turn into blast waves causing pressure pulse in the air. The dynamic pressure wave is very complex depending on parameters such as the shape and chemical composition of the charge, the characteristics of the container, the distance from the charge as well as the properties of the ground under the charge. The presence of other structures and obstacles also affect the free field blast waves due to refraction and reflection of waves on those structures. Despite the complexity of those parameters in blast-resistant design of ordinary buildings subjected to external car bomb blasts, it is assumed that the blast occurs in a still, homogeneous atmosphere and the source is semi-spherically symmetric charge placed on the surface of the ground. As a result the magnitude, wave length and velocity of the free-field pressure waves are assumed dependent only on the distance of the surface from the center of the charge, RG, and the weight of detonated charge, W.

In the equations and graphs used to establish blast pressure, the two parameters; W and RG are combined and defined as Scaled Ground Distance, ZG given by the following equation;

3/1/WRZ GG = (2.1)

Figure 2.11 shows time history of a typical free field blast pressure. The free field pressure is also called incident pressure. The most important parameter is the peak incident pressure Pso, which is the maximum pressure at the time of arrival of the pressure wave. As the time passes the incident pressure drops and after time To, becomes a negative pressure. For definition of terms, see “Notations” on Page 6.

Figure 2.12 shows the time histories of incident pressure for four values of TNT equivalent blast charges (Son and Astaneh-Asl, 2008). For all cases shown in this figure, the ground distance, RG , from the point where the incident pressure was measured to the center of blast, was 185 inches. As the figure shows, the peak incident pressure drops rapidly with the drop in the weight of the detonated charge W.

Figure 2.11. Time-History of Incident Pressure for Typical Blast Wave

Pso

po Pso(-)

Time, t Ta To To(-)

Negative pressure phase

Positive pressure phase

Inci

dent

Pre

ssur

e,

pso

, (ps

i)

Ambient air pressure


26

Figure 2.13 shows the time histories of incident pressure for a 100 pounds TNT equivalent blast charge placed at four different ground distances RG (Son and Astaneh-Asl, 2008). The figure shows that as the ground distance RG (i.e. the stand-off distance) increases the peak incident pressure for the same amount of blast charge drops. Figures 2.12 and 2.13 show the effects of two main parameters, the weight of the detonated charge W, and the ground distance RG on the peak incident pressure, Pso.

As mentioned earlier, since W and RG are the two primary parameters considered in establishing the incident peak pressure, quite often the combined parameter ZG=RG / W

1/3 is used to develop graphs used in establishing the peak incident pressure and other blast parameters such as impulse and peak reflected pressure as discussed below. Figure 2.14 shows an example of such graph from UFC 3-340-02 Manual (UFC/DoD, 2008). Notice that the horizontal axis is parameter ZG, the input parameter, and the vertical axis gives values of parameters such as peak incident or peak reflected pressures used in blast resistant design.

Figure 2.12 Time Histories of Incident Pressure Profiles for Different Weights W of TNT Equivalent Charges (Son and Astaneh-Asl, 2008)

Figure 2.13. Time Histories of Incident Pressure for the Same Charge at Different Distances RG (Son and Astaneh-Asl, 2008)

50 inches from the blast center




Time (sec)

Note: Distance from the center of the charge to the surface was 185 inches for all cases.

0.002 0.004 0

0

Inci

dent

Pre

ssur

e,

pso

, (ps

i)

100

200

300

400

500

600

Inci

dent

Pre

ssur

e,

pso

, (ps

i)

50

100

150

200

250 500 lb TNT

50 lb TNT

100 lb TNT

200 lb TNT

Note: Distance from the center of the charge to the surface was 185 inches for all cases.

0.002

0

0 Time (sec) 0.004


27

When a blast pressure wave reaches a surface of a building as incident pressure, it exerts pressure on the building surface. Depending on the geometry, size, mass, stiffness, damping and other dynamic characteristics of the building the incident pressure will reflect from the surface or diffract around the surface. The blast resistant design assumes the simplest case of reflection, which is the normal reflection of wave on a rigid infinitely large wall from a plane shock wave.

Although distribution of the actual reflected pressure over the exterior surface of the structure is not uniform, in blast design, the blast pressure acting on the building or its components is assumed to be uniformly distributed over the surface as shown in Figure 2.15.

Figure 2.14 Positive Phase Shock Wave Parameters for Hemispherical TNT Explosion on the Surface at Sea Level

(Adapted from: UFC/DoD, 2008, Formerly TM5-1300) (For definition of terms in the above graph see “Notations” on Page 6.)

Scaled Distance, 3/1/WRZ GG =

A B C D E F G H

A B C D E F G H

LEGEND:


28

Figure 2.16 shows both the pressure profile for incident pressure and reflected overpressure for the case of pressure wave hitting a flat rigid surface perpendicular to the direction of wave propagation. In blast-resistant design the reflected overpressure is applied to the building and its components. Blast resistant design manuals such as UFC 3-340-02 (UFC/DoD, 2008) and its predecessor TM5-1300 (Army, 1990) provide procedures on how to establish the reflected overpressure to be used in design.

Figure 2.14 from UFC 3-340-02 Manual (UFC/DoD, 2008) given before is a valuable tool to use when calculating the blast parameters, such as peak incident pressure, Pso, and reflected overpressure, Pro , by entering the scaled distance ZG=RG / W 1/3 in the horizontal axis and reading the value of the parameters on the vertical axis. Other parameters of blast such as ir (unit positive normal reflected impulse), is (unit positive incident impulse), tA, (time of arrival of blast wave), to (duration of positive phase of blast pressure), U (shock front velocity), and Lw (wavelength of positive pressure phase) can be established using the chart given in Figure 2.14 from the UFC/DoD Manual. The units for each parameter will be as indicated in the legend box for curves.

Figure 2.15. Incident Pressure Waves Affecting a Rigid Surface Perpendicular to the Wave Propagation Line and Uniform Pressure Assumed in Design

Incident Wave

5000 lb TNT

Figure 2.16. Typical Incident and Reflected Pressure-Time Profile for Blast Wave Affecting a Rigid Flat Surface

Pressure, pr

Pso

po

Pso(-)

Time, t Ta To

To(-)

Pr(-)

Pro

Reflected Pressure

Incident Pressure


29

The reflected overpressure is dependent on the angle of the direction from the center of the explosive to the structure. Figure 2.17 from the UFC 3-340-02 Manual (UFC/DoD, 2008) and TM5-1300 (Army, 1990) shows variation of the blast wave reflection coefficient , Crα , versus angle of incident for various values of peak incident pressure Po delivered to the surface by the air.

Value of reflected overpressure with an angle of incident α can be calculated from the following equation:

sorr pCp αα = (2.2)

where pso is the incident pressure obtained from Figure 2.14 and Crα is obtained from the chart in Figure 2.16 below, both charts are adapted from the UFC 3-340-02 Manual (UFC/DoD, 2008).

2.6. Blast Impulse

Impulse is the change in momentum and results from force acting over a period. Momentum is mass times velocity of a moving body. Impulse of a blast wave on a rigid surface is the area under the pressure-time curve. Damage done to a building due to blast is a function of pressure and impulse. In general, if the duration of the incident pressure is short relative to the natural periods of the component or the structure, then the response is primarily due to impulse. This occurs in high-pressure blast cases where a large amount of explosive is detonated close to the structure. In low-pressure detonations, where the amount of explosive is small or the distance from the blast to the building is large, the blast pressure will be the primary cause of damage. The impulse can be established by integrating the pressure-time curves. Figure 2.14 from UFC 3-340-02 Manual (UFC/DoD, 2008), given earlier, can be used to calculate the impulse values for a given scaled distance, ZG=RG / W

1/3.

Fig. 2.16. Reflected Pressure Coefficient versus Angle of Incidence (UFC/DoD, 2008) (Note that the Peak Incident Overpressure numbers are added to the graph here for clarity.

The numbers were not on the original graph in the UFC/DoD Manual))

Peak Incident Overpressure= 5000 psi

3000

2000

1000

500 400

0.2 05 1 2 5 10

20 30

50 70

100 150

200 300


30

3. Material Properties

Under High Strain Rates of Blast 3.1. Blast Effects on Material Properties During blast, loading the rate of applied strain is very high. Table 3.1 shows strain rates related to various types of loading (Paik and Thayambolli, 2003). The table gives creep in concrete as an example of the slowest strain rate applied with a strain rate of less than 0.00001 1/sec while it gives bombing as an example of the fastest strain rate at 10,000 in/in/sec. The table gives the impact and blast strain rate as 32 in/in/sec (i.e. 101.5) to 10,000 (i.e. 104).

Table 3.1 Dynamic Modes of Loading versus the Strain Rate (Paik and Thayambolli, 2003) Dynamic loading mode

Static or Quasi-static

Dynamic Impact Hyper- velocity impact

Strain rate <10-5 10-5 ~ 10-1 10-1 ~ 101.5 101.5 ~104 > 104 Examples Constant

loading machine

Dead or live loading

Impulse pressure effects on high-speed craft, wave breaking loads

Explosion, vehicle collision

Bombing

Depending on the rate of applied strain, material properties of steel and concrete change. Generally, yield stress and ultimate strength of steel increases under high strain rate while its ultimate elongation decreases. Modulus of elasticity of steel under high strain rate remains almost the same. In concrete, both compressive and tensile strength increase under high strain rate and the tensile strength can increase as much as 6 to 8 times. Following sections provide more information on the properties of steel and concrete under high strain rate.

Figure 3.1 shows typical stress-strain curves for steel under static and dynamic high strain rate loading.

Figure 3.1. Static and Dynamic (high strain rate ) Stress-Strain Curves for Steel

Creep


31

Yu and Jones (1991) state that currently there are no extensive test results on material properties under high strain rate nor extensive test data on fracture strain under high strain rates associated with blast loading. Therefore, there is a certain degree of conservatism in blast design to compensate for the lack of extensive data on material properties under blast effects.

3.2. Mechanical Properties of Steel under Blast Loa d Figure 3.2 shows a stress-strain curve for mild steel under high strain rate from Yu and

Jones (1991). It is important to note the fact that under high strain rate the yield stress measured on the tested specimen by strain gages is about 30% higher than the average yield stress calculated by dividing the force measured by the load cell in the test machine divided by the cross sectional area of the specimen. The modulus of elasticity obtained for the steel was 208 GN/m2

(30167 ksi). Figure 3.3 shows stress-strain curves for mild steel under various strain rates. The steel had a chemical composition as follows:

Figure 3.2. Stress Strain Curve for Mild Steel under a Strain Rate of 16.7 sec-1

Obtained by Yu and Jones (1991)

Curve based on applied load measured by strain gages on the specimen

Curve based on applied load measured by the load cell in the test machine

0 0.05 0.10 0.15 Strain

Stress, ksi

Specimen (Dimensions are in mm)

mm

100 50 0

Figure 3.3. Stress-Strain Curves for Mild Steel under Various Strain Rates

(Information from Yu and Jones (1991)

0 0.05 0.10 0.15 0.20 0.25 Strain

Stress, ksi

Strain Rate=0.0012/sec. (Almost static strain rate)

Strain rate=0.012/sec



Strain rate=0.012/sec (very high strain rate)

Specimen (Dimensions are in mm)

mm

100 50 0


32

The substantial rise in the yield stress under high rate of strain may have important ramifications for steel structures especially when a ductile failure mode is designed to be the governing failure mode. A good example is a tension member where the designer makes sure that the capacity of the net section in fracture is greater than the capacity of the gross area in yielding, thus ensuring that ductile yielding failure mode will govern over relatively brittle fracture failure mode. This happens in seismic design of members of “special” lateral force resisting systems such as Special Concentrically Braced Frames used in seismic design. However, if under high strain rate during blast, the yield stress Fy goes up considerably while the ultimate strength Fu increases only a small amount, it is quite possible that the fracture of net area will govern over the ductile yielding of gross area. Therefore, it is important to use realistic properties of steel under high strain rate in blast resistant analysis and design. This consideration is incorporated into blast-resistant codes by introducing Dynamic Increase factor (DIF) to be multiplied by specified static yield stress and ultimate strength of material to obtain realistic properties for steel under high strain rate of blast.

A frequently used stress-strain curve for steel is the Cowper and Symonds (1957) empirical expression given as:

q

y

yd

CF

F /1

0.1

+= ε& (3.1)

where, Fyd is the dynamic yield stress, Fy is the static yield stress (i.e. the specified yield stress of steel), and is the strain rate. Parameter C is the viscosity factor and q is the strain rate hardening parameter. Values of C and q are to be established by tests. Yu and Jones (1991) found values of C=1.05x107/sec and q=8.30 to make the Cowper-Symond’s equation to best fit their test results. Table 3.2 shows values of C and q for structural steel suggested by various sources.

Table 3.2. Values of Parameters C and q in Cowper and Symonds (1957) Equation

Type of Steel Value of C Value of q Source of Data Mild

Carbon Steel 40.4 5 Cowper and Symond (1957)

High Strength

Alloy Steel 3200 5 Paik and Thayamballi, (2003)

Figure 3.4 from Son and Astaneh-Asl (2008) shows variation of yield stress versus strain rate for low and high strength steels using Equation 3.1 and values of C and q given in Table 3.2. As seen, the increase in yield stress of high strength steel is not as much as the one in mild low strength steel. In case of concrete, the compressive strength, f’ c increases under high strain rate and more importantly, the tensile strength of concrete, which under slow rate of strain is quite small and ignored, also increases up to 8 times under high strain rate (Crawford, 2000). In the following, changes in the properties of steel and concrete under high strain rate of blast are discussed in more detail.

As seen in Figure 3.5 below, assuming a reasonable range of 100-5000 in/in/sec for strain rate during blasts, the increase in yield stress can be 2.3-3.4 for mild carbon steel and 1.4-1.8 for high strength alloy steel. Depending on the design, an upper level or lower level of the yield


33

Mild Steel High Strength Steel Approximate Region of High Strain Rate For Blast Loading

Figure 3.5. Range of Increase in Yield Stress of Mild (Low) and High Strength Steels

stress may be used in blast design. For example if a strength design is being done on a steel tension member and the goal is to ensure that the yield failure of the gross area governs over the fracture failure of the net area, we need to use the upper value of the yield stress. However, if we are checking the strength of a member subjected to an actual blast pressure load, to be conservative, we need to use the lower level of yield stress.

Some design guides, such as the Army Manual (Army, 1990) provide a single value for dynamic properties of steel under high strain rates.

In modeling the behavior of steel in blast simulation analyses, there is a need for a simple yet reliable model of stress strain curve under high strain rate. The isotropic elastic-strain hardening (bi-linear) model, in Figure 3.6, is often used in inelastic dynamic analysis of structures subjected to blast. Notice that in the model the value of modulus of elasticity E is 29,000 ksi, the same as for static loading. However, the values of yield stress, Fyd, and ultimate strength Fud are

Figure 3.4. Variation of Yield Stress versus Strain Rate for Mild (Low) and High Strength Steels (Son and Astaneh-Asl, 2008)

5/1

32000.1

+= ε&

y

yd

F

F

5/1

04.40.1

+=ε&

y

yd

F

F


34

dynamic values under high strain rate discussed earlier. The value of ultimate strain εu under blast load depending on strain rate and type of steel is given as 0.07 to 0.23 in the literature. For structural steel and the range of strain rate corresponding to blast resistant design a value of 0.15 seems reasonable and recommended (Son and Astaneh-Asl, 2008).

The UFC 3-340-02 Manual (UFC/DoD, 2008) recommends the use of simple bilinear model of Figure 3.6, in typical blast resistant design of steel structures and more sophisticated models for cases that a considerable strain hardening is expected.

In our simulation analyses of blast effect on buildings and bridges, (Son and Astaneh-Asl, 2008) and (Rutner, Astaneh-Asl and Son, 2005a), we found that using a bi-linear strain hardening model was necessary to capture more realistic response since the strains during blast were well into the strain hardening range and quite often reached the ultimate strain when the component fractured. The issue of what should be considered as a realistic value for ultimate strain of steel under high strain rates is a difficult one to resolve since the experimental data on the ultimate strain of structural steel such as A36, A572, A992 and A913 under high strain rate is almost non-existent. A literature survey only revealed that the ultimate strain of steel under high strain rate either remains almost the same or decreases when the strain rate is very high. Based on such information we decided to use an ultimate strain of 0.12-0.15 for structural steel. The lower value was used for very high strain rate cases when a large amount of explosive was detonated relatively close to the surface of bare steel directly exposed to the incident pressure waves and the higher value of 0.15 was used for typical blast cases.

3.3. Mechanical Properties of Concrete under Blast Load High strain rates during blast affects the mechanical properties of concrete. Generally, the

compressive strength of concrete increases under high strain rates. Its tensile strength, normally ignored in design under low strain rate such as gravity, wind and seismic loads, also increases substantially under high strain rates. As discussed in previous section, the high strength steel

Figure 3.6. Suggested Stress Strain Curve for Behavior of Steel under High Strain Rate

Fud

Fyd

εεεεyd

εεεεud


35

shows less increase in mechanical properties under high strain rate than the low strength steel. However, in concrete the trend is the reverse. Mechanical properties of high strength concrete increase under high strain rate more than the properties of low strength concrete.

Such changes in the compressive and tensile strength of concrete should be modeled in inelastic time history analysis of concrete and steel-concrete composite structures subjected to high strain blast loads to obtain realistic response of concrete to blast. In addition, in dynamic analyses of structures under extreme but short duration dynamic load effects such as blast, which lasts only a fraction of a second, any mechanism that dissipate energy and provides stiffness to the system should be considered (Son and Astaneh-Asl, 2008).

The blast-resistant design should carefully consider the increase in compressive and especially increase in tensile strength of concrete. In the case of composite structures, when calculating the strength of a composite member or connection, to be conservative, the tensile strength of concrete is ignored. However, the increase in compressive strength may be included in calculation of strength as allowed by blast-resistant design guidelines such as UFC 3-340-02 the Manual (UFC/DoD, 2008). In calculating the stiffness of a composite member or connection, the contribution of concrete under compression as well as tension is considered.

The information on the change of stiffness of concrete under high strain rate is very limited. It seems that at this time, without reliable data, the stiffness of concrete under high strain dynamic load application may be taken as equal to the stiffness under static load, which is given by ACI-318 (2008) as:

(3.2)

To establish the increase in compressive and tensile strength of concrete under high strain rate several models are found in the literature. Malvar and Crawford (1998) after reviewing the test data on the behavior of concrete under high strain rate suggested the following two models for the behavior of concrete under compressive or tensile stress in terms of strain rate.

3.3.a. Increase in Compressive Strength of Concret e under High Strain Rate

Compressive strength of concrete under high strain rate increases significantly. Figure 3.7 from Hasan et al (2010) shows the stress strain curves for concrete subjected to static as well as high strain rates of 350, 500 and 700 per second. Blast resistant design documents give such increases as Dynamic Increase Factor (DIF) defined as the ratio of dynamic stress to static stress. For concrete the DIF=f’ cd/f’ c .

Figure 3.8 from 3-340-02 Manual (UFC/DoD, 2008) shows the variation of compressive strength of concrete with the ultimate strength f’c between 2,500 and 5,000 psi. The UFC Manual has a separate graph for DIF of concrete with f’c of 6000 psi.

It should be noted that under high strain rates, as discussed earlier, the yield stress of steel rebar increases. Figure 3.9 from UFC 3-340-02 Manual (UFC/DoD, 2008) shows the increase in yield stress of various grades of steel rebars used in reinforced concrete and composite members and connections of the buildings.

psi 000,57 'cc fE =


36

In analysis of concrete components of structures subjected to high strain rates of blast, quite often there is a need to use precise values for dynamic properties of material. The increase in compressive strength of concrete is given by the CEB Model Code by following the equations. A comparison of the available test data with the predictions of the equations by Malvar and Crawford (1998) indicated that the CEB Model Code equations below properly fit the test data.

Figure 3.8. Design Curve for DIF for Ultimate Compressive Strength of Concrete (Adapted

from: UFC/DoD, 2008)

Figure 3.9. Design Curve for DIF for Yield Stress of Reinforcing Steel Bars

(UFC/DoD, 2008)

Concrete Reinforcing Steel

Figure 3.7. Stress-Strain Curves for Concrete under High Strain Rate Compression (Data from: Hasan et al., 2010)

Stress, ksi

10 5

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 Strain

Strain Rates: 700 sec-1

500 sec-1 350 sec-1


37

(3.3a)

(3.3b) Where,

log µ = 6.156 α – 2 α = 1 / (5 + 9 f’ c / fco)

'cf = static compressive strength of concrete

cdf = dynamic compressive strength of concrete fco = 10 kPa (1450 psi) ε& = strain rate

sε& = static strain rate (= 10-3 in/in/sec)

Figure 3.10 from Malvar and Crawford (1998) shows the above equations along with the test results.

3.3.b. Increase in Tensile Strength of Concrete un der High Strain Rate

For tensile strength, the CEB-FIP model code provides the following equations for strain rate dependency. The equations are:

1

3/1

1

026.1

'

30secεfor

sec30for /

−

−

>

=

≤

=

&&

&

&&

&

s

sccd ff

εεµ

εεε

α

Figure 3.10. Strain Rate Dependency of the Concrete Compressive Strength (Malvar and Crawford, 1998)


38

(3.4a)

(3.4b)

where, tf = static tension strength

tdf = dynamic tension strength

log β = 7.11δ-2.33

δ = 1 / (10 + 6 f’ c / fco)

Malvar and Crawford (1998) suggested a set of modified equations based on the experimental test data. Their equations are:

(3.5a)

(3.5b) where,

βlog = 6δ -2 (3.6) δ = ( )coc ff /81/1 '+ (3.7)

Figure 3.11 shows the above equations from Son and Astaneh-Asl (2008) along with the CEB-FIP curves.

1

3/1

1

016.1

sec30for

sec30for /

−

−

>

=

≤

=

εεεβ

εεε

δ

&&

&

&&

&

s

sttd ff

1

3/1

1

sec1for

sec1for /

−

−

>

=

≤

=

εεεβ

εεε

δ

&&

&

&&

&

s

sttd ff

f’c=7,250 psi (from CEB)

f’c=7,250 psi (from: Malvar)

f’c=2,900 psi (from: Malvar)

f’c=2,900 psi (from: CEB)


39

Figure 3.12 shows typical stress-strain curve for concrete both in tension and in compression. After concrete reaches its maximum stress, its unloading branch shows a negative stiffness. In the finite element analysis software such negative stiffness can cause singularity point. To avoid that, the slope of unloading curve in the actual stress-strain curve is assumed zero in the model as shown in Figure 3.12 with the dashed line. The elastic modulus of concrete in the analysis can be assumed the same for static and dynamic strain rates although as Figure 3.7 shows, modulus of elasticity (slope of stress-strain curve) increases slightly for high strain rate applications. The dynamic compressive and tensile strengths of concrete in this simple analytical model, when used in blast analysis, would be dependent on the strain rates and can be established using Equation 3.3 and 3.4, presented earlier, respectively. For definition of terms see “Notations” on Page 6 of this report.

Figure 3.11. Strain Rate Dependency of the Concrete Tensile Strength (Malvar and Crawford, 1998)

psi cc fE 000,57=

psi 5.6 ct ff =

Figure 3.12. Realistic and Bi-Linear Model of Concrete Behavior for Blast Analysis (Son and Astaneh-Asl, 2008)

Bi-linear Model

Actual Behavior

cdf

tdf

psi


40

4. Blast Protection of Steel and Composite Structures

4.1. Introduction Information and data on the effects of blast on typical civilian buildings are not extensive.

Most data come from military sources as the military makes efforts to protect its installations against accidental or intentional blasts. Since the 1980’s, when the U.S. military installations overseas came under terrorist car bomb attacks more focused research and development on blast protection of typical non-military buildings have been undertaken. Still, comprehensive research and development in the area of protection of civilian buildings against car bomb and other types of blast terrorist attacks are not in place.

Most of the military facilities used as bunkers, shelters, ammunition centers or command and control facilities are reinforced concrete structures. As a result, most of the available information on the design of blast resistant structures is on reinforced concrete structures. Until 2008 the Technical Manual TM5-1300 (Army, 1990), the predecessor to the current UFC 3-340-02 Manual (UFC/DoD, 2008), was the leading manual in design of blast resistant structures. Until 1990, the TM5-1300 had information on blast resistant design of reinforced concrete structures and very limited information- if any- on steel and composite structures. The current UFC Manual (UFC/DoD, 2008) has almost no information on composite structures other than some information on composite beams. Therefore, information on the behavior of steel and composite structures subjected to blast and design procedures on blast resistant design of steel and composite structures and their components are limited. The actual test results on the behavior of steel and composite structures subjected to blast loads are also very narrow. A few available tests unfortunately have been performed by subjecting non-representative specimens to unrealistic blast loading conditions rendering the applicability of these results to blast-resistant design questionable. Part of the reason for the problem is the lack of any standard procedures for conducting blast tests. As a result, the designers of blast-resistant steel and composite structures need to rely largely on intuition and good structural engineering rather than design concepts and robust modeling techniques that would have been specifically developed for steel and composite structures based on actual behavior during realistic tests.

One precise approach to investigate the behavior of structures subjected to blast is blast simulation analysis using high-end and powerful software such as Dytran and Nastran (MSC, 2010) or other commercially available blast simulation software. Such analyses can generate quite useful information on the behavior of steel and composite structures without the very high cost of doing the actual blast tests.


41

In addition, it is imperative to apply structural engineering principles with good intuition in order to improve blast resistance of buildings. Some of the principles are given in the literature especially in the publications by the General Services Administration (GSA, 2003) and the Department of Defense (UFC/DoD, 2008) and (UFC/DoD, 2009). In the remainder of this report, the author will focus on items that are hoped to help the designers in design of safe and economical steel and composite structures to resist blast loads and prevent injuries and death.

