Fatigue Testing of Galvanized and

Ungalvanized Socket Connections 

PUBLICATION NO. FHWA-HRT-14-066 SEPTEMBER 2014

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
FOREWORD

Wind-induced vibration of ancillary structures that support signs and lights for traffic operations 

has been known to cause fatigue cracks that sometimes lead to collapse. Since approximately 

1990, many researchers have called attention to this problem, and prior research has led to many 

changes in the American Association of State Highway Transportation Officials Standard 

Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. One 

particular detail of interest is the ring-to-tube connection used at the base of poles, commonly 

referred to as a socket connection. Typically, these connections have demonstrated 

comparatively reduced fatigue resistance. Much of the data on socket connection fatigue reveals

that poles which had been galvanized have reduced fatigue strength. However, not all owners 

galvanize their structures, meaning the fatigue resistance of non-galvanized structures is being 

unnecessarily discounted. This project specifically addressed the difference in socket connection 

fatigue resistance between galvanized and ungalvanized poles. This report will assist 

stakeholders, including State transportation departments, researchers, consultants, and industry 

representatives, with the design and review of ancillary sign structures.

Jorge E. Pagán-Ortiz

Director, Office of Infrastructure

Research and Development

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document.

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-14-066

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Fatigue Testing of Galvanized and Ungalvanized Socket 

Connections

5. Report Date

September 2014

6. Performing Organization Code: 

7. Author(s)

Justin M. Ocel, Ph.D., P.E.

8. Performing Organization Report No. 

9. Performing Organization Name and Address

Federal Highway Administration

Bridge and Foundation Engineering Team (HRDI-40)

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101

10. Work Unit No.

11. Contract or Grant No.

12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22101-2296

13. Type of Report and Period Covered

14. Sponsoring Agency Code

HRDI-40

15. Supplementary Notes

16. Abstract

The fatigue resistance of welded traffic signal support structure details is an ongoing research topic being 

addressed at multiple universities primarily through state funding mechanisms. Fatigue problems with these 

structures have plagued multiple states, and a handful of collapses are identified in the open literature. Some 

premature failures have been hypothesized to have been caused by cracking in the zinc metal bath during 

galvanizing. This led to a fatigue testing matrix of socket connection geometries in a galvanized and 

ungalvanized state. Specimens were attained from two different pole manufacturers and fabricated using their 

own techniques. This report describes how the specimens were tested along with the results showing the 

difference in fatigue life between galvanized and ungalvanized structures. Generally, the galvanized specimens

showed a one-category reduction in fatigue life versus identical specimens that were not galvanized.

17. Key Words

Sign pole, fatigue, socket connection, galvanizing,

wind vibration, tube-to-plate, ancillary structure

18. Distribution Statement

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161. 

http://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

43

22. Price

N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius o

C 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit o

F 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)

ii
TABLE OF CONTENTS

INTRODUCTION......................................................................................................................... 1

SPECIMENS ............................................................................................................................. 1

TESTING METHODS ................................................................................................................. 5

FATIGUE TESTING ............................................................................................................... 5

ULTIMATE LOAD TESTING ............................................................................................... 5

MATERIAL CHARACTERIZATION .................................................................................. 6

EXPERIMENTAL DATA............................................................................................................ 9

MATERIAL CHARACTERIZATION .................................................................................. 9

Chemistry of Galvanizing ..................................................................................................... 10

Fatigue Testing...................................................................................................................... 11

Ultimate Load Testing .......................................................................................................... 18

SUMMARY ................................................................................................................................. 23

APPENDIX: PICTURES OF FRACTURES ........................................................................... 25

REFERENCES............................................................................................................................ 37

iii
LIST OF FIGURES

Figure 1. llustration. Detailing of specimens from each fabricator ................................................ 3

Figure 2. Illustration. Schematic of socket connection testing frame............................................. 7

Figure 3. Illustration. ASTM E8 “plate-type” specimen ................................................................ 8

Figure 4. Graph. S-N plot of fabricator 1 fatigue data.................................................................. 15

Figure 5. Graph. S-N plot of fabricator 2 fatigue data.................................................................. 16

Figure 6. Photo. Peening evidence on Specimen 1U3.................................................................. 17

Figure 7. Graph. Moment versus displacement of test 1 .............................................................. 19

Figure 8. Graph. Moment versus displacement of test 2 .............................................................. 20

Figure 9. Graph. Moment versus displacement of test 3 .............................................................. 20

Figure 10. Graph. Moment versus displacement of test 4 ............................................................ 21

Figure 11. Photo. Specimen 1U1 fracture surface ........................................................................ 25

Figure 12. Photo. Specimen 1U2 fracture surface ........................................................................ 26

Figure 13. Photo. Specimen 1U3 fracture surface ........................................................................ 26

Figure 14. Photo. Specimen 1U4 fracture surface ........................................................................ 27

Figure 15. Photo. Specimen 1U5 fracture surface ........................................................................ 27

Figure 16. Photo. Specimen 1U6 fracture surface ........................................................................ 28

Figure 17. Photo. Specimen 1G2 fracture surface ........................................................................ 28

Figure 18. Photo. Specimen 1G3 fracture surface ........................................................................ 29

Figure 19. Photo. Specimen 1G4 fracture surface ........................................................................ 29

Figure 20. Photo. Specimen 1G5 fracture surface ........................................................................ 30

Figure 21. Photo. Specimen 1G6 fracture surface ........................................................................ 30

Figure 22. Photo. Specimen 2U1 fracture surface ........................................................................ 31

Figure 23. Photo. Specimen 2U2 fracture surface ........................................................................ 31

Figure 24. Photo. Specimen 2U3 fracture surface ........................................................................ 32

Figure 25. Photo. Specimen 2U4 fracture surface ........................................................................ 32

Figure 26. Photo. Specimen 2U5 fracture surface ........................................................................ 33

Figure 27. Photo. Specimen 2U6 fracture surface ........................................................................ 33

Figure 28. Photo. Specimen 2G1 fracture surface ........................................................................ 34

Figure 29. Photo. Specimen 2G2 fracture surface ........................................................................ 34

Figure 30. Photo. Specimen 2G3 fracture surface ........................................................................ 35

Figure 31. Photo. Specimen 2G4 fracture surface ........................................................................ 35

Figure 32. Photo. Specimen 2G5 fracture surface ........................................................................ 36

Figure 33. Photo. Specimen 2G6 fracture surface ........................................................................ 36

iv
LIST OF TABLES

Table 1. Fabricator 1 material data ................................................................................................. 9

Table 2. Fabricator 2 material data ............................................................................................... 10

Table 3. Galvanized coating chemistries...................................................................................... 11

Table 4. Fatigue results of fabricator 1 specimens ....................................................................... 13

Table 5. Fatigue results of fabricator 2 specimens ....................................................................... 14

Table 6. Static testing matrix ........................................................................................................ 19

v

INTRODUCTION

Fatigue of structural supports for overhead signs, traffic signals, and highmast light poles has 

received focused attention from researchers in the last 25 years because of failures that have been 

reported in welded details. The likely precipitating event was the close consecutive collapse of 

two cantilevered signal structures in Michigan in 1990, which resulted in property damage, 

injuries, and one fatality.(1) These lightweight, flexible structures are often susceptible to 

vibration from wind phenomena such as galloping, vortex shedding, and wind gusts. The 

dynamic response tends to excite the lower modes of vibration, and with very low damping 

ratios, large numbers of loading cycles quickly accumulate. These wind-induced vibrations can 

ultimately lead to fatigue cracking at welded details in the structure. One type of welded 

connection with poor fatigue resistance is the pole-to-base plate welds that often follow socket 

connection detailing practice. To make a socket connection, a hole is cut out of the base plate 

with a slightly larger diameter than the outside diameter of the pole. The tube is partially slid into 

the base plate, and a perimeter fillet weld adjoins the two pieces, essentially creating a hollow 

section with a thick stiffening ring welded to the end. The socket connection is the preferred 

connection method between a pole and a foundation because it can be quickly fabricated.

Fatigue testing of socket connection details began in the early 1980s at Lehigh University but 

had a resurgence once the 2001 American Association of State Highway Transportation Officials 

(AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaires, and 

Traffic Signals was published.(2,3) The 2001 specifications were the first to include fatigue 

design, which had not been considered in prior editions. In-service structures that historically 

never had fatigue issues were not meeting the 2001 fatigue provisions. To be compatible with the 

new provisions, States, designers, and fabricators had to drastically increase pole diameters and 

wall thickness to comply with the 2001 fatigue provisions. In response to concerns with this 

situation, several research projects addressed the fatigue resistance of traffic signal structure 

details, mainly with socket connection details. Some of these projects were conducted at the 

University of Wyoming, University of Texas–Austin (UT–Austin), Lehigh University, Purdue 

University, and the University of Minnesota. (See references 4–9.) One of the larger fatigue 

testing studies published in 2011 (National Cooperative Highway Research Program Project 10-

70) included only galvanized details, which provided the lower bound of fatigue resistance based 

on the research of Koenigs.(6,8) This decision ultimately led to specification changes that 

neglected any increased fatigue resistance of structures that are not galvanized.

