After-Fracture Reserve Strength of Two Four-Chord Aluminum Sign Trusses

Abstract

Many departments of transportation use four-chord aluminum tubular truss structures to support dynamic message signs (DMS). Inspections have often revealed that these structures contain cracks that likely occurred during or shortly after fabrication or were caused by fatigue while in service. Nearly all of these cracks are located at welded tube-to-tube connections, such as the diagonal-to-main chord fillet welded connections. Both weld toe cracks and surface-breaking root cracks have been observed. Destructive testing was conducted to evaluate the after-fracture reserve strength (AFRS) of 21.3-m-span and 16.8-m-span aluminum four-chord trusses. The dead weight of a DMS was simulated by attaching steel beams of the same weight eccentric to one exterior side of the truss near midspan. Wind load was simulated by hydraulic jacks and a whiffle-tree loading configuration. Simple steel frames were fabricated to mimic the sign truss end supports. Realistic boundary conditions, similar to the truss-to-end frame connections were provided by U-bolt connections and saddles found in the field. Fractures in up to two chords at the same cross section were simulated by cutting the members with a reciprocating saw. The testing revealed that both 21.3-m and 16.8-m span trusses have a considerable amount of reserve strength in the severely faulted (i.e., several members severed) condition. Testing of a main chord retrofit was conducted on the 16.8-m truss. Simple frame three-dimensional (3D) finite-element analysis was also conducted, and the predictions compared to the experimental data. Based on the results, recommendations are presented regarding how to respond if cracked members are discovered during inspection. The results are believed to be applicable to other similar trusses with span lengths up to 21.3 m.