Lateral stability and concrete strength requirements for precast, prestressed concrete components

The allowable level of temporary concrete compressive stress in precast, prestressed concrete components has been a source of debate in the concrete industry for many years. Traditionally, these stresses have been considered to originate only from the effects of prestress combined with the self-weight of a plumb component, evaluated about the major axis. The maximum compressive stress divided by the coefficient of the compressive stress limit determines the required concrete strength. Although these temporary stresses can occur at any time from fabrication through erection into the structure, the critical case is usually at transfer of prestress and subsequent lifting from the form. At this stage, the prestress force is higher and the concrete strength is lower than at any other point in the life of the component. At this early age, concrete is also more susceptible to damage from high compressive stress. As materials and fabrication capabilities in the precast, prestressed concrete industry advance, components are becoming longer and slenderer, particularly within the transportation sector. Stith components require serious consideration of lateral stability during handling, which introduces bending about the minor axis. This lateral bending will increase maximum tensile and compressive stresses at the extremities of the component. These localized stresses traditionally have not been used to determine the required concrete strength, and doing so at current stress limits can significantly increase the required concrete strength. This paper is intended to reconcile the interaction between the temporary concrete compressive stresses traditionally used to determine the required concrete strength and the requirements for lateral stability, primarily the additional stresses due to lateral bending. The assumption is made that compressive stresses govern the determination of the required concrete strength. It is normally not efficient to determine the required concrete strength based on tensile stresses, which can be satisfied in a different manner.