SPECIMENS

The desired specimen utilized a round tube with an 18-inch outside diameter at the socket 

connection, 0.25-inch wall thickness, 2-inch-thick base plate, and a 6-bolt, 2-inch-diameter 

anchor rod arrangement on a 24-inch-diameter bolt circle. A construction drawing was created 

and sent to two different fabricators. They were asked to provide shop drawings of how the 

specimen could best be fabricated using their typical processes. Four differences arose between 

the two manufacturers. Each used different weld profiles, and each had their own galvanizing 

sources. Fabricator 2 could not make perfectly round poles and instead made round-like tubes by 

press-braking a flat plate with multiple bends. Fabricator 1 made the tubes from ASTM A595 

steel; whereas, fabricator 2 bent ASTM A572 Gr.65 material into a tube. Figure 1 illustrates the 

1
detailing of the desired specimen and each fabricator’s shop drawing. Twelve specimens were 

acquired from each fabricator. Six were in an unfinished state, and six were hot-dip galvanized.

2
Figure 1. llustration. Detailing of specimens from each fabricator.

169.50"

2" THK. PL.

1" THK. PL.

3

8"

1" THK. PL.

2" THK. PL.

Ø30.00"

Ø24.00"

Ø18.00"

Ø2.25"

169.50"

20.00"

20.00"

Ø23.00"

Ø1.00"

5

16"

1" THK. PL.

2" THK. PL.

Ø30.00"

Ø24.00"

Ø2.25"

169.50"

20.00"

20.00"

Ø23.00"

Ø1.00"

Ø16.00"

Ø16.00"

Round tube

18.00" base diameter x 0.250 wall x 0.14 in/ft taper

Round-like (16 bends, 4.0" inner bend radius)

18.00" base diameter x 0.250 wall x 0.14 in/ft taper

7

16" x 1

2"

0.56" x 0.31

Ø30.00"

Ø24.00"

Ø18.00"

Ø2.25"

20.00"

20.00"

Ø23.00"

Ø1.00"

Supplied Construction Drawing

Shop Drawing from Fabricator 1

Shop Drawing from Fabricator 2

3

TESTING METHODS

Fatigue testing was performed in a specially fabricated load frame shown in figure 2. The 

methodology was based on a concept first performed at UT–Austin by bolting two pole 

specimens to a loading box and then simply supporting the ends of poles and loading through the 

loading box.(6) For this project, the loading box was replaced with a large concrete block. The 

block had dimensions of 45 by 45 by 72 inches and was cast of concrete, thus weighing just over 

12.6 kips. Six 2-inch by 4.5UNC B7 threaded rods passed through the entire length of the block 

and stuck out approximately 12 inches on each side. These threaded rods were used to bolt the 

pole specimens to the loading block with double-nut moment joints. The leveling nuts were 

spaced approximately one rod diameter off the concrete surface. The benefit of using the 

concrete block to load the poles was to accurately recreate the boundary conditions typical for 

socket connections bolted to a concrete foundation. The threaded rods were orientated such that 

one rod was at the point of maximum bending stress. Specimens were randomly selected for 

installation, and the installation orientation was also randomized, that is, no attempt was made to 

orient the tube seam weld into a beneficial configuration. Since the socket connections had a sixbolt pattern and the support ends had a four-bolt pattern, the specimen could be installed in only 

one of two orientations.

FATIGUE TESTING

During fatigue cycling the actuator would pull up on the loading block to induce a dead load 

stress in the pole. The actuator would cycle in between loads of 11.63 and 15.73 kips. This 

equated to a calculated mean (dead load) of approximately 19.3 ksi because the round and roundlike poles had slightly different moments of inertia. This load range induced a stress range of 

5.85 ksi in the fabricator 1 specimens and 5.73 ksi in the fabricator 2 specimens. The specimens 

were cycled either in load or displacement control. When running in load control, the actuator 

could not cycle faster than 0.8 Hz without going unstable. To expedite testing, most cycling 

occurred in displacement control with the actuator cycling at about 2.7 Hz, which was near the 

natural frequency of the system. The peak displacement targets were attained when the actuator 

had come to operating temperature and running in load control. Once a day, the displacement 

targets were verified and altered accordingly. However, once one of the two cycling specimens 

started to grow a crack, the test was run exclusively in load control to ensure a consistent stress 

range. Throughout the testing program, two poles never simultaneously reached failure; one pole 

always failed before another.

