stress concentration effect from poor support …wvuscholar.wvu.edu/reports/donald_kenneth.pdf ·...

Stress Concentration Effect from Poor Support Conditions

on Pultruded GFRP Composite Columns

Kenneth Michael Donald

Problem Report submitted to the

Benjamin M Statler College of Engineering and Mineral Resources at

West Virginia University in

partial fulfillment of the requirements

for the degree of

Master of Science

in

Civil Engineering

Approved by

Dr. Hota GangaRao, Chair

Dr. Udaya Halabe

Mark Skidmore

Department of Civil and Environmental Engineering

Morgantown, West Virginia

2013

Keywords: boundary support, pultrusion, GFRP, column

ii

ABSTRACT

Stress Concentration Effect from Poor Support Conditions on Pultruded GFRP Composite

Columns

Kenneth Donald

Constructed Facilities Center, West Virginia University

Noncorrosive Fiber Reinforced Polymer (FRP) composite materials are finding their way into

civil engineering projects in a variety of applications and higher volumes for a number of reasons

including their higher strength to weight ratio than conventional materials. However, these new materials

pose some problems due to the fact that these materials are not yet fully understood in terms of their short

and long term responses as thoroughly as conventional materials such as steel or concrete. This means

that the design, fabrication and erection must be carried out with proper understanding of their behaviors

on both the component and system levels to ensure proper performance.

The scope of this project was to determine causes of cracks that developed during construction of

a cooling tower in the bottom corners of Glass Fiber Reinforced Polymer (GFRP) composite tubular

columns. A field investigation was carried out to investigate the columns via load testing, plumb checks,

bearing evaluation and infrared thermography (IRT). Load testing revealed the stresses in the columns to

be reasonable, but the loads were sometimes much lower than the design dead load, indicating the load

distribution is not uniform. Although a high number of columns were found to be out-of-plumb, there was

no correlation between the plumb and cracking; thus it was determined to not be a major factor. The IRT

testing found that the cracks were not creating delaminations, but nothing else was found from IRT. The

major source of cracking was found to be poor contact conditions between FRP column base and the

ground. The field evaluation revealed four typical bearing conditions: 1) fully supported, 2) diagonal

support which has two consecutive sides supported, 3) C-shaped support with one side fully supported

and the half of the two adjacent sides supported, and 4) inner perimeter support, with the inner half of

each wall supported. The various support conditions were evaluated via FE analysis and by load testing in

the lab at both dead loads and to failure. Stress concentrations were also observed at discontinuities

between column base and ground and at fabric kinks in many corners, an unavoidable manufacturing

issue for the pultrusion process. It has been found that the kink stress concentrations are found to decrease

the capacity by 0.83, whereas the inner perimeter support by 0.31 and the diagonal support and C-shaped

support decrease the capacity by 0.2, more severe due to bending effects caused by the eccentric stresses,

though these factors are somewhat reflected in the current CTI code. For future cooling towers, it is

recommended to apply an additional reduction factor of 0.75 to account for improper bearing and stricter

field installation policies are enforced.

iii

ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Hota GangaRao for all his wisdom, advice, and

teachings during my time here at WVU both in the classroom and on the various projects I had

the opportunity to work on. I am very humbled to have had the opportunity to study under such a

great mind.

I would also like to thank Mark Skidmore and Dr. David Dittenber for all the help they

have given me. My time at WVU and on this project has been made much easier being able to

ask them for help with anything and everything.

Dr. Ruifeng Liang has helped me to test and analyze many samples throughout this

project and has been a great help. Similarly, Dr. Udaya Halabe assisted on this project and was

also kind enough to serve on my advising committee. Without their help I would not be where I

am.

Additionally, Jerry Nestor has assisted me in more laboratory tests than I am able to

count and I could not have completed this without him.

And finally, I must thank my Dad, my Mom, my Brother and the rest of my family and

friends for supporting me through this and always giving me the encouragement that I have

needed.

iv

TABLE OF CONTENTS

ABSTRACT ................................................................................................................................... ii

ACKNOWLEDGEMENTS ........................................................................................................ iii

TABLE OF CONTENTS ............................................................................................................ iv

LIST OF FIGURES ................................................................................................................... viii

LIST OF TABLES ...................................................................................................................... xii

CHAPTER 1 INTRODUCTION ........................................................................................... 1

1.1 Background ..................................................................................................................... 1

1.2 Objective........................................................................................................................... 2

1.3 Scope ................................................................................................................................ 3

1.4 Project Background ......................................................................................................... 3

1.5 Organization of Problem Report ..................................................................................... 6

CHAPTER 2 LITERATURE REVIEW ............................................................................... 7

2.1 Manufacturing Process ................................................................................................... 7

2.1.1 Issues with Pultruded Shapes ...................................................................................... 9

2.2 Base Support Conditions ............................................................................................... 10

2.3 Kink Effects ................................................................................................................... 10

2.4 Stress Concentrations .................................................................................................... 12

v

2.5 Buckling Effects ............................................................................................................ 12

2.5.1 Local Buckling .......................................................................................................... 13

2.5.2 Global Buckling ........................................................................................................ 13

2.5.3 Slenderness ............................................................................................................... 14

2.5.4 Boundary Conditions ................................................................................................ 15

2.6 Summary ........................................................................................................................ 16

CHAPTER 3 LABORATORY AND FIELD TESTING ................................................... 17

3.1 Introduction ................................................................................................................... 17

3.2 Field Testing .................................................................................................................. 18

3.2.1 Column Unloading .................................................................................................... 19

3.2.2 Plumb Test ................................................................................................................ 22

3.2.3 Grout Measurement .................................................................................................. 25

3.2.4 Thermal Imaging Testing .......................................................................................... 29

3.3 Lab Testing .................................................................................................................... 31

3.3.1 Compression Test...................................................................................................... 31

3.3.2 Thermal Imaging Testing .......................................................................................... 47

3.3.3 Impact Testing .......................................................................................................... 48

3.3.4 Bending Testing ........................................................................................................ 49

3.3.5 Shear Testing ............................................................................................................ 52

3.3.6 Pull Testing ............................................................................................................... 53

vi

3.3.7 Burn Out Testing....................................................................................................... 56

3.3.8 Differential Scanning Calorimetry (DSC) Testing ................................................... 58

3.3.9 Post Curing and Moisture Content Measurement Testing ........................................ 59

CHAPTER 4 FINITE ELEMENT ANALYSIS ................................................................. 61

4.1 Analysis of Different Support Conditions .................................................................... 61

4.1.1 Fully Supported Base ................................................................................................ 62

4.1.2 Diagonally Supported Base....................................................................................... 65

4.1.3 C-Shaped Supported Base......................................................................................... 68

4.1.4 Inner Perimeter Supported Base ............................................................................... 70

4.2 Kinked Corner Effect .................................................................................................... 73

4.2.1 Fully Supported Base with Kinked Corner ............................................................... 73

4.2.2 Diagonally Supported Base with Kinked Corner...................................................... 75

4.2.3 C-Shaped Supported Base with Kinked Corner........................................................ 78

4.2.4 Inner Perimeter Supported Base with Kinked Corner. ............................................. 80

CHAPTER 5 DATA ANALYSIS AND RESULTS ............................................................ 83

5.1 Calculation and Analysis of Theoretical Results ......................................................... 83

5.2 Comparison of Results .................................................................................................. 84

5.2.1 Material Property Test Results.................................................................................. 85

5.2.2 Field and Lab Testing Results................................................................................... 85

5.2.3 FE Results ................................................................................................................. 86

vii

5.2.4 Comparison of all Results ......................................................................................... 87

5.2.5 Summary of Results .................................................................................................. 89

CHAPTER 6 Conclusions and Recommendations............................................................. 92

6.1 Conclusion ..................................................................................................................... 92

6.1.1 Kink Effect ................................................................................................................ 92

6.1.2 Boundary Condition Effect ....................................................................................... 94

6.1.3 Summary ................................................................................................................... 96

6.2 Recommendations ......................................................................................................... 96

6.2.1 Future Research ........................................................................................................ 98

REFERENCES ............................................................................................................................ 99

viii

LIST OF FIGURES

Figure 1-1 GFRP Columns Supporting Cooling Tower ................................................................. 4

Figure 1-2 Image Showing size of Tower....................................................................................... 5

Figure 1-3 Ice Forming at the Base of the Tower ........................................................................... 5

Figure 2-1 Pultrusion Process [4 Barbero 84]................................................................................. 8

Figure 2-2 Image of Kink in Corner ............................................................................................. 11

Figure 2-3 Critical loads, Effective lengths and Effective Length Factors [Gere 2004] .............. 16

Figure 3-1 Base of Tower with Quadrant and Axis Labels .......................................................... 19

Figure 3-2 Column Unloading Set up ........................................................................................... 21

Figure 3-3 Mechanism used for Column Plumb Measurement .................................................... 23

Figure 3-4 Load versus Microstrain of Sample 1 Under Fully Supported Boundary Condition .. 32



Figure 3-7 Base Set-Up of Diagonally Supported Boundary Condition ...................................... 34

Figure 3-8 Failure of Diagonally Supported Column ................................................................... 35

Figure 3-9 Load versus Microstrain of Sample 1 Under Diagonally Supported Boundary

Condition....................................................................................................................................... 36


Condition....................................................................................................................................... 36


Condition....................................................................................................................................... 37

Figure 3-12 Base Set-Up of C-Shaped Supported Boundary Condition ...................................... 38

Figure 3-13 Failure of C-Shaped Supported Sample .................................................................... 38

ix

Figure 3-14 Load versus Microstrain of Sample 1 Under C-Shaped Boundary Support Condition

....................................................................................................................................................... 39


....................................................................................................................................................... 40


....................................................................................................................................................... 40

Figure 3-17 Base Set-Up of Inner Perimeter Boundary Support Condition ................................. 41

Figure 3-18 Failure of Inner Perimeter Supported Sample ........................................................... 42

Figure 3-19 Load versus Microstrain of Sample 1 Under Inner Perimeter Boundary Support

Condition....................................................................................................................................... 43


Condition....................................................................................................................................... 43


Condition....................................................................................................................................... 44

Figure 3-22 Vertical Gages of Column SE Z18 X6 ...................................................................... 45

Figure 3-23 Vertical Gages of Column SW Z90 X6 .................................................................... 46

Figure 3-24 Horizontal Gages of Column SE Z18 X6 ................................................................. 46

Figure 3-25 Horizontal Gages of Column SW Z90 X6 ................................................................ 47

Figure 3-26 SATEC BLI Impact Tester Used ............................................................................... 49

Figure 3-27 Samples Failed in Izod Impact Test .......................................................................... 49

Figure 3-28 Coupon Bending Test ................................................................................................ 50

Figure 3-29 Coupon Bending Test with “Normal” Orientation .................................................... 50

Figure 3-30 Coupon Bending Test with “Vertical” Orientation ................................................... 51

x

Figure 3-31 Shear Test Failed Sample #6 ..................................................................................... 52

Figure 3-32 Corner Pull Test Set-up Cross Section View ............................................................ 54

Figure 3-33 Corner Pull Set-up Test Side View ........................................................................... 55

Figure 3-34 Side Pull Test Set-up Cross Section View ............................................................... 55

Figure 3-35 Side Pull Test Set-up Side View ............................................................................... 56

Figure 3-36 Pre-Burn Out Test Samples in Oven ......................................................................... 57

Figure 3-37 Post-Burn Out Test Fiber Architecture ..................................................................... 58

Figure 3-38 DSC Test Machine .................................................................................................... 58

Figure 3-39 DSC Results for Sample 1......................................................................................... 59

Figure 4-1 FE Model of Fully Supported Boundary Condition .................................................... 63

Figure 4-2 Nodal Longitudinal (Z direction) Stress ..................................................................... 63

Figure 4-3 Nodal Transverse (X Direction) Stress ....................................................................... 64

Figure 4-4 Nodal Shear Stress in XY Plane.................................................................................. 64

Figure 4-5 FE Model of Diagonally Supported Boundary Condition .......................................... 65

Figure 4-6 Nodal Longitudinal (Z Direction) Stress ..................................................................... 66

Figure 4-7 Nodal Transverse (X Direction) Stress ....................................................................... 67

Figure 4-8 Nodal Shear Stress in XY Plane.................................................................................. 67

Figure 4-9 FE Model of C-Shaped Supported Boundary Condition ........................................... 68

Figure 4-10 Nodal Longitudinal (Z Direction) Stress................................................................... 69

Figure 4-11 Nodal Transverse (X Direction) Stress ..................................................................... 69

Figure 4-12 Nodal Shear Stress in XY Plane................................................................................ 70

Figure 4-13 FE Model of Inner Perimeter Boundary Support Condition ..................................... 71

