Nonlinear Dynamic Analysis of Soft-Story Light-Frame Wood Buildings

Many older multi-story woodframe buildings in the United States were not designed according to the earthquake engineering principles. Most of these older buildings have a soft first story due to the existence of large garage doors or retail store window openings. These soft-story buildings are prone to large lateral movements (displacements and rotations) and even pancake collapses in the first story during earthquakes. Analyzing the stability and seismic performance of soft-story buildings that undergo large deformations require a geometric nonlinear analysis. This paper presents a new three dimensional (3D) model for nonlinear dynamic analysis of woodframe buildings under large deformations. In the proposed 3D model, the floor and roof diaphragms are modeled using a 3D two-node twelve degrees of freedom (DOF) corotational frame element. In corotational formulation, the orientation of the frame elements is updated in each modeling time step. The use of corotational formulation and 3D frame element allows the numerical model accurately captures the in-plane and out-of-plane motions of the diaphragms under large rotations. Two-node six DOFs (three translations and three rotations) link elements which represent the shear walls and wall studs are used to connect the diaphragms in a 3D space. To reduce the computational demand, shape functions are utilized in the proposed 3D model to eliminate the DOFs of the link elements from dynamic analysis. In other words, the size of the building stiffness matrix for dynamic analysis does not increase as more link elements (shear walls) are added to the structural model. An example application of the proposed 3D model for modeling a three-story building with soft first story is presented.