| Steel moment-resisting frames consisting of welded flange-bolted web beam-to-column connections were widely used as lateral load resisting system of choice in regions of high seismic risk, because of their merits of architecture usability(such as no lateral strut required) and facilely fabrication. But during the 1944 Northridge Earthquake and 1995 Hyogoken-Nanbu Earthquake, beam-to-column connections experienced a great deal brittle fracture, which indicated that cognitions on the damage mechanism of this style joints were rather inadequate and the current seismic design specifications were not ultimately reasonable. In order to use moment-resisting frames reassurance and avoid similar damage in future, analyses of response and damage mechanism of welded-bolted beam-to-column connections and their modified style joints under cyclical load are not only signification from the theoretical aspect, but also important in practice.In order to predict the fracture mechanism of welded-bolted beam-to-column joints under seismic load, an updated Lagrange finite element formulation involving large deformation, large strain, steel hardening model under cyclical load in reality, as well as status nonlinear in order to predict high-strength bolts pretension and contact-friction in contact zone, is adopted in FEM Model. This FEM model involves steel ductility fracture criterions and brittle fracture index and reflects welded-bolted beam-to-column joints under cyclical load in reality. The approach adopted in this article is thought to be an original one.After verifications of the nonlinear FEM Model involving geometry, material and status nonlinear by test results, a parametric computation on the behavior of welded-bolted beam-to-column joints subjected to constant axial pressure on the top of column and cyclical load at the tip of beam is carried through, parametric such as: web thickness of panel zone, pressure on column, pretension of high-strength bolts,contact-friction in contact zone, beam depth, beam flange thickness, column flange width and thickness, ratio of beam length to depth, welding and steel strength, configurations of weld access hole, weld backing bar. After study, the damage mechanism are given out: shear force transfer to beam flange due to discontinuous configuration near connection and column flange highly constraint on the beam flange are major reasons of fracture, also the "crack" near the weld backing bar enlarged this role. As results, the article gives out modified strength design formulation on beam flange butt weld thinking of shear force and design recommendations in seismic load.On the basis of damage mechanism of the normal welded-bolted beam-to-column joints, the article designs modified joints and makes parametric study. The results are: the modified styles performance much better than normal joints under cycle load. The stress in beam butt weld in RBS connection reduces slight due to reduced beam, connections have probability of brittle fracture. Because of their unique conformations, large zone gap is in cover plate and beam flange in welded cover plate joints, when the cover plate is very long, "crack" effects of such gap is distinctness. Welded haunch joints performance best, but the flange of welded haunch is a weak part in this style connection.In conclusion, the FEM Model can well simulate the behavior of welded-bolted beam-to-column connections under cyclical load. The study is rather comprehensive and systematic. The conclusion and design recommendations are very useful for revising design specifications. This article play a basis role of more study about beam-to-column connections...
|
PDF Full Text Request