Figure 4-14 Nodal Longitudinal (Z Direction) Stress.................................................................. 71

xi














Figure 4-28 Nodal Shear Stress in the XY Plane .......................................................................... 82

Figure 6-1 - Kinked Corner........................................................................................................... 93

Figure 6-2 Longitudinal View of Kinked Corner ......................................................................... 94

xii

LIST OF TABLES

Table 3-1 Gage Location on Field Tested Columns ..................................................................... 20

Table 3-2 Results from Column Unloading Test .......................................................................... 21

Table 3-3 Out-of-Plumbness of Columns Measured at Full Height ............................................ 24

Table 3-4 Out-of-Plumbness of Columns Measured from Girt .................................................... 25

Table 3-5 Evaluation of Column Bearing Area ............................................................................ 28

Table 3-6 Non-Destructive Evaluation Results ............................................................................ 30

Table 3-7 Impact Strength of Coupon Samples ............................................................................ 48

Table 3-8 Coupon Bending Test Results ...................................................................................... 51

Table 3-9 Shear Test Results ........................................................................................................ 53

Table 3-10 Pull Test Results ......................................................................................................... 56

Table 5-1 Theoretical Stress Values ............................................................................................. 84

Table 5-2 Longitudinal Stress Ranges for Field and Lab Testing ................................................ 85

Table 5-3 FE Longitudinal Stress Range ...................................................................................... 86

Table 5-4 Comparison of FE Longitudinal Stress Range for Base Support Conditions Against

Fully Supported Base Condition with Normal and Kinked Corner .............................................. 87

Table 5-5 Longitudinal Stress Ranges for Testing, Analysis and Theory .................................... 88

Table 5-6 Load Reduction of each Support Condition ................................................................. 90

Table 5-7 Stresses and Eccentricity of Support Conditions.......................................................... 90

Table 5-8 Stress Increase Effect due to Kink ................................................................................ 91

1

CHAPTER 1 INTRODUCTION

1.1 Background

In the field of civil infrastructure applications, fiber reinforced polymers (FRP) composite

materials are gaining acceptance as structural materials that can be used. Possible uses can be

rehabilitation of existing structures such as, advanced composite wall overlays and retrofitting of

seismic columns [Van Den Einde, et. Al 2003, Mamlouk and Zaniewski 2011] or completely

replacing conventional materials in applications like bridge decks [Van Den Einde, et. Al 2003,

Mamlouk and Zaniewski 2011] and cooling tower support columns. FRP composites are made

up of two main constituents: 1) reinforcements like fibers, fabrics or mats which commonly

include glass, carbon, graphite etc., or natural fibers such as kenaf and jute, and 2) the matrix or

resin system. The matrix is comprised of multiple ingredients including a resin, used to bond

fibers and fabrics together, reactive diluents for viscosity, initiator to start the chemical reaction

to cure the resin so that it becomes a solid matrix and provides shear and compressive force

transfer, an inhibitor for prolonged shelf life and may also contain fillers or additives for cost

effectiveness and shrinkage control. The fibers provide most of the thermo-mechanical strength

in a composite.

There are two main types of resins that are used based on the needs of a project,

thermosets, such as vinyl esters and epoxy’s, and thermoplastics such as nylon and PVC. The

most notable differences between the two is that thermoplastics can be recycled whereas

thermosets cannot be reformulated. Thermoplastics have low resistance to thermal forces and are

susceptible to creep thus, a poor choice for most structural applications. Thermosets on the other

hand can be formulated for low viscosity, higher resistance to chemicals and temperature and

2

low creep, but have limited shelf life before manufacturing and are more brittle than

thermoplastics.

Advantages of using FRP composite materials compared to the more traditional civil

engineering materials (e.g. steel, concrete, and timber) are higher strength to weight ratio, better

corrosion resistance and nonconductive properties. But like all materials, they do have

limitations as well, such as difficulties in processing and low shear strength. Arguably the

biggest limitation of FRP composites is the extensive design requirements. Steel is isotropic,

meaning that its properties do not vary based on material orientation, whereas FRP composites

are orthotropic, or transversely isotropic in some cases i.e. they do have different strengths based

on axes of orientation (along the fiber direction versus perpendicular to the fiber direction).

Because the fibers are the primary load carrying constituents in a composite with the matrix

adding shear strength and force transfer capabilities to fibers through bond. It is obvious that

composites are loaded as far as possible along the main fiber direction resulting in much higher

load resistance in relation to the other two directions. This is particularly true for a unidirectional

composite. Strength increases in other directions can partially be enhanced by adding fibers

along the desired direction; however that involves a reduction in the percent of fibers in the main

direction to maintain the overall fiber volume fraction for a given thickness of a composite.

1.2 Objective

This research is being done in order to determine the main causes of cracks forming in

the base of FRP columns at loads much lower than the design loads. These columns, comprised

of glass fibers and vinyl ester resin, are manufactured through pultrusion process and are

currently supporting a large hyperbolic cooling tower.

3

1.3 Scope

To determine the causes of cracking, this research includes field testing and evaluation as

well as laboratory testing of pristine FRP box sections and in service (field) sections. The field

testing included checking for the compressive loads and resultant stresses applied to randomly

selected columns, checking for out of plumbness and contact area between the columns and

ground. In addition, images from infrared thermography of columns were collected extensively

to check for delaminations. The infrared testing was conducted by other researchers (Halabe,

GangaRao, and Kotha) and their results are summarized in this report. The lab testing focused on

simulating field contact conditions at column bases and checking column capacities for different

boundary contact condition at their bases with the ground. Other lab tests included checking for

the fiber architecture including fabric kink in manufacturing to ensure proper manufacturing,

coupon testing in shear, bending, impact, moisture up-take, and cure percentage.

To verify the findings in the lab, Finite Element (FE) analysis was conducted. The FE

analysis included the different contact at the column base as per our field evaluation. The FE

analysis was conducted with and without kinks at the corners of the FRP columns.

Theoretical calculations based on a mechanics of material approach are also done to

compare to the testing data and the results from the FE analysis.

1.4 Project Background

The columns that are being investigated are supporting a hyperbolic cooling tower in Ohio

and were installed from November 2011 to January 2012. The columns began showing cracks

during construction and before the full service loads are put on the columns. The dead load that

is expected to be applied to each of the columns is 6,170 lbs. This is the maximum load the

4

columns had been supporting when the cracks started to be noticed and at the time of field

testing. During their service life these columns will have the bottom few feet under water and

exposed to potentially harsh conditions. This could be causing freeze-thaw cycles to be reducing

the strength of the columns as well as the temperature could be making the columns more brittle

and adding to the cracking. One such example is shown in Figure 1.3 with large amount of ice

forming around the base of the tower preventing air from circulating in and helping to cool the

tower down. Figures 1.1 and 1.2 show the columns underneath the tower as well as a view of

how larger the tower is respectively. It should be noted that in Figure 1.1, the columns being

investigated are under the column and vertical and are not the large concrete X-braces along the

outer perimeter of the column.

Figure 1-1 GFRP Columns Supporting Cooling Tower

5

Figure 1-2 Image Showing size of Tower

Figure 1-3 Ice Forming at the Base of the Tower

6

1.5 Organization of Problem Report

Chapter 2 is dedicated to summarizing research found in literature on column response

under static loads. The literature review includes a description of the manufacturing process,

information about kink effects, stress concentrations and buckling of columns as a whole or as

components such as the web or flange.

Found in Chapter 3 is a description of all the testing that is done as part of this research

program. This includes a description of each test and the data that corresponds to it. Figures and

tables are included as support information for further analysis.

The FE analysis is discussed in Chapter 4 and includes the basics of the model and

presents the findings for each case.

Chapter 5 summarizes all the data from the field and lab testing to compare with FE

analysis and also to compare with other theoretical calculations.

All conclusions and recommendations are made in Chapter 6.

7

CHAPTER 2 LITERATURE REVIEW

In order to gain a better understanding of composite columns and their behavior, a literature

review is done. The main objective of this review is to find existing information, data, and

formulas for relevant topics to the main objective of this paper. This chapter presents the

manufacturing details of the pultrusion process including its strengths and limitations, different

effects that columns will experience due to support conditions, kinks, and stress concentrations

as well as column buckling behavior on the local and global scales and creep effects.

2.1 Manufacturing Process

Currently there are several different ways of manufacturing composite sections. These

include Hand Layup, Prepreg Layup, Autoclave Processing, Compression Molding, Resin

Transfer Molding (RTM), Vacuum Assisted Resin Transfer Molding (VARTM), Filament

Winding, Pultrusion, and many others [Barbero 2011]. The word pultrusion is a hybrid of the

words ‘pull’ and ‘extrusion’ indicating the basics of how the process works and shares

similarities with both of its parent words [Sotelino and Teng, 2002].

Pultrusion is a continuous process where the fibers and mats are pulled through a series of

guides, winder, injection chamber and heated dies, to create a constant cross section of any

desired length. Pultrusion is best suited and most economical when manufacturing the same cross

section multiple times. A basic pultrusion line begins with the fiber mat being pulled from the

creel and mat racks and passing through the performing guides into the winder. From here there

are two types of processes to impregnate the fibers with resin, the more common is the injection

of resin, but occasionally used is the open bath system. In the Injection system the reinforcement

fibers enter the injection chamber where it is saturated with the resin under pressure. In the open

8

bath system the fibers are submerged in, and pulled through a pool of the resin. No matter which

impregnation process is used, the resin wetted fibers are then pulled into the die which gives the

desired cross sectional shape. Heat is applied to aid the curing process. As the product cures, it

shrinks and is no longer attached to the walls of the die. The final product is then pulled by

multiple reciprocating pullers or a similar machine to allow for any desired length to be made. A

moveable saw attaches itself to the sample and finishes the process by cutting it to the specified

length [Palikhel, 2011, Ashley 1996, Barbero 2011]. A diagram of the pultrusion process can be

seen in Figure 2-1

Figure 2-1 Pultrusion Process [4 Barbero 84]

Open bath resin impregnation is not as common due to concerns over its environmental

effects including the emission of volatile organic compound (VOC) which have major health

issues for workers. Other issues include incorrect fiber orientation caused by altered alignments

9

initiated by the resin bath as well as the fibers not being coated completely or adequately

depending on the viscosity of the resin, and resin being wasted and needing to be disposed of

properly [Palikhel 2011].

Advantages of the injected resin systems include limiting the VOC emissions to almost

zero and void contents as low as 1% because of the fibers being properly and completely wetted.

It must be noted however that this is highly dependent on proper control of the resin injection

pressure. If the pressure is too high then excess resin will begin to leak out of the die and if it is

too low, then proper wetting of the fibers will not be achieved [Palikhel 2011]. The injection

pressure can range between 60-400 psi depending on the fiber density and geometric shape of the

finished composite [Ashley 1996].

2.1.1 Issues with Pultruded Shapes

Pultrusion does have certain limitations. One of the major limitations is that pultrusion

can only produce shapes of a constant cross section. This is because a custom die must be created

for each cross section and the cost of creating each die is high.

Another issue with pultruded shapes is the stress concentration effects that can occur in

corners. These concentrations can be at the corners of a closed cross section, or at web-flange

junction in an open cross section. At these locations, the fibers can spread apart or fold thus

forming a “kink”. This type of phenomenon causes a resin rich area, which by definition has low

fiber volume fractions and thus lower strength than in a uniform fiber distribution.

Because the resin must cure with the fibers to form the composite, the cure percent of the

resin must be monitored as this may have a large effect on the strength of the composite. Voids

must also be accounted for and monitored in pultruded FRP composites.

10

2.2 Base Support Conditions

As with any structure, the structural is support conditions will play a vital role in their

performance. For columns this support condition is especially important because an eccentric

stress can be induced from concentric column loads, causing higher stress concentrations due to

lower contact area at the column base which transfers forces to the ground, thus causing the

column to crack at much lower loads than the design loads.

Uneven support conditions can be caused by multiple reasons. These include construction

issues, such as uneven cutting of the column due to either poor craftsmanship or incorrect

measurements. Other possible issues can stem from grouting underneath the column, if the

contact surface between the column base and the floor system is uneven to begin with or because

of floor or foundation settlement.

2.3 Kink Effects

A kink in an FRP composite is an area with little to no reinforcement fiber to transfer load

to and resist applied forces and is therefore assumed to have lower mechanical properties than

the rest of the member. Figure 2.2 shows an example of a kink.

