Elimination of Prestressed Concrete Compression Limits at Service Load

摘要

This paper addresses the justification for removal of two compressive stress limits, called hereafter Limits 1 and 2, which are currently used in both the ACI 318 Building Code and AASHTO Bridge Specifications to limit concrete stresses due to effective prestress combined with applied loads. Removal of these limits provides relief in sizing of members that have small compression zones that are subjected to large bending moments, such as inverted tee bridge members in the positive moment zones and I-girder bridges made continuous for superimposed loads in the negative moment zone. Limit 7 is 0.6f'_(c'_) where f'_(c'_) is the specified compressive strength at service. It is imposed on concrete stresses due to the effective prestress combined with full dead plus live loads. Limit 2 is 0.45f'_(c'_) It is imposed on stresses due to the effective prestress combined with dead loads. It is proposed that strength be used as the primary design criterion to determine member capacity in compression at various loading stages, including prestress transfer, lifting, erection, deck weight, superimposed dead load and live load. Various serviceability checks can additionally be used as needed to control camber, deflection, vibration, and cracking. Design steps and numerical examples are given. Also, proposed changes to the ACI 318 Building Code and to the AASHTO-LRFD Bridge Design Specifications are given.