Evaluation of Multi-Post Large Sign Supports with Slip Base and Slip Hinge (621141/Task Order AP)

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TTI Research Supervisor:
Roger P. Bligh, Ph.D., P.E.
Senior Research Engineer
Texas A&M Transportation Institute
Texas A&M University System
TAMU 3135,
College Station, Texas, 77843-3135
(979) 317-2703
[email protected]
  Pooled Fund Technical Representative:
Scott Jollo, P.E.
State Traffic Structures Engineer
Oregon Department of Transportation
4040 Fairview Industrial Dr SE, MS#5 Salem, Oregon 97302-1142
(503) 510-2204
[email protected]

Background (and Problem Statement)

Full-scale testing experience of large guide sign systems in accordance with the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH) is limited (1). Testing to date has been performed on systems that incorporate a fuse plate below the sign panel to permit rotation of the impacted post relative to the sign assembly. In some tests, the fuse plate has not ruptured and activated as a hinge mechanism as intended (2). The activation of the fuse plate relies on variables such as plate thickness, cross-sectional area, and the mechanical properties of the steel (e.g., ultimate tensile strength). The activation of the fuse plate is also dependent on the stiffness of the sign panel and the strength of the connections between the sign panel and support posts. A survey of Pooled Fund members under another project noted differences in the thickness and cross-sectional area of the fuse plates for the same size support post in their multi-post sign support standards (2). Other types of hinge mechanisms are used by some states. For example, Oregon DOT has had good field experience with a slip-style hinge mechanism (see Figure 1). The slip hinge functions similarly to a slip base system. A clamping force is provided through the tension developed in the slip bolts when they are tightened. Such a system may provide more consistent and reliable activation in an impact compared to fuse plates. Testing has also shown that the sign assembly can be damaged and require extensive and expensive repair if the impacted lower portion of the support post remains hinged to the upper portion of the support post. If the hinge connection between the upper and lower portions of the support post can be designed to readily disengage, the level of repair may be significantly reduced. A slip hinge design may offer such an advantage.  

Objective

The project objective is to evaluate the performance of multi-post sign support systems that incorporate a slip base at the groundline and slip hinges below the sign panel to determine if they comply with MASH criteria and offer performance advantages compared to fuse plate hinge mechanisms.

Products

The TTI research team will provide a final report that will include the results of the testing and evaluation performed under the project. The report will contain recommended slip hinge design details for multi-support breakaway sign support systems, and document any performance differences observed between the slip-style hinge mechanism and fuse plate hinge designs.

Work Plan

The work plan for this research includes four tasks. Details of these tasks are described below.

Task 1: Literature Review and State Survey:

The research team will review previous and ongoing research projects to determine multi-post sign support systems that have met MASH requirements with various breakaway base and hinge types. A survey instrument will be distributed to the Roadside Safety Pooled Fund members to identify the design characteristics of their multi-post sign support systems.

Task 2: Engineering Analysis

The research team will analyze the data obtained in Task 1 to determine common features and critical configurations of multi-post sign support systems to evaluate under this project. For example, it is anticipated such features might include extruded aluminum sign panels, W-section posts, and 4-bolt slip base. The hinge design will be a focal point of the research effort. Design details regarding the slip hinge will include number of bolts, bolt size, and bolt tightening procedure. The analyses performed under this task will be used to recommend design configurations for further evaluation through full-scale crash testing.

Task 3: Full-Scale Crash Testing

The research team will evaluate two critical configurations of multi-post sign support systems according to MASH TL-3 criteria for breakaway support structures. The configurations may differ by size of support posts and the sign panel details that bracket the impact performance of the slip hinge mechanism and other key design characteristics. The MASH TL-3 test matrix for support structures consists of three tests: Test 3-60 (low-speed impact with passenger car), Test 3-61 (high-speed impact with passenger car), and Test 3-62 (high-speed test with pickup truck). Critical tests will be identified and performed to evaluate the impact performance of each of the selected multi-post sign support configurations. Two passenger car tests and two pickup truck tests have been budgeted for this task. The test results will be used to evaluate MASH compliance of the multi-post sign support configurations.

Task 4: Deliverables

The research effort will be documented in a final report. The report will document all aspects of the research including literature review, survey, and full-scale crash testing. The report will include recommended design details for slip hinge mechanisms, and discuss observed performance differences between the slip-style hinge and fuse plate hinge designs.  


REFERENCES

  1. American Association of State Highway and Transportation Officials, Manual for Assessing Safety Hardware – Second Edition, AASHTO Subcommittee on Bridges and Structures, Washington, D.C., 2016.
  2. Silvestri-Dobrovolny, C., Bligh, R.P., Schroeder, W.J.L., and Kuhn, D.L., Testing and Evaluation of Large Signs Slip Base Support On Slope at MASH Test Level 3 Impact Conditions, Research Report 612261-04-1, -05-1, & – 05-2, Texas A&M Transportation Institute, College Station, TX, January 2023