4.2. BEHAVIOR AND BLAST DESIGN OF STEEL COMPONENTS

This section provides information and notes on the behavior and design of steel members and connections of buildings subjected to blast. Later sections will focus on composite members and connections.

Recently, Crawford and Magallanes of Karagozian & Case released a paper on their analysis of an 8-story steel structure (six stories above the ground with two basements) subjected to blast loads. The building was designed and built in the 1970’s presumably following the AISC Specifications of the time. Therefore, it is a reasonable representative of the steel structures today. Although the model did not include floor slabs, still there is some useful information in the paper (Crawford and Magallanes, 2009).

The paper in its summary states that “The analysis of the steel framed structure presented in Section 3.0 indicates that the frame is not likely to collapse as a result of the blast effects. Although the primary column was significantly damaged, the structural system was robust enough to resist a progressive collapse. It should be reiterated, that this model did not account for brittle fractures or failures in the connections which may preclude the ductile response computed here.” The important note on the conclusion is that the studies did not represent the actual behavior of the particular building under blast load since the brittle failure modes were not included in the study. The authors correctly have made it clear that one should not overlook the presence of brittle failure modes in the connections. However, the study is still very useful showing that if we prevent the brittle failure modes from becoming the governing failure modes and if we only have ductile yielding failure modes in our structure-which the researchers assumed in their model-the structure will survive the blast and will not progressively collapse.

The issue of avoiding the brittle failure modes is one of the most important concepts used in blast protection of structures and blast resistant design of structures. This is especially important for steel structures since steel as a material has abundant ductility and avoiding the occurrence of brittle fracture failure modes leads to a better and desirable performance of the structure subjected to blast effects.

In the remainder of this chapter, there is an emphasis on this point and suggestions on how to avoid brittle failure modes in blast resistant design of both steel and composite structures.

4.2.a. Steel Columns Subjected to Blast Effects

When blasts happen near a column, the blast subject the column to very large and short duration pressures. The lateral pressure acting on the column results in bending of the column


42

while the axial gravity load is still acting on it. Due to bending of the column and the resulting lateral deflections, which are generally large, the column also experiences considerable P-∆ effects due to presence of the axial load. Blast pressure waves acting on a steel column also create high strain effects on the material, generally raising the yield stress and ultimate strength but causing no change or slight reduction in the modulus of elasticity and ultimate strain. Because of the application of lateral blast load, even an axially loaded gravity column in a steel structure ends up being a beam column with relatively large bending moment and shear applied to it.

4.2.b. Blast Damage to the Columns of the World T rade Center Tower Caused by the 1993 Car Bomb Attack

Probably the most important case of a blast attack on a steel column was the February 26, 1993 terrorist truck bomb attack on the now destroyed north tower of the World Trade Center. Figure 4.1 shows the location of the car bomb used in the attack. The damage to the floors was extensive as shown in Figure 4.2 from USFA (1993). The main structural damage was the destruction of four floors of the parking structure located adjacent to the north tower. The car bomb was parked in front of the Column No. 324, which was a main perimeter column of the North Tower. In the parking structure due to the loss of floors and beams, 16 steel columns, including the columns at the north end of the Vista Hotel, were left un-braced over a number of floors (Ramabhushanam and Lynch, 1994).

Figure 4.1. World Trade Center and the Location of the 1993 Car Bomb

Explosion at the Basement B2

Graphics courtesy of Mark P. Thomas

N

Column 324

Vista Hotel

N

North Tower

South Tower

PLAN

North Tower

South Tower

Copyright © 2010 by Abolhassan Astaneh-Asl. All rights reserved.


43

The most important damage, as reported by Ramabhushanam and Lynch (1994) as well as USFA (1993) was the damage to the structural elements of the tower itself. Based on the information in these two sources, the author has assembled the sketch in Figure 4.3 showing the south facade of the North WTC tower where the explosion occurred in front of the Column No. 324. The explosion caused fractures of a bracing member, and removed and bent another bracing member. Both bracing members were connected to Column 324 as shown in Figure 4.3. In addition, according to Ramabhushanam and Lynch (1994), the blast bowed-in several spandrel plates at B1 and B2 levels, and cracked the welded steel beam-column connections in nearby framing.

The fractured main column is shown in Figure 4.4. There are no more details available on the crack in the column to establish the cause of the crack. Studying the crack pattern shown in

Figure 4.2. Damage to World Trade Center due to 1993 Car Bomb Explosion


44

Figure 4.4, as well as considering that the blast occurred on the south side of the column, one might conclude that the crack in the column was most likely caused by shear fracture of heat affected zone of the weld in the column splice.

Figure 4.3. Damage to World Trade Center Main Column and Braces due to 1993 Car Bomb Explosion (Information on the dame from Ramabhushanam and Lynch (1994))

Figure 4. 4. Damage to Steel Box Column 324 of WTC Due to 1993 Car Bomb Attack (Information on the damage from Ramabhushanam and Lynch (1994))

Crack

Brace gone

Brace bent


Notes: 1. The information on crack location is from Ref. Ramabhushanam and Lynch (1994),

2. The geometry (irregularities) of the cracks is for demonstration purpose and does not appear to represent the

actual crack shape in the actual column.

The upper 100 floors had a “Bearing Wall Structure

The lower 10 floors had a traditional framing system

Location of column splices at Elev. 87.5 m.

Cracks

PL 544x71 mm

PL 818x71 mm

East Face

North Face

324

324 324 324 324

East Face North Face West Face South Face


45

The blast created very high strain rate shear as well as normal stresses in the heat affected zone of the welded splice. The cracks do not appear to have been caused by tensile stresses since, due to gravity load, these axially loaded columns had considerable compressive stress in them during the blast which would cancel some of the tension created by bending due to the blast. Second, the blast was located on the south side of the column which would create large tension on the opposite side , i.e., the north side of the column and compression in the front plate. But the north side plate which was subjected to tension due to blast had only cracks on its edges. Also, it is unlikely that the rebound action could have caused the cracking of the column since the stresses during the rebound were much smaller than the initial impact stresses and as a result could not have fractured the south plate which was under tension during the rebound.

The case of the damage to the sructure of the World Trade Center during the 1993 car bomb attack, especially the cracking of the column reminds us of the need to study the performance of steel elements , especially welded elements, under the effects of high strain rate loading created by blast. There is an urgent need for data on high strain rate failure modes of steel, welds, bolts, shear studs and other elements of steel and composite structures.

It must be mentioned that the structure of the World Trade Center was a unique one. The lower 10 floors , which included six basements and four floors above the ground, had a traditional steel framing system with concentrically braced core as shown in Figure 4.3. However, the 100 stories above those ten floors did not have framing system. The structure of the upper 100 stories was “Steel Bearing Wall” system, where the stiffened steel plate on four sides of the towers as well as the gravity columns in the core were carrying the gravity load. The steel bearing walls on the four sides were also responsible for providing lateral stiffness and strength to resist the wind loads. The floors of the office area around the core were supported on steel truss joists which in turn were supported on the seat angles welded to the outside steel bearing walls. More information on the structure of the collapsed World Trade Center towers and their collapse can be found in Astaneh-Asl (2002).

4.2.c. Blast Effects on Open Shape Steel Columns

In bare steel open shape columns, or columns with no significant encasement or non-structural cover, the elements of cross section such as flanges or web, when subjected to direct pressure of blast, bend, as shown in an exaggerated position in Figure 4.5. Such bending of unstiffened elements (i.e. flanges) and stiffened elements (i.e. web) result in significant change of cross section geometry and its properties such as moment of inertia, section modulus and radius of gyration. The changes in geometrical properties, due to application of lateral blast load to bare steel, added to the changes in the material properties due to high strain rate of blast, result in significant changes in the load carrying capacity of a steel column. As discussed in Chapter 2, high rates of strain increase the yield stress of steel considerably while adding slightly to the modulus of elasticity. As a result, under high strain rates, the possibility of local buckling increases. The potential of steel cross section elements to buckle locally depends on the square root of Fy/E where Fy is the yield stress of steel and E is the modulus of elasticity. By increasing Fy and maintaining the value of E, the value of (Fy/E)0.5 will increase resulting in increasing the possibility of local buckling. A survey of literature did not produce published data on the local buckling behavior of steel sections subjected to blast effects. Due to the importance of local buckling to


46

stability of columns in multi-story buildings, there is a great need for realistic tests of steel columns, especially open shape columns under the combined effects of axial gravity and lateral blast loads.

The effect of an increased yield and ultimate strength on the overall buckling of columns subjected to combined gravity and blast effects also is not well understood. A survey of the literature did not result in publications on the actual and realistic blast tests of beam columns. In the following a summary of a blast test of a steel wide flange member, positioned vertically and subjected to lateral blast load done by Magallanes et al. (2006) is provided.

4.2.d. Blast Test of Steel Open Shape Member

Magallanes et al. (2006) reported on a blast test of a steel wide flange member. Figure 4.6. shows a photo of the test set-up from Magallanes et al. (2006) with bare steel member and with steel member covered with brick masonry prior to the blast test. The tested W14 wide flange had a depth of 16 inches with a nominal web thickness of 1.045 inches, and nominal flange width and thickness of 15.865 and 1.688 inches respectively. It appears that the tested specimen was a W14x233. Even though throughout the report the tested member is called a column, a close look at the details of the test and the way the specimen was loaded reveals that the tested member could not be called a column since there was no axial load applied to the member during the blast test. Therefore, the results of this test cannot be used for columns or beam columns since by definition a column is a member that has axial compression. Even with this shortcoming in the test, given the scarcity of the actual blast test data, the author will continue summarizing the test here and discuss what we may learn from it. There was also a lack of instrumentation, such as strain gages and displacement transducers, to record the strains and deflections at the critical locations further limiting what can be learned from this test.

Figure 4.7 shows the specimen after the tests with the brick masonry totally blown away by the blast. The damage to the wide flange consisted of what appears from the photo to be a shear yielding of the hinge at the bottom of the member as well as folding of the flanges, which

Figure 4. 5. Deformation of a Wide Flange Shape under Blast Loads



47

can be seen in Figure 4.7. Again, one should be careful not to extend the behavior of this specimen to the behavior of steel columns under blast effects since the member that was subjected to blast load was not a column and did not have axial compression in it at the time of the blast test.

Figure 4.7. Wide Flange Steel Member, With No Axial Load in It after the Blast Test (from Magallanes et al. ,2006).

Figure 4.6. Wide Flange Steel Member Prior to the Blast Test. (from Magallanes et al. ,2006). Note that there is no Axial Load Applied to the Member

No axial load is applied to the wide flange steel member


48

4.2.e. Analytical Studies of Open Shape Columns Su bjected to Blast Effects

An analytical study of the behavior of steel wide flange column sections was done by Lee, Kim and Kim (2009). They subjected W14x211, W24x162 and W33x152 wide flange sections with a height of 3.6 meters (11ft-10in) to a rectangular charge of 500 kg (1100 Pounds) equivalent TNT placed at a distance of three meters from the column with the center of explosive mass being 1.5 meters above the ground. The wide flange sections satisfied the compactness requirements of the AISC Seismic Provisions(AISC, 2005b). The wide flange members were subjected to blast load acting on their web resulting in their bending about the weak axis. The end connections of the columns were either fixed-fixed or pin-pin at the top and bottom. The cross sections were considered by the authors to be representing the columns of perimeter moment frames. However, no axial load was applied to the members that were analyzed. Designating a member with no axial load as a column causes confusion in the users. It is also not a conservative design since the presence of axial load in those members with large lateral deformations creates large P-∆ effects that cannot be ignored. Therefore, a better designation would be wide flange members instead of columns. Another shortcoming of the study is that the material stress strain curve was modeled as bi-linear, which may be acceptable, but, with infinite value for ultimate strain the bi-linear stress strain curve ignores the fracture of steel. In other words, the steel in these sections is assumed capable of elongating to infinity without fracture, which is quite unrealistic for blast applications where the fracture of materials including steel is a likely failure mode. Despite these shortcomings of the study, the author still finds it valuable to share the paper and its results with the readers of this Steel TIPS report.

The main findings of the study of wide flange members (with no axial load) subjected to lateral blast pressure were:

1. The blast pressure was not uniformly felt throughout the length of the member, as currently is assumed in single-degree of freedom dynamic analysis of members, but the pressure at the ends was less than the pressure in the middle portion of the member.

2. Under the blast pressure, the H-shape cross sections deformed inelastically and ended up having a folded geometry as shown in Figure 4.1 earlier. This deformation placed a large strain demand on the k-areas of the wide flange shape, where the flanges are connected to the web making these k-areas susceptible to fracture.