The specimens were cycled at a low stress range to ensure the threaded rods were below their 

fatigue threshold. If the threaded rods had failed, a new loading block would have to be 

fabricated. This was the fundamental drawback to using this concrete loading block versus the 

steel loading box used in the UT–Austin research. 

ULTIMATE LOAD TESTING

Strength tests were performed on eight of the cracked pole sections. This was done to assess the 

remaining capacity of the cracked section, for instance, as a way to determine the risk associated 

with keeping a knowingly cracked pole in service. The same loading system was used in the 

strength tests as in the fatigue tests, except the tests were run monotonically until failure. 

5
Two parameters were investigated in the strength tests: loading rate and temperature. The 

loading rate was analyzed in terms of the applied displacement from the actuator. The first two 

tests were conducted at 0.002 and 0.04 inches/s at room temperature. The first represented a true 

static loading rate and the second represented a rate closer to that of a wind loading event on a 

pole. The second two tests were conducted at 0.02 and 0.2 inches/s at -30 °F, which is the 

AASHTO Zone 2 lowest anticipated service temperature. To attain -30 °F, a rigid foam box was 

built around each connection and injected with liquid nitrogen through a solenoid valve 

controlled by a temperature controller and a thermocouple attached to the base plate of the 

connection. The difference in the loading rates was accidental. The first tests were performed

using old analog controllers, while the second tests were run with digital controllers, which were 

upgraded through the duration of the project. The different loading rates were an artifact of not 

understanding the new digital controller functionality. All the poles tested statically were from 

fabricator 2.

The actuator had only a 6-inch stroke. In the tests to failure, the system had to be unloaded

frequently, spacers had to be added at the support, and the system reloaded until failure occurred. 

Failure was defined as attaining a peak load or a fracture of the cross section.

MATERIAL CHARACTERIZATION

Tensile coupon testing was used to characterize the steel plate used to make the specimen 

supplied by each of the fabricators. Testing was performed according to the ASTM E8

Specification.(10) The coupons were machined according to the schematic shown in figure 3. All 

pretest measurements and markings were also performed in accordance with ASTM E8.

Testing was performed on a four-post universal testing machine with hydraulic wedge grips and 

a 220 kip capacity. The testing machine was controlled with a digital controller. Strain 

measurements were made using a clip-on extensometer with a 1-inch measurement range but 

fitted with an extension bar to measure strain over an 8.000-inch gauge length. 

Specimens were initially loaded at a rate of 0.0003 inch/s. Once the specimen had yielded, the 

static yield was attained by pausing the loading for a period of 90 seconds in three locations 

along the yield plateau. Once strain hardening had begun, the loading rate was gradually 

increased to 0.01 inch/s until the specimen fractured. Typically, it took approximately 20 min to 

fracture a specimen. 

6
Figure 2. Illustration. Schematic of socket connection testing frame.

7
Figure 3. Illustration. ASTM E8 “plate-type” specimen.

8
EXPERIMENTAL DATA

MATERIAL CHARACTERIZATION

In total, 20 coupons were tested, five from each set of galvanized and bare specimens. Table 1

and table 2 show the results of testing for each of the fabrication sources. The averages for each 

set of specimens are shown in the bolded line. Fabricator 1 had average 0.2-percent yield 

strengths of 51.5 and 48.7 ksi for the respective galvanized and bare coupons. Fabricator 2 had 

average 0.2-percent yield strengths of 69.3 and 64.7 ksi for the respective galvanized and bare 

coupons.

Table 1. Fabricator 1 material data.

Specimen 

ID

0.2%

Offset 

Yield 

Stress

(ksi)

Static 

Yield 

Stress

(ksi)

Tensile 

Strength

(ksi)

Elongation

(percent)

Area 

Reduction

(percent)

Galv1 50.5 48.7 63.6 22 59

Galv2 52.5 49.8 64.8 20 64

Galv3 50.4 48.3 62.4 23 63

Galv4 51.3 49.2 63.7 22 62

Galv5 52.8 50.7 64.8 23 66

Average 51.5 49.3 63.9 22 63

Bare1 47.2 45.6 58.9 23 58

Bare2 48.8 47.3 58.9 26 57

Bare3 49.3 47.7 60.3 26 62

Bare4 49.3 47.9 61.5 27 56

Bare5a 43.1 42.8 61.2 26 61

Averageb 48.7 47.1 59.9 25 58 a Indicates specimens where fracture occurred outside the original gauge length marks or was 

located less than 25 percent of the elongated gauge length from either of the original gauge 

length marks.

b Specimens failing ASTM acceptance criteria were not included in statistical analysis.

