Investigation on Vierendeel mechanism in steel beams with circular web openings

In the current design method for steel I beams with circular web openings, the load carrying capacities of the beams is assumed to be limited by the formation of plastic hinges at the top tee-sections at the low moment side of the web opening. Moreover, a linear interaction formula is used to assess the moment capacity of the tee-sections above and below the web openings under co-existing axial and shear forces. The method is regarded as conservative since the formation of plastic hinges in the top tee-sections at the low moment side of the web openings does not always cause failure. The beams are capable of carrying additional load until four plastic hinges at critical locations of the perforated sections are developed to form a Vierendeel mechanism. Moreover. the reduction in the moment capacities of the tee-sections under coexisting axial and shear forces is less severe than that anticipated by the linear interaction formula. The paper presents an investigation of the Vierendeel mechanism in steel beams with circular web openings based on analytical and numerical studies. The current design method is examined in detail with plastic hinges formed at the low moment side (LMS) and the high moment side (HMS) of the web openings separately. A finite element model is then established with both material and geometrical non-linearity so that load redistribution across the web openings may be incorporated. Moreover, the moment capacities of the tee-sections above and below the web openings may be properly evaluated in the presence of co-existing axial and shear forces in the finite element model. The load carrying capacities of typical universal steel beams with circular web openings art: also presented and discussed. An empirical shear moment interaction curve at the perforated sections is also suggested for practical design of steel beams with circular web openings against the Vierendeel mechanism. It is found that shear yielding in steel beams with circular web openings is very important as it promotes the plastic hinge formation at the high moment side of the web opening. Such effect is less significant in steel beams with rectangular web openings where the bending moment is often dominant. (C) 2001 Elsevier Science Ltd. All rights reserved.