Span-to-height ratio limits for prestressed concrete members

This paper reports the main results of a parametric analysis which was performed to define appropriate limits for the span-to-height ratio of prestressed concrete beams or slabs that are in compliance with the deflection control required by international standards (Eurocode 2, Model Code 1990) for concrete buildings structures. Two deflections were considered: the total deflection and the active deflection that occurs after construction, in both cases under the quasi-permanent load combination. The active deflection has to be considered when fragile elements such as partitions or glazing are present on the structure. Four structural systems were considered: simply supported fully or partially prestressed beams with an I-section and simply supported or fixed-end slabs with a rectangular cross-section. The load history first included the application of the prestress and the dead load and then the application of the remaining quasi-permanent load. Taking into account the effects of imposed deformations the structure was designed to obtain the geometrical properties strictly necessary for the ultimate limit state (ULS) and for stress limitation in service under quasi-permanent and characteristic load combinations. The results allow the influence of some of the variables that most affect the deflection to be evaluated, such as the span and the height of the beams, the concrete strength, the permanent load and the variable-to-permanent load ratio.