11

Figure 2-2 Image of Kink in Corner

The strength properties of the resin are significantly lower than the fiber and therefore act

as a weak spot. These kinks can be caused by inadequate quality control during a manufacturing

process or an unavoidable issue in the process, i.e. a corner, where the fabric must cover a longer

length toward the outer radius and smaller length along the inner radius. This poses a problem

and can result in too much fabric gathering along the inner radius causing folds, or not enough

fiber reinforcement along the outer radius leading to lower fiber volume fractions. Both have

detrimental effects on the overall strength of the column. To illustrate the decrease in strength in

a resin rich area, the modulus of elasticity of a vinyl ester resin is approximately 493 ksi (kips

per square inch) [Barbero 2011] with the longitudinal modulus of elasticity of the entire

composite (glass fiber with vinyl ester resin) being in the area of 3200 ksi. A kink will not reduce

the mechanical properties all the way to equal that of the resin, however it will be much lower

than that of the rest of the section.

Kink

12

2.4 Stress Concentrations

One of the most important things to consider when exploring the failure of any material is

possible stress concentrations that could have affected it. A stress concentration is an area of a

member that has higher stresses than anticipated and predicted by basic mechanics of materials

approach formulas, i.e. σ = P/a or σ = Mc/I. There are many conditions that could be causing

these calculations to be higher, some of the most common items include; abrupt changes in a

section, contact stresses at the point of application of external loads, discontinuities in the

material itself, initial stresses in a member and cracks that exist in a member [Boresi and

Schmidt 2003]. These stress concentrations affect all materials and FRP composites are no

different.

Some of the stress concentrations that are applicable to this project include, the ratio of

corner radius to the wall thickness, discontinuities in the material (resin rich areas), contact

stresses at the point of application (insufficient contact between base of the column and the

ground), and cracks that exist in the member (possible delaminations as well as the cracks in the

corners). When accounting for the stress concentrations, it is usually done by finding a stress

concentration factor that is defined by either a mathematical approach or from laboratory testing

and applying that to the stress formula to lower the allowable stress.

2.5 Buckling Effects

Buckling is the basic concept that a column will experience some bending forces even if it

is meant to be strictly axial. This can be due to a force not being applied directly over the

centroid of a column and therefore causing an eccentric load, or material and geometric

13

imperfections causing weak spots. Typically long columns fail due to compressive and flexural

loads.

Buckling is just one form of column failure and can be broken up into local (flange)

buckling as well as global (Euler) buckling. Each of these has its own definition as well as the

effects, tolerances and considerations. Two of the main causes of buckling are the slenderness of

the member as well as the boundary conditions.

2.5.1 Local Buckling

The concept of local buckling is that a localized area of a column will fail, but is not

necessarily catastrophic however can lead to the failure of the entire member [Blanford 2010].

Local buckling usually appears as a “wrinkle” and can appear in either the flange or the web. A

major cause of this type of failure is high stresses that are induces by excessive local

deformation. Another cause is material imperfections that cause weak points which will result in

local failure before the entire column fails [Blanford 2010].

2.5.2 Global Buckling

Global buckling occurs in columns that are considered slender. This buckling mode

consists of an out of plane deflection but does not alter the cross section of the member itself

[Barbero et. al. 2000]. In design, global buckling is the buckling failure mode that is accounted

for more often due to its predominant in column failure. Excessive deflection of a column is

typically the controlling factor in global buckling and thus more common in columns of longer

lengths [Blanford 2010]. As the length of a column increases, its slenderness also increases and

is related to the buckling load of the column. The fundamental formula used to predict the

critical buckling load of a long or slender column is the Euler buckling equation, expressed as:

14

2.1

Where PE is the critical load at which the column is expected to buckle, E is the modulus

of elasticity, I is the moment of inertia, k is the boundary coefficient, and L is the length of the

column [Barbero et. al 2000].

2.5.3 Slenderness

The slenderness of a column is the ratio of its length, or effective length, to the radius of

gyration about an axis. The effective length is often used in this formula, as it accounts for the

boundary conditions of the column. The effective length of a column if found by simply

multiplying the actual length, L by the boundary coefficient, k which will be discussed in Section

2.5.4. Columns that are more slender are less desirable as their critical buckling load is lower. In

order to increase the load carrying capacity of a column, the effective length should be

decreased, either by choosing a cross section with a larger radius of gyration or modifying the

boundary conditions to give a larger ‘k’ value.

Several studies have been done to determine different slenderness parameters for

pultruded FRP columns which are necessary due to their high ratio of longitudinal modulus of

elasticity to shear modulus of elasticity values [Lee and Hewson 1978]. A new formula was

proposed by Lee and Hewson (1978), and its accuracy was later verified by Zureick and Scott

(1997). This formula and the Euler buckling formula only slightly overestimate the actual

buckling load. The average value of the ratios of experimentally determined load to the predicted

load for the Euler equation is 0.92 and the average of the Lew and Hewson equation is 0.94

15

[Zureick and Scott, 1997]. By combining these equations with a classic nondimensional

slenderness parameter, Zureick and Scott came up with the following equation to describe the

slenderness for pultruded FRP columns.

2.2

Where λ is the slenderness parameter, Leff is the effective length of the column, r is the

radius of gyration, FLc is the average ultimate compressive stress from coupon sample tests, EL

c

is the longitudinal compresses modulus of elasticity, ns is the form factor for shear (2 for hollow

box columns as are studied in this investigation), and GLT is the shear modulus of elasticity

[Zureick and Scott 1997].

2.5.4 Boundary Conditions

The different boundary condition that a column has (pinned, fixed or free) are used to

determine the boundary coefficient, or ‘k’ factor, that has previously been mentioned. The

boundary coefficient is used as a multiplier to measure the length that is needed to represent the

buckling shape of an ‘ideal’ column. An ideal column has both ends pinned. As presented by

Gere (2004), Figure 2.3 gives the critical buckling formula, boundary coefficient factor and

effective length of the most common combinations of column supports.

16

Figure 2-3 Critical loads, Effective lengths and Effective Length Factors [Gere 2004]

2.6 Summary

The research and formulas presented in this chapter give background knowledge of columns

and the issues that go along with them. This information will be used in the analysis of the

research done in this investigation and will help provide insight into the current problem being

experienced. There has been very little research on columns with partial contact between the

base of the column and the ground or how the loads will be transferred due to this partial contact

surface.

17

CHAPTER 3 LABORATORY AND FIELD TESTING

3.1 Introduction

With the goal of finding the reason for the cracking in the corners of in-service columns,

this project relies heavily on a wide variety of testing, both in the field to determine the current

conditions of the columns as well as experiments in the lab that attempt to replicate the cracks

found in the field. The field testing was done first, and includes unique static load tests to

determine the induced loading on randomly selected columns (six in number). These columns

additionally were subjected to thermal imaging and digital tap hammer tests (36 columns total) to

determine if delamination’s are present, and if so, how large they are. Additionally, tests are run

to determine the straightness of 50 randomly selected columns and see if any out-of-plumbness

was present, potentially causing bending stresses and uneven bearing stresses. Finally, the grout

was measured under 35 select columns to see if there is adequate contact between the base of the

column and the ground to transfer the loads.

After the completion of the field testing, samples were brought back to the lab for testing.

These samples include both virgin columns that have not been exposed to any loads and in-situ

columns cut from the field visit. The ultimate goal of the laboratory testing was to be able to

replicate the cracks found in the field and this was done by systematically ruling out possible

causes until the root cause was found. One such test was a compression test that applies load to a

stub column until failure; this test was repeated with a modified contact area between the column

and the ground to simulate the different base conditions that are previously found in the field.

Tests were run on coupon samples to check the impact resistance, bending strength, shear

strength, and a unique “pull” test that has been designed specifically for this project. Other

18

testing that was done is to determine the material properties, and fiber architecture of the column

was found through burn off, post curing and moisture absorption tests.

In the subsequent sections, these tests will be described in detail and the results will be

presented and discussed. Further discussion, comparisons, and recommendations will be made in

Chapters 5: Data Analysis and Results and in Chapter 6: Conclusions and Recommendations.

All of the tests performed in sections 3.3.3 to 3.3.9 have been performed by Dr. Ray

Liang.

3.2 Field Testing

For the field testing, the columns are identified in a basic X-Z coordinate system with the

X-axis representing East-West directions and the Z-axis representing the North-South directions.

Also due to the layout of the base of the cooling tower, four quadrants are labeled with four

directions as NE (north-east), NW (north-west), SE (south-east) and SW (south-west). All

following column labels follow this basic naming criteria being defined by the quadrant they fall

in and their Z-X location. Figure 3.1 shows the plan view of the tower base and the quadrants

and labels.

19

Figure 3-1 Base of Tower with Quadrant and Axis Labels

3.2.1 Column Unloading

The first field test was to determine the in-situ load on the columns and check to see if it

matched the design assumptions. In order to do this, 6 columns were selected and 16 strain gages

have been attached to each column. The location of the gages can be seen in Table 3.1. These

locations are maintained on all 6 columns tested at the site. The columns were chosen at random,

making sure that there was at least one in each of the quadrants. After all gages were installed for

the column, two actuators with extender poles were placed next to the column to lift the beams

and remove the load from the column. A column ready for testing can be seen in Figure 3.2.

20

Table 3-1 Gage Location on Field Tested Columns

Gage Number Orientation

Width Location Face Height

1 Vertical Centered South 1 in

2 Vertical Centered South 6 in

3 Vertical Centered South Halfway to

Girt

4 Vertical Centered South Below Girt

5 Vertical Centered South above Girt

6 Vertical Centered East 1 in

7 Vertical Centered West 1 in

8 Vertical Centered North 1 in

9 Horizontal NE North 2 in

10 Horizontal SW South 3 in

11 Horizontal SW West 4 in

12 Horizontal NE East 5 in

13 Horizontal NE North 6 in

14 Horizontal SW South 6 in

15 Horizontal SW West 6 in

16 Horizontal NE East 6 in

The strain is measured when the load on the column is completely removed. The load is

assumed to be completely removed when the strains stop increasing and level off at a constant

value, and visible in the field when the column lifted off the ground. The load is removed by

uplifting the girts (horizontal tie lines) that serve as intermediate braces on the columns to

prevent buckling and resist lateral loads. At that point the load is found from the two load cells in

between the beams and the extender poles. The stresses, average deflections and load at which

the columns are considered fully unloaded can be seen in Table 3.2. These results are discussed

in Section 5.2.2.

21

Figure 3-2 Column Unloading Set up

As can be seen in Table 3-2, most of the stresses are relatively low and range to only 400

psi in either tension or compression (positive values signify tension and negative values

compression). It would be expected that gages 1-8 would be in compression as they were

installed vertically (longitudinal direction). Gages 9-16 can be in tension or compression

depending on the bearing, cracking or other issues but are expected to be low as they were

installed horizontally (transverse direction). Because the stressed are not as expected, the loads

are likely not being applied as expected. As the magnitude of the loads shown as the ‘additive

load’ vary more than expected, the loads from the tower re not being distributed evenly.

Extender Pole (typ).

Actuator (typ.)

Column

Load Cell (typ).

22

Table 3-2 Results from Column Unloading Test

Stress at Fully Released Load [psi]

Gage Number SW Z6

X54 SW Z6

X42 NW Z54

X66 NE Z90

X42 SE Z90

X78 SE Z102

X78

1 845 -21 -10 2470 -10 1859

2 159 -28 246 1414 742 925

3 71 130 400 822 589 586

4 18 376 502 541 355 400

5 -62 -8 -51 -74 -182 -243

6 112 361 278 349 1034 797

7 -220 42 867 -19 42 3

8 -189 42 80 -288 -10 0

9 82 8 531 -32 -118 64

10 638 -285 -170 -138 -109 102

11 100 -236 -176 179 35 6

12 792 -157 154 -285 -125 -134

13 126 -178 61 29 -205 -99

14 170 99 182 -310 -262 -141

15 118 93 128 -173 -243 -118

16 205 -163 51 -16 -275 -61

Additive Load [Kips] 1.557 3.475 6.251 6.882 5.021 6.773

Average Deflection [in] 0.004 0.019 0.028 0.015 0.016 0.049

The “Additive Load” from Table 3-2 is the measured load that was needed to bring the

column to the point where it is completely unloaded.

3.2.2 Plumb Test

One of the hypothesized reasons for the column cracking is an out of plumbness causing

an eccentric load. In order to measure this in the columns, a unique measuring device was made.

This device involves two parts, “the drop mechanism” and “the base”. The drop mechanism has

a fishing reel with a plumb bob on the end of the line as the main component, and two boards

that are connected at 90 degrees. The base is similar with two boards connected at 90 degrees,

but also has a third board that is perpendicular to them with a computer generated graph on it to

23

measure the out-of-plumbness. The lines of the graph are spaced at 1/16” of an inch. Both

mechanisms are attached to the same corner of a column, one at the bottom and one at a

specified height. The fishing line has a carabiner on the end and is dropped down to the bottom

of the column where the plumb bob is then attached to the end of the fishing line. The location of

the point of the plumb bob was then recorded in Cartesian coordinate system. The attached

mechanism to a short column can be seen in Figure 3.3.

