Accelerated Construction of the Highway Steel Overhead Sign Truss (SOST) through the Implementation of U-Bolt Connections

Project Information

Link to Latest Report: September 2023 Progress Report


The load capacities of U-bolt connections used in Iowa DOT steel overhead sign trusses are not known because they are used in ways that do not match available manufacturer data. The previous phase of this research work determined the load capacity of these U-bolt connections based on the results from the limited laboratory tests and the parametric studies performed on the finite element (FE) models. It was pointed out that additional laboratory tests should be performed on high-strength U-bolts subject to static loading in multiple directions. In order to resolve the concerns left from the previous phase of this research, this proposal is drafted with a goal to investigate the structural behavior of the U-bolt connections and evaluate the capacity of the U-bolt connections subject to various loading directions and validate the analytical results developed in the previous research. To achieve the proposed objectives, a five-task 12-month research plan consisting of multiple stages of laboratory tests was developed. The research results will assist contractors in effectively and efficiently designing overhead sign support sign structures without concern for the capacity and safety of the U-bolt connections.


The objectives of this project will be built on the work conducted by Phares and Liu (2019). The objectives of the proposed research are to:

1. Investigate the structural behavior of the U-bolt connections and evaluate the capacity of the U-bolt connections subject to various loading directions

2. Validate the analytical results developed in Phares and Liu (2019).

Consistent with the work conducted by Phares and Liu (2019), two types of critical U-bolt connections will be evaluated: 1) Type A U-bolt connection − used to anchor the bottom chords of the horizontal space truss to supporting columns at each end of the space truss, and 2) Type B U-bolt connection ─ used to attach vertical sign-support members to the front-top and front-bottom chords of the space truss. The details and dimensions of Type A and Type B specimens are shown in Figures 1 and 2, respectively. As indicated in Figure 1, the Type A specimen consists of a W-shaped steel beam, a saddle assembly, a steel pipe, two steel plates, and U-bolt components. As indicated in Figure 2, the Type B specimen consists of an L-shaped steel angle, a steel pipe, two steel plates, and U-bolt components.

This budget was created based on a research plan consisting of a series of material properties test and 7 laboratory static ultimate-load tests. The proposed research will consist of five main tasks, described in detail below.

Task 1 – Literature Review

The completion of the previous phase (Phares and Liu 2019) of this project has produced a comprehensive literature review. An additional literature search will be conducted to collect information that was not uncovered in the previous phase research, mostly focusing on the review on the state DOT’s use of the same or similar system.

Task 2 – Laboratory Tests

The proposed laboratory tests consist of two components: material property tests (Task 2.1), additional static tests to those performed in research Phase I (Task 2.2). Table 1 shows the test matrix for each phase. All specimens tested in Task 2 will be appropriately designed with the identical material and geometric properties as used in the actual SOST design standards.

Table 1 Test Matrix in Task 2

Task I.D. Test type Load type No. of tests
Task 4.1 Material Property Tests U-bolt Coupon Static Tension 3
Tube Coupon Static Tension At least 2
Task 4.2 U-bolt Static Tests Type A-S1 45° 1
Type A-S2 135° 1
Type A-S3 180° 1
Type B-S1 1
Type B-S2 45° 1
Type B-S3 135° 1
Type B-S4 180° 1

* The intent here is to run 3 million cycles per specimen, the test will stop if the system fails during the test or if nothing in the system fails after 3 million cycle loads.

Task 2.1 – Material property tests

The goal of the material property tests is to capture the stress-strain relationship of the tested U-bolt and the tube material when they are subject to the static tensile loading. These material properties are needed in order to understand the overall structural behavior of the U-bolt connections. In these tests, three specimens that are made with the same material as used on the tested U-bolt will be prepared and tested following ASTM A370 to capture the static stress-strain behavior. At least one tension coupon will be tested from each tube size (assuming all full-scale test specimens come from the same mother tube), and the Type B angles. All these coupons tested in this task will be in the “galvanized” condition.

Task 2.2 – Additional static tests

Although Phares and Liu (2019) provided the capacities of the U-bolt connection when the load is in different directions, most of the results were predicted by the analytical approach and never validated by the experimental tests. It was found that analytical simulation results show different failure modes when the loads are in different directions. Additional experimental validation was recommended for loading in the 45°, 135° and 180° directions. See Figure 3 for the loading orientations. The objective of this step is to conduct additional static tests to validate the predication results in Phares and Liu (2019). In total, seven tests will be conducted in this step including three on the Type A connection with loading directions of 45°,135° and 180° and four on the Type B connection with loading directions of 0°, 45°, 135° and 180°. Each test will be performed on a new U-bolt setup and loaded until either ultimate capacity or the desired capacity is achieved.

Before testing, all the specimens will be galvanized since this is a specified practice performed on the SOST structures. Similar to the work conducted by Phares and Liu (2019), the U-bolt response will be measured during each test. In order to compare the results with those in Phares and Liu (2019), a similar instrumentation plan used in the previous research phase will be adopted in this task. Five uniaxial strain gages will be attached along the exterior of each U-bolt. Displacement transducers will be installed to measure the displacement at the loading point and a load cell will be used to measure the loading increment. Since the previous research indicated that the rosette gages installed on the saddle (Type A connection) shows minimal response, no instrumentation will be installed on the saddles in this step.

Task 3 – Validation of Interaction Diagrams

In the previous research phase (Phares and Liu 2019), interaction diagrams were developed with limitations for design usage to estimate the capacity of the U-bolt connections with different material types and load directions. These diagrams were developed based on a parametric study performed on the calibrated FE models. However, these results were not validated through laboratory tests.
In this task, the capacity results obtained from Task 2.2 will be used to compare with the prediction from the interaction diagrams developed by Phares and Liu (2019). The difference between the experimental results and the results predicted by the interaction diagrams will be discussed if any discrepancies are found.

Task 4 – Recommendation Development

Based on results from the laboratory tests, the recommendation for the design subsequently will be derived to accurately estimate the capacity of the U-bolt connections under different loading conditions for the two types of U-bolts connections.

Task 5 – Final Report

A final report will be prepared as the final product of this project. Prior to report finalization all the review comments properly addressed.

Research Team:
Principal Investigator: Zhengyu Liu
Co-Principal Investigator: Brent Phares
Research Assistant: Abdalla Alomari

Previous Progress Reports: