33. Iowa Sign Truss

Donated by: Iowa Department of Transportation
Original Location:  Over I-80 in the state of Iowa
Emergency Repair: Repair strategies developed in the research for Iowa DOT
Removal from Service: Removed from service preemptively in 2006/2007
Donated to SBRITE: Summer 2007

Structure Type: Aluminum Space Truss
Number of Spans: One

Destructive testing of a 70 ft and 55 ft aluminum four chord truss was conducted at Purdue University.  The dead weight of the DMS sign was simulated by bolting scrap steel beams to one exterior side of the truss near mid-span.  The scrap steel had nearly the same weight as the reported weight of the DMS sign.  The wind load was simulated by hydraulic jacks and a wiffle-tree loading configuration.  The force transmitted by the hydraulic jacks was monitored with load cells.  Simple steel frames were fabricated for the purpose of mimicking the sign truss end supports.  The U-bolt connections and saddles were arranged to provide boundary conditions that were similar to the truss-to-end frame connections that are found in the field. A fracture in a member was simulated by cutting the cross section of the tube with a reciprocating saw. 

Testing of a main chord retrofit was conducted on the 55 ft truss.  Frame analysis using SAP 2000 Version 11 was conducted and the analysis predictions were compared to the experimental data.  Samples of crack faces were examined for the purpose of ascertaining the cause of cracking.  Recommendations for the practicing engineer’s response to cracked aluminum trusses are given at end of this document.  The truss that is currently housed at S-BRITE was a third truss provided by the Iowa DOT for use in future training and inspection exercises.