Figure 3-3 Mechanism used for Column Plumb Measurement

This process is repeated for 50 columns. 28 columns were measured at a height of 36 feet

(full height), however the battery of the lift died and due to time constraints it was decided to

measure the final 22 columns from the second girt at a height of 14.5 feet from the ground which

can be reached with a ladder. Upon completion, the overall out of plumbness is calculated for

each column. The data can be seen in Table 3.3 for the columns checked at full height and Table

3.4 for the columns checked at the girt. The construction specifications allowed for an out of

plumbness of 3/16th

inch for every 10 feet of column length. This means that for the columns

Drop Mechanism

Plumb Bob

Base

24

checked at full height, anything above 0.628 inches is above the allowable out-of-plumbness and

above 1.256 in is more than twice the allowable out-of-plumbness. Likewise, for the columns

checked at the girt 0.271 inches is the allowable out-of-plumbness and 0.543 inches is twice the

allowable..

Table 3-3 Out-of-Plumbness of Columns Measured at Full Height

Column ID [in] [in]

Number Quadrant Z X N-S E-W

1 NW 18 126 2.375 -0.25

2 NW 18 114 2.5 -0.875

3 NW 30 102 2.3125 0.375

4 NW 30 90 1.9375 0.125

5 NW 18 66 1.4375 -0.3125

6 NW 18 54 0.5625 -0.625

7 NW 30 42 -0.125 0.3125

8 NW 42 42 0.4375 0.4375

9 NW 54 54 0.4375 -0.0625

10 NW 54 66 1.375 0.3125

11 NW 42 78 1.625 0.0625

12 NW 42 90 2.3125 -0.125

13 SW 18 126 2 -0.5625

14 SW 18 114 2.25 -0.4375

15 SW 6 54 0.5 0.25

16 SW 6 42 0.0625 0.25

17 SW 18 42 0.3125 -0.1875

18 SW 18 30 0.125 -0.25

19 SW 54 42 0.1875 0.125

20 SW 54 54 0.125 -0.375

21 SW 66 54 0.25 -0.25

22 SW 66 42 -0.625 -0.25

23 SW 54 30 -0.25 0.25

24 SW 54 18 -0.375 0.125

25 SW 90 66 0.6875 0.1875

26 SW 102 66 0.5625 -0.8125

27 SE 102 78 0.9375 -0.4375

28 SE 90 78 0.875 -0.4375

25

Table 3-4 Out-of-Plumbness of Columns Measured from Girt

Column ID [in] [in]

Number Quadrant Z X N-S E-W

29 NE 90 42 -0.125 -0.125

30 NE 78 42 -0.125 0

31 NE 90 66 -1.0625 0.5625

32 NE 90 78 -0.125 0.4375

33 NE 66 78 0.8125 0

34 NE 45 78 0.75 0.0625

35 NE 30 66 0.4375 0.0625

36 NE 18 66 0.375 -0.0625

37 NE 18 42 0.4375 0.625

38 NE 18 30 0.8125 -0.375

39 NE 18 114 0.625 0.0625

40 NE 18 102 0.5 -0.125

41 SE 30 114 0.5625 0.375

42 SE 54 114 0.1875 -0.75

43 SE 54 102 0.5625 -0.3125

44 SE 54 66 0.5625 0.3125

45 SE 54 54 0.4375 -0.25

46 SE 66 54 0 -0.5625

47 SE 66 42 -0.25 -0.3125

48 SE 42 30 1 -43.75

49 SE 30 30 0.125 -0.0625

50 SE 30 18 -1.75 -0.25

Although a large number of columns were out of plumb by more than the specifications

(66%), the cracking rates were very similar for the plumb vs out-of-plumb columns, thus this is

not a contributing factor.

3.2.3 Grout Measurement

When the columns were installed, there was a small amount of space between the base of

the column and the bearing pad supporting it due to poor cuts on the column and the non-

uniform concrete floor of the cooling tower. In order to provide a solid bearing, grout was placed

under the columns by first placing a bead of caulk around the base of the column and then

26

pouring the grout into the center of the column via drain holes at the column base. The premise

was that the grout would seep under any voids under the column and thus provide sufficient

bearing along the entire column base. The grout strength is adequate to support loads that are

transferred to it from the column into the ground. However, the caulk is a very flexible material

that offers no load bearing ability. It was found that the caulk was often applied into the gaps

under the column thus preventing the grout from supporting the column. In order to measure the

grout underneath the column the caulking was removed surrounding the base of each column

tested. A ruler was then slid underneath each side at four locations per side, close to each corner

and at roughly third points of the column face. These measurements were taken to the closest

1/16th

of an inch. The results of this evaluation can be seen in Table 3.5. Due to cracking

discovered previously, some of the existing columns had been cut off 6 inches above the ground.

A new concrete base was then poured to make up the elevation difference and new bearing plate

was installed and grouted as with the other columns. These retrofits were referred to as “grout

pads” as seen in Table 3.5.

The goal of this is to see the amount of effective support the column has and to see if it

relates to cracking rates. The columns are able to be separated into five major groups based on

support conditions. These boundary conditions have been replicated in the lab testing as well as

finite element analysis to see if the cracks could be replicated and to try and get similar results to

the field testing. The five major categories are:

1. Fully supported – This support condition is for columns that are completely supported by

adequate grout. 16 columns fall into this category.

27

2. Diagonally supported – This condition has two connected sides completely

supported by grout while the other two are not. 7 columns are represented

by this support condition.

3. C-Shaped supported – Columns in this category have one side completely

supported and half of the two connected sides also supported forming a “C”

shape. 4 columns were found to have this type of boundary support.

4. Inner Perimeter Supported – 5 columns were found to have roughly half of

their base supported, but only on the inner half of the perimeter.

5. Undeterminable –The remaining 3 columns that do not fall into any of the

above categories.

Of the 35 columns tested, 37% were missing 10% of the bearing or less, 37% were missing 10 %

to 50% and 26% were missing more than 50% of the grout. The more grout that was missing also

tended to have higher cracking rates, indicating that this is a significant cause of cracking.

28

Table 3-5 Evaluation of Column Bearing Area

Column ID

Number Quadrant Z X Area

Supported Cracked?

1 NE 90 90 100% Yes

2 SE 90 78 100% No

3 SE 30 66 100% Yes

4 SE 54 66 99% Yes

5 NE 78 6 98% Yes

6 NW 30 102 97% No

7 SE 66 54 97% Yes

8 SE 6 54 97% Yes

9 NW 18 114 97% Yes

10 NE 90 42 96% No

11 NW 54 66 95% No

12 NW 54 18 94% Yes

13 SE 102 78 92% No

14 NE 42 54 89% No

15 NE 66 42 89% Yes

16 SE 18 18 87% Yes

17 NE 42 66 85% Yes

18 SW 6 54 77% Grout Pad

19 SW 18 126 65% Grout Pad

20 SW 90 6 60% No

21 SW 54 30 58% Yes

22 NW 90 78 56% Grout Pad

23 NW 54 30 53% Grout Pad

24 NE 66 30 52% Grout Pad

25 SW 30 90 52% Yes

26 SW 18 42 51% No

27 SW 42 66 48% No

28 NW 90 30 47% Yes

29 SW 6 42 44% No

30 SW 18 114 43% Yes

31 NE 18 126 40% Yes

32 SE 18 6 30% Yes

33 SE 102 66 27% Yes

34 SW 66 54 27% Yes

35 NW 6 18 12% Yes

29

3.2.4 Thermal Imaging Testing

This section summarizes the results of the thermal imaging and digital tap hammer

testing that was conducted by other researchers (Halabe, GangaRao, and Kotha).When testing

the columns in the field, two different non-destructive testing (NDT) techniques are used,

Infrared Thermography (IRT) and Digital Tap Hammer Testing (DTHT). Each of these tests has

its own strong points and weak points in how it works and the results each testing technique

shows. The IRT is able to measure an area and compares the difference in thermal behavior of

the area. DTHT on the other hand uses an echo signal to detect subsurface imperfections and is

more accurate because it is directly correlated to the material stiffness and density, however it

can only do this for a point, not an area. Therefore, columns are initially checked using the IRT

then, if a potential delamination or subsurface crack is detected and then DTHT is used to verify

it.

The field portion of the NDT is conducted over two days where the mean ambient

temperature is 26°f and 47°F on each day respectively. A total of 36 columns are examined. Of

the columns examined, the base is looked at, varying sides with a height of roughly 18 inches

using IRT, if potential delaminations or microcracking are found, then DTHT is conducted to

verify its existence. The columns selected at random in the field and are listed in Table 3.6. Table

3.6 also shows the results found from the NDT broken down by quadrant.

30

Table 3-6 Non-Destructive Evaluation Results

Column ID

Quadrant Z X Visual Observations NDT Results

NW 6 126 North Face corners - Cracks < 6" No Delamination found

NW 18 90 North Face Corners - Cracks <6"; South Face

Corners - Cracks >6" South Face Crack Zone -

Subsurface Delamination

NW 18 126 No Cracks No Delamination found





NW 54 90 NE Corner - Crack <1"; NW Corner - Crack <6";

South Face Corners - Cracks <6" No Delamination found

NW 54 102 NW Corner - <6" Crack; SW Corner - <6" Crack No Delamination found


SW 6 42 No Cracks No Delamination found


SW 18 90 SW Corner - Crack <1" No Delamination found

SW 30 90 SW Corner - Crack <1" No Delamination found




SW 54 30 All Corners - Cracks <6" No Delamination found

SW 78 30 NW Corner - Crack <1"; SE Corner - Crack <1" No Delamination found

SW 90 6 South Face Bent Inward No Delamination found

NE 18 30 No Cracks No Delamination found








SE 6 54 SE Corner - Crack <1" No Delamination found

SE 30 30 No Cracks No Delamination found



SE 54 66 NE Corner - Crack <1" No Delamination found

SE 66 54 SE Corner - Crack <1" No Delamination found



31

3.3 Lab Testing

After the field testing was complete, the data was analyzed to narrow down the

hypotheses for verification based on the probable reasons for the crack initiation. Tests were

designed and performed to replicate the cracks as well as to verify the material properties. The

boundary conditions selected for additional testing include the lack of support under the base of

the column. Also, to verify mechanical properties of FRP shapes, shear, bending, impact, cure

percent, thermal imaging, and burn off tests were conducted.

3.3.1 Compression Test

The first set of laboratory tests that were conducted were compression loading. These

tests were done on samples that are 12 inches for the fully supported samples and 15 inches in

length for the diagonal, C-shaped, and inner perimeter samples. The lengths were chosen to

eliminate buckling effects and maximize the number of samples that can be tested based on the

available length of column. Additionally, three in-service columns were cut from the base of the

tower and brought to the lab to verify the field results, as well as additional virgin columns that

have not been subjected to field loads. These columns were tested based on four of the support

conditions that have been found in the field. The fully supported condition was done by simply

applying a compressive load to the column length. To form the diagonal and C-shaped support

condition, a steel plate was placed at the bottom of the sample in the respective shape. The inner

perimeter support condition was created by machining a piece of steel to support the inner 50%

of the column. Three samples were tested for each different support condition. Each side of the

sample was given a cardinal direction to better compare between samples and ease of

identification. Visual monitoring was done on the samples in an attempt to see when and where

cracking begins. A full discussion of the results is in Chapter 5

32

3.3.1.1 Fully Supported Base

This test set was used as the base line to compare all the other tests against. For

simplicity, the columns were given numbers (1-3) for each of the support cases. For the columns

used in this test, four strain gages were installed on Sample 3, one on each face, all vertical to

measure the strain in the longitudinal direction. Samples 1 and 2 had two gages installed

vertically on the North and South sides that measured the strain. Due to limits of the test machine

used, none of these samples were taken to failure. Another lab ran the same tests on a higher

capacity test machine, and found that to completely fail the column it takes 353 kips. The Instron

test machine used by WVU has a max load capacity of 220 kips. A Load versus Microstrain

chart for each of the samples can be seen in Figure 3.4 through 3.6. It can be seen that all the

gages follow a close to linear path to the maximum applied load.

