Transmission length and shear capacity in prestressed concrete hollow core slabs

Abstract

The transmission length Lt of bonded tendons in prestressed concrete is crucial to the shear-tension capacity VRd,c, in particular in precast hollow core floor slabs that do not have shear reinforcement. Defining and measuring Lt is difficult and variable, and has steadily increased to almost 100 diameters for indented wire in the present-day Eurocode 2 (EC2). Because de-tensioning of hollow core slabs takes place where the bond has already fully developed, it is not certain whether the code equations reflect their manufacturing process. Full-scale tests were used to measure Lt and relate it to the design length lpt as well as to VRd,c with the aim of proposing a modification to the design equation that links lpt to VRd,c. The ratio of Lt to the predicted length was from 0·99 to 1·21 and the ratio of Lt to the EC2 design value was from 0·51 to 0·67. Furthermore, a 26% reduction in Lt at the ‘cut' versus ‘cast' ends of T-beams adds weight to the argument that the code equations do not reflect manufacturing methods. Ultimate test shear capacities varied greatly due to different geometries, but the test/predicted ratio was 1·09 to 1·17 and the test/design ratio was 1·95 to 2·13, practically doubling the code equation. By comparing the shear loads with Lt, if Lt is divided by 1·5, the predicted and tested shear loads are equal. This paper proposes an equation for prestressed hollow core units manufactured using long-line techniques only. The effect of this on a typical slab design is to increase VRd,c by about 10% and reduce the zone of transmission around holes by about 350 mm, which in some circumstances may benefit the failure criteria at holes.