Three-dimensional steel building response to near-fault motions

The seismic behavior of a three-story steel framed building designed by con sultants for Los Angeles seismic conditions was assessed using 3D dynamic i nelastic time-history analyses with near-fault as well as code design level earthquake records. It is shown that 3D frame drifts due to simultaneous s trike-normal and strike-parallel shaking were sometimes greater and other t imes less than for a 2D frame subjected to strike-normal shaking only. The ability of the 30% rule, Square Root of the Sum of the Squares, and Sum-of- Absolute-Values methods to assess building drifts for bidirectional shaking effects is dependent on the reference axes chosen. In come cases, horizont al shaking orthogonal to the principal shaking direction increased drifts i n the principal shaking direction by up to 84%. This occurred as a result o f biaxial flexural yielding at column bases so that even the Sum-of-Absolut e-Values method underestimated the actual drifts. Also, gravity columns, wh ich were pinned at the base, yielded at the top of the first story due to t he severe near fault shaking.