Figure 3-4 Load versus Microstrain of Sample 1 Under Fully Supported Boundary Condition

0

50

100

150

200

250

-6000 -5000 -4000 -3000 -2000 -1000 0

Load

[K

ips]

Microstrain

Full - Sample 1

North - Vertical

South - Vertical

33



0

50

100

150

200

250

-6000 -5000 -4000 -3000 -2000 -1000 0

Load

[K

ips]

Microstrain

Full - Sample 2

North - Vertical

South - Vertical

0

50

100

150

200

250

-8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0

Load

[ ki

ps]

Microstrain

Full - Sample 3

North - Vertical

South - Vertical

East - Vertical

West - Vertical

34

3.3.1.2 Diagonally Supported Base

When testing the samples that were diagonally supported, because of the possibility of

more failure types and induces strains, additional strain gages were put on the three diagonal

samples. There are a total of 4 gages placed on each diagonally supported sample. There were

two vertical gages placed on the North and South faces that measured the longitudinal strain and

bending of the samples. Also two horizontal gages were placed over the Northeast and

Southwest corners that measured the strain at the corners and attempt to be able to pinpoint when

the cracking began. Figure 3.7 shows the base of one of the samples and location of two of the

gages as well as the orientation of the steel plate. These samples were taken to failure. The

average of the three failures is 70.7 kips. Figure 3.8 shows a typical failure of the diagonal

samples.

Figure 3-7 Base Set-Up of Diagonally Supported Boundary Condition

35

Figure 3-8 Failure of Diagonally Supported Column

The vertical strain gages measured comparatively low strains compared to the fully

supported case at the same load, usually under 500 microstrain with one exception, the South

side of Sample 2 went to approximately 1000 microstrain at failure and all load-strain variations

were somewhat linear. The horizontal gages at the corners were also somewhat linear until about

40 kips in most cases, then the strains began to increase quickly. This could be where the

cracking begins. Load-strain graphs can be seen in Figures 3.9 through 3.11.

36

Figure 3-9 Load versus Microstrain of Sample 1 Under Diagonally Supported Boundary Condition


0

10

20

30

40

50

60

70

80

90

-2000 -1000 0 1000 2000 3000 4000

Load

{ki

ps]

Microstrain

Diagonal Support - Sample 1

North - Vertical

South - Vertical

NE Corner - Horizontal

SW Corner - Horizontal

0

10

20

30

40

50

60

70

80

90

-2000 -1000 0 1000 2000 3000 4000

Load

[K

ips]

Microstrain

Diagonal Support - Sample 2

North - Vertical

South - Vertical



37


3.3.1.3 C-Shaped Supported Base

The three samples tested with the C-shaped boundary condition had four gages installed.

Two were placed vertically on the North and South sides to measure the longitudinal strain and

bending and two were placed horizontally on the East and West sides above the edge of the plate

to try and measure when cracks occur. Sample 3 also has two additional horizontal gages placed

on the North and South sides. The steel plate is orientated in a way that fully supported the South

side as well as the south half of the East and West sides. Figure 3.12 shows the base of a sample

with the gage location and the plate location. These samples were taken to failure with an

average failure load of 71.1 kips with a typical failure shown in Figure 3.13. It should be noted

that this is a bending failure. The bending was caused by the load being applied eccentrically due

to the column only being supported along half of its base.

0

10

20

30

40

50

60

70

80

90

-1000 0 1000 2000 3000 4000

Load

[ki

ps]

Mircostrain

Diagonal - Sample 3

North - Vertical

South - Vertical



38

Figure 3-12 Base Set-Up of C-Shaped Supported Boundary Condition

Figure 3-13 Failure of C-Shaped Supported Sample

39

Figures 3.14 through 3.16 show the Load versus Microstrain plots of the three samples. It

can be seen from these plots that the strain on the South side is very close to zero until failure

which is expected for an unsupported side. For samples 2 and 3, the strain on the South side

tends to dip down toward -600 microstrain until approximately 40 kips at which point the slope

changes to positive and moves back toward zero microstrain at failure. Sample 1 shows a similar

path but only approaching -50 microstrain and turning to a positive slope around 20 kips. The

strain on the North side of the column is very close to zero throughout because that side is

unsupported and therefore not resisting any load. The South side is originally dominated by

compressive forces causing the negative strain but then the strain turns back toward zero when

the bending forces become large and begin to counteract the compressive strain. The horizontal

gages over the edge of the steel plate increase until approximately 50 kips at which point the

strain makes a abrupt jump, indicating a likely point where cracks form.


0

10

20

30

40

50

60

70

80

90

-1000 0 1000 2000 3000 4000

Load

[K

ips]

Microstrain

C-Support - Sample 1

North - Vertical

South - Vertical

East - Horizontal

West - Horizontal

40



0

10

20

30

40

50

60

70

80

90

-1000 0 1000 2000 3000 4000

Load

[K

ips]

Microstrain


North - Vertical

South - Vertical

East - Horizontal

West - Horizontal

0

10

20

30

40

50

60

70

80

90

-1000 0 1000 2000 3000 4000

Load

[K

ips]

Microstrain


North - Vertical

South - Vertical

East - Horizontal

West - Horizontal

North - Horizontal

South - Horizontal

41

3.3.1.4 Inner-Perimeter Supported Base

Of the three different partially supported conditions tested, the inner perimeter support

condition had the highest average load to failure of 110.9 kips. This support condition had a

different steel plate used than the diagonal and C-shaped tests, in that a plate was cut to an area

of 4.8125 square inches to be able to support the inner half of all four walls. This can be seen in

Figure 3.17. Five gages were installed on each of the three samples. There was one horizontal

gage placed on a corner (NE for Sample 1, NW for Samples 2 and 3). The North and South sides

had vertical gages on the outside, similar to the other tests, but also had vertical gages mirroring

the location on the inside of the column wall. The reason for having the inner gages was to

measure the difference in strain on the inner and outer faces of the column. Figure 3.18 shows a

typical failure of the column base splitting out at the base.

Figure 3-17 Base Set-Up of Inner Perimeter Boundary Support Condition

42

Figure 3-18 Failure of Inner Perimeter Supported Sample

As can be seen in the plots of Load versus Microstrain graphs in Figures 3.19 through

3.21, the inner gages show a very large compressive strain while the outer gages show very small

compressive or even tensile strains. This is expected as the outer gages are measuring a wall that

is unsupported and tensile forces are expected. The corner gage shows relatively small strains

compared to the inside gages possibly indicating that cracking is not occurring at the gage

location and showing that the transverse strains are only a fraction of the longitudinal

compressive strains.

43

Figure 3-19 Load versus Microstrain of Sample 1 Under Inner Perimeter Boundary Support Condition


0

20

40

60

80

100

120

140

-6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000

Load

[K

ips]

Microstrain

Perimeter - Sample 1


North - Vertical

South - Vertical

North - Inside

South - Inside

0

20

40

60

80

100

120

140

-7000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000

Load

[K

ips]

Microstrain


NW Corner - Horizontal

North - Vertical

South - Vertical

North - Inside

South - Inside

44


3.3.1.5 In-Service Columns

The three column samples that were cut from in service columns were tested with the

base plate and grout still intact from the field instillation. The sealant was removed to measure

the grout and see which of the five categories they fell into as well as how much bearing area is

actually missing. A total of 13 strain gages were installed to mimic the field tested columns

excluding the gages at the girder height and half the girder height as the height of the in service

cuts do not allow for them. Each of the three columns was chosen on site for a different reason

and had different issues. Column SW Z90 X6 was found to have a “mushroom” effect in the

field as one of the faces was bending out of plane. Column SE Z18 X 6 was chosen because it

had large existing cracks (5 and 6 inches) at the base. Column SW Z18 X42 was found to have

cracks at the girts. The columns are found to have missing bearing area between roughly 40%

and 70%. The tops of the columns were cut as straight as possible in the lab but are still not

0

20

40

60

80

100

120

140

-6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000

Load

[K

ps]

Microstrain


NW Corner - Horizontal

North - Vertical

South - Vertical

North - Inside

South - Inside

45

perfectly square. This is because an old concrete saw was used to cut the columns with the

column base plate still attached during the cutting, which made it difficult to make an accurate

cut. The columns were tested with the base plate and grout intact and found to fail at an average

load of 111.8 kips. Figures 3.22 through 3.25 show the Load versus Microstrain graphs for each

case, being identified in terms of vertical and horizontal gage readings.

Figure 3-22 Vertical Gages of Column SE Z18 X6

0

20

40

60

80

100

120

-5000 -3000 -1000 1000 3000 5000

Load

[K

ips]

Microstrain

Z18 X6 - Vertical Gages

South 1

South 6

East 1

West 1

North 1

46

Figure 3-23 Vertical Gages of Column SW Z90 X6

Figure 3-24 Horizontal Gages of Column SE Z18 X6

0

20

40

60

80

100

120

140

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000

Load

[ki

ps]

Microstrain

SW Z90 X6 - Vertical Gages

South 1

South 6

East 1

West 1

North 1

0

20

40

60

80

100

120

-5000 -3000 -1000 1000 3000 5000

Load

[K

ips]

Microstrain

SE Z18 X6 - Horizontal Gages

North 1 NE

East 1 NE

South 1 SW

West 1 SW

North 6 NE

East 6 NE

South 6 SW

West 6 SW

47

Figure 3-25 Horizontal Gages of Column SW Z90 X6

3.3.2 Thermal Imaging Testing

The thermal imaging testing was done in the lab by other researchers (Halabe, GangRao,

and Kotha) in order to prove its accuracy and verify its use in the field testing. Before field

testing, a few pre-damaged samples were tested using IRT and DTHT. Tests were conducted in

three phases to simulate the different conditions that would be met in the field. The first set of

tests done was on a dry sample at room temperature, followed by a sample that has been

submerged in water for 20.5 hours and at room temperature. The third test wasa of a sample that

has been frozen for 4.5 hours to mimic the weather of the day of the field testing. Both the IRT

and DTHT were able to accurately detect the delaminations of the columns in all three different

environmental conditions. This justifies the accuracy of the data collected during field testing.

0

20

40

60

80

100

120

140

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000

Load

[K

ips]

Microstrain

SW Z90 X6 - Horizontal Gages

North 1 NE

South 1 SW

West 1 SW

North 6 NE

East 6 NE

South 6 SW

48

3.3.3 Impact Testing

In order to obtain the energy required to break a coupon sample, an Izod impact test was

performed in accordance with ASTM D256, however a coupon length of 3.75 inches is used

instead of the recommended 2.5 inches. Also, the coupons did not have a notch in them as

specified by the ASTM D256 standard. A SATEC BLI Impact Tester with a weight of 8 ft-lb (as

can be seen in Figure 3.26) was used which means the scale value that is given will be four times

lower than the actual impact resistance. Table 3.7 shows the dimensions and results of each of

the samples. It should be noted that Samples 1, 2 and 3 have a CSM fabric layers from the

outside face to the inside face and therefore have much higher failure values due to the added

strength. The averages were computed including these results, then the “modified average” was

computed excluding these results for more accurate results. Even excluding the samples with the

additional reinforcement, it can be seen that the average energy per unit width of 8.97 ft-lb/in.

Figure 3.27 shows all the failed samples of this test.

Table 3-7 Impact Strength of Coupon Samples

Sample Width [in] Thickness

[in] Scale [ft-

lb] Actual Strength [ft-

lb] Energy/unit width [ft-

lb/in]

1 0.535 0.380 1.845 7.380 13.794

2 0.530 0.388 1.895 7.580 14.302 3 0.530 0.389 1.360 5.440 10.264 4 0.504 0.371 1.100 4.400 8.730 5 0.512 0.381 1.000 4.000 7.813 6 0.535 0.370 0.941 3.764 7.036 7 0.530 0.371 1.060 4.240 8.000 8 0.507 0.391 1.680 6.720 13.254

Average 0.523 0.380 1.360 5.441 10.399

Modified Average*

0.518 0.377 1.156 4.625 8.967

49

Figure 3-26 SATEC BLI Impact Tester Used

Figure 3-27 Samples Failed in Izod Impact Test

3.3.4 Bending Testing

In order to find the flexural stress at failure, a three point bending test was performed on

6 coupon samples cut in the longitudinal directions of the column of length 8 inch with an

effective length of 6 inches and cross section of 0.5 inches wide by 0.38 inches thick. This test

corresponds with ASTM D790. This test was set up to check two different variables, the

50

condition of the samples as they were received versus ones that were post cured and also the load

orientated being “normal” with the load applied on the outside surface or “vertical” with the load

applied along the thickness of the sample. A photo of the test set up can be seen in Figure 3.28

while examples of the normal placement and the vertical placement can be seen in Figures 3.29

and 3.30 respectively and the results are presented in Table 3.8.