9
Table 2. Fabricator 2 material data.

Specimen 

ID

0.2% 

Offset 

Yield 

Stress

(ksi)

Static 

Yield 

Stress

(ksi)

Tensile 

Strength

(ksi)

Elongation

(percent)

Area 

Reduction

(percent)

Galv1a 69.1 67.5 76.0 14 55

Galv2 68.2 65.5 76.3 14 76

Galv3 66.7 64.7 73.5 15 73

Galv4 73.1 70.6 80.6 14 69

Galv5a 72.9 70.5 79.9 15 85

Averageb 69.3 66.9 76.8 14 73

Bare1 65.4 62.5 75.6 16 74

Bare2 65.9 64.5 74.6 12 76

Bare3 63.3 61.4 74.0 15 71

Bare4 65.6 63.4 76.3 16 72

Bare5 63.5 61.9 74.3 15 76

Average 64.7 62.7 75.0 15 74 a Indicates specimens where fracture occurred outside the original gauge length marks or was 

located less than 25 percent of the elongated gauge length from either of the original gauge 

length marks.

b Specimens failing ASTM acceptance criteria were not included in statistical analysis.

Chemistry of Galvanizing

Neither of the specimen fabricators would provide the chemistry of their zinc baths, pleading it 

was proprietary information. Therefore, a core plug was removed from four randomly selected 

specimens, two from each manufacturer. The core plugs were sent to a lab for chemical analysis

of the zinc coating. Table 3 shows the results of the chemistry in terms of percent by weight. No 

conclusions will be drawn on the chemistry results, although elevated levels of some elements 

have been known to cause cracking. They are presented here for information purposes only.

10
Table 3. Galvanized coating chemistries.

Element 1G5 1G6 2G6 2G5

Copper 0.015 0.013 0.018 0.018

Cadmium 0.001 0.001 0.004 0.005

Aluminum 0.081 0.081 0.080 0.080

Magnesium 0.003 0.003 0.003 0.003

Lead 0.35 0.31 0.56 0.55

Tin <0.001 <0.001 0.011 0.013

Iron 1.24 1.23 0.65 0.73

Nickel 0.016 0.017 0.078 0.077

Zinc Balance Balance Balance Balance

Fatigue Testing

For this research, failure was considered a 12-inch-long crack around the perimeter of the tube. 

The UT–Austin researchers defined failure as a 10-percent decrease in the stiffness of the 

system. For this research the stiffness criterion was evaluated, but it was not believed to be 

accurate, since only one pole would fail at a time, while during the UT–Austin tests, poles would 

always fail in pairs. Generally once the crack was 12 inches long (21 percent of the perimeter),

the remaining life was small in comparison to the cycles to reach failure. In addition, once the 

crack became that long, the stress range in each anchor rod around the intact portions of the pole 

would increase, thus increasing the risk that they could develop fatigue cracks themselves. The 

12-inch crack length criterion was not strictly followed, particularly in the beginning of the 

testing with fabricator 1 specimens when the 10-percent stiffness reduction criteria was being 

evaluated. Initially the 10-percent stiffness decrease rule was used, but it was determined to be 

not working after the fourth specimen; then the 12-inch-long crack rule was adopted.

Table 4 and table 5 outline the fatigue data for the 24 socket connections tested. Specimens were 

assigned a 3-character, alphanumeric naming designation xyz where:

• x is 1 or 2 representing the two fabricators.

• y is either a U or G representing either unfinished or galvanized coatings.

• z represents the individual specimen (1 through 6).

Therefore, specimen 2G5 represents the fifth galvanized specimen from fabricator 2. Table 4 and 

table 5 also show the length of the crack on each tube’s perimeter at failure. The appendix 

contains photos of each fracture surface showing the crack’s shape, length, and area reported 

(figure 11 through figure 33). 

An anomaly in the fatigue data requires further explanation. At one point in the program,

Specimen 1U6 was being fatigue cycled along with Specimen 1G1. Specimen 1U6 had existing 

cracks from previous cycling, and Specimen 1G1 was virgin. After 616,158 cycles, there was an 

accidental overload applied to the system and evidence that the actuator applied approximately 

11
32 kips of load to the system based on the peak/valley indicator on the controller. This overload 

destroyed the 1U6 specimen but caused no visible damage to the 1G1 specimen. The fact that 

Specimen 1G1 accumulated more than 15 million cycles with no cracks is not coincidental 

considering that the overload must have plastically deformed the tube at the weld toe, thus 

erasing the residual stresses from the welding and enhancing fatigue life.

