policy guide to steel moment frame conctructions

8/7/2019 Policy Guide to Steel Moment Frame Conctructions

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A Policy Guide toSteel Moment Frame

Construction


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DISCLAIMER

This document provides information on the seismic performance of steel moment-frame structures and theresults and recommendations of an intensive research and development program that culminated in a seriesof engineering and construction criteria documents. It updates and replaces an earlier publication with thesame title and is primarily intended to provide building owners, regulators, and policy makers with summary

level information on the earthquake risk associated with steel moment-frame buildings, and measures thatare available to address this risk. No warranty is offered with regard to the recommendationscontained herein, either by the Federal Emergency Management Agency, the SAC Joint Venture, theindividual Joint Venture partners, or their directors, members or employees or consultants. Theseorganizations and their employees do not assume any legal liability or responsibility for theaccuracy, completeness, or usefulness of any of the information, products or processes included inthis publication. The reader is cautioned to review carefully the material presented herein andexercise independent judgment as to its suitability for specific applications. This publication has beenprepared by the SAC Joint Venture with funding provided by the Federal Emergency Management Agency,under contract number EMW-95-C-4770.

Cover Art. The background photograph on the cover of this guide for Policy Makers is a cityscape of aportion of the financial district of the City of San Francisco. Each of the tall buildings visible in thiscityscape is a steel moment-frame building. Similar populations of these buildings exist in most otherAmerican cities and many thousands of smaller steel moment-frame buildings are present around theUnited States as well. Until the 1994 Northridge earthquake, many engineers regarded these buildings ashighly resistant to earthquake damage. The discovery of unanticipated fracturing of the steel framingfollowing the 1994 Northridge earthquake shattered this belief and called to question the safety of thesestructures.


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A Policy Guide toSteel Moment-frame Construction

SAC Joint Venturea partnership of:

Structural Engineers Association of California (SEAOC)Applied Technology Council (ATC)

California Universities for Research in Earthquake Engineering (CUREe)

Prepared for SAC Joint Venture byRonald O. Hamburger

Project Oversight Committee

William J. Hall, Chair

Shirin AderJohn M. Barsom

Roger FerchTheodore V. Galambos

John GrossJames R. HarrisRichard Holguin

Nestor IwankiwRoy G. Johnston

Len JosephDuane K. Miller

John TheissJohn H. Wiggins

SAC Project Management CommitteeSEAOC: William T. Holmes

ATC: Christoper RojahnCUREe: Robin Shepherd

Program Manager: Stephen A. MahinProject Director for Topical Investigations:

James O. MalleyProject Director for Product Development:

Ronald O. Hamburger

SAC Joint Venture

Structural Engineers Association of Californiawww.seaoc.org

Applied Technology Councilwww.atcouncil.org

California Universities for Research in Earthquake Engineeringwww.curee.edu

November, 2000


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THE SAC JOINT VENTURE

SAC is a joint venture of the Structural Engineers Association of California (SEAOC), the AppliedTechnology Council (ATC), and California Universities for Research in Earthquake Engineering (CUREe),formed specifically to address both immediate and long-term needs related to solving performance

problems with welded, steel moment-frame connections discovered following the 1994 Northridgeearthquake. SEAOC is a professional organization composed of more than 3,000 practicing structuralengineers in California. The volunteer efforts of SEAOCs members on various technical committeeshave been instrumental in the development of the earthquake design provisions contained in the UniformBuilding Codeand the 1997 National Earthquake Hazards Reduction Program (NEHRP) RecommendedProvisions for Seismic Regulations for New Buildings and Other Structures. ATC is a nonprofitcorporation founded to develop structural engineering resources and applications to mitigate the effects ofnatural and other hazards on the built environment. Since its inception in the early 1970s, ATC hasdeveloped the technical basis for the current model national seismic design codes for buildings; the de-factonational standard for post earthquake safety evaluation of buildings; nationally applicable guidelinesand procedures for the identification, evaluation, and rehabilitation of seismically hazardous buildings; andother widely used procedures and data to improve structural engineering practice. CUREe is a nonprofitorganization formed to promote and conduct research and educational activities related to earthquakehazard mitigation. CUREes eight institutional members are the California Institute of Technology,Stanford University, the University of California at Berkeley, the University of California at Davis, theUniversity of California at Irvine, the University of California at Los Angeles, the University of California atSan Diego, and the University of Southern California. These university earthquake research laboratory,library, computer and faculty resources are among the most extensive in the United States. The SACJoint Venture allows these three organizations to combine their extensive and unique resources,augmented by consultants and subcontractor universities and organizations from across the nation, intoan integrated team of practitioners and researchers, uniquely qualified to solve problems related to theseismic performance of steel moment-frame structures.

ACKNOWLEDGEMENTS

Funding for Phases I and II of the SAC Steel Program to Reduce the Earthquake Hazards of Steel

Moment-Frame Structures was principally provided by the Federal Emergency Management Agency, withten percent of the Phase I program funded by the State of California, Office of Emergency Services.Substantial additional support, in the form of donated materials, services, and data has been provided bya number of individual consulting engineers, inspectors, researchers, fabricators, materials suppliers andindustry groups. Special efforts have been made to maintain a liaison with the engineering profession,researchers, the steel industry, fabricators, code-writing organizations and model code groups, buildingofficials, insurance and risk-management groups, and federal and state agencies active in earthquakehazard mitigation efforts. SAC wishes to acknowledge the support and participation of each of the abovegroups, organizations and individuals. In particular, we wish to acknowledge the contributions provided bythe American Institute of Steel Construction, the Lincoln Electric Company, the National Institute ofStandards and Technology, the National Science Foundation, and the Structural Shape ProducersCouncil. SAC also takes this opportunity to acknowledge the efforts of the project participants themanagers, investigators, writers, and editorial and production staff whose work has contributed to thedevelopment of these documents. Finally, SAC extends special acknowledgement to Mr. MichaelMahoney, FEMA Project Officer, and Dr. Robert Hanson, FEMA Technical Advisor, for their continuedsupport and contribution to the success of this effort.


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1

INTRODUCTION

The Northridge earthquake of January 17, 1994,caused widespread building damage throughoutsome of the most heavily populated communities

of Southern California including the SanFernando Valley, Santa Monica and West LosAngeles, resulting in estimated economic lossesexceeding $30 billion. Much of the damagesustained was quite predictable, occurring intypes of buildings that engineers had previouslyidentified as having low seismic resistance andsignificant risk of damage in earthquakes. Thisincluded older masonry and concrete buildings,but not steel framed buildings. Surprisingly,however, a number of modern, welded, steel,moment-frame buildings also sustainedsignificant damage. This damage consisted of abrittle fracturing of the steel frames at the welded

joints between the beams (horizontal framingmembers) and columns (vertical framingmembers). A few of the most severely damagedbuildings could readily be observed to be out-of-plumb (leaning to one side). However, many ofthe damaged buildings exhibited no outwardsigns of these fractures, making damagedetection both difficult and costly. Then, exactlyone year later, on January 17, 1995, the city ofKobe, Japan also experienced a largeearthquake, causing similar unanticipateddamage to steel moment-frame buildings.

Following discovery of hidden damage inLos Angeles area buildings, the potential forsimilar, undiscovered damage in SanFrancisco and other communities affected bypast earthquakes was raised.

Ventura Boulevard in the San Fernando

Valley. Many of these buildings had hiddendamage.

Prior to the 1994 Northridge and 1995 Kobeearthquakes, engineers believed that steelmoment-frames would behave in a ductilemanner, bending under earthquake loading, butnot breaking. As a result, this became one ofthe most common types of construction used formajor buildings in areas subject to severeearthquakes. The discovery of the potential forfracturing in these frames called to question theadequacy of the building code provisions dealingwith this type of construction and created a crisisof confidence around the world. Engineers didnot have clear guidance on how to detectdamage, repair the damage they found, assessthe safety of existing buildings, upgradebuildings found to be deficient or design newsteel moment-frame structures to performadequately in earthquakes. The observeddamage also raised questions as to whetherbuildings in cities affected by other pastearthquakes had sustained similar undetecteddamage and were now weakened and potentiallyhazardous. In fact, some structures in the SanFrancisco Bay area have been discovered to

have similar fracture damage most probablydating to the 1989 Loma Prieta earthquake.