Figure 3-28 Coupon Bending Test

Figure 3-29 Coupon Bending Test with “Normal” Orientation

51

Figure 3-30 Coupon Bending Test with “Vertical” Orientation

Table 3-8 Coupon Bending Test Results

Sample Condition Placement Width

[in] Thickness

[in] Failure Load

[lbs] Failure Stress

[psi]

1 As

Received Normal 0.502 0.386 862.14 103.739

2 As

Received Vertical 0.504 0.388 895.88 81.809

3 As

Received Vertical 0.506 0.389 910.24 82.252

4 Post

Cured Normal 0.510 0.390 828.38 96.111

5 Post

Cured Normal 0.507 0.390 811.98 94.766

6 Post

Cured Vertical 0.508 0.387 924.97 83.355

As can be seen in Table 3.8, vertical placement has an overall higher stress at failure,

which is expected due to the difference in the moment of inertia however, the percent difference

in largest and smallest in stress value based on the placement, is still close (9% for the normal

orientation and 2% for the vertical orientation). The difference between the condition of the

samples, (as received versus post cured) does not appear to affect the stress at failure.

52

3.3.5 Shear Testing

This test is simply a bending test that is designed to fail due to shear by creating a small

(effective) length to depth (thickness) ratio, or L/D. An L/D ratio of 7 was chosen for these

samples an effective length of 2.8 inches (0.4 inch thickness x 7 = 2.8 in length) samples were

cut transversely from the column to dimensions of 3.75 in long by 0.5 in wide by 0.4 in thick.

The shear test was also performed on an Instron test machine with two different placements,

“normal” orientation, with the outer wall being loaded, and “upside down” orientation, with the

inside wall being loaded. It is found that the different placements have no effect on the failure

load. Figure 3.31 shows failed sample #6 and Table 3.9 provides the results.

Figure 3-31 Shear Test Failed Sample #6

Looking at Figure 3.31, it can be seen that there is a large piece of CSM fabric that goes

from the inner to the outer face which is given the sample added strength in resisting the shear

forces. For this reason, Table 3.9 gives an average failure load and stresses and a “modified

average” load and stresses excluding sample 6 for more accurate results. Additionally, the

average and modified average standard deviation and coefficient of variation are calculated.

53

Table 3-9 Shear Test Results

Sample Width [in] Thickness [in] Failure Load [lbs] Failure Stress [psi]

1 0.513 0.388 241.00 1211

2 0.534 0.371 245.68 1240

3 0.534 0.372 244.63 1231

4 0.533 0.386 217.57 1058

5 0.534 0.372 207.03 1042

6 0.534 0.386 378.97 1839

Average 0.530 0.379 255.81 1270

Modified Average

0.530 0.378 231.18 1156

Std. Dev 0.0085 0.0083 62.38 291.95

Modified Std. Dev

0.0093 0.0084 17.72 98.01

COV 0.016 0.022 0.244 0.230

Modified COV

0.018 0.022 0.077 0.085

Samples 1-5 failed at a similar load however sample 6 failed much higher. This is due to

sample 6 having the large piece of CSM fabric running through it (as seen in Figure 3-31) which

increased the strength, where are Samples 1-5 did not have this added mat through it.

3.3.6 Pull Testing

In another attempt to recreate the corner cracking that was observed in the field, a new

test was designed, called the “Pull Test”. This test was conducted on an Instron test machine and

had a unique fixture, build specifically for this project to attempt to recreate the corner cracking.

This test had two set ups, a corner pull and a mid-section, or side pull. For both of the test set

ups, two samples were cut from a column, one is 0.5 in wide and the other is 1.0 in wide. For

comparison purposes, the results are reported in load per unit width [lb/in]. Figures 3.32 and 3.33

show the set up for the Corner Pull from two different angles and Figures 3.34 and 3.35 show the

54

same angles for the side pull test. Table 3.10 gives the max failure results of each of the tests.

Samples 1 and 3 were both 0.5 inches wide and samples 2 and 4 were 1 inch wide. Sample 1 had

a multi stage failure, first breaking at ~88 lbs before it ultimately failed at ~97 lbs.

Figure 3-32 Corner Pull Test Set-up Cross Section View

55

Figure 3-33 Corner Pull Set-up Test Side View

Figure 3-34 Side Pull Test Set-up Cross Section View

56

Figure 3-35 Side Pull Test Set-up Side View

Table 3-10 Pull Test Results

Sample Load Type

Width [in]

Thickness [in]

Max Failure Load Failure Stress [psi] lbs lbs/in width

1 Corner 0.515 0.379 96.66 187.69 495

2 Corner 0.991 0.375 189.2 190.94 509

3 Side 0.517 0.379 220.09 425.71 1123

4 Side 0.995 0.375 404.33 406.36 1084

Table 3.10 shows that when compared on the lb/in width scale, the results are very close

for both the corner and side pull tests, only varying by 2% and 5% respectively.

3.3.7 Burn Out Testing

A burn out test was conducted to examine the fiber architecture of the column. In order to

do this ASTM D 2584-02 had been followed. Three samples were cut from the column in a

57

transverse manner to a length of 4 inches. The samples were then placed in individual crucibles

and heated for 6 hours at a temperature of 575 °C to ensure all the resin was removed. After the

heating was complete, the fiber architecture was found by cautiously picking up the layers one at

a time. It was found that the architecture is: (from outer face to inner face) chopped strand mat

(CSM), 90° rovings, 0° rovings, 90° rovings, 0° rovings, CSM. There was also clay filler found

from the burn out test. Figure 3.36 shows the three samples sitting in the oven before the test.

Figure 3.37 shows the layout of the fibers after the resin is burned off. Each row of Figure 3.37

represents a sample.

Figure 3-36 Pre-Burn Out Test Samples in Oven

58

Figure 3-37 Post-Burn Out Test Fiber Architecture

3.3.8 Differential Scanning Calorimetry (DSC) Testing

To check if the cure percent of the resin is indeed 100%, and to verify the glass transition

temperature, a DSC test was performed on six samples using a DSC Q series Model Q100 from

TA Instruments Inc. The DSC machine can be seen in Figure 3.38.

Figure 3-38 DSC Test Machine

59

DSC testing involves finding the transition temperature of materials by measuring the

heat flows and temperatures with respect to time. This allows for insight into the heat capacity

change, as well as the endothermic or exothermic processes that may be occurring in a material.

Figure 3.39 gives the temperature versus heat flow graph that was produced for a sample

and shows the transition temperature of 105°C, which is typical for glass. Also, no major

chemical reactions are found, showing that the resin is approximately 100% cured.

Figure 3-39 DSC Results for Sample 1

3.3.9 Post Curing and Moisture Content Measurement Testing

Three coupon samples were cut from a column received from the field and were used to

determine the amount of water that was absorbed by the coupons and to test them as “post cured”

samples in bending (See section 3.2.5 for results). The “as received” samples were dried for 24

60

hours at 140°F and were found to absorb, by weight, 0.07% water. This water absorption value is

similar to other GFRP samples [Kawada and Kobiki 2003] and therefore most likely is not

contributing to the premature cracking. Another possible issue that has been discussed as to the

cause of the corner cracking is water getting inside the columns and freezing and causing

damage. This is a possibility, however it is not likely as the water uptake percent is very small,

meaning that the columns are not absorbing enough water to be causing the amount of cracking

that is being seen.

61

CHAPTER 4 FINITE ELEMENT ANALYSIS

Finite Element (FE) Analysis is becoming more popular and accurate in correctly

representing the reaction of different shapes and members under many different loadings. FE

analysis can be used to predict both static and dynamic responses of structures. FE analysis can

also show exactly the location of stress concentrations and their magnitude. This type of analysis

has a large variety inputs and options.

Finite Element Analysis was conducted to compare to the theoretical data as well as the data

found in the Lab and in the Field. The FE software ANSYS version 14.0 had been used. For this

analysis, a length of 36 inches was used for the total length of the column to maximize the ability

of the program and also to minimize the edge effects of the actual columns. As presented in the

following sections, although the columns are 36 inches long, only about 3 inches are actually

shown in order to focus on the applicable data and remove any edge effects.

A load of 6,170 pounds was applied to the end of the column in one of four ways simulating

the four different boundary support conditions. The load was evenly distributed over the nodes

forming the boundary condition. The base had been fully restrained in the longitudinal (Z-

direction) as well as one side in the X-direction and one side in the Y-direction. Only the outer

edge of each of the two transversely restrained sides had been restrained in order to allow for

response of the columns under bending, swelling, and displacements.

4.1 Analysis of Different Support Conditions

As was done with the field testing, three different support conditions are hypothesized to

be contributing to cracking in the columns. In order to compare with the field and lab values, as

well as values calculated by astrengths of materials approach, The FE analysis utilizes these

62

same base support conditions and have been analyzed in ANSYS 14.0. To get baseline values, a

fully supported condition had also been analyzed. The other three have 50% of the base being

supported in various ways including two connecting sides supported, forming a “diagonal” type

loading, one side being fully supported with the two connecting sides being supported half way

creating the “C-shape” support, and the “perimeter” support which has the inner perimeter of the

column supported. These supports conditions have been chosen based on similar support

conditions that were found in the field testing.

4.1.1 Fully Supported Base

In order to appreciate the stresses found for different support conditions, a baseline must

be established for comparison. Theoretically, all of the columns should be fully supported in the

field, in order to perform as they are designed to, but this is not a realistic field condition. Figure

4.1 shows the nodes of the idealized column cross section that are all supported. The load of

6170 pounds was distributed evenly over the entire base of the column, as expressed in red in

Figure 4.1

Figure 4.2 is the nodal longitudinal stresses that are produced because of this boundary

condition. As it can be seen, the range of stresses is very small (-875 psi to -878 psi), which is

expected because there are no stress concentrations or bending forces in a fully supported

column. Figure 4.3 shows the nodal transverse stresses which are very close to zero. The shear

stresses in the XY plane are shown and similar to the transverse stresses they are very close to

zero and are shown in Figure 4.4.

63

Figure 4-1 FE Model of Fully Supported Boundary Condition

Figure 4-2 Nodal Longitudinal (Z direction) Stress

64

Figure 4-3 Nodal Transverse (X Direction) Stress

Figure 4-4 Nodal Shear Stress in XY Plane

65

4.1.2 Diagonally Supported Base

The second base support arrangement was with two consecutive sides being supported

while the two opposite side are not. This is referred to as the “diagonal” base support. It is

thought that this may be causing the cracking in the corners because of the shear forces that are

created by the sudden loss of support at the corners, causing a stress concentration. As can be

seen in the Figures 4-5 through 4-8 below, the 6170 pounds is distributed over roughly half of

the column base. Because of unsupported nodes, the diagonal boundary condition not only has

the axial compressive reactions, but also bending reactions increasing the total stresses. Because

of these bending stresses that are not seen in the fully supported boundary condition, in addition

to the axial stresses, produces a much larger stress range than the fully supported base did in the

longitudinal and both transverse directions. Figure 4-5 shows the loaded nodes in red (the left

side and the bottom side).

Figure 4-5 FE Model of Diagonally Supported Boundary Condition

66

The longitudinal stresses are shown in Figure 4.6 and range from -1641 psi to 979 psi.

This range, as expected, is much larger than the fully supported condition. The transverse and

shear stresses also increase substantially from near zero to a range of -139 psi to 154 psi and -56

psi to 55 psi respectively and can be seen in Figure 4.7 and Figure 4.8. These increased stress

ranges are caused by multiple reasons including the lack of support under half of the column and

the bending stresses. The lack of support is causing the compressive stresses to be twice as high

on the area that is still supported, because compression stresses is the force divided by the area,

and the area being reduced by half, the stresses should double (875 to 1641 is approximately

double) and also causing bending. These bending stresses are induced because the missing

support caused the load to become eccentric, producing bending in the column. The bending

stresses are the cause of the positive (tensile) stresses that can be seen in Figures 4.6 through 4.8.

Figure 4-6 Nodal Longitudinal (Z Direction) Stress

67



68

4.1.3 C-Shaped Supported Base

Another support that has been found in the field and replicated in the lab and through FE

analyses is the “C-shaped” support. The C-shaped boundary is one side fully supported, and half

of both of the connected sides also being supported, forming into the shape of a C as is shown in

figure 4.9 with the red nodes (left side and left half of top and bottom sides) carrying the load. At

first glance, it may not be expected for this to cause cracking in the corners, but similarly to the

diagonal support, this also creates bending stresses and has a drop off in support, increasing the

shear forces at in the column. This was also used to verify similar findings in the field. This did

not have as big of an impact as the diagonally supported base condition does, however the

longitudinal stress range shown in Figure 4.10 is found to be -1410 psi to 271 psi with the

transverse stress range being shown in Figure 4.11 being -160 psi to 79 psi and the shear stresses

going from -30 psi to 16 psi as shown in Figure 4.12.

