Office of Bridges and Structures

Research Project


Investigation of Sign Support Truss Structure


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Project status


About the research

Background: A sign support truss over eastbound I-80 about 2 miles west of the west I-35/I-80/I-235
interchange near Des Moines, IA was removed in the spring of 2000. A routine
inspection of the truss found that one of the bottom chord members was severely
deteriorated. Corrosion of the bottom chord has caused significant thickness loss of cross
section area of many members. This deterioration led to a concern about the structural
adequacy of the truss.

The truss is a galvanized steel, four chord truss with a span of approximately 60 ft and
carries a single 14 ft x 40 ft sign and associated lighting.

The Iowa DOT initiated this investigation to evaluate the structural behavior of sign
support trusses that have structural deficiencies and to assess the urgency to remove or
retrofit these trusses.

Objective: The primary objectives of this investigation are:
(1) Perform a load test of a damaged truss to assess the impact of the damaged members
on the overall performance of the truss.
(2) Compare the load test results with the results of structural analysis of the truss with
the intent of assessing the accuracy of predicting the structural behavior of damaged
and undamaged sign support trusses.
(3) Provide recommendations for managing damaged trusses.