In response to the many concerns raised bythese damage discoveries, the FederalEmergency Management Agency (FEMA)sponsored a program of directed investigationand development to identify the cause of thedamage, quantify the risk inherent in steelstructures and develop practical and effective


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engineering criteria for mitigation of this risk.FEMA contracted with the SAC Joint Venture, apartnership of the Structural EngineersAssociation of California (SEAOC), aprofessional association with more than 3,000members; the Applied Technology Council

(ATC), a non-profit foundation dedicated to thetranslation of structural engineering research intostate-of-art practice guidelines; and theCalifornia Universities for Research inEarthquake Engineering (CUREe), a consortiumof eight California universities withcomprehensive earthquake engineeringresearch facilities and personnel. The resultingFEMA/SAC project was conducted over a periodof 6 years at a cost of $12 million and includedthe participation of hundreds of leadingpracticing engineers, university researchers,industry associations, contractors, materialssuppliers, inspectors and building officials from

around the United States. These efforts werecoordinated with parallel efforts conducted byother agencies, including the National ScienceFoundation and National Institute of Standardsand Technology (NIST), and with concurrentefforts in other nations, including a largeprogram in Japan. In all, hundreds of tests ofmaterial specimens and large-scale structuralassemblies were conducted, as well asthousands of computerized analyticalinvestigations.

As the project progressed, interim guidance

documents were published to provide practicingengineers and the construction industry withimportant information on the lessons learned, aswell as recommendations for investigation,repair, upgrade, and design of steel moment-frame buildings. Many of theserecommendations have already beenincorporated into recent building codes. Thisproject culminated with the publication of fourengineering practice guideline documents.These four volumes include state-of-the-artrecommendations that should be included infuture building codes, as well as guidelines thatmay be applied voluntarily to assess and reducethe earthquake risk in our communities.

This policy guide has been prepared to provide anontechnical summary of the valuableinformation contained in the FEMA/SACpublications, an understanding of the riskassociated with steel moment-frame buildings,and the practical measures that can be taken to

reduce this risk. It is anticipated that this guidewill be of interest to building owners and tenants,members of the financial and insuranceindustries, and to government planners and thebuilding regulation community.

Program

toReducetheEarthquakeHazardsof

SteelMomentFrameStructures

FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 353 / July, 2000

Recommended Seismic

Design Criteria For

New Steel Moment-

Frame Buildings

Program



FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 352 /July, 2000

Recommended Seismic

Design Criteria ForNew Steel Moment-

Frame Buildings

Progr

am



FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 351 /July, 2000

Recommended Seismic

Design Criteria For

New Steel Moment-

Frame Buildings

Program

toRe

ducetheEarthquakeHazardsof

SteelM

omentFrameStructures


Recommended Seismic

Design Criteria For

New Steel Moment-

Frame Buildings

FEMA 350 Recommended Seismic DesignCriteria for New Steel Moment-FrameBuildings

FEMA 351 Recommended Seismic Evaluationand Upgrade Criteria for Existing WeldedSteel Moment-Frame Buildings

FEMA 352 Recommended Post-earthquakeEvaluation and Repair Criteria for WeldedSteel Moment-Frame Buildings

FEMA 353 Recommended Specifications andQuality Assurance Guidelines for SteelMoment-Frame Construction for SeismicApplications


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AN OVERVIEW OF THE PROBLEM

What is a steel moment-frame building?

All steel-framed buildings derive basic structural

support for the building weight from a skeleton(or frame) composed of horizontal steel beamsand vertical steel columns. In addition to beingable to support vertical loads, including theweight of the building itself and the contents,structures must also be able to resist lateral(horizontal) forces produced by wind andearthquakes. In some steel frame structures,this lateral resistance is derived from thepresence of diagonal braces or masonry orconcrete walls. In steel moment-framebuildings, the ends of the beams are rigidly

joined to the columns so that the buildings can

resist lateral wind and earthquake forces withoutthe assistance of additional braces or walls.This style of construction is very popular formany building occupancies, because theabsence of diagonal braces and structural wallsallows complete freedom for interior spacelayout and aesthetic exterior expression.

Columns

Beams

Beam/ColumnConnection

VerticalForce

LateralForce

A steel moment-frame is an assembly ofbeams and columns, rigidly joined togetherto resist both vertical and lateral forces.

Construction of a modern steel framebuilding in which the ends of beams arerigidly joined to columns by weldedconnections.

Are all steel moment-frame buildingsvulnerable to the type of damage thatoccurred in the Northridge earthquake?

The steel moment-frame buildings damaged in

the 1994 Northridge earthquake are a specialtype, known as welded steel moment-frames(WSMF). This is because the beams andcolumns in these structures are connected withwelded joints. WSMF construction first becamepopular in the 1960s. In earlier buildings, theconnections between the beams and columnswere either bolted or riveted. While these olderbuildings also may be vulnerable to earthquakedamage, they did not experience the type ofconnection fractures discovered following theNorthridge earthquake. Generally, welded steelmoment-frame buildings constructed in the

period 1964-1994 should be consideredvulnerable to this damage. Buildingsconstructed after 1994 and incorporatingconnection design and fabrication practicesrecommended by the FEMA/SAC program areanticipated to have significantly less vulnerability.


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What does the damage consist of?

The damage discovered in WSMF buildingsconsists of a fracturing, or cracking, of thewelded connections between the beams andcolumns that form the frame, or skeleton, of thestructure. This damage occurs most commonlyat the welded joint between a column and thebottom flange of a beam. Once a crack hasstarted, it can continue in any of several differentpatterns and in some cases has been found tocompletely sever beams or columns.

Steel backing

Column flange

Beam bottom flangeWeld

Fracture

Column flange

Beam bottomflange

Damage consists of fractures or cracks thatinitiate in the welded joints of the beams tocolumns.

Damage ranges from small cracks that aredifficult to see, to much larger cracks. Here,a crack began at the weld and progressedinto the column flange, withdrawing a divotof material.

What does the damage look like?

There are several common types of damage,each of which looks somewhat different. Themost common cracks initiate in the weld itself or

just next to the weld. These cracks often arevery thin and difficult to see. In a few cases,cracks cannot be seen at all. In some cases,cracks cause large scoop-like pieces of thecolumn flange, called divots, to be pulled out. Instill other cases, the cracks run across the entirecolumn, practically dividing it into twounconnected pieces.

What is the effect of the damage?WSMF buildings rely on the connectionsbetween their beams and columns to resist windand earthquake loads. When the welded jointsthat form these connections break, the buildingloses some of the strength and stiffness it needsto resist these loads. The magnitude andsignificance of this capacity loss depends on theunique design and construction attributes ofeach building, as well as the extent and type ofdamage sustained. Few buildings weredamaged so severely in the Northridgeearthquake that they represented imminent

collapse hazards. However, significantweakening of some buildings did occur. Oncethe welded joints fracture, other types of damagecan also occur including damage to bolted joints.Damage that results in the complete severing ofbeams or columns or their connections poses aserious problem and could result in the potentialfor localized collapse.

Fracturing of welded connections can lead todamage to the bolted connections that holdthe beams onto the columns, creatingpotential for localized collapse.


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Why did this damage occur?

We now understand that the vulnerability ofWSMF structures is a result of a number of inter-related factors. Early research, conducted in the1960s and 70s suggested that a particular styleof connection could perform adequately.Designers then routinely began to specify thisconnection in their designs. However, theparticular style of connection tends toconcentrate high stresses at some of theweakest points in the assembly, and in fact,some of the early research showed somepotential vulnerability. As the cost ofconstruction labor increased, relative to the priceof construction materials, engineers adopteddesigns that minimized the number ofconnections in each building, resulting in largermembers and increased loads on theconnections. At the same time, the industry

adopted a type of welding that could be used tomake these connections more quickly, butsometimes resulted in welds that were moresusceptible to cracking. Although building codesrequired that inspectors ensure the quality of thiswelding, the inspection techniques andprocedures used were often not adequate.Finally, the steel industry found new ways toeconomically produce structural steel with higherstrength. Although the steel became stronger,designers were unaware of this and continued tospecify the same connections. Often, theseconnections did not have adequate strength to

match the newer steel material and weretherefore, even more vulnerable. In the end, thetypical connections used in WSMF buildingswere just not adequate to withstand the severedemands produced by an earthquake.

Fractures commonly initiate at the weldedjoint of the beam bottom flange to column.

BeamBeam

ColumnColumn

WeldsWelds

The typical connection used prior to 1994.Severe stress concentrations inherent in its

configuration were not considered in thedesign.

How widespread was this damage?