3. Although the relative web displacement of pin-pin or fixed-fixed members were very similar, the effective (Von Mises) plastic strain at the k-line for pin-pin member was almost twice as much as that of the fixed-fixed member. The high effective k-line strain in the pin-pin member results in the fracture of its k-line area much earlier that of the fixed-fixed member. This finding is very important in blast-resistant design showing an additional advantage in making members continuous instead of simply support.

4. The deep sections (W24 and W33) deformed much more than the shallow section. (W14). This, according to the authors was due to the larger exposed web area of the deep sections and their smaller bending strength about the weak axis of the cross section.


49

4.2.f. Preventing Flange Folding in Open Steel Cros s Sections (Wide Flanges)

Folding of flanges and bending of the webs (web dishing) is a major problem for steel open sections, such as wide flanges, when subjected to blast pressures. To study this phenomenon and to find an efficient and economical solution Goodson and Astaneh-Asl (2004) conducted an exploratory analytical research. The main objectives of the study were (a) to establish the structural response of a steel wide flange member representing column sections when subjected to lateral blast pressure and (b) to explore possible improvements to prevent flange folding. The study was supported in part by the Structural Steel Educational Council (SSEC), which is the producer of the Steel TIPS reports including this one, in the form of a student scholarship to Emma Goodson.

In the study the wide flange members were assumed to be pin-ended to allow for a worst case scenario in terms of bending deformations. The center of the blast was at the mid-length of the member and at a standoff distance of 10 ft away from the member’s mid-height. The distance was almost the same as in the columns of the reinforced concrete Murrah Building in Oklahoma attacked by a terrorist car bomb in 2005, which partially collapsed.

The levels of axial load in the study were 60%, 30% and 0% of axial capacity of the wide flange. The case of 0% axial load (i.e. no axial load) was included to observe the tensile fracture of the leeward flange reported here.

The member studied was a W14x145, ASTM A912 steel wide flange section. The length of the wide flange member was 14 ft. T o study the flange folding phenomenon, we subjected the members to lateral blast loads. The same wide flange sections were encased in pre-fabricated concrete encasements made of two L-shaped elements, Figure 4.8, and were subjected to the same blast loads to study the effectiveness of encasement in preventing flange folding in the steel wide flange member.

The wide flange member as well as its encased version, shown in Figure 4.8, was modeled using shell elements. The finite element analysis software Dytran, developed by the MSC Software Corporation (www.mscsoftware.com), (MSC, 2003) was used to simulate the blast

10 ft

L-shaped R/C

L-shaped R/C Steel Wide Flange

(a) (b)

Figure 4.8. (a) Bare Steel Wide Flange and Encased Members Studied, and; (b) Finite Element Model


50

effects and to apply them to the member. The shell elements used in modeling solid material such as steel and concrete were 4-Noded Quadrilateral Elements with assigned thicknesses corresponding to the column web and flange thicknesses. The 4-Noded “QUAD4” elements in Dytran generally perform well when the thickness of the structural element is much smaller than the length, which is the case for flanges and web of a steel wide flange member.

Dytran allows Gas-to-Solid Interaction with Euler elements for air gas to carry blast pressure, Lagrange elements for solids (structure) and interface elements for Gas to Solid Interaction. With explicit code in Dytran, no stiffness matrix is to be inverted allowing fast results for short duration events such as blast. Models also undergo large deformations and rotations as expected during the blast. The materials of steel and concrete were modeled as non-linear allowing plastic behavior and deformations as well as fracture at the end of plastic range.

Figure 4.9 shows the deformed shape of bare steel wide flange and the encased members during the blast. The fracture is indicated by separation of material. The main conclusions of the study were: (a) flange folding is a likely failure mode in wide flanges subjected to lateral dynamic pressure created by blast, and; (b) the precast reinforced concrete encasement was quite efficient in preventing flange folding.

It must be mentioned that the idea of encasing steel wide flanges in precast concrete encasement was given to the author by Congressman Vernon J. Ehlers of Michigan at a meeting in the congressional representative’s office in Washington D.C. in 2002. Upon presenting the research as part of a NSF research delegation to Capitol Hill, Congressman Ehlers, with previous physics research background, suggested adding precast concrete plates to existing steel wide flange sections to prevent their flange folding. The study done by Goodson and Astaneh-Asl (2004) clearly showed that this solution was very efficient in preventing flange folding.

Figure 4.9. (a) Bare Steel Wide Flange undergoing, yielding flange folding and fracture of tension flange, and; (b) Wide Flange Encased in R/C undergoing bending, yielding of steel with no flange folding and finally fracture of the Encased Member (From: Goodson and Astaneh(2004))

(a) Steel Wide Flange (b) Encased in Precast R/C Encasement


51

4.3 FAILURE MODES OF COLUMNS SUBJECTED TO BLAST

Figure 4.10 shows major failure modes of a bare steel column with fixed base and pin top. The sketch is prepared using data from Chen and Liew (2005). The failure modes can be listed as:

a. Overall Buckling- under combined axial compression and bi-axial bending

b. Local Buckling- under axial strain and lateral blast pressure

c. Distortion of Cross Section (Flange Folding or Web Dishing)- due to blast pressures causing folding of flanges and dishing of web.

d. Shear Failure of Column Ends- under primarily the combined effects of shear and bending

e. Fracture of Tension Flange- more likely for columns with small axial compressive load or those with tension uplift

f. Failure of Column Splices- under combined bending, shear and possible uplift

g. Failure of Base Plate/ Anchor Bolts Assembly- primarily under shear effects and possible uplift due to blast

h. Fracture of Foundations- more likely for spreads footings without piles and those with small axial compressive load or with tension uplift.

Figure 4.10. Failure modes of a Steel Column Subjected to Axial Load of Gravity and Lateral Blast Load

Reflective Pressure of Blast

Copyright © 2010 by Abolhassan Astaneh-Asl. All rights reserved

Failure Modes

Local Buckling

Shear Yielding

Connection Failure

Failure of Base Plate/anchor bolts

Failure of column splices, if used

Folding of Flanges

Overall Buckling/Bending Failure as a Beam-Column

Foundation Failure


52

4.3.a. Notes on Overall Buckling of Steel Columns u nder Blast Loads

Since a blast charge can be placed anywhere around the column, the blast pressure on the column can be in any direction, in general, creating biaxial bending about both the weak and strong axes. Biaxial bending of the column under blast loads is amplified by relatively large P-∆ effects since during the blast columns are expected to undergo relatively large, inelastic, lateral deformations. If the column is slender, i.e. KL/r > 4.71(E/Fy)

0.5 as per the AISC Specification (AISC, 2005a), and buckles almost elastically, its buckling capacity will primarily depend on modulus of elasticity. Since the modulus of elasticity remains almost constant during high strain rate loadings, the overall buckling capacity of slender member will not increase much because of the increase in the material property under high strain rate. On the other hand if the column is not slender, i.e. KL/r <4.71(E/Fy)

0.5 as per the AISC Specifications (AISC, 2005a), then the buckling capacity of the column will depend primarily on Fy instead of E. Since the yield stress of steel, Fy

increases under high strain rates of blast loads, the buckling capacity of stocky (i.e. non-slender) columns will increase under blast loads due to the increase in the Fy of steel.

Compared to reinforced concrete columns, steel columns generally have the benefit of not releasing secondary fragments during blast, such as spalled concrete, to cause injuries to people and damage to other structural elements that further deteriorate the structural integrity and strength. However, due to the relatively thin segments used in steel sections, the folding of steel cross section elements, such as shown in Figure 4.7 earlier, and/or local buckling of steel segments of a column may result in premature loss of capacity during the blast as discussed further in the following section.

4.3.b. Notes on Local Buckling of Steel Columns und er Blast Loads

Local buckling failure in steel columns during blast can result in premature overall buckling of the column, which then results in the gravity load of the failed column being shed to the neighboring columns. Such load shedding has the potential to cause progressive collapse if the columns, beams and connections around the lost column are not capable of resisting the gravity load of the lost column. Therefore, local buckling failure mode of bare steel columns under high strain rate blast load should be given a higher importance than the cases such as gravity, wind and seismic where the load is applied at a much slower rate.

One of the solutions to the problem of local buckling and distortion of steel cross sections under blast loading is to use composite columns discussed later in this chapter. By using concrete filled tube or box columns or by encasing open shape cross sections such as wide flanges in concrete, local buckling of cross section under blast loads can be prevented.

The information on local buckling behavior of steel cross section under blast loads is very limited and no data is publicly available on the results of blast tests done to study local buckling phenomenon in steel members when subjected to blast. During blast, the steel is subjected to very high strain rates and its yield point increases while the modulus of elasticity remains almost the same. Axial strains that can initiate local buckling in a steel cross section element are related to square root of modulus of elasticity divided by yield stress. If the yield stress increases during the blast and the modulus of elasticity remains almost the same, the axial strain that can cause local buckling during high strain blast loading will be lower than the strain causing local buckling under


53

static condition. Therefore, it is expected that steel sections will undergo local buckling during the blast under smaller axial strain than the same section locally buckling under static strains. The premature local buckling of steel columns under blast load is also mentioned by Zhang et al (2008).

Chen and Liew (2005) conducted an analytical study of the behavior of bare steel columns under initial axial load and lateral dynamic blast pressure. The wide flange used in the analysis was European section UC254x254x89 that is very close to W10x60 with a depth of 10.2 inches, a nominal web thickness of 0.42 inches, and nominal flange width and thickness of 10.2 and 0.68 inches respectively. The lateral blast pressure was applied to the flange surface as a dynamic (time dependent) uniform pressure. An important finding of the investigation was that under high strain rate loading during blast, local buckling of columns with compact section could occur prematurely and reduce the buckling capacity of the column. In the AISC Specifications (AISC, 2005a), the b/t ratio limits are expressed in the form of:

yF

EC

t

b ≤ (4.1)

Where, b and t are the length and thickness of the segment respectively, E is modulus of elasticity and Fy is the specified yield stress of steel. Parameter C depends on boundary condition of the segment being stiffened or unstiffened and the type of normal compressive stress acting on it.

To establish b/t ratios for blast high strain rate application we replace Fy , which is the yield stress under slow strain rate with Fyd , the dynamic yield stress under high strain rate. The value of Fyd is established for any given strain rate using Equation 3.1 in Chapter 3.

For design purposes, if we assume a 20% rise in the yield strain under high strain rate of blast, the limit of b/t ratio of steel section given in the AISC Specifications (AISC, 2005a) should be multiplied by 1/√1.2= 0.90. Such reduction makes the minimum b/t ratio for columns in blast resistant design comparable to limits set in the AISC Seismic Provisions (AISC, 2005b) for seismically compact sections to be used in special ductile frames. Therefore, until more research on local buckling of steel cross sections subjected to blast is conducted, it seems reasonable to use b/t limits given in the AISC Seismic Provisions (AISC, 2005b) for seismically compact sections in blast resistant design. This limit of b/t in seismic design is also recommended by the UFC 3-340-02 Manual (UFC/DoD, 2008) although primarily to ensure sufficient ductility for members to allow large rotations of plastic hinges.

However, it should be noted that all of the b/t ratio limits in current codes, including limits for seismic design, were established based on tests done on steel cross section where the locally buckled segment was subjected to axial strain only. In the case of blast, when a steel section is directly exposed to blast pressures, the segments of the cross section are subjected to axial strain combined with bending strains created in the segment by the lateral pressure of the blast. This complex local buckling behavior needs further analytical research and actual realistic blast test results. Based on the findings of such investigations, rational local buckling rules and limits of b/t ratios for blast condition can be established.


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4.3.c. Notes on Distortion of Column Sections under Blast Loads

As mentioned earlier, when a bare steel section is subjected to blast, the relatively thin elements of the cross section bend and distort under blast pressure, see Figure 4.5 earlier for distortion of the cross section if blast pressures are acting along the principle axes. In general, blast can hit the column in any direction resulting in a very irregular distortion such as shown in Figure 4.11.

4.3.d. Notes on Shear Failure of Steel Columns unde r Blast Loads

An important item in design of steel columns for blast resistance is the large amount of shear generated in the column during blast. This item, among others, was reported by Zhang et al. (2008), Astaneh-Asl, Heydari, and Zhao (2003) for building columns, and Rutner, Astaneh-Asl and Son (2005a) for bridge piers subjected to blast loads.

Zhang, Duan and Zhang (2008) conducted a realistic analytical study of the performance of a steel multi-story building subjected to a car bomb. In the study, a 2-bay by 4-bay 5-story steel structure was subjected to a car bomb blast of 500 kg (1,100 pounds) equivalent TNT explosion placed 12m (≅40 feet) from the column. The column flange was directly in front of the charge. The structural system consisted of moment frames on all frame lines in both directions (i.e. a space moment frame with all connections being moment connections). All columns were a European section close to a W16x36 and all beams were close to a W18x97. The material of steel was very close to U.S. A36 steel. In the studies, the high strain rate effects were considered using Cowper and Symonds (1957) model, which was discussed earlier in Chapter 2 of this report. The analytical studies indicated that “columns subjected to blast load may fail by shear yielding rather than flexural deformation” (Zhang, Duan and Zhang, 2008) and recommended further research.

