Is the "Staggering Concept" of Shear Design Safe?
Thomas T. C. Hsu
Appears on pages(s):
beams (supports); loads (forces); models; plasticity; reinforced concrete; shear properties; stresses; stirrups; structural design; trusses; web reinforcement.
For beams subjected to uniformly distributed load, shear design according to a "staggard" shear diagram has been proposed in Europe and imported recently to North America. This "staggard concept" of shear design can be derived from the plasticity truss model, which assumes complete plasticity of materials and no bond between steel and concrete when design load is reached. As a result, thje force in a stirrup is assumed to be uniform throughout its length. A compatablity truss model to study the equilibrium and the steel forces in a beam element with uniformly distributed load is presented. It was found that the distribution of stirrup forces was nonuniform and maximum stirrup force near the bottom stringer varies longitudinally according to the conventional shear diagram. Design according to this diagram will be safe because it does not require the destruction of bond between steel and concrete when the design load is reached. Comparision of the design based on the staggrad shear diagram and conventional shear diagram shows that the former gives an upper-bound solution, while the latter provides a lower-bound solution. Therfore, it is suggested that shear stirrup design using the staggered shear diagram is unsafe, and shear stirrups should continue to be designed according to the conventional shear diagram.