Although no comprehensive survey of all of thesteel buildings affected by the Northridgeearthquake has been conducted, the City of LosAngeles did enact an ordinance that requiredmandatory inspection of nearly 200 buildings inareas that experienced the most intense groundshaking. Initial reports from this mandatoryinspection program erroneously indicated that

nearly every one of these buildings hadexperienced damage and in some cases, thatthis damage was extensive. It was projectedthat perhaps thousands of buildings had beendamaged. It is now known that damage wasmuch less widespread than originally thoughtand that many of the conditions that wereoriginally identified as damage actually wereimperfections in the original construction work.Of the nearly 200 buildings that were inspectedunder the City of Los Angeles ordinance, it nowappears that only about 1/3 had any actualearthquake damage and that more than 90% ofthe total damage discovered occurred within asmall group of approximately 30 buildings.Therefore, although this damage was significant,and does warrant a change in the design andconstruction practices prevalent prior to 1994, itappears that the risk of severe damage tobuildings is relatively slight, except under veryintense ground shaking.


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Engineers expect steel to be ductile, capable

of extensive bending and deformationwithout fracturing, as shown in this testspecimen. The development of cracks in thesteel at relatively low levels of loading wasunexpected.

Why was this damage a surprise?

In its basic form, steel is a very ductile material,able to undergo extensive deformation anddistortion before breaking. This is the type ofbehavior desired for earthquake resistance, soengineers believed WSMF buildings would bequite earthquake resistant. Following the 1971San Fernando earthquake, the 1987 WhittierNarrows earthquake, and the 1989 Loma Prietaearthquake, there were few reports of significantdamage to steel buildings, especially ascompared to other types of construction,confirming this general belief. However,relatively few WSMF buildings were subjected tosevere ground motion in these events. As theindustrys confidence in the ability of WSMFbuildings to resist earthquake damage grew,design practice, material production processes

and construction techniques changed, addingunexpected vulnerability. The 1994 Northridgeearthquake was the first event in which a largenumber of recently constructed WSMF buildingswere subjected to strong ground motion.

Have other earthquakes caused similardamage?

When damaged WSMF buildings were firstdiscovered following the Northridge earthquake,there was speculation that this was a result ofsome peculiar characteristic of the earthquakeitself or of local design and constructionpractices in the Los Angeles region. It has nowbeen confirmed that similar damage occurred tosome buildings affected by the 1989 Loma Prietaearthquake and also the 1992 Landers and BigBear earthquakes. In 1995, the Kobeearthquake resulted in damage to severalhundred steel buildings, and the collapse of 50older steel buildings. Japanese researchershave confirmed problems similar to thoseexperienced in the Northridge earthquake.

Engineering researchers around the world haverecognized this behavior as a common problemfor welded steel moment-frame buildingsdesigned and constructed using practicesprevalent prior to the Northridge earthquake andhave been working together to find solutions.

Many steel moment-frame buildings weredamaged in the 1995 Kobe earthquake. Thebuilding shown here, lying on its side, is one

of more than 50 older steel buildings thatcollapsed.


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How many WSMF buildings are there?

Thousands of WSMF buildings have beenconstructed in all regions of the United States.One of the factors contributing to theseriousness of this problem is that WSMFconstruction is often used in many of the nationsmost important facilities, including hospitalbuildings, emergency command centers andother federal, state and local government officebuildings. It is commonly used for commercialoffice structures. Most high-rise buildingsconstructed in the United States in the last 30years incorporate this type of construction.WSMF construction has also frequently beenused for mid-rise and to a lesser extent, low-risecommercial and institutional construction,auditoriums and other assembly occupancies,and has seen limited application in industrialfacilities. It is relatively uncommon in residential

construction, although some high-risecondominium type buildings are of thisconstruction type.

Many of the tallest buildings in our majorcities are of WSMF construction.

Are existing WSMF structures safe?

No structure is absolutely safe and, if subjectedto sufficiently large loads, a structure made ofany material will collapse. Building codes in theUnited States permit structures to be damagedby strong earthquakes, but attempt to preventcollapse for the most severe levels of groundmotion which can be expected at the site. No

WSMF structure in the United States has evercollapsed as a result of earthquake loading.However, research conducted as part of theFEMA/SAC project confirms that WSMFstructures with the style of beam-columnconnection typically used prior to the Northridgeearthquake have a higher risk of earthquake-induced collapse than desired for new buildings.

How does the risk of WSMF structurescompare to other types of buildings?

Many other types of buildings present muchgreater risks of collapse than do WSMFbuildings. Based on observations from pastearthquakes, buildings that present greater risksinclude buildings of unreinforced masonryconstruction, buildings of concrete and masonry

construction that pre-date the mid-1970s,buildings of precast concrete construction andeven some types of wood frame construction.However, many WSMF buildings are large andhouse many occupants. The earthquake-inducedcollapse of one such structure could result in avery large and unacceptable life loss.

Older concrete frame, tilt-up and masonry buildings are generally more prone to earthquake-induced collapse than WSMF buildings.


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AFTER THE NEXT EARTHQUAKE

Can one tell if a WSMF building isdamaged?

Some damaged WSMF buildings permanentlylean to one side after an earthquake and havesevere damage to finishes, providing a clearindication that the building has sustainedstructural damage. However, many WSMFbuildings do not exhibit any obvious signs ofdamage. To determine if these buildings aredamaged, it is necessary to conduct anengineering inspection of the framing andconnections. To conduct these inspections, it isnecessary first to remove building finishes andfireproofing. These inspections can be disruptive

of occupancy and very expensive, ranging froma few hundred dollars to more than onethousand dollars per connection. Largebuildings may have several thousandconnections. The presence of asbestos,sometimes used in fireproofing in buildingsconstructed prior to 1979, substantially increasesthe inspection costs.

The most severely damaged buildings willoften lean to one side. However, there maybe no obvious indications of the damage insome buildings.

Is it safe to occupy a damaged building?

If a building is severely damaged, there is a riskthat aftershocks or subsequent strong winds orearthquakes may cause partial or total collapseof the building. The more severe the damage,the higher this risk. Most buildings damaged bythe Northridge earthquake were judged to besafe for continued occupancy while repairs weremade. FEMA 352provides engineeringprocedures that can be used by building officialsand engineers to quantify the risk associatedwith continued occupancy of damaged buildings.FEMA 352 also provides recommendations as to

when a building should be deemed unsafe forfurther occupancy.

How does an owner know if a building issafe?

Following an earthquake, building departmentswill typically perform rapid inspections of affectedbuildings to identify buildings that may beunsafe. Once such an inspection has beenperformed, the building department will typicallypost the building with a placard that indicateswhether the building is safe for occupancy.FEMA 352provides procedures that buildingofficials can use to conduct these rapidinspections. If the building department doesntprovide this service, building owners can retain

private engineers or inspection firms for thispurpose.


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Is the building department inspectionenough?

Rapid inspections conducted by buildingdepartments after an earthquake are intended to

identify those buildings at greatest risk ofendangering the public safety. They are notadequate to detect all damage a building hassustained. If a building has experienced strongground motion, the building owner should retainan engineer to conduct more thoroughinspections in accordance with FEMA 352, evenif the building department posts the building assafe.

After an earthquake, is it necessary toinspect every WSMF building?

The amount of damage sustained by a buildingis closely related to the severity of ground motionat the building site. Unless a buildingexperiences strong ground motion, it is unlikelythat it will sustain significant damage. FEMA 352recommends that all buildings thought to haveexperienced ground motion in excess of certainlevels be subjected to detailed inspections todetermine if they sustained significant damage.Because detailed inspections can be timeconsuming and costly, FEMA 352alsorecommends that an initial, rapid investigation beperformed to look for obvious signs of severedamage that could pose an immediate threat to

life safety.

Is it necessary to inspect everyconnection in a building?

The only way to ensure that all damage in astructure is found is to inspect all of theconnections. However, as previously discussed,connection inspections are costly and disruptive.Therefore most owners would prefer not toinspect every connection, if possible. Becausedamage tends to be distributed throughout astructure, it is possible to inspect a sample of the

total number of connections in a building andmake judgments as to how widespread damageis likely to be throughout the entire building.FEMA 352provides recommendations forselecting an appropriate sample and drawingconclusions on the condition of the building,based on the damage found in the sample.

Program

toReducetheE

arthquakeHazardsof



Recommended Post-earthquake

Evaluation and Repair Criteria

for Welded Steel Moment-Frame

Buildings

FEMA 352 contains recommendedprocedures for identifying earthquakedamage in WSMF buildings and the risk ofcontinued occupancy.

Is it possible to repair any damagedbuilding?

