Transmission length and shear capacity in prestressed concrete hollow core slabs

The transmission length L_t of bonded tendons in prestressed concrete is crucial to the shear-tension capacity V_Rd,c, in particular in precast hollow core floor slabs that do not have shear reinforcement. Defining and measuring L_t 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 L_t and relate it to the design length l_pt as well as to V_Rd,c with the aim of proposing a modification to the design equation that links l_pt to V_Rd,c. The ratio of L_t to the predicted length was from 0·99 to 1·21 and the ratio of L_t to the EC2 design value was from 0·51 to 0·67. Furthermore, a 26% reduction in L_t 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 L_t, if L_t 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 V_Rd,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.