A study of blast effects on 3-story steel and reinforced concrete structures by Astaneh-Asl, Heydari and Zhao (2003) indicated that large base shear forces are generated in the columns

Figure 4.11. (a) Simplified Blast Pressures on a Wide Flange, and; (b) Exaggerated Sketch of Possible Distortion of Cross Section

Distorted Section



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subjected to blast loads and that such large shear forces can cause failure of the base connections of columns.

Test of a wide flange shape by Magallanes et al. in 2006 showed a failure mode that appears to be primarily shear failure. This test was briefly discussed earlier in Section 4.2.d and photos of the test specimen before and after the test were shown in Figures 4.6 and 4.7 earlier.

4.3.e. Notes on Fracture of Tension Flange of Colum ns under Blast Loads

In columns that have light axial compressive load and in cases where the blast results in uplifting a column, it is quite possible that the flange on the leeward side of the column, due to combined effects of axial load and bending, fractures. In such cases, it may be necessary to check the tensile strain on the tension side of plastic hinges to ensure that the stresses are less than the ultimate strain. In columns that are subjected to high tensile uplift loads due to blast, the column splices, if any, as well as the base connection of column and the foundation need to be checked for tension uplift failure mode. These two items are discussed in the following sections.

4.3.f. Notes on Columns Splices under Blast Loads In columns that are directly subjected to blast effects column splices must be avoided. These are perimeter columns of ground floors and possibly one or two floors above the ground floors. As discussed earlier, one of the most important failures of a critical column occurred during the 1993 car bomb attack on the World Trade Center where the steel box column cracked near the splice (Ramabhushanam and Lynch, 1994)

If columns subjected to direct impact have to be spliced, it is suggested that the splice be designed such that the M-P interaction strength of column under axial and bending moment is greater than M-P interaction of the column section in both principle directions. In considering the axial load, the possibility of column developing tension during the blast should be considered, especially for low and medium rise buildings where the columns may not have very large gravity compression load in them. Rittenhouse, DiMaggio and Ettouney (2001) recommend that direct bearing strength of column sections in the splice should not be relied upon and the splice be designed to develop full strength of the column section.

A literature survey did not produce published results of the experimental or analytical studies of behavior of bolted, or welded steel column splices subjected to blast effects. In the absence of such data, it is prudent to design splices such that the welds and bolts in the splice remain elastic while the cross section of the column develops the (M-P)dyn interaction curve. To achieve this, the forces used to design brittle failure modes of the elements of the splice should be increased by a factor of 1.5 as recommended by UFC/DoD (2008). To design the components of the splice, such as splice plates, where yielding failure modes govern the design, the force used in the design of the splice is multiplied by 1.25. This results in a column splice being 1.50 times stronger than the column for brittle failure modes and 1.25 times stronger for ductile failure modes involving yielding of steel.

The concept of making the splice stronger than the column is shown in Figure 4.12 where (M-P)splice-brittle curve is the curve representing M-P capacity of the splice if brittle failure modes


56

govern the design and (M-P)splice-ductile curve is the curve representing M-P capacity if the ductile failure modes govern. The M-P curve is the bending moment-axial load interaction curve of the column section. Figure 4.12 is for bending of column about one axis. For bi-axially bending beam column similar concept for P-Mx-My may be constructed.

4.3.g. Notes on Failure of Column Base Plate/Anchor Bolts Assembly

Steel column base connections are one of the most critical locations of failure under blast loads. The problem becomes more serious if there are no basements in the building and the exterior columns of the ground floor are supported on base plates and foundations at the ground floor level. In these cases, the base plates are subjected to combination of large axial load (tension or compression), bending as well as shear. The forces due to blast, especially shear, are dynamic in nature and have very high frequency content if the center of blast is close to the affected column.

Base plates, anchor bolts and all elements connecting the column base to the foundation should be designed to have a ductile failure mode of component, such as yielding, be its governing failure mode. Brittle failure modes such as fracture should be designed to have 50% larger strength than the strength of the ductile failure modes. For anchor bolts, the use of upset anchors is strongly recommended to avoid brittle failure of threaded areas under very high frequency dynamic load of blast. In upset bolts or threaded rods (used as anchor bolts) shown in Figure 4.13, the shank of the bolt or threaded rod is machined down to a diameter such that the tensile yield strength of the shank is smaller than the fracture strength of the area under the threads. This makes the ductile tension yield failure of the shank the governing failure mode of the tension bolt or threaded rod. The upset anchor bolts have been used in many bridges and buildings in the past and earlier versions of the AISC Manual of Construction (AISC, 1957) had information on properties of “upset” anchor bolts, which is reproduced in Appendix to this report. Table 4.1 is

Figure 4.12. M-P Curves of Splice and the Column Section

(M-P)splice-brittle of Splice for Brittle Failure Modes

(M-P)splice-ductile of Splice for Ductile Failure Modes

(M-P) of Column Section

P / Pn

M / Mn 1.0

1.0

1.25

1.25

1.50

1.50


57

reprinted from the American Institute of Steel Construction, Steel Construction, Fifth Edition (AISC, 1957) p. 137.

Figure 4.13. Reduced Section in Upset Bolts or Threaded Rods

Table 4.1. Properties of ‘Upset” Bolts and Threaded Rods (from: AISC, 1957)

d

T

K D

T d D


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For welds in the column bases, while avoiding relatively brittle partial joint penetration (PJP) welds, the use of fillet welds are strongly encouraged. The use of full joint penetration welds in some large column base connections may be unavoidable. In such cases, guidelines for welding of special moment frames given in the AISC Seismic Provisions (AISC, 2005b) or later editions should be used to prevent brittle failure of welds.

Finally, due to the presence of dynamic effects and stress concentrations at the base of columns even if yielding failure mode is the governing failure mode for column base connection components, still the base connection M-P and shear capacity is suggested to be 25% more than the corresponding values for the column section. The M-P curve of column should be established for the plastic hinge forming at the base of column just above the base connection.

Figure 4.14. shows three main failure modes of base plates subjected to column uplift. The failure mode on the right involves yielding of base plate while anchor bolts are essentially elastic. Note that Case III in Figure 4.14 is the most ductile while Case I is the most brittle failure mode unless upset anchor bolts are used. In design of base plates, that could be subjected to uplift due to blast, yielding of base plate should be made the governing failure mode of the column base connection. The column base connections are expected to experience uplift due to blast if the explosive device is quite close to the column and the gravity load of the column is light.

For more information on design of column base plates subjected to dynamic loads the reader is referred to two Steel TIPS reports by Honeck and Westphal (1999) and by Astaneh-Asl (2008). Both Steel TIPS reports are primarily for seismic effects, but some of the design concepts and detailing issues discussed there are equally applicable to blast dynamic loads.

One of the cost effective measures to increase blast resistance of column base connections is to encase the bottom of the column and base plate with a concrete pedestal (Denio, 2010). In most cases, where there is no basement, there is usually concrete surrounding the base of the steel column of the first floor as part of the typical construction geometry. Such pedestal can be reinforced to provide shear strength to the base of column as shown in Figure 4.15. Another important item to increase blast resistance of column base plates is to increase uplift ductility of

Figure 4.14. Three Cases of Failure of Base Plates.

Case I- Anchors Yielding Case II-Anchors & B. PL. Yielding Case III- Base Plate Yielding


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the column base. In many cases, the blast pressure pushing against the floor slabs in an upward direction results in uplift of columns. Such a dynamic uplift force can result in fracture of anchor bolts through the threads. To prevent such undesirable fracture of anchor bolts under uplift tension, the use of “upset” anchor bolts, shown in Figure 4.13, is strongly recommended.

4.3.h. Failure of Foundations

If the foundation is close to the surface where the explosion occurs, it can be directly subjected to blast effects and needs to be designed for such effects. If the foundation is located deep underground, i.e. when there is a basement, the foundation may not be subjected to direct blast effects. However, the foundation of a column subjected to blast effects should be designed to resist the combined effects of loads transferred to it by the base plate. Again, a ductile failure mode should be the governing failure mode. If piles are used, steel H-piles are a very good choice since they have shown to possess significant ductility in both tension and compression. The steel H-piles, are designed following the provisions in the AISC Seismic Provisions (AISC, 2005b) . For more information on cyclic behavior and ductility of H-piles and their use in seismic applications, the reader is referred to Astaneh-Asl and Ravat (1997 and 1998).

Figure 4.15 Suggested Base Plate Detail to Increase Blast Resistance

Bracket Plates Welded to Column and Base Plate

Pedestal or Grade Beam

Rebars

Shear Studs

Column

Anchor Bolts (Use of “Upset” Anchor Bolts Is Recommended.)

Base Plate

All Welds Fillet Welds (Avoid Partial Joint Penetration Welds)


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4.4. NOTES ON COMPOSITE COLUMNS SUBJECTED TO BLAST EFFECTS

Composite columns are a very viable option in blast-resistant design. As discussed earlier, several failure modes of open sections, such as flange folding, web dishing and local buckling, make bare steel wide flange columns susceptible to those undesirable failure modes. By using composite columns, we can eliminate or substantially delay those failure modes. Figure 4.15 shows some suggested composite column sections that appear to be quite suitable to resist blast effects in a very economical manner.

4.5. NOTES ON DESIGN OF STEEL AND COMPOSITE BEAMS U NDER BLAST LOADS

Beams in steel structures are generally I shaped wide flanges. When a blast occurs outside the building, the beams exposed to the direct pressure of the blast will undergo distortion of the cross section and may experience severe flange folding and web dishing deformations. Since beams are local elements supporting the gravity load from one floor only, their damage and even loss is not as critical and catastrophic as the loss of a column. As long as the damage to steel beams and girders due to blast does not cause the collapse of the floor under the gravity load after the blast, such damage should be acceptable. This is because as long as the floors do not collapse, the chances of serious injuries and loss of lives of the occupants are very low and successful rescue efforts can follow.

Figure 4.16. Suggestions for Composite Columns used to Harden Open Shape (Wide Flanges) Column Sections to Better Resist Blast Effects

Steel Column with Concrete-Filled Tube Jacket

Encasing steel

Column in R/C

Steel Column with Concrete-Filled Box Jacket

Steel Column within a Concrete-Filled Tube or Box Jacket. The inner tube is either hollow or filled with viscous damping material.

Steel Column


Steel Pipe

Steel Box

Two Steel Pipes


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In addition to supporting the gravity load of the floor slab and walls on the floor, beams have a secondary function of bracing the columns end. The blast damage to beams must be limited such that the damaged beam still provides stability bracing to the columns attached to it although in many cases, the steel deck/concrete slab of the floor, if in place, will be able to provide the necessary stability bracing to the columns.

4.5.a. Steel Beams under Blast Loads

As mentioned earlier, a study of the performance of steel structures subjected to blast effects was conducted by Crawford and Magallanes (2009). Although the model of the study did not include floor slabs, still some valuable lessons can be deduced from this study.

Figure 4.17 shows the model and view of the deflections in the ground floor column and two side spans directly in front of the blast. Note the flange folding of girders. It appears that unfortunately the researchers did not include the gravity loads in the analysis. If the gravity loads were present on the girders, they most likely would have collapsed after sustaining such a large flange folding deformations .

4.5.b. Composite Beams and Shear Studs

The undesirable flange folding of the girders exposed to blast pressures may be prevented by making them partially encased composite beams or concrete-filled boxes as shown in Figure 4.18 below. Such solutions do not need to be applied to all girders of the exterior floors, but only to girders of the first floor on the sides of the building exposed to car traffic where a blast may occur directly in front of the girders.

Figure 4.17. The Steel Structure and a Close up of the Ground Floor Column and Side Girders Exposed to Blast

(Information and Graphics from: Crawford and Magallanes (2009)).

Flange Folding


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4.6. NOTES ON DESIGN OF STEEL AND COMPOSITE CONNECT IONS

UNDER BLAST LOADS

4.6.a. Notes on Welds under Blast Effects

The behavior of welds used in the connections of steel structures even under static loading is quite complex. Under dynamic and very high strain rate conditions, which occur during a blast, the behavior becomes even more complex. As pointed out by Rittenhouse, DiMaggio, and Ettouney (2001), in steel connections, the high frequency dynamic blast pressure can excite element of connections as well as move them in an out-of-phase manner resulting in additional stresses in the welds within the connections. These dynamic effects add even more to the complexity of the state of stress in the welds. Added to the problem is the fact that the experimental data on the behavior of structural welds under blast load is extremely limited given the type and configuration of welds found in structural steel connections. Given the unknowns on the behavior of welds under blast loads the prudent approach once again, as in seismic design, is to use the ductility tool but this time to apply it at the local level.