Figure 4 and figure 5 plot the fatigue data along with the AASHTO S-N curves for fabricators 1 

and 2, respectively. The colored, dashed lines represent the lower bound limit of the two data 

sets as the mean minus two standard deviations from linear regression analysis and an assumed 

slope of -3 to 1 on the log-log scale. The dashed blue line represents the ungalvanized specimens, 

and the dashed red line signifies the galvanized specimens. 

For fabricator 1 the data are highly scattered for both the galvanized and ungalvanized 

specimens. For both specimen types, the lower bound resistance is similar and much less than 

Category E'. The scatter in the data can be explained by considering the preparation and quality 

of the socket weld from fabricator 1. Specimens with fatigue lives of more than 2 million cycles 

had welds with a low entry angle into the tube. In addition, evidence indicated that the weld toes 

were peened. 

Figure 6 shows the peened surface of the weld from Specimen 1U3. (Note that the white line at 

the weld toe is residual developer from dye penetrant testing.) The speckled surface on the weld 

suggests that it was needle peened, and in some cases the weld toe was also treated. All of the 

fabricator 1 welds appear to have been needle peened, but the weld toes were not treated in all 

cases. The specimens with the lowest lives had equal leg welds and bad undercutting in some 

instances with cracks developing at multiple locations at each undercut.

The data from fabricator 2 are much more pronounced, showing the difference in fatigue strength 

between galvanized and ungalvanized specimens, with two distinct scatter bands for each 

specimen type. The lower bound of the ungalvanized specimens did plot slightly above 

Category E'; whereas, the galvanized specimens were much below Category E', similar in 

strength to the specimens made by fabricator 1.

12
Table 4. Fatigue results of fabricator 1 specimens.

Specimen Finish

Stress Range

(ksi)a

Cycles to 

Failure

Crack Length 

on Tube 

Perimeter

(inches)

1U1 Unfinished 5.85 2167227 6.90

1U2 Unfinished 5.85 1602406 7.59

1U3 Unfinished 5.85 3846508 6.19

1U4 Unfinished 5.85 8555356 8.59

1U5 Unfinished 5.85 924948 12.10

1U6 Unfinished 5.85 3835237b 8.06

1G1 Galvanized 5.85 15015310c No crack

1G2 Galvanized 5.85 4461772 6.72

1G3 Galvanized 5.85 3067630 10.86

1G4 Galvanized 5.85 1229060 11.25

1G5 Galvanized 5.85 1360291 12.09

1G6 Galvanized 5.85 2928887 12.00 a Stress range calculated using a moment arm distance of 174 inches, moment of inertia of 549.14 inches, 

and an extreme fiber distance of 9 inches. b The actuator accidently full-scaled (possibly applied ~32 kips of load) and destroyed this specimen before 

the failure criterion was reached.

c Specimen was declared a runout. Extreme life was hypothesized to be from accidental overload when 

tested together with Specimen 1U6.

13
Table 5. Fatigue results of fabricator 2 specimens.

Specimen Finish

Stress Range

(ksi)a

Cycles to 

Failure

Crack Length 

on Tube 

Perimeter

(inches)

2U1 Unfinished 5.73 3738417 13.99

2U2 Unfinished 5.73 4873910 12.58

2U3 Unfinished 5.73 7000983 11.63

2U4 Unfinished 5.73 4411691 14.06

2U5 Unfinished 5.73 3409173 11.76

2U6 Unfinished 5.73 5631182 12.61

2G1 Galvanized 5.73 1171624 13.77

2G2 Galvanized 5.73 878218 12.68

2G3 Galvanized 5.73 639952 13.56

2G4 Galvanized 5.73 1864066 12.48

2G5 Galvanized 5.73 700310 12.64

2G6 Galvanized 5.73 748184 11.42 a Stress range calculated using a moment arm distance of 174 inches, moment of inertia of 561.61 inches, 

and an extreme fiber distance of 9.02 inches.

14
Figure 4. Graph. S-N plot of fabricator 1 fatigue data.

105 106 107 108

1

10

100

Stress Range (ksi)

Cycles to Failure

Fabricator 1 Unfinished

Fabricator 1 Galvanized

Cat. A

Cat. B

Cat. C

Cat. E'

Cat. D

Cat. E

15
Figure 5. Graph. S-N plot of fabricator 2 fatigue data.

105 106 107 108

1

10

100

Stress Range (ksi)

Cycles to Failure

Fabricator 2 Unfinished

Fabricator 2 Galvanized

Cat. A

Cat. B

Cat. C

Cat. E'

Cat. D

Cat. E

16
Figure 6. Photo. Peening evidence on Specimen 1U3.

17
Ultimate Load Testing

The static tests were meant to be conducted to two different loading rates and two different 

temperatures. The two loading rates were meant to be extremely slow to represent static loading, 

and the faster rate was intended to represent a strain rate from a dynamic wind event. During the 

duration of the project, however, a new actuator control system was set up, and a mix-up 

occurred in interpreting the loading rates from one system to the next. Therefore, the rates do not

necessarily represent the intention of the testing. Two temperatures were investigated: room 

temperature (approximately 70 °F) and the AASHTO Zone 2 lowest anticipated service 

temperature for fracture assessment (-30 °F). 

