Report No.: CCEER-99-4

Title: Shear Strength and Shear Fatigue Behavior of Full-Scale Prestressed Concrete Box Girders

Authors: A. Buzick and M. Saiidi

Date: April 1999

Sponsoring Agency: National Science Foundation

Performing Organization:
Department of Civil Engineering/258
University of Nevada, Reno
Reno, NV 89557April 1999


The effect of repeated loads on prestressed concrete beams is becoming of more importance as older bridges sustain large number of traffic cycles. Previous studies of the fatigue characteristics of prestressed concrete indicated that beams that would fail in flexure under monotonic loading could fail prematurely in shear under fatigue loading. Therefore, fatigue failure is an important consideration in the evaluation of existing bridges and in the design of new bridges.

Two full-scale, prestressed concrete box girders with identical properties were tested. The girders had a shear-span of 13ft - 10in (4.22m), approximately five times the depth of the girders. The loads were placed so that shear rather than flexure would govern the failure mechanism. The first girder was tested in order to evaluate the ultimate shear strength under monotonic loading. The second girder was subjected to one-half million cycles of repeated loading and nine static overloads before its ultimate strength test. The test program of the second girder was designed in order to be able to compare the results to the findings of a previous study investigating similar girders subjected to flexure controlled loading. The measured shear strength was evaluated according to current code ACI/AASHTO and AASHTO LRFD shear procedures and to the Compression Field, Modified Compression Field and the truss model shear resistance theories.

All of the current shear procedures included in this study yielded conservative estimates for the shear strength. The results indicated that the effect of repeated loading on the shear strength of a shear dominated girder is less significant than that of a flexure dominated girder.