At this time, without extensive data available on the performance of structural welds under blast effects, one might recommend that , to design the weld, with some modification to material properties using the same methodology that we currently apply to design of welds for static (gravity) or quasi-static (seismic) loads . For the weld itself since we are dealing with the ultimate strength, we may use the static values (i.e. the specified weld strength) without considering any increase due to the blast high strain rate effects. This is a reasonable assumption since as shown in earlier sections on material, the ultimate strength of steel does not go up as much as its yield stress. For welds, with non-homogeneities, stress concentrations, 3-D plain strain effects, micro cracks in heat affected zones and hardening of the steel, the dynamic ultimate strength may not be much above the static ultimate strength. The dynamic ultimate strength may even be less than the

Figure 4.18 (a) Blast Pressures on Girders, (b) Flange Folding of Bare Steel Girder, and (c) Partially Encased Composite Girder resisting Blast Pressures.


Beam will be subjected to lateral and uplift pressures.

(a) (b) (c)


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static ultimate strength. Until we have better test data on these complex parameters, as suggested, we might use the static, specified strength of welds (e.g. 70 ksi for E70xx welds) in blast resistant design.

Since the yield and ultimate strength of base metal rises under the high strain rate of blast, we need to design the welds for larger forces than the capacity of the base metal based on its static yield stress and ultimate strength. It is suggested that the welds be designed for forces that are 1.5 times greater than the yield capacity of the base metal.

To design welds, and until more data on the behavior of welds under blast loads are available, it is suggested that the strength reduction factor φw for welds and base metal is taken as the values currently given in the AISC Specification (2005a).

4.6.b. Notes on Bolts and Rivets under Blast Effect s

A survey of literature did not yield much information on the actual testing of structural bolted connections under blast effects. There is an urgent need for such tests done under realistic condition of blast. Until then, the brittle failure modes of bolted connections need to be avoided. It is suggested that the bolted connections directly subjected to blast effects be designed such that the strength of the bolts is at least 1.5 times the strength of the base metal yielding, forcing the ductile failure mode of the base metal yielding to be the governing failure mode. In calculating the yield capacity of the base metal, the dynamic yield stress and not the specified static yield stress should be used. For dynamic yield stress of steel, refer to Chapter 2. The reason for the factor of 1.5 is that the bolt has a stress concentration at the thread roots making it vulnerable to high frequency dynamic effects due to blast.

4.6.c. Steel Moment Connections under Blast Effects

Due to the importance of columns in preventing a progressive collapse, similar to special moment frames used in resisting seismic effects, the use of connections stronger than the connected member is suggested. In addition, moment frames subjected to blast effects should be designed to have strong columns and weak beams.

The moment connections in special moment frames, used in seismic design, are designed to develop the strength of the girder. One of the exceptions is the welded Reduced Beam Section (RBS connection), where instead of making the connection stronger than the beam, the beam is made weaker than the connection by cutting parts of the flanges at the location of plastic hinges. Although the RBS moment connections are used frequently as a reliable and economical connection in special moment frames resisting seismic effects, their good performance may not be extended to blast effects.

Rittenhouse, DiMaggio, and Ettouney (2001) correctly point out two problems with cases where the beam cross section is weakened to ensure that a plastic hinge forms in the beam instead of in the connection or in the column. Such is the case with the reduced beam section (RBS) connections developed after the Northridge earthquake and currently used in many Special Moment Resisting Frames. The first problem is that the weakened area, under lateral blast pressure develops premature plastic hinges and the beam fails prematurely. The second serious


64

problem that the authors correctly point out, is the limited ductility of the beams with reduced beam sections to resist large axial catenary tension forces.

Let us consider a scenario where a car bomb blast destroys a column of the exterior moment frame. After the column is eliminated, the gravity load of the floors above the eliminated column will push the upper portion of this column down. The girders on the sides of the column will develop large catenary forces as the column pushes them down as shown in Figure 4.19.

Figure 4.20(a) shows a close-up of the girders in one floor before removal of the column below the girders. When the column below is removed, Figure 4.20(b), as the column above the girder pushes the girders down forming a V-shape deflected configuration, relatively large catenary forces develop in the girders. This catenary tension acting along the length of the girders will cause the RBS zone of the girder to yield under the combined effects of axial tension and bending moment as shown in Figure 4.20(b). Since the length of the girder outside the RBS zone remains elastic, all the inelastic elongation of the girders due to catenary action has to come from elongation of the RBS zone. Since the length of RBS zone is usually a fraction of the total length of the span, the amount of inelastic elongation of the RBS zone will be a fraction of what it would have been if there were no RBS in the girder and the entire length of the span would yield. The result of having only a relatively short length of the beam within these weakened areas yield is very large tensile inelastic strains developing there quite possibly causing tensile fracture of the catenary beam at one of the weakened RBS locations.

As shown in Figure 4.20(c), if no RBS exists in the girder, the catenary tension force will yield the entire length of the girder resulting in the supply of large amounts of inelastic elongation of the girder without premature fracture.

Figure 4.19. (a) Elimination of Ground Floor Column by Blast, and; (b) Development of Catenary Tension Forces in Girders

(a) (b)


65

/

/

Based on the above discussion it may not be prudent to use moment frames with weakened girders, such as the current RBS system, in the perimeter frames of the structures that are designed to resist blast. The members must be involved as catenary elements in preventing progressive collapse after removal of a column. However, if moment frames with RBS are the

(Photo courtesy of Prof. M. Engelhardt)

Figure 4.20. Inelasticity in Girders with and without Reduced Beam Section (RBS)

(a) Frame prior to removal of column

(b)-Girders with RBS in them have very limited ductility for yielding under

catenary tension force

Elastic Region

(c) Frame with no RBS in the girders

Plastic Region

Plastic Region

Length of RBS Zone

Limited yielding zone


66

selected system to be used throughout the structure, the author suggests that the location of the RBS hinge be reinforced as shown in Figure 4.21(a) for outside moment frames exposed to blast. The suggested web reinforcement results in making the axial yield capacity of the RBS area greater than the axial yield capacity of the beam section. As a result, when the middle column is removed and large axial catenary tension forces develop in the girders, the “reinforced” RBS area remains elastic, Figure 4.21(b), forcing almost the entire length of the beam outside the RBS zones to yield in tension instead of just the short lengths of the RBS zones.

Figure 4.21. Modification Suggested by A. Astaneh-Asl for Reduced Beam Section (RBS) To Prevent Their Fracture Failure under Catenary Tension Forces

Length of RBS Zone

Horizontal Web Plates on Neutral

Axis

Plastic Region

Plastic Region

(a) Frame prior to removal of column

(Original Photo courtesy of Prof. M. Engelhardt, Addition of reinforcement by A. Astaneh-Asl)

Web Horizontal Plates to Reinforce RBS to Resist Catenary Tension

(b) Frame with “Reinforced” RBS after removal of column


67

Another option to prevent the premature tension fracture of the weakened RBS girders under catenary tension forces is to use moment connections that are stronger than the girder section with no need for the RBS in the girder. A very versatile system in this regard is the “Sideplate” moment connections. For more information on this connection and tests showing its blast and progressive collapse resistance, the reader is referred to (www.sideplate.org). As stated in the acknowledgements, the author had no involvement with the development, production, marketing or sale of the Sideplate connections and has no financial or other interests on the product itself or the company that holds the patent on this product. The mention of this product here is only based on the author’s opinion, formed by a review of test results available at this time , on the good performance of this product under structural loads especially seismic and blast effects.

4.7. EXTERIOR WALLS AND FACADES An important function of the exterior walls and facades in cases of close-in high-impulse

blasts is to prevent the primary fragments generated by the device from entering the building causing injury to occupants and damage to the building components, especially the gravity load carrying structural members and connections. An equally important role of the exterior walls and facades is to bloc the secondary fragments from penetrating the wall and entering the building. According to UFC 3-340-02 Manual (UFC/DoD, 2008), “…fragment protection is usually accomplished by increasing the element thickness to resist fragment penetration or by providing supplementary fragment protection”.

Currently in many cases very thick reinforced concrete walls are used as fragment protection walls. However, composite shear walls and even bare steel plate shear walls provide excellent fragment protection walls not only for the exterior walls but also for the interior safe rooms that many government buildings, embassies, banks and other critical buildings have. In composite shear walls , a steel plate is welded or bolted to the boundary beams and columns and a reinforced concrete wall is placed in front of the steel plate, on the exterior side of the wall and connected to the steel plate by shear studs or bolts. In the following sections notes on the behavior and design of steel and composite walls subjected to blast loads are provided and the performance of composite and reinforced concrete walls subjected to blast loads are compared.

4.8. COMPOSITE SHEAR WALLS SUBJECTED TO BLAST LOAD S Composite shear walls consist of a concrete wall with steel plates attached to one side or both sides or placed in the middle of the wall. Figure 4.22 shows typical composite shear walls.

Composite shear walls, shown as Type (a) on Figure 4.22 were studied and tested by Astaneh-Asl and Zhao (2002) and results were published in references listed under Zhao and Astaneh-Asl. For more information on the design of composite shear walls, the reader is referred to a Steel TIPS by the author on “Seismic Behavior and Design of Composite Shear Walls”, (Astaneh-Asl, 2002).

Rutner, Astaneh-Asl and Son (2005b) conducted a comparative study of performance of reinforced concrete and composite shear walls subjected to blast loads. Figure 4.23 shows


68

configuration of shear walls studied. The composite shear wall consisted of a six inch concrete wall attached to a ½-inch thick steel plate. The reinforced concrete wall was an 18-inch thick reinforced concrete wall. The walls were designed to have comparable shear strength and satisfy provisions of the AISC Specifications and ACI-318 code for composite and R/C walls respectively.

Shear Connectors

Steel Plate Concrete Wall

Reinforcement

(a)

(b)

(c)

(d)

Figure 4.22. Typical Composite Shear Walls

10ft

12ft

29 ft

columns: 24x24x1 beams: W24x68

Figure 4.23. Composite and Reinforced Concrete Shear Walls Subjected to Blast Effects and Studied

(Rutner, Astaneh-Asl and Son, 2005b)

Composite Shear Wall RC- Shear Wall section

6 inch 1/2 inch 18 inch


69

The main conclusions of the study were:

1. When subjected to 1000 pounds of equivalent TNT blast at a distance of 10 feet, the reinforced concrete behaved in a brittle manner and developed large hole releasing large amounts of debris, which entered the building with high velocity. Such highflying debris is extremely dangerous to occupants and the main cause of casualties and injuries during explosions. Figure 4.24 shows a time history of effective, von Mises stresses in the outer and inner solid elements of R/C wall at the center of the wall. The blast was equivalent 200 pounds TNT. Figure 4.25 shows the extensive damage to the R/C wall due to 1000 and 2000 pounds equivalent TNT blasts.

Damage to R/C Wall Due to 1000 TNT Equivalent

Figure 4.25. Damage to R/C Wall due to Large Blasts (Rutner, Astaneh-Asl and Son, 2005b)

Figure 4.24. Von Mises Effective Stress in the Concrete Elements in the Shear Wall Subjected to 200 pounds TNT Charge at 10 feet Ground Distance


0 0.005 0.010 0.015 0.020 0.025 0.030 0.035

Von-Mises Effective Stress,

(psi)

Time, (sec.)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000 Outer Solid Element in R/C Wall

Inner Solid Element in R/C Wall

Damage to R/C Wall Due to 2000 TNT Equivalent


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2. The composite wall, subjected to 200 pounds equivalent TNT blast at a distance of 10 feet behaved in an almost elastic manner with effective, von Mises stresses, in the steel wall barely exceeding yield stress of steel (50 ksi) while the concrete wall part of the composite shear wall was shattered. However, since the steel plate, placed on the inside surface of the composite wall, did not fracture, the debris from shattering of the concrete wall were not able to penetrate the steel plate and enter the building. As a result, for the case of composite shear wall, little or no injuries would occur inside the building due to the impact of high-speed flying debris. Figure 4.26 shows time history of the effective stress on the two surfaces of the steel plate of the composite shear wall.

4.9. USE OF COMPOSITE SHEAR WALLS IN BLAST HARDENIN G OF BUILDINGS

To study using the composite shear walls to harden buildings against terrorist car bomb attacks, an analytical study was undertaken by Astaneh-Asl, Zhao and Heydari (2003) supported by the Applied Technology Institute. For information on composite shear walls and their tests and applications to resist seismic forces, the reader is referred to Astaneh-Asl, 2000, and 2002) and Zhao and Astaneh-Asl, (2003, 2004, 2007 and 2008). In the following, the focus is on a comparative study of the behavior of reinforced concrete and composite shear walls against blast effects.

The main objective of this study was to demonstrate the potential of using composite shear walls in protecting typical buildings of the Army against local damage and partial or full progressive collapse in the event of an attack using explosives. Such attacks are launched by organized enemy army units or by terrorists. The typical building considered in this case study is shown in Figure 4.27, which was a generic 3-story steel structure representing typical barracks or other DoD buildings. The building also represented a typical civilian low-rise building designed

Figure 4.26. Von Mises Effective Stress in the Steel Plate of Composite Shear Wall Subjected to 200 pounds TNT Charge at 10 feet Ground Distance


0

10

20

30

40

50

60

0 0.005 0.01 0.015 0.02 0.025 0.03

Time, (sec.)