Table 6 shows the matrix of tests together with the fatigue crack length, measured loading rate, 

and the peak moment attained for each test. For the mode of failure, in the two room temperature 

tests, the fatigue cracks extended in a ductile manner with through-thickness yielding of the tube. 

The two tests conducted at the cold temperature exhibited stable and ductile extension of the 

existing fatigue cracks with one exception. In test 3, one tube had a pop-in fracture where the 

fatigue crack suddenly extended 2 inches at a 45-degree angle relative to the fatigue crack 

propagation.

Figure 7 through figure 10 show the moment versus displacement plots for each of the four tests. 

In each plot, dashed and dotted lines indicate the plastic moments of the uncracked and cracked 

sections using the measured yield strength of the tube. The cracked section plastic modulus was 

calculated assuming a 12-inch-long crack on the outside perimeter of the tube. On average, the 

beginning crack lengths were 12 inches for the eight specimens tested. No attempt was made to 

calculate the cracked plastic section modulus for each tube individually.

The peak moments attained for each set of tubes were very close to the theoretical cracked 

section plastic moment capacity. This would indicate that if an owner did have a cracked pole in 

service, and could accurately assess the shape and length of the crack, a cracked plastic section 

analysis could be performed to determine if the pole was susceptible to collapse under design 

loads. 

It should also be noted that this project did not attempt to measure the fracture resistance of the 

tube. Since each of the tests was able to attain the cracked plastic moment capacity, the tube 

must have had adequate fracture resistance to the approximately 12-inch-long fatigue cracks.

18
Table 6. Static testing matrix.

Test

Specimen 

Designations

Crack 

Length 

Estimate

(inches)

Temperature 

(°F)

Loading 

Rate

(inch/s)

Peak 

Moment

(kip-inch)

1

2G4 12.48

70 0.002 3,311a

2G6 11.42

2

2G2 12.68

70 0.04 3,410a

2G1 13.77

3

2G5 12.64 -30 0.02 3,342a

2G3 13.56

4

2B5 11.76 -30 0.2 3,566b

2B6 12.61 a Specimens have a full-section plastic moment capacity of 5,353 kip-inch (Z = 80.01 inch3

). b Specimens have a full-section plastic moment capacity of 5,017 kip-inch (Z = 80.01 inch3

).

Figure 7. Graph. Moment versus displacement of test 1. 

0

1000

2000

3000

4000

5000

6000

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Applied Moment at Weld Toe (kip-inch)

Actuator Displacement (inches)

Test 1

Galvanized Plastic Moment

Galvanized Cracked Section Plastic Moment

Peak Moment

19
Figure 8. Graph. Moment versus displacement of test 2.

Figure 9. Graph. Moment versus displacement of test 3.

0

1000

2000

3000

4000

5000

6000

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Applied Moment at Weld Toe (kip-inch)

Actuator Displacement (inches)

Test 2

Galvanized Plastic Moment

Galvanized Cracked Section Plastic Moment

Peak Moment

0

1000

2000

3000

4000

5000

6000

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Applied Moment at Weld Toe (kip-inch)

Actuator Displacement (inches)

Test 3

Galvanized Plastic Moment

Galvanized Cracked Section Plastic Moment

Peak Moment

20
Figure 10. Graph. Moment versus displacement of test 4.

0

1000

2000

3000

4000

5000

6000

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Applied Moment at Weld Toe (kip-inch)

Actuator Displacement (inches)

Test 4

Ungalvanized Plastic Moment

Ungalvanized Cracked Plastic Moment

Peak Moment

21

SUMMARY

The fatigue tests on specimens from fabricator 1 showed how workmanship greatly influences

the fatigue strength of socket connections and may govern over any effect galvanizing may have. 

The tests revealed that the poor performing welds had equal legs with undercuts, while the better 

performing welds had unequal legs and evidence of peening. This may speak to larger issues of

quality control or the need for industry specifications for fatigue control in sensitive structures. 

The test data from fabricator 2 show a pronounced difference in fatigue strength between 

galvanized and ungalvanized specimens, with galvanizing producing about a one category 

fatigue life reduction. However, current AASHTO specifications do not recognize the increase in 

fatigue life of ungalvanized specimens, and most of the fatigue data used to create the

specifications were based on galvanized specimens.