Figure 4-9 FE Model of C-Shaped Supported Boundary Condition

69



70


4.1.4 Inner Perimeter Supported Base

The final support condition that has been found in field testing that is replicate-able in the

lab and FE analysis is the “Perimeter” support. This support has the inner 50% of the cross

section supported. This does not create bending stresses, however it was hypothesized that this

will cause larger stress concentrations leading to delamination within the center of each wall as

well as the corners due to the abrupt drop of contact at the support. Figure 4.13 shows the loaded

nodes in red. Because of the inability to reduce the edge effects for this boundary condition, it

produced the largest stress ranges by far in all three of the reduced support areas. Figure 4.14

shows the longitudinal stress rangnig from -8651 psi to 6804 psi. The stress range for the

transverse direction is from -5546 psi to 2071 psi as seen in Figure 4.15. Figure 4.16 shows the

shear stress range of -1917psi to 1827 psi.

71

Figure 4-13 FE Model of Inner Perimeter Boundary Support Condition


72



73

4.2 Kinked Corner Effect

After looking at all the analyses of the columns, a “kinked” corner was added. The kink

represents a phenomenon that occurs in FRP composites where two plate members (sides) meet

and are connected (e.g. the corner of a closed section or the intersection of a flange and web). A

kink can lead to a resin rich area with mechanical properties differing from the matrix. To

accurately represent the kink phenomenon in this analysis, elements in one corner of the column

are selected and the longitudinal modulus of elasticity is modified. The new value of the

longitudinal modulus of elasticity is reduced but cannot be assumed to be equal to the modulus

of elasticity of the resin because there is still some fiber reinforcement providing strength. To

account for this, the ‘kinked’ value of the longitudinal modulus of elasticity was set equal to the

transverse modulus of elasticity, therefore giving some additional strength from the fibers while

still being reduced from the rest of the column. The selected elements represent approximately

three vertical inches in the column. The kink was added in the corner for accuracy because the

samples tested in the field and lab were found to have a kinked area at the corners of the

members.

4.2.1 Fully Supported Base with Kinked Corner

In terms of the boundary conditions, loading and restraining of nodes, and geometric

properties, both the kinked corner FE analysis and unkinked corner FE analysis were identical.

The only difference between the two analyses was the modified longitudinal modulus of

elasticity in the selected corner elements for the kinked corner FE analysis. Figure 4.17 shows a

large increase in longitudinal stresses ranging from -1168 psi to -212 psi. This shows the

immediate impact of the kinked corner compared to the unkinked corner. Similarly, Figures 4.18

and 4.19 show increases in the stress ranges due to the kink.

74



75


4.2.2 Diagonally Supported Base with Kinked Corner

For the FE analysis of the diagonally supported boundary condition, two options are

explored: 1) placing the kink in one of the partially supported corners (i.e. where the support

ends) and 2) placing the kink in the corner that is fully supported. It is found that the second

option yields a larger increase in the stresses, therefore, this option explored and option 1 is left

out. Similar to the fully supported boundary condition, the stress ranges increase in all three of

the aspects being looked at almost entirely in the kinked corner, again proving its effect.

76



77


Figure 4.20 shows the longitudinal stress range which has increased to -1915 psi to 617

psi with a lower stresses in the kinked corner. The transverse and shear stresses that are shown in

Figures 4.21 and 4.22 respectively show an increase in the range of stresses to -331 psi to 549 psi

and -100 psi to 190 psi, again being concentrated in the corner. These stresses are higher

compared to the unkinked diagonally supported FE analysis because of the effect that the kink is

having. The kinked elements have different stress in them because they cannot support the same

amount of stress as the unkinked elements due to the reduced properties. This is the reason that

the columns are cracking in the corners, because the kink is causing a stress concentration. The

lowered resistance to stress caused by the kink, in conjunction with the increased stresses caused

by the missing support conditions is likely the cause of the cracking in the corners.

78

4.2.3 C-Shaped Supported Base with Kinked Corner

The C-Shaped base support was altered only in the half of the base that is supported.

Similar to the diagonal support condition, it was explored having the kink in two different

corners, a supported corner and an unsupported corner. Also similar to the diagonal boundary

condition, when the kinked corner was supported, a larger increase in stresses is yielded

compared to the kink in an unsupported corner. Figure 4.23 demonstrates the effect that the

kinked corner has on the longitudinal stress range, and clearly shows that the kink has a different

reaction to the applied load than the C-Shaped support without the kink does.


79



80

Figure 4.24 models the transverse stress and how it changes with the kinked corner

increasing the stress range to -255 psi to 451 psi with the stresses being more or less constant

throughout the column except in the kinked corner. The shear stresses act similarly i.e., relatively

constant except in the kinked corner where the stresses are ranging from -73 psi to 149 psi. When

comparing the stresses of the unkinked C-shape case and the kinked C-shape case, the stresses

are found to be higher in the kinked case due to the stress concentration of the kink.

4.2.4 Inner Perimeter Supported Base with Kinked Corner.

The inner perimeter support condition has provided extremely large stress ranges

(compared to the fully supported, diagonally supported and C-shape supported boundary

conditions) in the unkinked corner and kinked corner FE analyses because it was presenting

edge effects that cannot be minimized as they were in the other support conditions. These edge

effects made it difficult to observe any stress concentrations that may be created in the kinked

corner. However, an increase in stress can still be seen in the kinked corner case versus the

unkinked corner case in the longitudinal, transverse and shear stress ranges.

Figure 4.26 is showing a longitudinal stress range of -8342 psi to 6844 psi. The

transverse stress range is increased to -8718 psi to 2115 psi and the shear stress ranges from -

1763 psi to 2792 psi. All three stress ranges that have been evaluated, show that large stress

concentrations are occurring in all the corners making it difficult to draw accurate conclusion of

the impact of the kinked corner for the inner perimeter support condition however because the

stress ranges do increase, the kink does still have an effect on the column.

81



82

Figure 4-28 Nodal Shear Stress in the XY Plane

83

CHAPTER 5 DATA ANALYSIS AND RESULTS

5.1 Calculation and Analysis of Theoretical Results

As well as all the testing done, a basic mechanics of materials approach was taken to

analyze columns under different support conditions to verify the experimental data validity with

theoretical predictions. For the fully supported base conditions, the basic axial stress formula is

used:

(5.1)

Where σa is the axial stress, P is the applied load and A is the cross sectional area. To

keep consistent with the field and lab testing as well as the FE analysis, the load is taken as 6170

lbs, and the cross sectional area is taken as 7.24 in2.

The diagonal and C-shaped support conditions have not only the axial induced stress but

also a bending induced stress. To include these, the bending stress formula is added to the axial

stress formula to yield the total stress formula:

(5.2)

Where σt is the total stress, P is the applied force of 6170 lbs, A is the cross sectional area of 7.24

in2, M is the resulting moment equal to the force multiplied by the length of the base to give

16042 lb-in, c the distance from the center of the base to the outside wall, 2.6 in, and I is the

moment of inertial equal to 28.25 in4. Equation 5.2 has two constants in it, one in the axial stress

84

calculation (2) and one in the bending stress calculation (x). The 2 in the axial part of the

equation is due to the area being reduced by half. The bending constant is not calculated as

simply. The constant, x, will vary somewhere between 2 and 4. On the low end, the moment of

inertia can be assumed to be half, therefore making the constant 2. However because the moment

of inertia equation has a cubic term in it, this value may be reduced by as much as a factor of 8,

but the c value will also be reduced by half, therefore making the highest possible value for the

constant equal to 4. There will be bending on the sides, one positive and one negative, thus

justifying adding and subtracting to find both high and low values. Table 5.1 gives results of

each boundary support condition.

Table 5-1 Theoretical Stress Values

Base Support Condition

Axial Stress [psi]

Bending Stress [psi]

Total Stress [psi]

Axial – Bending

Axial + Bending

Full -852.21 - -852 - Diagonal -1704.42 +/-2952 -4657 -1248

C-Shaped -1704.42 +/-2952 -4657 -1248 Inner Perimeter -1704.42 - -1704 -

5.2 Comparison of Results

After completing all the field and lab tests, as well as the FE analysis and utilizing

mechanics of materials formulas to predict the strength, the results are compared with each other

as shown in this chapter. The goal of the comparison is to try and find the cause of the cracking

in the corners. This will be done by using the available data to look at which tests, theories and

analyses are most closely predicting how the columns are actually responding to the applied

loads.

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5.2.1 Material Property Test Results

A large portion of the lab testing had been dedicated to verifying that the FRP columns

meet the design requirements. This was done to ensure that the cracking was not caused by a

flaw in the manufacturing. After the burn off, DSC, post curing check, and moisture uptake tests,

it appears that none of these played a factor in the performance of the columns. Additionally, the

bending, shear, impact and pull test all yielded results that are comparable to similar FRP

composite materials and thus are also determined to be meeting the design requirements and not

influencing the corner cracking.

5.2.2 Field and Lab Testing Results

The most probable causes of the cracking as found in the field and laboratory testing are

the missing support between the column base and the ground, and the kinked corners. Table 5.2

shows the minimum and maximum stress values in the longitudinal direction that have been

observed from the test data.

Table 5-2 Longitudinal Stress Ranges for Field and Lab Testing


Field Testing [psi] Lab Testing [psi]

Min Stress Max Stress Min Stress Max Stress

Full -2470 288 -2324 -4 Diagonal NA NA -87 396 C-Shaped -844 219 -409 44

Inner Perimeter NA NA -112 1420

There are no results for the diagonal and inner perimeter support condition from the field

testing because the different base support conditions were developed after the field testing was

complete. Only 6 randomly selected columns were tested in the field limiting the possible

different support conditions that are analyzed. Two of the field tested columns had C-shaped

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support conditions while the other four had fully supported base conditions. It has been found

that even with the reduced support in the lab, the columns still do not show signs of cracking

until a load of approximately 190kips for fully supported, 47 kips for diagonally supported, 50

kips for C-shaped (on center of face with drop off in support) and no visible cracks were found

until failure on the inner perimeter supported columns.

5.2.3 FE Results

The finite element analysis gave a good comparison of the effects that the different

support conditions have as well as showing the effect of the kinked corner versus unkinked

corner. The results in the following tables are observed from approximately 1 inch from the

bottom of the column. This is done in order to have comparable values against the lab and field

data as the gages in those tests are installed 1 inch above the base. Table 5.3 gives the

longitudinal stress ranges for the different support conditions for the kinked and unkinked corner.

Table 5-3 FE Longitudinal Stress Range

Base Support Condition Normal Corner [psi] Kinked Corner[psi]

Min Stress

Max Stress

Min Stress

Max Stress

Full -877 -876 -955 -742

Diagonal -1641 688 -1915 617

C-Shaped -1410 271 -1617 233

Inner Perimeter -1782 1652 -3279 1782

Table 5.3 compares the three 50% support conditions versus the fully supported case for

both kinked and unkinked corners. This shows more accurately the increase caused by the lack of

support with a kink against what it should be without a kink.

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Tables 5.3 and 5.4 are both showing the effects of the two major issues that are causing

the corner cracking, the missing support under the columns and the kinked corner. The missing

support, as displayed by the three different boundary conditions is clearly showing an increase in

stress ranges for the three support conditions that are explored. The kink effect is also causing

increased stresses in every case versus the unkinked corner and as shown in chapter 4, the

stresses are concentrated in the corners where that kink is found. Table 5.4 is comparing each of

the partial support conditions against the fully supported case and taking the difference. This is

done to show how much each partial support condition increases the stress from the fully

supported case.

Table 5-4 Comparison of FE Longitudinal Stress Range for Base Support Conditions Against Fully

Supported Base Condition with Normal and Kinked Corner


Normal Corner [psi] Kinked Corner[psi]

Min Stress Max Stress Min Stress Max Stress

Full -877 -876 -955 -742

Diagonal -1641 688 -1915 617

Difference 764 -1564 960 -1359

Full -877 -876 -955 -742

C-Shaped -1410 271 -1617 233

Difference 533 -1147 662 -975

Full -877 -876 -955 -742

Inner Perimeter -1782 1652 -3279 1782

Difference 905 -2528 2324 -2524

5.2.4 Comparison of all Results

To validate our results, the data from the lab and field are compared to the FE analysis

and the strengths of materials formula results. The longitudinal stress ranges for all are presented

in Table 5.5

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Table 5-5 Longitudinal Stress Ranges for Testing, Analysis and Theory


Field Testing [psi]

Lab Testing [psi]

Finite Element Kinked [psi]

Theoretical [psi]

Min Max Min Max Min Max Min Max

Full -2470 288 -2324 -4 -955 -742 -852 -852 Diagonal NA NA -87 396 -1915 617 -4657 1248 C-Shaped -844 219 -409 44 -1617 233 -4657 1248

Inner Perimeter NA NA -112 1420 -3279 1782 -1704 -1704

When comparing the stress ranges for the FE kinked corner and the theoretical approach,

it can be seen that the fully supported case matches up well, however there is a larger difference

in the other three support cases. For the diagonal and C-shaped support conditions, the values

differ so largely because the application of the load is eccentric. Planning for the worst case

scenario, the largest possible value of the eccentricity is used (i.e. half of the column width) to

maximize the bending stress. If a lower value is used for the eccentricity then the results will be

closer. For the inner perimeter comparison of the FE kinked corner versus the theoretical

approach, the FE values are much larger due to the edge effects the program causes that cannot

be minimized in the analysis.