Center Element Bottom Element

Von-Mises Effective Stress,

(ksi)


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Figure 4.27. A Typical Modern 3-Story Building and the Generic Model Used in the Study

following the U.S. building codes. Therefore, the results of this study are equally applicable to ordinary low rise steel buildings designed and constructed according to U.S. building design codes such as Uniform Building Code (ICBO, 1997) , the International Building Code (IBC, 2006) and the AISC Specification (AISC, 2005a).

4.9.a. Introduction to Composite Shear Walls

The main function of structural walls in buildings is to act as the lateral load resisting system and resist horizontal story shear. Until the 1970’s, the lateral load resisting shear walls were reinforced concrete. Since 1970, steel shear walls have also been used in important buildings in Japan, Canada, the United States and China (Astaneh-Asl, 2001). A third type of shear walls, used occasionally in the past is the composite shear wall, which is a combination of reinforced concrete and steel plate shear walls. During the 1999-2006 period , Qiuhong Zhao and the author studied and tested composite shear walls subjected to in-plane cyclic loading representing earthquake effects (Zhao and Astaneh-Asl, 2003) and developed design guidelines and recommendations (Astaneh-Asl, 2002). Figure 4.28(a) shows the elements of the composite walls studied and tested. Figure 4.28(b) shows a typical composite wall subjected to blast loads.

Steel Plate Welded to Steel Structure

Reinforced Concrete Panel Connected to Steel Plate

Steel Plate Membrane Action.

Concrete Panel Bends but cannot penetrate the building to cause death and injuries

a. Components of the Proposed Wall b. Composite Wall Subjected to Blast Pressures

Figure 4.28. Components of Composite Shear Wall and Blast Pressure on the Wall


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In order for the steel plate and concrete to work in a composite manner, they must be connected to each other. The connection is usually steel welded shear studs for new construction and for cases where the cast-in-place walls are used. For existing buildings where cast in-place concrete walls are used, bolts should be used instead of welded shear studs. Figure 4.29 shows these two ways of connecting steel plate to reinforced concrete wall. Due to the presence of the steel plate in the composite shear wall there is no need to have heavy reinforcement in the concrete wall. For lateral load resistance the reinforcement is only minimal to avoid shrinkage and creep-related cracks.

Astaneh-Asl and Zhao, (2002), in a project sponsored by the National Science Foundation, studied and tested traditional composite shear walls and developed an innovative composite shear wall system for seismic applications. The traditional and the innovative system performed extremely well reaching drifts in excess of 4%. Figure 4.30 shows the composite shear walls studied and a photo of the specimen during the test. Figure 4.31 shows the base shear- drift cyclic hysteresis curves for the composite shear wall specimen shown in Figure 4.29. As shown, the composite shear wall could easily reach cyclic drift values of 0.05. For more information on cyclic behavior and seismic design of composite shear walls, the reader is referred to the literature including Astaneh-Asl (2002).

Figure 4.29. Bolted and Shear Stud Connection of Steel Plate to Concrete Wall

Steel Plate

Shear Studs for New Buildings

Concrete

Washer Plate: 4x4x3/8” Min.

Bolts for Retrofit Cases

Components of Composite Shear Wall

Steel Plate

Concrete Wall

Steel Columns and Beams

Bolts

Figure 4.30. Composite Shear Walls Studied by Zhao and Astaneh-Asl (2003 and 2008)

(a) (b)


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4.9. b. Analysis of a 3-Story Case Study Building S ubjected to Blast

Objective of the Study- The main objective of the study was to investigate the performance of typical reinforced concrete facade walls subjected to blast and compare it to the performance of the same walls but with a steel plate added to the interior to make it a composite wall.

Explosives Used in the Study-The blast effects considered in these studies are the result of blasts due to a certain amount of condensed high explosives such as TNT. It is customary to convert other types of explosives, such as fertilizer-based devices used by terrorists in the past to equivalent TNT. The explosive device considered in these studies was a small device placed at a standoff distance of 10 feet representing small car bombs hidden in the trunk of a car and parked on the curb in front of a building as shown in the figure below.

Energy of Explosion-According to Kinney and Graham (1985), the energy released from a standard gram of TNT is 4610 Joules. In reality and depending on the method of measurement, the energy may be slightly different. In our studies, we used the above value of energy for TNT or TNT equivalent explosives. The analytical studies consisted of building a 3-dimensional model of the case study building with traditional reinforced concrete walls and composite walls and subjecting the models to various sizes of explosives. In the following, the case study building is discussed and its details are provided.

Architectural Aspects of Study Building: The study building is a typical 3-story building for office or residential use. The building is considered representative of many low-rise structures in civilian or military use as residential buildings and barracks, office buildings, hospitals, command and control centers and other facilities. Although the study building was only three stories tall, since the main damage due to car bomb is in the lower three floors of the building, the results are somewhat useful to shed light on the performance of the lower 3 floors of taller buildings.

Figure 4.31. Base Shear-Drift Curves for Composite Shear Wall Specimen in Figure 4.30 (Zhao and Astaneh-Asl, 2003)

Story Drift, ( ∆∆∆∆/L)

-0.006 -0.004 -0.002 0 0.002 0.004 0.006

1.0

0

-1.0

Shear, (V/Vmax)

5% Drift

5% Drift


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A typical floor plan is shown in Figure 4.32. The exterior walls are pre-cast concrete panels with windows in some of the panels as shown while other panels are solid infill panels with no openings. The interior walls around the stairwell are permanent masonry walls. The floor area has some partitions.

Structural Aspects of Study Building: Foundations are spread footings supported on soil. The building is a steel framed structure with HSS tube columns. The steel structure consists of perimeter moment frames and interior gravity columns. The floors consist of typical 6-inch concrete slab on steel deck floors. The shear connections of simply supported beams to columns are shear tabs. The moment connections of perimeter frames are welded connections. The base plates for columns are axially loaded base plates with four anchor bolts and act as pin connections. The lateral load resisting system is the perimeter moment frame.

Modeling of Material -The steel used in our study had material properties similar to A572-Gr. 50 steel given in Table 4.2.

Table 4.2. Material Properties of Steel Used in the Study

A572 Gr. 50 Properties

US (ksi) SI (N/mm2)

E 29000 199955

Eh 72 497 Modulus

Eh /E (%) 0.25 0.25

Fy (0.2%) 50 345 Stress

Fu 65 448

yε 0.00172 0.00172 Strain

uε 0.21 0.21

Figure 4.32. Plan View and a Cross Section of the Study Building (Astaneh-Asl, Heydari, Zhao 2003)

a

Walls with Large Windows

Partitions

Utilities

& Stairs

a

N

Walls with No Opening

15 ft. (Typ.)

Section a-a

3@14 ft.


75

Modeling of Explosives -the amount of explosive corresponding to a regular bag bomb and a car bomb was used in the studies. The standoff distances for the explosives are given in Figure 4.33.

MSC Dytran software (MSC, 2010) was used to simulate the explosive and study the structural response. The sophisticated software program is a three-dimensional simulation code developed by MSC Software Corporation (www.mscsoftware.com) for analyzing the dynamic, nonlinear behavior of structures, gases and fluids. It uses explicit time integration and incorporates features to simulate a wide range of material and geometric nonlinearities.

Explicit time integration is particularly suitable for analyzing short duration transient dynamic effects such as explosives and blast waves that involve large deformation, failure and fluid-structure interaction. MSC.Dytran (MSC, 2010) couples a finite strain, large deflection structural finite element idealization (Lagrange) with a finite volume fluid-gas flow simulation (Eulerian). The Eulerian calculations solve the finite volume equations in integral form over a constant volume, conserving mass, momentum and total energy at all time steps.

To model the problem, two Eulerian regions were defined to investigate the blast dynamics inside and outside the structure. The outer Eulerian region included the air outside the building as well as the explosive while the inner Eulerian region included only the air inside the building as an initial condition. A technique called the “blast pressure” method was used to simulate the explosive by initializing a certain area inside the outer Eulerian region with high density and specific internal energy. Once the explosive is detonated, the pressure wave is propagated within the Eulerian volume by transporting mass, momentum and energy from one element to the next until it reaches the structure which acts as coupling surface between the two Eulerian domains. The Eulerian regions were defined large enough to accommodate for varying the distance of the explosive with respect to the building and allow pressurized air to interact freely with the structure. By changing the initial conditions of the outer Eulerian region, different amounts of explosives were simulated at close proximities of the structure.

Modeling of the 3-story Building-The case study building was designed using a traditional steel structure and reinforced concrete or composite exterior walls. Both buildings were subjected to the same explosion and their performances were compared. Two levels of explosion were considered; a bag bomb and a car bomb

3 ft

10ft

Figure 4.33. Position of Explosives Relative to the Study Building


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The building with the traditional reinforced concrete exterior walls and the similar building using composite wall system were modeled inelastically. The models were subjected to realistic simulations of explosives placed at a close standoff distance of 10 feet horizontal and 3 feet vertical as shown in Figure 4.33.

Structural members of the case study buildings were modeled using shell elements available in MSC Dytran. The beams, columns, slabs, the reinforced concrete and the masonry alls were modeled using homogenous shell elements with homogeneous material, while the composite shear wall was modeled using composite shell elements with composite material in which two layers of material, steel and concrete wall, were put together and their composite action was considered. All homogeneous materials were modeled as bi-linear (elastic-strain-hardening) material.

4.9 c. Analysis Results of a 3-Story Case Study Bui lding Subjected to Blast

Behavior of 3-story Buildings under Bag Bomb- The traditional R/C exterior walls failed when subjected to a bag bomb. The flying rubble from the shattered concrete exterior wall entered the building with very high velocity. Such performance is obviously unacceptable since the flying debris entering the building can seriously injure or kill the occupants. Figure 4.34 shows the maximum effective stress on the outside surface of the exterior walls and structural members. The figure on the left corresponds to the 3-story building with reinforced concrete exterior walls while the figure on the right represents the building with exterior walls made composite by adding steel plate to the interior face of the R/C walls.

When the case-study building with composite exterior walls was subjected to the same bag bomb, the performance was much improved. The composite walls did not yield under the bag bomb and although the outside concrete wall was crushed under the pressure of the blast, no projectile entered the building. This was because the steel plate behind the concrete wall did not fracture and it kept the debris outside protecting the occupants from injury and death.

Failure of R/C Wall

MSC Software Corporation MSC Software Corporation

(a) Reinforced Concrete Exterior Wall (b) Com posite (Steel Plate + Concrete) Exterior Walls

Figure 4.34. Behavior of R/C Walls and Composite Walls Subjected to Bag Bomb (Astaneh-Asl, Heydari and Zhao, 2003)


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Behavior of 3-story Buildings under Car Bomb-When the case study building with reinforced concrete exterior walls was subjected to a “car bomb, the reinforced concrete facade walls in all three floors near the explosion totally shattered as shown in Figure 4.35 (a). The flying rubble from the shattered concrete wall entered the building with very high velocity.

The composite exterior walls only yielded and developed a very small fracture at the base when subjected to a car bomb, Figure 4.35(b). No projectile entered the building in this case, which means the injuries would be minor if any. Table 4.3 shows a summary of the performance of the buildings with traditional reinforced concrete walls and proposed steel composite walls subjected to the same blast effects of a bag bomb and a car bomb.

Table 4.3. Comparison of Behavior of Buildings Subjected to the Same Amount of TNT Charge

Steel Plate Concrete Wall Charge

Case Name Thickness

(in) Fy

(ksi)

Thickness

(in) f`c

(ksi) Equivalent TNT (lb)

Summary of Performance

3ST_20_S0050_C44

No steel plate used

50 4 4 (20 lb)

Bag bomb Concrete wall crushed. Minor yielding of steel column.

3ST_20_S5050_C44

0.5 50 4 4 (20 lb)

Bag bomb Steel plate remained elastic. Concrete wall did not crush. Minor yielding in steel column

3ST_700_S0050_C44

No steel plate used

50 4 4 (700 lb)

Car bomb Concrete wall totally shattered. Steel frame fractured and buckled.

3ST_700_S5050_C44

0.5 50 4 4 (700 lb)

Car bomb

Steel plate yielded but not fractured. Concrete wall crushed. Column between the walls fractured at bottom.

Figure 4.35. Behavior of R/C Walls and Composite Walls Subjected to Car Bomb (Astaneh-Asl, Heydari, Zhao, 2003)

Yielding of steel plate and minor fracture in steel plate. There was no flying debris

since concrete debris stayed outside the building.