Both sets of fatigue test data showed the galvanized specimens had a resistance much less than 

Category E'; however, the new AASHTO Standard Specifications for Structural Supports for 

Highway Signs, Luminaires, and Traffic Signals, 6th edition, still predicts the tested geometries 

have Category E' resistance.(11) There still appears to be a disparity in the new specifications, 

either in the determination of allowable stress range or in the workmanship standards to which

these structures are fabricated. 

A plastic cracked section analysis can accurately assess the remaining moment capacity of poles 

with cracks. To implement this analysis, one should also demonstrate confidence in the fracture 

toughness of the plate material. This project did not evaluate the fracture toughness, but except 

for one of the specimens, the fracture toughness was certainly high enough to sustain 12-inchlong cracks at temperatures to -30 °F.

23

APPENDIX: PICTURES OF FRACTURES

The pictures of the fracture surfaces shown in figure 11 through figure 33 are highlighted with 

green hatching to represent the area of fatigue crack growth. Two dimensions are provided for 

each hatched area: the length of the crack along the outer tube perimeter (in inches) and area of 

the crack (inches2).

Figure 11. Photo. Specimen 1U1 fracture surface.

25
Figure 12. Photo. Specimen 1U2 fracture surface.

Figure 13. Photo. Specimen 1U3 fracture surface.

26
Figure 14. Photo. Specimen 1U4 fracture surface.

Figure 15. Photo. Specimen 1U5 fracture surface.

27
Figure 16. Photo. Specimen 1U6 fracture surface.

Figure 17. Photo. Specimen 1G2 fracture surface.

28
Figure 18. Photo. Specimen 1G3 fracture surface.

Figure 19. Photo. Specimen 1G4 fracture surface.

29
Figure 20. Photo. Specimen 1G5 fracture surface.

Figure 21. Photo. Specimen 1G6 fracture surface.

30
Figure 22. Photo. Specimen 2U1 fracture surface.

Figure 23. Photo. Specimen 2U2 fracture surface.

31
Figure 24. Photo. Specimen 2U3 fracture surface.

Figure 25. Photo. Specimen 2U4 fracture surface.

32
Figure 26. Photo. Specimen 2U5 fracture surface.

Figure 27. Photo. Specimen 2U6 fracture surface.

33
Figure 28. Photo. Specimen 2G1 fracture surface.

Figure 29. Photo. Specimen 2G2 fracture surface.

34
Figure 30. Photo. Specimen 2G3 fracture surface.

Figure 31. Photo. Specimen 2G4 fracture surface.

35
Figure 32. Photo. Specimen 2G5 fracture surface.

Figure 33. Photo. Specimen 2G6 fracture surface.

36
REFERENCES

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Michigan Department of Transportation, Lansing, MI.

2. Miki, C., J. Fisher, and R. Slutter (1981). Fatigue Behavior of Steel Light Poles, 

Lehigh University, Fritz Engineering Laboratory, Report No. 200.81.714.1, 

Bethlehem, PA.

3. AASHTO (2001). Standard Specifications for Structural Supports for Highway Signs, 

Luminaires, and Traffic Signals, 4th edition, American Association of State and 

Highway Transportation Officials, Washington, DC.

4. Hall, J.H. III (2005). The Effect of Baseplate Flexibility on the Fatigue Performance of 

Welded Socket Connections in Cantilevered Sign Structures, Master’s Thesis, Lehigh 

University, Bethlehem, PA.

5. Hamilton, H.R., et al. (2004). Traffic Signal Pole Research, Wyoming Department of 

Transportation, Report No. FHWA-WY-04/03F, Laramie, WY.

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7. Ocel, J.M. (2006). The Behavior of Thin Hollow Structural Section (HSS) to Plate 

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8. Roy, S., et al. (2011). Cost-Effective Connection Details for Highway Sign, Luminaire, 

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(http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_w176.pdf).

9. Stam, A., et al. (2011). Fatigue Life of Steel Base Plate to Pole Connections for Traffic 

Structures, Texas Department of Transportation, Report No. FHWA/TX-11/9-1526-1, 

Austin, TX.

10. ASTM E8-13 (2013). Standard Test Methods for Tension Testing of Metallic 

Materials, Book of Standards Volume 03.01, ASTM International, West

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11. AASHTO (2013). Standard Specifications for Structural Supports for Highway Signs, 

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Highway Transportation Officials, Washington, DC.

37
HRDI-40/09-14(WEB)E