Analysis and Design for End Effects in Twisted Double Tees

Abstract

Prestressed concrete double tees are sometimes set on non-parallel supports to facilitate drainage; this practice induces twisting in the members. When double tees are twisted through sufficiently large angles, they develop longitudinal cracks at the web-flange junction. This article reports on a study that identifies the important modes of deformation and presents an analysis of the stresses and deformations caused by twisting. Local distortions of the cross section near the member ends are shown to play a pivotal role in bending and cracking of the flanges of double tees. The authors develop a new theory of torsion that includes those deformations, and report on a parametric study that they carried out to show the effect of variations in the dimensions of the member. The article includes several graphs that facilitate the computation of the twist angle that causes cracking in a double tee of common dimensions. The authors conclude that the flange cracking at the ends of twisted double tees appears to be caused by flange bending stresses, and not by St. Venant torsional shear stresses. The member depth and flange thickness have only moderate effects on flange bending stresses.