Shattering of R/C wall and release of dangerous high speed flying concrete

debris

a. Reinforced Concrete Wall b. Composite (Steel Plate + Concr ete) Walls

MSC Software Corporation MSC Software Corporation


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4.9.d. Summary of Blast Studies of a 3-story Buildi ng

Both R/C and the composite exterior walls studied had the same 4 inch thick concrete wall. The composite wall had a ½ inch thick steel plate connected to the 4-inch concrete wall. Both walls were designed for the same shear capacity following the ACI and AISC design codes respectively. In the R/C walls, concrete and reinforcement provided the shear resistance while in composite walls, the shear force was resisted by steel plate alone. The concrete in composite shear wall had only nominal amount of reinforcement and was only used to provide bracing for the steel plate and prevent its premature buckling before the steel plate reached its shear yielding. For more information on design of composite shear walls see the Steel TIPS report by the author (Astaneh-Asl, 2002). Two 3-story buildings, one with R/C and the other with composite exterior walls were subjected, analytically, to “bag bomb (20 pounds TNT equivalent charge) and “car bomb” (700 pounds TNT equivalent charge) and their behavior and level of damage were studied.

Based on the results of those studies, the following conclusions were reached:

1. By using the composite exterior walls, the blast resistance of the building increased several times compared to the case of building with reinforced concrete walls.

2. The reinforced concrete exterior walls at the ground floor failed when subjected to a bag bomb while the composite wall remained essentially elastic under the same bag bomb blast.

3. The reinforced concrete exterior wall was shattered by the car bomb while in the composite wall, although the concrete portion of the wall on the outside failed, the steel plate on the inside face of the wall only yielded and did not fracture.

4. In the case of the reinforced concrete wall, the car bomb blast shattered the wall in the ground floor, caused significant damage in two floors above and created high-speed flying debris. Such high-speed flying objects are the main cause of serious injury and possible death to the occupants during blasts. In the case of the composite wall, the car bomb did shatter the R/C wall in front of the steel plate and yielded the steel plate but the blast failed to fracture the steel plate. As a result, the high velocity flying concrete pieces were prevented by the steel plate from entering the building and causing injuries to the occupants or damage to the material inside.

5. As mentioned above, in the case of R/C exterior wall, the car bomb blast not only shattered the R/C walls in the ground floor but the R/C walls in the first and 2nd floors also were severely damaged and failed. With the loss of a column in the ground floor and the walls in all three floors during the blast, the floor beam in the first floor in this case was the only element to redistribute the floor loads of the lost column to adjacent columns. However, in the case of the composite wall, although the car bomb did shatter the R/C walls in front of the steel plate, it did not fracture the steel plates and did not shear off the column in front of the blast. After the blast, the steel plates in the first and second floor could easily transfer the gravity load of the bent column to the adjacent columns in shear and prevent a progressive collapse.

6. In the case of the R/C wall, the car bomb blast inelastically bent the column directly in front of the car bomb (see Figure 4.35 (a)) and sheared off the base of the column above the base plate. The base plate connection was designed to develop the shear capacity of


79

the column. But, for the case of the composite wall, although the blast did bend the column directly in front of the car bomb the column did not shear off (see Figure 4.34(right) above).

7. The final conclusion is that in this 3-story case study building, the reinforced concrete façade wall failed when subjected to a bag bomb (20 pounds TNT equivalent charge) while the proposed steel composite wall resisted much larger blast effects of a car bomb(700 pounds of TNT equivalent charge) without fracture.

4.10. COMPARISON OF CONSTRUCTION OF R/C AND PROPOSED BLAST RESISTANT COMPOSITE SHEAR WALLS

Figure 4.37 shows the main steps to be taken in construction of R/C and composite shear walls. Rough-order-of-Magnitude estimate of 3-story case study building indicated that by using steel composite walls the cost of construction is almost the same but the case study buildings with composite exterior walls had a blast resistance of more than 30 times that of the building with R/C exterior walls. Following are some of the advantages of using steel composite shear walls not only in seismic applications but also for blast resistance:

1. By using the proposed composite wall system, a significant amount of form work is eliminated because of the presence of steel plate, so there is no need to dispose of the forms into the environment or use resources to recycle the forms. According to a knowledgeable structural engineer, familiar with design and construction of steel, concrete and composite buildings, the overall cost of the building using composite steel plate shear walls is expected to be quite similar to that using traditional reinforced concrete shear walls. However, according to him the composite walls themselves would be probably 40% to 50% more expensive compared to the similar reinforced concrete shear wall.

2. The design of the proposed composite shear wall system is based on well-established principles of design of composite systems practiced today and incorporated into currently governing codes; namely AISC-LRFD Seismic Provisions (AISC, 2005b) and ACI-318 (ACI, 2008). Therefore, such systems can be readily applied by the structural engineers in design of hardened buildings.


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Figure 4.37. Steps in Construction of an R/C Wall (top) and a Composite Wall (Bottom) Within a Steel Structure (Astaneh-Asl, Heydari and Zhao, 2003)

Graphics by Mark Thomas


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About the Author: Abolhassan Astaneh-Asl, Ph.D., P.E., is a professor of structural engineering at the University of California, Berkeley. He is the winner of the 1998 T. R. Higgins Lectureship Award of the American Institute of Steel Construction. During the period 1978–1982, he completed his M.S. and Ph.D. in structural engineering, both at the University of Michigan in Ann Arbor. Since 1982, he has been a faculty member of structural engineering involved in teaching, research, and design of both building and bridge structures in the United States and abroad He has conducted several major projects on seismic design and retrofit of steel long-span bridges and tall buildings. Since 1995, he has also been studying behavior of steel and composite structures, both buildings and bridges, subjected to blast and impact loads. He has been involved in testing and further development of technologies and design concepts to reduce blast damage and to prevent progressive collapse of steel and composite building and bridge structures subjected to terrorist blast (car bombs) or impact (planes and rockets) attacks. After the September 11, 2001, tragic terrorist attacks on the World Trade Center and the collapse of the towers, armed with a research grant from the National Science Foundation, he conducted a reconnaissance investigation of the collapse and collected perishable data. As an expert, he later testified before the Committee on Science of the House of Representative of the U.S. Congress on his findings regarding the collapse of the World Trade Center towers. His current research includes studies of blast effects on steel and composite long-span bridges and elevated freeways with the aim of developing technologies to prevent progressive collapse of these important transportation links. Since 2004 he has been doing research on earthquake hazard reduction in the Middle East, particularly he has focused on developing low-cost steel frame buildings for rural areas of seismic countries in the Middle East. His recent projects include investigation of the April 29, 2007 collapse of the two spans of the MacArthur Maze elevated intersection in Northern California due to fire, funded by the National Science Foundation and investigation of the tragic progressive collapse of the Interstate 35W in Minneapolis. His current research includes protection of bridges and buildings against blast and impact of projectile, progressive collapse prevention of steel and composite bridges and buildings, seismic behavior and design of steel and composite structures and connections of steel and composite buildings and bridges. Since 1967, he has been involved in design of numerous structures and in providing consulting to structural engineering teams both in bridges and in buildings. He is currently the lead engineer in structural design of the Century development in Doha, Qatar being done by the Chicago Design Consortium International (CDCi). The tallest structure is a steel/composite 48 story 2-tower high rise with top of the towers connected to each other with a sky-bridge restaurant. Contact information for the author: Phone: (925) 946-0903, E-mail: [email protected] Web: http://www.astaneh.net and http://www.ce.berkeley.edu/~astaneh


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List of Published Steel TIPS Reports -------------------------------------------------------------------------------------------------------------------------------

The following reports have been published by the Structural Steel Educational Council (SSEC). The reports have been peer reviewed by members of the SSEC and other professionals. Copies of the Steel TIPS reports can be downloaded from www.steeltips.org, free of charge by California and Nevada residents and for a nominal fee for others. Funding for Steel TIPS publication is provided by the California Field Iron Workers Administrative Trust, a Union Trust Fund.

04.2010 > Gusset Plates in Steel Bridges-Design and Evaluation

04.2010 > Steel Plate Shear Walls: An Option for Lateral Resistance in High-Rise Core Wall Buildings

12.2009 > Economy of Steel-Framed Buildings For Seismic Loading

10.2008 > A Comparison of Frame Stability Analysis Methods in AISC 360-05

09.2008 > Quality Assured Steel Bridge Fabrication and Erection

06.2008 > Seismic Behavior and Design of Base Plates in Braced Frames

04.2008 > Cost-Effective Steel Bridge Fabrication and Erection

06.2007 > Early California Accelerated Steel Bridge Construction

06.2007 > Design of RBS Connections for Special Moment Frames

05.2007 > Progressive Collapse Prevention of Steel Frames with Shear Connections

12.2006 > Seismic Detailing of Gusset Plates for Special Concentrically Braced Frames

08.2006 > Alfred Zampa Memorial Steel Suspension Bridge

07.2006 > Buckling and Fracture of Concentric Braces Under Inelastic Loading

08.2005 > Notes on Design of Double-Angle and Tee Shear Connections for Gravity and Seismic Loads

06.2005 > Design of Shear Tab Connections for Gravity and Seismic Loads

07.2004 > Seismic Design of Buckling-Restrained Braced Frames

05.2004 > Special Concentric Braced Frames

12.2003 > Steel Construction in the New Millennium

08.2002 > Cost Considerations for Steel Moment Frame Connections

06.2002 > Use of Deep Columns In Special Steel Moment Frames

05.2002 > Seismic Behavior and Design of Composite Steel Plate Shear Walls

09.2001 > Notes on Design of Steel Parking Structures - Including Seismic Effects

07.2001 > Seismic Behavior and Design of Steel Shear Walls

06.2001 > Metal Roof Construction on Large Warehouses or Distribution Centers

03.2001 > Large Seismic Steel Beam-to-Column Connections

10.1999 > Welded Moment Frame Connections With Minimal Residual Stress

08.1999 > Design of Reduced Beam Section (RBS) Moment Frame Connections

07.1999 > Practical Design and Detailing of Steel Column Base Plates

12.1998 > Seismic Behavior and Design of Gusset Plates

03.1998 > Compatibility of Mixed Weld Material

08.1997 > Dynamic Tension Tests of Simulated Moment Resisting Frame Weld Joints

04.1997 > Seismic Design of Steel Column-Tree Moment Resisting Frames

01.1997 > Quick Reference Guide for Structural Steel Welding Practices

12.1996 > Seismic Design Practice For Eccentrically Braced Frames

11.1995 > Seismic Design of Special Concentrically Braced Steel Frames


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07.1995 > Seismic Design of Bolted Steel Moment-Resisting Frames

04.1995 > Structural Details to Increase Ductility of Connections

12.1994 > USE OF STEEL IN THE SEISMIC RETROFIT of HISTORIC OAKLAND CITY HALL

12.1993 > Common Steel Erection Problems and Suggested Solutions

10.1993 > Heavy Structural Shapes in Tension Applications

03.1993 > Structural Steel Construction in The 90's

10.1992 > Economical Use of Cambered Steel Beams

08.1992 > Value Engineering and Steel Economy

07.1992 > SLOTTED BOLTED CONNECTION ENERGY DISSIPATERS

06.1992 > What Design Engineers Can Do To Reduce Fabrication Costs

04.1992 > Designing for Cost Efficient Fabrication

01.1992 > STEEL DECK CONSTRUCTION

03.1991 > LRFD-COMPOSITE BEAM DESIGN with METAL DECK

12.1990 > Design of single Plate Shear Connections

11.1990 > Design of Small Base Plates for Wide Flange Columns*

05.1989 > The Economies of. LRFD in Composite Floor Beams

03.1989 > EXTERIOR WALL CONNECTIONS TO STEEL FRAMING

01.1987 > COMPOSITE BEAM DESIGN WITH METAL DECK

02.1986 > UN FIRE PROTECTED EXPOSED STEEL PARKING STRUCTURES

11.1976 > STEEL HIGH-RISE BUILDING FIRE

01.0 > Design Practice to Prevent Floor Vibrations

01.0 > Fireproofing Open-Web Joists & Girders


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Funding provided by the California Field Iron Worke rs Administrative Trust

A Union Trust Fund

Tel. (925) 631-1313 Fax. (925) 631-1112

Fred Boettler, Administrator

Steel TIPS may be viewed and downloaded at www.steeltips.org

STRUCTURAL STEEL EDUCATIONAL COUNCIL

Steel

Participating Members of SSEC

ABOLHASSAN ASTANEH-ASL, Ph.D., P.E.; UNIVERSITY OF CALIFORNIA, BERKELEY

MICHAEL COCHRAN, S.E.; WEIDLINGER ASSOCIATES, INC.

RICH DENIO, S.E.; RUTHERFORD & CHEKENE

RICH DEVEAU; ALBANY STEEL, INC.

JEFFREY EANDI, P.E.; EANDI METAL WORKS, INC.

PATRICK M. HASSETT, S.E.; HASSETT ENGINEERING, INC.

WILLIAM KAVICKY; TRANS BAY STEEL CORP.

JOHN KONECHNE, P.E.; CALIFORNIA ERECTORS, INC

BRETT MANNING, S.E.; SCHUFF STEEL CO.

KEVIN MOORE, S.E.;CETUS CONSULTING INC.

JAY MURPHY; MURPHY PACIFIC CORPORATION

RICHARD PERSONS; PERSONS & ASSOCIATES

PAUL RANGEL; PDM STEEL SERVICE CENTERS, INC.