Modeling and behavior of prestressed concrete spandrel beams.

Abstract

Spans for precast and prestressed concrete spandrel beams (or spandrels) are typically as long as 30 to 48 ft. In the near future, precasters might begin to produce spandrels with longer spans (up to 60 ft) and thinner webs (as little as 8 in.), which facilitate more cost-effective parking garages and advance the competitiveness of the precast concrete industry. However, there are likely to be spandrel dimensions beyond which excessive deformations make such designs unfeasible. In particular, lateral deformations developing at the top and bottom of a slender spandrel beam need to be investigated for short-term and long-term loading effects. Spandrels in parking garages typically support double-tee beams (or double tees) on ledges, spot corbels, or pockets, all of which introduce lateral eccentricity as the double tee loads are transferred to the spandrels. Upon connecting the spandrels to the double tees using deck ties, these connections serve as lateral restraints for the spandrels due to the axial stiffness of the deck ties. Thus, lateral deflection of the spandrel is prevented at the connection points. However, given that the deck ties are thin steel plates, they are unable to eliminate twisting of the spandrel section. Thus, subsequent deformation of the spandrel will include both vertical and lateral deflections. Excessive lateral deflections might cause undesirable serviceability problems in the structure or even collapse of the concrete deck. For this reason the study of lateral deflections is the focus of the research presented in this dissertation. Two approaches are pursued to study the lateral deflection response of the slender spandrels under static loads: analytical and computational. First, an approximate analytical solution for the lateral deflections of spandrels is presented. Next, the effects of various parameters on the results from finite element analyses are examined. This is followed by a detailed discussion of modeling suggestions under which computational results can be sufficiently reliable. The best possible modeling approach, deduced from such parametric studies, is used for a three-dimensional nonlinear finite element analysis of spandrels. Finally, time-dependent (long-term) lateral deflections in spandrels having various span lengths and cross sections are investigated.