Effect of girder flexibility on the dynamic response of buildings subjected to ground motion

An investigation is made of the effect of the flexibility of girders on the dynamic response of multistory buildings to lateral motions of the base. The response quantities examined are story displacements, interfloor deformations , and story shears; A numerical analysis is made on a realistic building designed in accordance with AC I code recommendations for constant floor masses and seismic shears as recommended by the Structural Engineers Association of California. Rotary inertia and vertical inertia of the joints are neglected. The building is approximated by two models. These are a frame model having flexible girders and extensible columns and the shear-beam model having infinitely rigid girders and inextensible members. A comparison is made of the behavior of the two models when excited by a half-cycle displacement pulse. The response quantities are shown in graphical form for a wide range of fundamental frequencies. It is found that the fundamental frequency of the building is substantially different in the two representations. The shears are found to be greatly underestimated by the shear-beam model for high frequencies. Displacements and deformations differ most markedly for the two models in the first story and are almost. identical in the top stories