It is technically possible to repair almost anydamaged building. However, if a building isseverely damaged, the cost of repair mayexceed the replacement cost. Repair may beparticularly difficult and costly if a building hasexperienced large, permanent sidewaysdisplacement, sometimes called drift. In suchcases, the structure may even be unsafe foroccupancy even for repair. In these cases, itmay make more sense to demolish a buildingand replace it rather than to repair it. Followingthe 1994 Northridge earthquake, the owner ofone building elected to do this.


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How is the fracture damage repaired?

Each type of fracture requires a somewhatdifferent repair procedure. FEMA 352providesrecommendations for many types of repair.Generally, repairs consist of local removal of thedamaged steel using cutting torches or electricarcs, and welding new, undamaged materialback in its place. To do this cutting and weldingsafely, it is necessary first to remove anycombustible finishes from the work area andalso to provide ventilation to remove potentiallyharmful fumes. In some cases, it may benecessary to provide temporary shoring of thedamaged element while the repair work is done.These operations are disruptive to normaloccupancy of the immediate work area and alsoquite costly. Typical connection repair costs canrange from $10,000 to $20,000. Additional costsare associated with the temporary loss of use of

floor space during repair operations.

Flange removal and replacementper Figure 6-6, if required

Weld access holes as requiredfor weld terminations

FEMA 352 provides detailed repairprocedures for different types of damage.One repair procedure is shown here as anexample.


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DEALING WITH THE RISK OF EXISTING BUILDINGS

Is it possible to upgrade an existingWSMF building?

It is technically feasible to upgrade an existingsteel moment-frame building and improve itsprobable performance in future earthquakes.The most common methods are upgrades of theindividual connections, addition of steel braces,concrete or masonry walls, or addition of energydissipation systems. Each of these approachesmay offer advantages in particular buildings.

Buildings can be upgraded by modifyingconnections, adding braces and othermethods.

Connections can be upgraded by welding orbolting new plates onto the beams andcolumns to change the connection shapeand reduce stress concentrations. In somecases, it may also be appropriate to replacedefective welds and welds with lowtoughness with welds with improved

toughness and workmanship.

What is a connection upgrade?

Connection upgrades are the most directapproach to improving the seismic performanceof an existing steel moment-frame structure. Aspreviously discussed, existing connections canfracture because their shape results in thedevelopment of large stress concentrations;some welds have large defects that reduce their

strength and the weld metal itself may be of lowtoughness and unable to resist the largestresses imposed on it. Connection upgradesaddress these problems directly by modifying theshape of the connection to reduce the stressconcentrations. In addition, depending on theapproach used, the upgrade may includereplacement of defective welds and welds withlow toughness with new welds that haveimproved toughness and better workmanship.FEMA 351 provides information and designcriteria for a number of alternative methods ofupgrading connections.


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How does the addition of braces or wallsupgrade a building?

Connection fractures occur when the forcedelivered by the earthquake exceeds the

connection strength or when connectionelements experience low-cycle fatigue. This issimilar to what happens to a paper clip, when itis bent back and forth repeatedly. As the metalis bent back and forth, it dissipates energy, butalso becomes damaged. Eventually, so muchdamage occurs that the metal breaks. If aconnection is bent through a large angle ofdeformation, or is relatively weak or brittle, it mayonly be able to withstand one or two cycles, thatis, be bent back and forth once or twice.However, if the bending deformation is relativelysmall, or if the connection material has hightoughness, the connection may be able to

withstand a number of such cycles.Earthquakes produce many cycles of motion.

When steel braces or masonry or concrete wallsare added to a building, they stiffen it and reducethe amount that the building will sway in anearthquake. This reduces the amount ofbending on the connections. Guidance on theuse of these techniques is already available insuch publications as FEMA-273, NEHRPGuidelines for Seismic Rehabilitation of Buildingsand, therefore, is not repeated in FEMA 351.

Energy dissipation systems are typicallyinstalled in buildings as part of a verticalbracing system that extends between thebuildings floors.

Program




Recommended Seismic

Evaluation and Upgrade

Criteria for Existing Welded

Steel Moment-Frame

Buildings

FEMA 351 provides engineering proceduresfor evaluating the probable performance ofbuildings in future earthquakes. Theseprocedures address the safety as well asfinancial aspects of earthquake performance.

What is an energy dissipation system?

Energy dissipation systems reduce the amountthat buildings sway in an earthquake byconverting the earthquakes energy into heat.Several types of energy dissipation devices areavailable. One type is a hydraulic cylinder,similar to the shock absorbers in an automobile.Other types dissipate energy through friction. Inorder for these systems to be effective, one endof the device must move relative to the otherend. The most common way to do this is toinstall the dissipation devices as part of abracing system between the building floors. As

the building sways in an earthquake and onefloor moves relative to another, this drives thedevice and dissipates the energy as heat.Detailed guidance on the design of energydissipation systems is already available in suchdocuments as FEMA 273 NEHRP Guidelines forSeismic Rehabilitation of Buildingsand,therefore, is not covered in detail in the steelmoment-frame criteria documents.


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What type of upgrade is best?

No one type of upgrade is best for all buildings.Basic factors that affect selection of an optimalalternative include the individual buildingscharacteristics, the severity of motion anticipatedat the building site, desired buildingperformance, cost of the upgrade, the feasibilityof performing upgrade work while the buildingremains occupied, and the effect of the upgradeon building appearance and space utilization.Addition of diagonal bracing may be the leastcostly alternative; however, its effect on buildingappearance and functionality may be viewed bysome owners as unacceptable. Addition ofenergy dissipation devices will result in betterperformance of the building, but may cost more.Modification of individual connections wouldhave the least effect on building appearance, but

would inconvenience the existing tenants morethan some other approaches and may cost moreto implement.

How does an owner pick an appropriateupgrade approach?

To determine the best method of upgrading abuilding, an owner should retain an engineer toevaluate the buildings probable earthquakeperformance. An upgrade should be consideredif the probable performance is unacceptable. Inthe absence of ordinances that require upgradeof buildings, it will be necessary for each ownerto decide what constitutes acceptableperformance. The engineer can assist theowner in understanding the various options.Once a performance objective is selected, theengineer can prepare preliminary designs forvarious upgrade approaches, together withestimates of probable construction cost. Theowner can then select the most appropriatedesign considering the cost, structural

performance improvements, aesthetics andother impacts of each upgrade approach on thebuilding.

Should all existing WSMF buildings beupgraded?

A building should be upgraded only if the risk oflosses associated with its probable performance

in future earthquakes is deemed unacceptable.Individual owners may make this judgement, orin some cases, the local community may decidethat risk is unacceptable. The severity ofearthquake risk associated with a buildingsperformance in future earthquakes, as well asthe acceptability of this risk, must be determinedon a building-specific basis. The earthquakeperformance of some WSMF buildings housingcritical occupancies and located in zones likely toexperience frequent intense ground motion maybe unacceptable, whereas many other WSMFbuildings will be capable of performingadequately in the levels of ground motion they

are likely to experience. To determine if abuilding should be upgraded, an owner shouldretain the services of an engineer to evaluate thebuildings probable performance in futureearthquakes, permitting the owner to balance theassessed risks against the cost of mitigation.

What is the cost of upgrading a WSMF?

Upgrade costs are dependent on a number offactors including the upgrade approach selected,the structural configuration of the building, thespecific seismic deficiencies present, theseverity of earthquake motion used as a basisfor design, the intended performance of theupgraded building, the buildings occupancy, andthe nature of tenant improvements andfurnishings. Costs can range from about $10per square foot of building floor area to as muchas $30 or more. This can be compared to thetypical costs associated with new buildingconstruction. Exclusive of the costs of landacquisition, planning and design, theconstruction cost of a new steel frame buildingshell will range from about $70 to $125 persquare foot. The cost of tenant improvements,

including interior partitions, ceilings, lighting, andfurnishings can equal the building constructioncosts.


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When is the best time to upgrade?

Much of the cost of performing a seismicupgrade relates to the need to demolish andrestore architectural finishes and to displace

tenants temporarily from construction workareas. In many commercial buildings thesecosts are incurred on a regular, periodic basis asleases expire and tenants in a building change.Therefore, the most economical time to performan upgrade is during a change of tenantoccupancy. Most buildings have many tenantswith staggered lease periods. If upgrade work isto be done when tenant space is being changedfrom one leaseholder to the next, it will usuallybe necessary to phase the work over a period ofyears. If such an approach is taken, theprobable performance of the building in the

event that an earthquake occurs while it is onlypartially upgraded should be evaluated for eachof the potential stages of completion, to ensurethat an unsafe condition is not createdinadvertently. Another beneficial time to performupgrade work is at the time of property transfer.If upgrade work is done as part of a buildingownership transfer, financing can includeadditional funding to perform the upgrade work.

