Load transfer of longitudinal pipe forces in penstocks through friction

In a recently completed research project, the load transfer of axial loads in penstocks via friction between the steel pipe and the surrounding concrete and rock, without thrust rings, was studied. Thrust rings are traditionally used to transfer the axial loads in steel pipes into the surrounding concrete, and further into the rock. This paper deals with a summary of the measurements on a branch of a penstock to evaluate appropriate coefficients of friction between the steel pipe and the concrete surrounding. Parametric studies are then presented to demonstrate the principal behaviour of load transfer along the pipe, with and without thrust rings. Based on this, a simplified analytical model was developed for practical design, including regions without friction. In addition, the effects of the important boundary conditions, longitudinally free or fixed, of the steel pipe within the rock are presented.