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Determination of U-Bolt Connection Load Capacities in Overhead Sign Support Structures


The service, fatigue, and ultimate load capacities of U-bolt connections used in overhead sign support structures are not known because the U-bolts are used in ways not originally intended by manufacturers and therefore do not match the available manufacturer data. Although these U-bolt connections appear to have performed satisfactorily in the past, in recent years there has been a growing safety concern because of the need for support structures with larger signs and greater span lengths.

In 2015, the Bridges and Structures Bureau of the Iowa DOT began to investigate if the U-bolt connections have adequate strength to safely perform in current sign support structures as well as in future ones that will need to resist even greater loads. After an initial literature search failed to yield any useful information, the Iowa DOT distributed a questionnaire through the AASHTO Committee on Bridges and Structures. Fourteen state DOTs responded to the questionnaire, confirming that little U-bolt load capacity research and theoretical design had been done.

In 2018, researchers at the Iowa State University Institute for Transportation started work to determine the static load capacity of U-bolt connections used on standard four-chord space trusses in Iowa. The results from both laboratory tests and analytical solutions indicate that different failure modes occur when the loading is in different directions. The results from the analytical study show a relatively low yield capacity but indicate that the details have good ductility before reaching failure. Although this preliminary work represents a major step toward developing a better understanding of the behavior and design of U-bolt connections, additional static load capacity research and fatigue capacity research are needed.


The main research goal is to determine the load capacities of overhead sign support structure U-bolt connections when they are subject to static load and fatigue load. To achieve this goal, the research team will accomplish the following two objectives:

1) Investigate the fatigue behavior of the U-bolt connections and evaluate the fatigue capacity of the U-bolt connections used on steel overhead sign trusses

2) Validate the analytical results in the initial (i.e., Phase I) research, Phares (2019)


The proposed research will consist of five main tasks, described in detail below:

Task 1: Technical Advisory Committee (TAC)

The research team will meet with the project’s Technical Advisory Committee (TAC) to review the project scope and work plan.

Task 2: Literature Review

A brief literature review will be conducted on relevant topics including:

1) Material fatigue behavior of different types of steel

2) Evaluation of the number of wind load cycles experienced by overhead sign support structures

3) Evaluation of the magnitude of wind load experienced by overhead sign support structures

4) Structural fatigue behavior of U-bolt connections

The results from the literature will provide guidance for the work in the subsequent tasks.

Task 3: Laboratory Tests

Laboratory testing will be conducted to understand the material properties of the U-bolt material, fatigue performance of the U-bolt connections, and the static performance when U-bolts are subject to loading in the directions not tested in the Phase I research. Consistent with the work conducted by Phares (2019), two types of critical U-bolt connections (Type A and Type B) will be tested in Phase II. The goal of the material property test is to capture the stress-strain relation of the tested material when it is subject to the static load, fatigue load, and the ultimate tensile strength. The fatigue performance will be investigated on four specimens (two Type A and two Type B). The magnitude of the cyclic loading and the number of cycles will be chosen based on the results from the literature and input from TAC members. For each connection type, one specimen will be tested in the vertical direction (0°) and the other one will be tested in the horizontal direction (90°). The data collected from this step will be used to understand the actual U-bolt loading behavior/failure modes and to validate the finite-element models. Additional static tests will be conducted to validate the predication results in Phares (2019). In total, seven tests will be performed: three on Type A with loading directions of 45°, 135°, and 180° and four on Type B with loading directions of 0°, 45°, 135°, and 180°.

Task 4: Analytical Study

The purpose of the analytical study is to interpret the fatigue test results and gain a better understanding of the structural behavior of the U-bolt connections when subject to fatigue loading. A finite-element model will be developed to simulate the fatigue behavior of the U-bolt when it is subject to the cyclic loading and validated against the test data. In addition, a parametric study will be conducted utilizing the calibrated model to predict the fatigue load capacity for different loading directions and material properties, etc. Based on results of the parametric study, design guidance will be provided. This guidance will include interaction diagrams enabling designers to estimate U-bolt connection capacity when the material type and loading direction are known.

Task 5: Final report

At the end of the project a concise final report will be developed to be approved by the TAC.

Sponsoring Committee:AKB10, Innovative Highway Structures and Appurtenances
Research Period:12 - 24 months
Research Priority:High
RNS Developer:Brent Phares, Iowa State University
Date Posted:01/07/2022
Date Modified:01/19/2022
Index Terms:Load tests, Bolts, Overhead traffic signs, Sign supports,
Cosponsoring Committees: 
Bridges and other structures

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