Is upgrading economically feasible?

The feasibility of an upgrade depends on the

individual economic circumstances of eachbuilding, its tenants and owners. Earthquakesare infrequent events. Even in areas of highseismic risk, like California, most buildings willexperience at most only one or perhaps twodamaging earthquakes over their lives. Seismicupgrades can greatly reduce the financial andlife losses that occur as a result of suchearthquakes. However, because the probabilityof occurrence of a damaging earthquake isusually small, the present value of these avoidedlosses rarely exceeds the initial cost of theupgrade unless there are large life loss oroccupancy interruption costs associated with thepotential damage. If upgrades are performedconcurrently with other building renovation work,such as remodeling or asbestos removal, theupgrade work will cost less and produce a moreattractive return on the investment.

How can an owner evaluate a buildingsprobable future performance?

To determine the probable performance of abuilding in future earthquakes, an owner should

retain an engineer. FEMA 351 providesstructural engineers with several methods forevaluating the probable performance of buildingsin future earthquakes. These methods includeSimplified Loss Estimation, Detailed LossEstimation and Detailed PerformanceEvaluation, a procedure that is compatible withmodern performance-based design approaches.

How can an owner tell if a building willbe safe in future earthquakes?

FEMA 351 presents engineering procedures thatcan be used to estimate the probability that abuilding will experience life-threatening damagein future earthquakes. The ability of a building toresist earthquakes without endangering life isdependent on two primary factors, the capacityof the building and the intensity of futureearthquake ground motion. It must beremembered that any building has the potentialto experience life-threatening damage, if itexperiences sufficiently intense ground motion.In typical regions with significant earthquake risk,earthquakes that produce low levels of groundmotion may be felt relatively frequently, perhaps

one time every ten to twenty years. Such groundmotion rarely causes damage. Intense groundmotion, capable of causing severe damage tomodern buildings, will occur much lessfrequently, perhaps only one time in a fewhundred to a few thousand years. Theprocedures of FEMA 351 allow determination ofthe probability that a building will experienceeither partial or total collapse, if earthquakeground motion of a specified intensity occurs. Touse this method, the owner and engineer mustagree upon an appropriate return period for theground motion to be used as a basis for the

evaluation. The return period is the averagenumber of years, for example, 100, 500, etc.,between damage-causing earthquakes.


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What is the Simplified Loss EstimationMethodology?

The Simplified Loss Estimation Methodologycontained in FEMA 351 consists of a series of

graphs that indicate the probability that WSMFbuildings will experience various levels ofdamage if they are subjected to certain levels ofground motion. These graphs were compiledfrom data obtained for buildings in the LosAngeles area following the 1994 Northridgeearthquake. To use these graphs, an engineermust estimate an intensity of ground motion towhich a building will be subjected. Using thegraph, an engineer can obtain an estimate of thepercentage of the buildings connections that willbe damaged and the probable cost of connectionrepairs, if the building experiences anearthquake of a certain intensity. These graphs

do not take into account the individualcharacteristics of a building, except in anapproximate manner and, therefore, do notprovide precise estimates. Rather, they providean indication of the potential range of damageand repair costs and the probability that damageand repair costs will exceed certain amounts,based on the behavior of the buildings that wereaffected by the Northridge earthquake.

The Simplified Loss Estimation Methodologyuses a series of graphs to relate probableearthquake losses to ground motionintensity.

What is the Detailed Loss EstimationMethodology?

In addition to the Simplified Loss EstimationMethodology, FEMA 351 also presents a

procedure for developing building-specific,damage and loss estimates for WSMF buildings.This method can be implemented in HAZUS,FEMAs nationally applicable loss estimationmodel, to explore the potential benefits to acommunity of requiring upgrade of steelbuildings. The method can also be used todevelop loss estimates for individual buildings toassist owners in making upgrade decisions.Loss estimates generated using this techniquetake into consideration the specificcharacteristics of individual buildings and,therefore, provide a more accurate basis forcost-benefit studies for seismic upgrades.

However, the method is somewhat complex andrequires specialized expertise and training toimplement.

How safe should an existing buildingbe?

There is no single commonly accepted minimumlevel of safety for existing buildings. However, itis always useful to compare the safety of anexisting building with that intended for newbuildings as well as with that of other existingbuildings. New buildings are designed to providea high level of confidence, on the order of 90%,that they will survive the most severe groundmotion likely to be experienced, every 1,000 to2,500 years, without collapse. Therefore, it isunlikely that most new buildings would everexperience earthquake-induced collapse. Mostexisting buildings will not be capable of providingthe same performance as a new building. Anexisting building may be acceptably safe if it canprovide high confidence that it will resist collapseunder ground motion intensities likely to occurevery 500 years or so. The selection of anacceptable level of safety depends on a number

of factors including the number of occupants inthe building, and its use. FEMA 351 presentsevaluation procedures that can be used toestimate the probability that a building willcollapse in future earthquake ground motion andto associate a confidence level with thisestimate.


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Will it be possible to reuse a buildingafter an earthquake?

In addition to safety concerns, many owners andtenants in buildings are concerned with their

ability to return to a building after an earthquake,and continue to live or work in it. FEMA 351provides an evaluation procedure that may beused to determine the probability that a buildingwill experience so much damage that it shouldnot be reoccupied following an earthquake. Touse this procedure, it is necessary for the ownerand the engineer to select an appropriate returnperiod for the ground motion upon which thisevaluation will be based and the performancethat will be acceptable if this ground motionoccurs.

Is it cost effective to upgrade?

While earthquakes can cause catastrophicdamage, in most cases the probability that abuilding will be affected by such an event is low.Even in the most seismically active regions ofCalifornia, ground motions likely to causesubstantial damage to WSMF buildings arecurrently projected to affect a building site onlyone time every hundred years or so. Given thatmany investors retain ownership in a buildingasset for a relatively small number of years, thelikelihood that a loss will actually occur during an

ownership period is low. Therefore, whenowners balance the cost of a seismic upgradeprogram against the probable economic benefitto be gained during their ownership period, agood return on investment is often found to existonly if post-earthquake occupancy of a buildingis perceived to be critical for economic or otherreasons, or if life loss is anticipated and theeconomic value of such loss is included in theevaluation.

Why is building performance expressedin probabilities?

The amount of damage that a building willsustain in an earthquake depends on a number

of factors including the configuration andstrength of the building, the quality of itsconstruction, and the specific characteristics ofthe ground motion produced by the earthquake.Although engineers can develop estimates ofeach of these factors, it is not possible to predictany of these things precisely. Therefore,engineers must express future buildingperformance in probabilistic terms.

Should communities adopt mandatoryupgrade programs for steel frame

buildings?Given the low benefit/cost ratio for seismicupgrade of steel frame buildings, it is unlikelythat many owners will perform such upgradesvoluntarily. Mandatory upgrade ordinances canbe an effective measure to assure that thesebuildings are upgraded. For example, a numberof California cities, including Los Angeles andSan Francisco, have successfully implementedordinances requiring upgrade of unreinforcedmasonry buildings, a type of building that cancollapse in even moderate earthquakes.However, similar ordinances requiring upgrade

of WSMF buildings may not be an effectiveapplication of the limited resources available to acommunity. It is likely that other financial andhealth risks faced by the community are moresignificant than the hazards posed by theearthquake performance of steel framebuildings. Indeed, many communities have largeinventories of buildings that are far morehazardous than typical steel frame buildings,including unreinforced masonry buildings andolder concrete buildings. Before adopting amandatory upgrade ordinance for steel framebuildings, communities should carefully consider

the costs and benefits on a community-widebasis and weigh these against other potentialapplications of the limited resources available.FEMAs HAZUS loss estimation model isavailable to communities and can be used toprovide guidance in deciding on the advisabilityof adopting a mandatory upgrade ordinance.


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CRITERIA FOR NEW BUILDING DESIGN

How should new buildings be designed?

