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Publication Number:  FHWA-HRT-14-066    Date:  September 2014
Publication Number: FHWA-HRT-14-066
Date: September 2014


Fatigue Testing of Galvanized and Ungalvanized Socket Connections

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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


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.


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Technical Report Documentation Page

1. Report No.


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. (TRAIS)

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


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.

19. Security Classification
(of this report)


20. Security Classification
(of this page)


21. No. of Pages


22. Price


Form DOT F 1700.7 Reproduction of completed page authorized


SI* (Modern Metric) Conversion Factors


Table of Contents


List of Figures

List of Tables


Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000
Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101