Limit states of cracking in beam-and-block floor systems using pretensioned ribs

Abstract

Beam-and-block floor systems using pretensioned ribs enable very efficient industrialised production. By ensuring that the ribs are produced under a strict quality assurance programme and that they are safely transported, assembled and properly detailed, an equally monolithic structural performance is achieved. However, calculation methods to control cracking in these structural elements must ensure the durability according to the surrounding environment's severity and predefined exposure classes. The design of beam-and-block floor systems is particularly affected by serviceability limit states. This fact is closely linked to the verification conditions for crack control. Therefore, new design models have been developed to take into account the peculiarities and characteristics of these floor systems. The aim is to find practical and effective crack-control methods for composite beam-and-block floor systems with pretensioned ribs or beams. The verification of the limit state of crack widths has so far been disregarded in the design of this type of lightweight slab. In fact, the current procedure is simply to compare the tensile stress in the lower fibre of the beams with the characteristic strength of the rib's concrete. This paper presents a method to assess crack control. The procedure implies the quantification of a limit bending moment depending on the physical and geometric characteristics of the sections. Simplified calculation methods and verification rules are presented, which allow the establishment of tables and design charts for the indirect verification of this limit state through bar size and spacing limits.