Seismic Rehabilitation Of Steel Moment-Resisting Connections With Initial Loading

Steel Moment-Resisting Connections with Welded-Flanges-Bolted-Web have been widely applied in of high-rise steel structures. In the Northridge earthquake, however, many of this type of connections failed due to the brittle fracture at the weld between beam and column flanges and cannot achieve the design demand"strong connection weak components". It is urgent to conduct seismic rehabilitation onto the existing connections which cannot resist the expecting earthquakes.Many experimental and theoretical investigations have been carried out to understand the reinforced connections. It has been observed that the cover-plate reinforced connections show excellent performance under cyclic loads and can be taken as an integrated part of a beam section in design. But in practical rehabilitation process, it is possible that the structure is not sufficiently shored and may not be completely unloaded. In this circumstance, the connection would be retrofitted while resisting some initial loads. As well, the application of the welding heat in the rehabilitation process may also impose a significant influence on the behavior of the retrofitted connections. Further researches on these cases are urgently needed.In this dissertation: (1) Finite element analysis is employed to investigate the loading behavior of the rehabilitated connection with initial loading. Parametric analysis, including the initial loads and the geometrical parameters of the cover plates, was carried out to obtain the moment distribution among the original connection and the cover plates. It is indicated that the initial loads are resisted only by the original connection and the additional loads are shared by the original connection and the cover plate with the same proportion as the newly erected reinforced connection. The distribution proportion is only affected by the thickness, but not the length of the cover plate. (2) The welding process of the cover plates to the column flange is simulated by thermo-stress coupling finite element model. The temperature fields and residual stresses are obtained. The influence of welding residual stresses on the retrofitted connection has been discussed. (3) Based on the stressed distributionsbetween the cover plates and the original beam obtained from the finite element analysis, a simplified theoretical model is suggested. This simplified model further confirms the results of parametric analysis of the finite element model, and the numeric expressions of all dominant parameters are established. Finally, a design procedure of the rehabilitation with initial loading using cover plates is proposed.The results above have provided some valuable information for the development of the theory and method on the seismic rehabilitation design of steel moment connection.