Buildings must be designed to meet minimum

criteria specified in building codes. Modelbuilding codes are developed on a national basisby professional organizations representingengineers, architects, building officials and firemarshals, and are adopted, sometimes withmodification, by individual cities, counties andstates. Prior to the 1994 Northridge earthquake,the building codes contained prescriptiverequirements for the design and construction ofsteel moment-frame buildings. Theserequirements included specification of minimumpermissible strength and stiffness for resistingearthquake loading, and specific requirements

for connections between beams and columns.The Northridge earthquake demonstrated thatsome of these prescriptive requirements werenot adequate. Following that discovery, theprescriptive requirements were removed frombuilding codes used in regions of highearthquake risk and replaced with a requirementthat designs include test data to show thatreliable performance could be achieved for eachnew building. These new requirements wereboth costly and difficult to enforce and resulted ina decrease in the number of steel moment-frame buildings constructed.

Studies conducted under the FEMA/SACprogram to reduce earthquake hazards inwelded moment-resisting steel frames confirmthat some of the design requirements containedin the building codes prior to the Northridgeearthquake were inadequate. In the time since,some of these inadequate requirements havebeen improved based on interim findings andrecommendations of the FEMA/SAC program.Now that the program is complete, FEMA 350presents a series of additional criteria for thedesign and construction of steel moment-framebuildings that should make performance of these

structures in future earthquakes much morereliable. It is recommended that the buildingcodes adopt these new recommendations andthat, until this occurs, engineers and ownersvoluntarily adopt these criteria when developingnew buildings.

What types of changes to design andconstruction practice does FEMA 350

recommend?The FEMA 350recommendations affect nearlyall phases of the design and constructionprocess. The recommendations address thetypes of steel used, the way in which columnsand beams are connected, the way the steel isfabricated, the type of welding that is performed,and the techniques that are used to assure thatthe construction work is performed properly.

In addition, FEMA 350provides engineers with aseries of performance-based design criteria thatcan be used to design and construct buildings to

resist earthquakes reliably while sustaining lessdamage than anticipated by building codes.

Program




Recommended Seismic

Design Criteria For

New Steel Moment-

Frame Buildings

FEMA 350 provides recommendations fordesign and construction of new steelmoment-frame buildings. Theserecommendations affect the materials ofconstruction, design and constructionprocedures, and methods of qualityassurance.


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What are the recommended changes tothe types of structural steel that shouldbe used?

In the last few years, the steel industry in theUnited States has undergone rapid change with

old rolling mills phased out and new mills, usingmore modern technologies, coming on line.Although the chemical composition and physicalproperties of steel have changed rapidly duringthis period, industry standard specifications anddesign practice largely neglected this. Workingwith the FEMA/SAC project, the AmericanInstitute of Steel Construction (AISC) developednew industry standard specifications forstructural steel material. These newspecifications provide better control of the criticalproperties that are important to earthquakeperformance. FEMA 350recommends the use

of these more modern steels and alsorecommends design procedures that properlyaccount for the high strength of modern steelmaterials.

Methods of steel production have undergonedramatic change in recent years and industry

standard design and material productionspecifications did not adequately reflect thematerial currently produced. FEMA 350updates the code requirements to assurespecification of proper materials and propertreatment in design.

If inadequately constructed, welded joints inmoment-frames can be weak links that failprematurely. FEMA 350 recommends carefulcontrol of the materials, workmanship andprocedures used to make these joints andverify their adequacy.

Are there any changes to weldingrequirements?

FEMA 350and its companion document, FEMA353, present extensive new recommendationsfor welding of moment-resisting connections in

frames designed for seismic applications. Thenew recommendations address the types ofmaterials that may be used for this work, thewelding processes and procedures that shouldbe followed, the environmental conditions underwhich welding should be performed, and thelevel of training and workmanship required ofwelders and inspectors engaged in this work. Inaddition, FEMA 353provides detailedrecommendations for quality control, qualityassurance and inspection procedures thatshould be put into place to assure that thiscritical work is properly performed. These newrecommendations affect all segments of the

design and construction process and requiredesigners, producers, fabricators, weldingelectrode manufacturers, welders andinspectors, among others, to adopt newpractices. These recommendations have beensubmitted to the American Welding Society forincorporation into the structural welding code.


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What is different about the new designprocedures?

Prior to the Northridge earthquake, most steelmoment-frame designs used a standard

connection that was prescribed by the buildingcode. We know now that the configuration ofthese connections was problematic and resultedin unreliable connection performance. Followingthis discovery, the building codes were changedto require testing of each new connectiondesign. This was very costly and difficult toimplement. FEMA 350presents a series of newconnection configurations that arerecommended as prequalified for use inmoment-frames conforming to certainlimitations. The use of these prequalifiedconnections will greatly simplify the designprocess and make the construction of new

moment-frame buildings more economical andmore reliable.

1

23

General:

Applicable systems OMF, SMF

Hinge location distance sh dc/2

Critical Beam Parameters:

Depth Up to W36 (OMF)

Up to W30 (SMF)

Minimum Span OMF: 15 ft.

SMF: 20 ft.

bf/2tf of f lange 52Fy ; 35Fy recommended minimum

Flange thickness Up to 1-1/4 (OMF)

Up to (SMF)

Permissible Material Specifications A36, A572 Grade 50, A992

Critical Column Parameters:

Depth Not Limited

Permissible Material Specifications A572, Grade 50; A913 Grade 50

Minor Axis Connection Pre-

qualified

OMF: Yes

SMF: No

Beam/Column Relations:

PZ strength Section 3.3.3.2 for SMF; Cpr=1.2; Rv/Ru < 1.2 (recommended)

Column/beam bending strength Section 2.8.1; Cpr=1.2

Connection Details

Web connection Section 3.5.3.2. Welding QC Level 2.

Continuity plate thickness Section 3.3.3.1

Flange welds Section 3.5.3.1: Welding QC Level 1.

Weld electrodes CVN 20 ft-lbs at -20oF and 40 ft-lbs at 70

oF

Weld access holes Not Applicable

FEMA 350 provides prescriptive criteria forprequalified beam-to-column connectionscapable of reliable performance.

The Reduced Beam Section, or dog boneconnection is one of 12 different types ofprequalified connections that engineers canspecify without project-specific testing.

How many different types ofconnections are prequalified?

FEMA 350contains design criteria for twelvedifferent types of prequalified, welded and bolted

beam-to-column connections. Eachprequalification includes specification of thelimiting conditions under which the connectiondesign is valid, the materials that may be used,and the specific design and fabricationrequirements.

What is the basis for the newprequalified connections?

The new prequalified connections contained inFEMA 350are based on extensive laboratoryand analytical investigations including more than

120 full-scale tests of beam-to-columnconnection assemblies and numerous analyticalstudies of the behavior of different connectiontypes. Connections were prequalified only aftertheir behavior was understood, analytical modelswere developed that could predict this behaviorand laboratory testing demonstrated that theconnections would behave reliably.


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What is the best type of connection touse?

Each of the prequalified connections offerscertain advantages with regard to design

considerations, fabrication and erectioncomplexity, construction cost, and structuralperformance capability. No one connection typewill be most appropriate for all applications. It islikely that individual engineers and fabricatorswill develop preferences for the use of specifictypes of connections, and that certainconnections will see widespread use in someparts of the country but not others.

Do the prequalified connections coverall possible design cases?

The prequalifed connections should beapplicable to many common design conditions.However, they are not universally applicable toall structures and, in particular, are limited inapplicability with regard to the size, types andorientation of framing members with which theycan be used. If a design requires the use oftypes or sizes of framing that are not included inthe ranges for a prequalified connection, FEMA350recommends that supplemental testing ofthe connection be performed to demonstrate thatit will be capable of performing adequately.

Is it possible to use types ofconnections other than those that areprequalified?

FEMA 350contains information on several typesof proprietary connections that are notprequalified and nothing in FEMA 350preventsengineers from developing or using other typesof connection designs. However, FEMA 350does state that, when other types of connectionsare used, a testing program should beconducted to demonstrate that theseconnections can perform adequately. FEMA 350also presents information on the types of testingthat should be conducted and how to determineif connection performance is acceptable.

Will designs employing these newrecommendations cost more?

The cost of a steel frame building is generallydependent on the amount of labor required to

fabricate and erect the steel and the totalnumber of tons of steel in the building frame.The new design recommendations bothformalize and simplify design procedures thatgenerally have been in use by engineers sincethe 1994 Northridge earthquake. They do notresult in an increase in steel tonnage orsignificantly greater labor for fabrication anderection. However, more extensive constructionquality assurance measures are recommendedand these will result in some additionalconstruction cost. It is anticipated that thisadditional cost will amount to less than 1% of the

total construction cost for typical buildings.

Will buildings designed to the newrecommendations be earthquake proof?

