Vertical distribution of force-controlled seismic responses in multistory RC buildings

This study investigates the vertical distribution of force-controlled seismic demands of multi-story buildings by means of a 12-story shear wall building. Since structures with slender walls for their lateral force resisting system are expected to undergo considerable higher mode demands, a method incorporating the Fourier Transform of the response time histories is adapted to decompose the response into its modal components. Using this method, shear (V), diaphragm force (F), moment (M) and inter-story drift ratio (IDR) response envelopes using 50-design spectrum compatible MCE earthquakes are decomposed into first three modes of vibration. It is found that M and IDR demands are mostly controlled by the first mode response, whereas V and F are controlled by higher modes in many instances. The results also indicate that the Equivalent Lateral Force (ELF) design procedure used by current US building code unconservatively predicts the seismic demands and the actual vertical distribution of demands significantly differ from that of design patterns. Strikingly large acceleration induced diaphragm force demands are found to not only occur at the top floor, but also at the first-floor level, indicating unexpected damage along the height may be induced and this may result in unacceptable failure in the structure.