Determination of Fatigue Resistance of Coupler Connections in Aluminum Overhead Truss Structures

Abstract

Overhead truss sign structures (OHTSS) are widely used over highways across the nation. An overhead truss sign structure is comprised of a truss and two supporting frames at each end, and can be made using aluminum or steel. Aluminum overhead truss sign structures are generally more prone to vibration issues due to their light weight (Fouad, et al. 2003). Before 2015, aluminum overhead truss sign structures constructed in Kansas used an identical type of coupling assembly to provide connection between vertical poles and horizontal trusses. There are approximately 450 sign structures that use this type of connections over active highways in the State of Kansas. The coupler connection was designed in the early 1970s. At that time, experimental tests were conducted to determine the static strength of the connection (McCollom 1973), however, no prior research has been conducted to evaluate the fatigue performance of the coupler connection. Many of these connections have now been in service for 30-40 years and research is needed to evaluate the fatigue performance of the connections. This report presents a study aimed at evaluating the fatigue performance of the coupler connections used in bridge-type overhead truss sign structures. It consists of two parts. The first part describes a series of finite element (FE) analyses that were used to determine the behavior of the coupler connections in expected real use. The results indicated that among all loading cases analyzed in this study, the out-of-plane responses of the sign structures were more significant than in-plane responses. The coupler connections behaved like idealized pinned connections, with little to no capability to transfer moments; therefore, the rivets connecting the interior two couplers were found to undergo mostly direct tension and shear. A rational experimental testing plan was developed according to the findings of the finite element analyses. The second part of this study included performing 22 fatigue tests on newly-fabricated coupling assemblies obtained from Steve Johnson Fabrication, Inc. (Wichita, KS), the company that manufactures most OHTSS in Kansas. The experimental tests were conducted to characterize the fatigue performance of the coupler connection, utilizing AASHTO S-N curves. The fatigue tests were conducted in three directions, such that the rivet was loaded in: 1) tension, 2) vertical shear, and 3) horizontal shear. These loading conditions on the rivet represented, respectively: 1) truss out-of-plane shear, 2) truss in-plane shear, and 3) truss chord axial force. The testing program revealed that the coupler connection has much better fatigue resistance when loaded in shear than in tension. Despite poor tensile fatigue performance, fatigue failure is still considered to be unlikely in OHTSS applications, due to very low stress demands found from finite element analyses.