When comparing the data collected from the field and lab testing versus the FE kinked

analysis, it can be seen that the field and lab stresses fall within the ranges provided by the FE

kinked analysis for the diagonal, C-shaped and inner perimeter support cases. This gives a good

indication that the assumptions made for the FE analysis are valid. The much larger compressive

values for the fully supported case in the lab are due to the largest strain reading being much

higher than the rest (i.e. a possible outlier). Excluding this reading, the next value give a stress of

-918 psi, which is very close to the values found in the FE analysis and the strengths of materials

approach. Similarly, for the field data, the maximum value is much higher than expected, but

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when the average (excluding the max value which is likely an outlier) is taken, the corresponding

stress value is -958 psi, again this is in line with the expected value.

5.2.5 Summary of Results

Two comparisons are done with all the complied data, one comparison at failure and one

at the dead load. When comparing the loads at failure, the data collected in the lab is used as

well as the predicted values using the strengths of materials approach. The strengths of materials

approach will provide numbers that do not account for the stress concentration due to the kink.

Comparing this data against data obtained in the lab will allow for a reduction factor related to

the kink to be found. Also the test data found for each of the partially supported cases will be

compared against the data found for the fully supported case therefore providing a reduction

factor for the poor boundary conditions. The analysis done at the dead load will use the data

found from the lab tests with both FE analyses (kinked and unkinked) to also determine how the

stresses are affected due to the kink and the poor support conditions.

Given that the fully supported columns are found to fail at 353 kips and this is verified as

the crushing strength of the columns as stated by the manufacturer, this is used as the baseline for

the ultimate failure load and stresses of the columns. With a cross sectional area of 7.24 in2, this

gives the ultimate failure stress of the columns as 48.76 ksi. Comparing the loads at failure of the

partially supported columns against the fully supported columns gives the factor by which the

actual loads are reduced, as can be seen in Table 5.6. Table 5.6 shows that by reducing the area

supporting the column (from fully supported to any of the partially supported conditions) can

reduce the failure load down as low as 20% of what is should be.

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Table 5-6 Load Reduction of each Support Condition

Base Support Conditions

Failure Load [kips]

Reduction Factor

Full 353 1

Diagonal 70.7 0.20

C-Shaped 71.1 0.20

Inner Perimeter 110.9 0.31

By comparing the stresses at the failure load, the amount of axial and bending stresses

can be found. The axial stresses are calculated using the 2*P/A formula. It can be seen in Table

5.7 that these are reduced by up to 60% (1-19.53/48.76). Because the columns are failing at this

point, the axial stress and the bending stresses must sum to equal the failure stress. Therefore the

total bending stress for each support condition is as given in Table 5.7. This maximum bending

stress is then used to determine the eccentricity of the applied load. The eccentricity will fall

between the two given values based on whether the factor is 2 or 4 as discussed in section 5.1

Table 5-7 Stresses and Eccentricity of Support Conditions

Base Support Conditions

Failure Stress [ksi]

Axial Stress [ksi]

Bending Stress [ksi]

Eccentricity of load at failure [in]

min max

Full 48.76 48.76 0.00 0.00 0.00

Diagonal 48.76 19.53 29.23 1.12 2.25

C-Shaped 48.76 19.64 29.12 1.11 2.22

Inner Perimeter 48.76 30.64 18.12 0.44 0.89

In order to find out the effect that the kink is having on the columns, the FE analysis is

used and compared to the lab test data. It has been shown that the kinked FE analysis yields

similar results to the lab data showing that the assumptions made in the FE analysis are accurate.

Similarly, the strengths of materials formulas are providing similar results to the unkinked FE

analysis again showing that the assumptions made in the FE analysis are accurate. Therefore the

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difference found in the stresses between the kinked FE analysis and the unkinked FE analysis

will show the effect of the kink on the increase in stresses. Table 5.8 gives the stresses found

from the FE analyses and the increase factors that are found based on the kink. As can be seen in

the increase for the fully supported, diagonally supported and C-shaped supported columns is

comparable at approximately 1.2. The Inner Perimeter support is found to have a much higher

increase of 1.84. This is because the stress as provided by the FE analyses is much higher in the

inner perimeter due to the inability to reduce the edge effects of the support conditions. This will

be the largest possible increase factor and the actual one will likely be smaller and closer to the

1.2 as found in the other support conditions.

Table 5-8 Stress Increase Effect due to Kink

Base support Conditions

FE Stress Ranges [psi] Increase factor Unkinked Kinked

Full 877 955 1.09

Diagonal 1641 1915 1.17

C-Shaped 1410 1617 1.15

Inner Perimeter 1782 3279 1.84

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CHAPTER 6 Conclusions and Recommendations

6.1 Conclusion

With the goal of finding the cause of the corner cracking of fiber reinforced plastic

composite columns at loads less than the design loads, tests were conducted in the field and in

the lab. A finite element analysis was also carried out, as well as common strength of materials

formulas. All of these results have been presented and compared in Chapter 5. It has been

determined that the ultimate reason for the cracking around the base of the columns is the lack of

support and therefore poor load transfer from the column base to the ground. Of the three

different support shapes that have been found and explored, it is the diagonal and C-shaped

support conditions that have the most effect on the cracking, however all are found to have

caused cracks. However, it is not solely the poor support conditions; the fabric kink in the corner

that is created by the pultrusion process is shown to weaken the strength in the corners. These

two issues combined are causing corner cracking of the columns at loads up to 5 times lower

than predicted.

6.1.1 Kink Effect

FRP columns can be manufactured in multiple different ways, including pultrusion. The

pultrusion process is best suited for projects where the same cross section is being produced

again and again because of its economic value. Pultrusion is an economical mass production

process, but does cause fabric kink issues at web-flange junctions during production. This kink is

an area with lower mechanical properties than the rest of the cross section because of folds in the

fabric causing an area with a lower fiber volume fraction. The FE analyses that have been done

93

for this project show just how big of an effect this kink can have. This cannot be verified from

lab or field testing because all of the columns tested have kinks in them making it impossible to

establish a baseline for an unkinked corner. From strengths of materials and FE analyses it is

found that this kink is causing the columns to fail at stresses 1.17 times lower than expected, i.e.

a reduction factor of approximately 0.85. This is determined using the Finite Element analyses

andcomparing the kinked corner and the unkinked corner results. This can be done because the

FE results are in line with what has been observed with the lab testing and found from basic

strengths of materials formulas.

Figure 6.1 shows what a kink actually looks like on a cross section. The fold in the fabric

can be clearly seen causing the lower fiber volume fraction. Figure 6.2 shows the longitudinal

view of the same corner and a darker area can be seen running up the column. This darker area is

where the kink is and the buildup of resin in that area is causing the discoloration.

Figure 6-1 - Kinked Corner

Kink

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Figure 6-2 Longitudinal View of Kinked Corner

6.1.2 Boundary Condition Effect

One of the major issues found from the field testing was the lack of complete contact

between the base of the column and the ground. This is causing poor load transfer from the

column to the ground. The poor contact is caused from several issues, improper cutting of the

column causing the column base to not be flat, unevenness of the concrete floor of the tower and

inadequate grouting. The grout was installed to provide a proper base by filling the gap between

the column base and the ground, however in multiple cases, the grout did not fully seep under the

full column base due to the caulk being placed improperly. From measurements taken in the

Kink Kink

95

field, these base support conditions are found to fall into four different categories. These

categories are fully supported, diagonally supported (two consecutive sides supported), C-shape

supported (on side fully supported with half of the two connected sides being supported as well)

and the inner perimeter supported (the inner half of all the walls being supported). The fully

supported base is used as the baseline (accounting for the kink effect) and each of the partially

supported base conditions are compared against it to determine the effect they are having.

6.1.2.1 Diagonally Supported

The diagonally supported case is found to have stress ranges much higher in the lab and

in the FE analysis than the column is designed to have at dead loads. The larger induced stress is

caused by bending in the column from the load being applied eccentrically. Using the strengths

of materials approach, it is determined that the diagonally supported boundary condition is

failing at a load of approximately 5 times lower than predicted after accounting for the kink

effect. This translates into the need for a reduction factor of 0.2

6.1.2.2 C-shape Supported

The C-shaped support, similar to the diagonally supported columns are found to have

much higher stress ranges than it is designed for because of the bending forces that are caused by

the eccentric load. In the lab testing, these columns are also found to fail due to bending. It is

predicted using the strengths of materials approach that a factor of 5 should be used to account

for the poor load transfer caused by the C-shaped support case. Therefore, similar to the

diagonally supported columns, a reduction factor of 0.2 is needed.

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6.1.2.3 Inner Perimeter Supported

The inner perimeter support case does not have an eccentric load like the diagonal and C-

shaped support cases, but bending still needs to be accounted for. This is because as the column

begins to fail, one (or more) of the sides will begin to bow out causing bending in the column.

The main load application is still axial which is why the columns take more load to failure than

the diagonal and C-shaped supported columns, but ultimately bending will add some stress.

Using the strengths of materials approach, a factor of 0.31 is needed to account for this base

support for the load because the load at failure is approximately 3.2 times lower than the failure

load of a fully supported column.

6.1.3 Summary

It can be concluded that the lack of support for a pultruded GFRP column coupled with a

kink caused by the pultrusion process can play a major role in the overall strength of the column

and cause premature cracks to form in the corners. This conclusion has been made by

observations of the higher stresses caused by the lack of support, but also by ruling out other

potential causes such as material flaws, improper design and manufacturing, out-of-plumbness of

the columns, moisture uptake, and cure percentage of the columns.

6.2 Recommendations

Now that the cause of the cracking is understood, preventative measures must be taken to

ensure this does not happen again. It is recommended that a factor be introduced into the code (or

a current factor be modified) that is currently being developed to account for the stress

concentration issues caused by the lack of support under these columns. It has been found that

97

the worst case causes the columns to fail at a load 5 times lower than expected. Therefore the

reduction factor should be 0.2. These columns have been designed according to the Cooling

Tower Institute Standard 152 (CTI-STD-152) so this reduction factor should be compared to

what is already in the code to see how it matches up. Currently the safety factors in this code

include 2.0 for column buckling, 2.5 (min) for flexure, 3.0 for compression and 4.0 for bearing

(CTI). Because the diagonal and C-shaped supported columns have a bending type failure, the

reduction of 5 is compared to the minimum recommended value of 2.5. It can be assumed that at

least half of this reduction is accounted for in this CTI factor. However, this still implies that the

stresses are 2.5 times higher than expected. This gives a reduction factor of 0.4 (1/2.5) which is

still relatively severe. However, knowing that the temperature and moisture uptake did not play

a major role in the cracking, but reduction factors have been applied for each of 0.85 and 0.65

respectively in accordance with CTI-STD-152, which account partially for the stress reduction.

For example, the temperature and moisture factors of 0.85 and 0.65, respectively, when

multiplied together give 0.5525. By dividing this by the 0.4 factor given above yields a result of

1.38, meaning that this is the safety factor required for the lack of boundary condition which

translates into 0.72 (1/1.38). Accounting for all this the recommended reduction factor for the

lack of complete support should be 0.75. This reduction factor is less conservative as cracking

due to inadequate column support conditions has not been widely found in similar projects.

Comparing the inter perimeter supported columns, the safety factor is 3.2 and because

these columns failed primarily in compression (although some bending is induced) the CTI-STD

152 safety factor is 3.0. Because these values are so close and this seems to be an isolated

incident no additional reduction is needed for the inner perimeter supported columns.

98

This leads to the second recommendation, that during and after the installation process,

the columns be checked to ensure they have the proper support. This includes, proper cutting of

the columns (i.e. the base be cut squarely), lengths of the column be specified carefully to

account for variations in the tower base, and any grout material be installed so that the grout can

be visibly inspected around the entire column perimeter. If caulk is used to dam the grout, it is

recommended that the grout is applied ½ inch away from the perimeter of the column to ensure it

does not penetrate under the column.

6.2.1 Future Research

To further investigate the cracking in column corners, possible continued research may

include columns manufactured with different processes to see the effect that this can have with

the kink in the corner, testing of longer columns to see how buckling of slender columns will be

affected by the poor base conditions, and using different cross sectional shapes to determine how

that is affecting the loads at which the corners crack. Other possible ideas for future research are

different amounts of support for columns to see how that will affect the reduction of load (i.e.

75% support).

99

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