Report No.: CCEER-92-3

A STUDY OF PRESTRESS CHANGES IN A POST-TENSIONED BRIDGE DURING THE FIRST 30 MINUTES

Authors: M. "Saiid" Saiidi and Eric Hutchens

Date: April 1992

Sponsoring Agency: Nevada Department of Transportation

Performing Organization:

Department of Civil Engineering/258

University of Nevada, Reno

Reno, NV 89557

Abstract:

A post-tensioned, simply-supported, concrete bridge structure in northern Nevada was instrumented in 1988 during construction, and the variation of its response was monitored over a 30-month period. The measured data consisted of tendon strain on four strands, concrete surface strain on two girders, and the deflection of the mid span relative to the ends of the superstructure. The measured losses on the tendons were due to creep and shrinkage. The data showed that the actual total creep and shrinkage losses are 30 percent higher than those predicted using a time-step analysis and 60 percent higher than those predicted by AASHTO. The results also indicate that the prestress forces continue to change due to seasonal variation in temperature and humidity, but the average stress becomes nearly stable (Abstract by authors).