It is theoretically possible to design and constructbuildings that are strong enough to resist severeearthquakes without damage, but it would not beeconomical to do so and we could not afford toconstruct many such buildings. The objective ofmost building codes is to design buildings suchthat they might be damaged by severeearthquakes but not collapse and endanger

occupants in any earthquake they are likely toexperience. The FEMA 350recommendationsadopt this same design philosophy. It isanticipated that there would be a very low risk oflife threatening damage in buildings designedand constructed in accordance with the FEMA350recommendations. However, they mayexperience damage that will require repair,including permanent bending and yielding of theframing elements and possible permanent lateraldrift of the structure. Although it is possible thatsome connections designed using the newprocedure may fracture, it is anticipated that this

will not be widespread.


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Will the new recommendations beincluded in the building codes?

The seismic provisions of current building codesare largely based on the NEHRP Recommended

Provisions for Seismic Regulations of Buildingsand Other Structures, supplemented by standardspecifications developed by industryassociations, including the American Institute ofSteel Construction (AISC) and the AmericanWelding Society (AWS). Many of the designrecommendations contained in FEMA 350havealready been incorporated into the standarddesign specifications developed by AISC and aredirectly referenced by the 2000 InternationalBuilding Code. The remainingrecommendations are being proposed forincorporation into later editions of the AISC andAWS specifications. This material is also beingincluded in the NEHRP RecommendedProvisions for Seismic Regulations of NewBuildings and Other Structures. Some of theearlier material has already been included in the1997 edition (FEMA 302/303) and the remainderof the material will be incorporated into the new2000 edition (FEMA 368/369), due to bereleased in spring 2001. It is anticipated thatmany jurisdictions around the United States willadopt building codes based on these criteria asthe basis for building regulation in theircommunities.

Is it possible to design to the newrecommendations before the newbuilding codes are adopted?

Many of the new design recommendations arecompatible with existing building coderegulations that are already in force around theUnited States. It should be possible forengineers to utilize the new recommendations,on a voluntary basis, as a supplement to theexisting building code requirements. However,the local building official is the ultimate authorityas to the acceptability of this practice. Engineersshould verify that the building official will acceptthe new documents prior to using them to designbuildings.

Is it possible to design for betterperformance?

As an option, FEMA 350includes a methodologywhich engineers can use to design for superior

performance relative to that anticipated by thebuilding code. This approach is similar to theprocedures contained in FEMA 351 for theevaluation and upgrade of existing buildings. Inthese procedures, a decision must be made asto what level of performance is desired for aspecific level of earthquake motion. As anexample, one of the available performancelevels, termed Immediate Occupancy, results insuch slight damage that the building should beavailable for occupancy immediately followingthe earthquake. The performance-basedprocedures of FEMA 350can be used to designa building such that there is a high probability the

building will provide Immediate Occupancyperformance for any level of earthquake intensitythat is desired. As a minimum, however,buildings should be designed to satisfy theprescriptive criteria of the applicable buildingcode, which are intended to protect life safety.


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CONSTRUCTION QUALITY CONTROL AND ASSURANCE

What is construction quality control?

Construction quality control includes that set of

actions taken by the contractor to ensure thatconstruction conforms to the specified materialand workmanship standards, the requirementsof the design drawings and the applicablebuilding code. Construction quality controlincludes hiring workers and subcontractors thathave the necessary training and experience toperform the construction properly; making surethat these workers understand the projectrequirements and their responsibility to executethem; and performing routine inspections andtests to confirm that the work is properlyperformed. Some contractors may retain

independent inspection and testing agencies toassist them with their quality control functions.

What is construction quality assurance?

Construction quality assurance is that set ofactions, including inspections, observations andtests, that are performed on the owners behalfto ensure that the contractor is conforming to thedesign and building code requirements. Inessence, construction quality assurancerepresents a second line of defense andsupplements the contractors own quality control

program. Building codes specify minimum levelsof quality assurance for different types ofconstruction. If the contractor on a project doesnot perform adequate quality control it may beappropriate to provide more quality assurancethan required by the applicable building code.Quality assurance tasks are typically performedby design professionals and special inspectionand testing agencies specifically retained by theowner for this purpose. It is recommended thatthe engineer assist the owner in determining thelevel of quality assurance appropriate to aspecific project, and also that the engineer be

retained by the owner to actively monitor andparticipate in the quality assurance process.

Why is construction quality control andquality assurance important?

Investigations performed after every earthquakeindicate that much of the damage that occurs isa result of structures not being constructed inaccordance with the applicable building code orthe designers intent. This problem has affectednearly every type of building construction,including wood frame, masonry, concrete andsteel buildings. Construction errors andconstruction work that is improperly performedcreate weak links where damage in structurescan initiate. Earthquakes can place extremeloading on structures and, if structures containimproper construction, damage is likely to occur

where that poor construction occurs.

Was inadequate construction quality asignificant factor in the damagesustained by steel buildings in theNorthridge earthquake?

One of the contributors to the damage sustainedby steel moment-frame buildings in theNorthridge earthquake was construction that didnot conform to the applicable standards. In

particular, investigations conducted after theearthquake revealed that critical welds of beamsto columns were not made in accordance withthe building code requirements. A number ofdefects were commonly found in the constructionincluding: welds that had large slag inclusionsand lack of fusion (bonding) with the steelcolumns, welds that were placed too quickly andwith too much heat input into the joint; andwelding aids including backing and weld tabsthat were improperly installed. Theseconstruction defects resulted in welded jointsthat had lower strength and toughness thanshould have been provided, resulting in a

propensity for damage.


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Who is responsible for constructionquality control and assurance?

Each of the participants in building construction,including the design engineer, the contractor, the

building official, and special inspectors, plays acritical role in assuring that construction meetsthe appropriate standards. The engineer mustspecify the applicable standards and the qualityassurance measures that are to be used. Thecontractor must perform the work in the requiredmanner and perform inspections to verify thatthis occurs. Building codes require that theowner retain a special inspection agency toperform detailed inspections and assure that thecontractor is executing the work properly. Theresponsible building official typically monitors theentire process and makes sure that each of theparties fulfills their individual responsibilities.

Program




Recommended Specifications

and Quality Assurance

Guidelines for Steel

Moment-Frame Construction

for Seismic Applications

FEMA 353 provides recommendedspecifications for fabrication, erection, andquality assurance of steel moment-framestructures designed for seismic applications.

Where can one find recommendationson appropriate quality control andquality assurance procedures?

FEMA 353provides detailed recommendations

for procedures that should be followed to assurethat steel moment-frame construction complieswith the applicable standards. It includesinformation that is useful to engineers, buildingofficials, contractors and inspectors. Thisinformation is presented in the form ofspecifications that can be included in the projectspecifications developed by engineers forspecific projects. It also includes commentarythat explains the basis for the recommendationsand methods that can be used to implementthem.

Are the new quality recommendationssignificantly different than thoserequired in the past?

Many of the recommendations contained inFEMA 353are a restatement and clarification ofrequirements already contained in building codesand in the standard AISC and AWSspecifications. There are also some importantnew recommendations. These include newrequirements intended to assure that weld fillermetals and welding procedures are capable ofproviding welded joints of adequate strength andtoughness. In addition, comprehensiverecommendations are provided as to the extentof testing and inspection that should beperformed on different welded joints. In part,these new requirements are based on findingsthat the inspection methods traditionally used inthe past to assure weld quality are incapable ofreliably detecting nonconforming construction. Itis anticipated that many of these newrequirements will be incorporated into the AWSstandards in the future.


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PREPARING FOR THE NEXT EARTHQUAKE

What should communities do before thenext disaster occurs?

Before the next disaster strikes, communitiesshould conduct a thorough examination of theirvulnerability to natural hazards and the risks theymay present to their citizens. In addition to apotential earthquake threat, communities may bevulnerable to other significant hazards, such asflooding, high winds, hurricanes, tornadoes,tsunami, etc. While this publication addressesthe potential earthquake risk of steel moment-frame buildings, communities may have moresignificant risks from other more hazardoustypes of buildings, such as unreinforcedmasonry, non-ductile concrete frame or tilt-upconcrete buildings. Communities should carefullyconsider all of these potential risks and how theymay affect the different types of constructionpresent.

Regardless of the risk posed by existingconstruction, before the next disaster occurscommunities should adopt reliable buildingcodes and building regulation practices that willensure that new buildings are adequatelydesigned and constructed to resist the futureearthquakes and other disasters that willinevitably occur. Building codes that incorporatethe latest edition of the NEHRP Recommended

Provisions for Seismic Regulation for Buildingsand Other Structuresare stronglyrecommended.

