Evaluation of Different Approaches to Estimate Seismic Input Energy and Top Displacement Demand of Moment Resisting Frames

Energy-based seismic design (EBSD) has many advantages over the conventional design approaches (force- and displacement-based) provided by the current seismic design codes since it accounts for duration, frequency content and pulse-type of the ground motion. Cumulative damage potential of the earthquake excitation is also taken into consideration by this approach. In order to obtain a satisfactory design, energy capacity of a structure should exceed the energy demand from an earthquake in EBSD. The structural damage indicators like inelastic top displacement demand (δ_top) of MDOF system, which can be used as a crucial input data for general nonlinear static analysis procedures, are claimed to be accurately predicted using the input energy. Primarily, seismic input energy per unit mass (E_I/m) imparted into a MDOF frame system during an earthquake is determined by only using modal properties of the system and input energy time series of earthquake ground motion on its equivalent SDOF systems. Using the determined seismic input energy per unit mass, δ_top is predicted by literature equations. Effects of spectral matching on the success of seismic input energy and inelastic displacement demand estimations were also investigated. Evaluation of the predictions of δ_top was achieved by comparing with the results of nonlinear time history analyses (NLTHA) on two distinct three-story moment resisting frames. It was observed that the relative differences between analyses results and the predicted values were calculated as 18% (input energy prediction) and 30% (top displacement prediction) for the original records whereas they were 12 and 20% for the spectrally matched ground motions.