Communities may wish to encourage buildingowners to upgrade their existing hazardousbuildings to minimize potential damage,economic and life loss. FEMA 351 can be usedas a technical basis for both voluntary andmandatory upgrade programs. In addition,communities should put emergency responsesystems into place. Building departments shouldtrain their personnel to conduct building

inspections. In large cities, building departmentswill quickly become overwhelmed by thisresponsibility. Before the next earthquake, thebuilding department should make arrangementsfor assistance from outside the affected area.Some states have agencies that will facilitatethis. Communities should also consider adoptingordinances to govern the post-earthquakebuilding inspection and repair process.Experience has shown that some building

owners will not act responsibly to inspect andrepair buildings, unless required by law to do so.FEMA 352 can be used as a basis for

developing post-earthquake inspection andrepair ordinances.

Is help available?

FEMA has developed a number of tools thatcommunities can use to identify their exposure tonatural disasters, including both earthquake andflood hazard maps. FEMA has also developedstandardized, GIS-based computer software thatcan help a community to assess the risk thatthese hazards present. This software package,called HazardsUS, or HAZUS, presently

addresses earthquake risk, however modulesare currently under development that will alsoaddress the risk from floods and high winds.FEMA has also published a series of guidancedocuments for both technical and non-technicalaudiences to assist in addressing specifichazard-related issues. All of these aids areavailable, without charge, from FEMA.

Once the hazard and risk are known, FEMA hasa community-based initiative called ProjectImpact to help encourage the implementation ofpre-disaster prevention, or mitigation, activities.This initiative, which may include a one-timeseed grant, encourages the formation ofcommunity-based partnerships between publicand private stakeholders. For more informationon this and other programs, visit FEMAs websiteat www.fema.gov.

Project Impact assists communities to formpublic and private partnerships to reduce thepotential for natural disaster losses.


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THE FEMA/SAC PROJECT AND DISSEMINATING ITS RESULTS

Why did FEMA elect to undertake suchan effort?

FEMA, as part of its role in the NationalEarthquake Hazards Reduction Program(NEHRP), is charged with reducing the ever-increasing cost of damage in earthquakes.Preventing losses before they happen, ormitigation, is the only truly effective way ofreducing this cost. In this light, the role of thisnation's building codes and standards in mitigatingearthquake losses is critical, and FEMA iscommitted to working with this nation's seismiccodes and standards to keep them among thebest in the world. Because the damage thatresulted from the Northridge earthquake called

into question the design assumptions and buildingcode requirements associated with steel moment-frame construction and because of the unknownlife safety hazard of the damaged buildings, it wascrucial that adequate repair and retrofittingprocedures be quickly identified. This wasparticularly critical for FEMA, because many of thedamaged steel buildings were publicly owned andtherefore the cost of damage repair was eligiblefor funding under the public assistance provisionsof the Robert T. Stafford Disaster Relief andEmergency Assistance Act. To fully respond tothis need, development of reliable and cost-

effective methods for use in the various buildingcodes and standards that address the design ofnew construction and the repair and/or upgradingof existing steel moment-frame buildings wasnecessary.

The solution of this problem required acoordinated, problem-focused program ofresearch, investigation and professionaldevelopment with the goal of developing andvalidating reliable and cost-effective seismic-resistant design procedures for steel moment-frame structures. This work involvedconsideration of many complex technical,

professional and economic issues includingmetallurgy, welding, fracture mechanics,connection behavior, system performance, andpractices related to design, fabrication, erectionand inspection.

How were related policy issuesaddressed by the FEMA/SAC project?

As part of the project, an expert panel withrepresentation from the financial, legal,commercial real estate, construction, andbuilding regulation communities was formed toreview social, economic, legal and politicalissues related to the technicalrecommendations. This panel was charged withevaluating the potential impact of therecommended design and construction criteriaon various stakeholder groups, identifyingpotential barriers to effective implementation ofthe recommendations and advising the projectteam on potential ways of making the guidelines

more useful and effective. The panel held aworkshop in October 1997 that brought togetherrepresentatives of these constituencies, andtheir recommendations were considered duringthe development of technical recommendationsand in the preparation of this publication.

Why did it take so long to develop thenecessary information?

The damage experienced by steel moment-frame structures in the Northridge earthquakecalled to question the entire portions of building

codes that addressed this type of construction,and which had been developed over a period ofmore than 20 years. This project basically hadto start from scratch and either revalidate all ofthe existing criteria, or when this criteria wasfound to be inadequate, develop new designcriteria and construction standards that could berelied upon. This involved a complete re-evaluation of the properties of structural steelsand welding materials, the assumptions used indesigning and evaluating different types ofmoment-connections, and the methods used ininspecting and evaluating these connections and

their behavior. In the process, all of this materialhad to be developed and presented in a rigorousand scientifically defensible manner so that itwould be recognized as reliable and used by thebuilding design, construction and regulatorycommunities. This project accomplished in sixyears what originally evolved over 20 years.


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How will this information be put untopractice?

The new FEMA guidance documents are nowbeing widely disseminated to the structural

engineering and building regulationcommunities. Three of these publicationsprovide recommended criteria for design of morereliable new construction (FEMA 350), upgradingexisting structures to minimize the potential forfuture damage (FEMA 351), and evaluating andrepairing buildings after an earthquake (FEMA352). The fourth provides technicalspecifications and quality assurance guidelines(FEMA 353). Much of the information containedin these publications, especially for newconstruction, is being incorporated into both theNEHRP Recommended Provisions for SeismicRegulation of Buildings and Other Structuresandthe latest standard design specificationpublished by AISC. Together, these publicationsserve as the consensus design standard forsteel structures nationwide. This standard isroutinely adopted by reference into the modelbuilding codes where it is in turn used as part ofstate and/or local building codes. AISC has alsopublished a design guide, developed jointly withNIST, for upgrade of existing structures, andwhich compliments the recommendationscontained in FEMA 351.

How can one obtain additionalinformation?

The four FEMA publications described abovecan be ordered free of charge by calling 1-800-

480-2520. In addition, a series of state-of-the-artreports have been developed which summarizethe current state of knowledge that forms thetechnical basis for the recommended criteriapublications. These reports are available fromFEMA in CD-ROM format (FEMA 355).Individual reports on the technical investigationsperformed in support of the FEMA/SAC projectare available as well. These technical reportsand FEMA 355may be purchased in hard copyformat from the SAC Joint Venture. The SACJoint Venture will also continue to maintain apublicly accessible site on the World Wide Webat www.sacsteel.org. Future information and

training opportunities will be available from theAmerican Institute of Steel Construction (AISC)and other organizations. Consult AISCs WorldWide Web site at www.aisc.org for additionalinformation.


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INDEX

American Institute of Steel Construction (AISC), 2,

18, 21, 23, 26

American Welding Society (AWS), 21, 23

building code, 1, 15, 17, 19, 20, 21, 22, 23, 25

collapse, 4, 7, 8, 14, 20

connection, 3, 5, 8, 9, 10, 11, 12, 15, 19, 20, 25

construction, 20

cost, 2, 5, 9, 13, 14, 15, 16, 19, 20, 25

crack, cracks, cracking 4, 5, 6

damage, i, 3, 4, 5, 6, 8, 9, 10, 12, 14, 15, 16, 17, 20,

21, 22, 25, 26

design, 1, 2, 4, 5, 6, 11, 12, 13, 14, 17, 18, 19, 20, 21,

23, 25, 26

FEMA 350 Recommended Seismic Design Criteria

for New Moment Resisting Steel Frame Buildings,

2, 17, 18, 19, 20, 21, 26

FEMA 351 Recommended Seismic Evaluation and

Upgrade Criteria for Existing Welded Steel Frame

Buildings, 2, 11, 12, 14, 15, 16, 21, 26

FEMA 352 Recommended Post Earthquake

Evaluation and Repair Criteria for Welded Steel

Moment Frame Buildings, 2, 8, 9, 10, 24, 26

FEMA 353 Recommended Specifications and Quality

Assurance Guidelines for Steel Moment Frame

Construction for Seismic Applications, 2, 18, 23,

26

fracture, 1, 4, 10, 20, 25

inspection, 5, 8, 9, 23, 24, 25

Kobe, 1, 6

Loma Prieta Earthquake, 1, 6

materials, 19

Nati

policy guide to steel moment frame conctructions

Documents