INFLUENCE OF DIAPHRAGM FLEXIBILITY ON RESPONSE MODIFICATION FACTOR

Response modification factors are used in building seismic design to reduce design forces safely to economic levels based on the structures ability to deform and dissipate energy. The relationship between the response modification factor (R), structural ductility (μ) and structural period (T), known as the R-μ-T relationship, has been investigated by many researchers. In this past research, the R-μ-T relationship was studied using single-degree-of-freedom models representing rigid diaphragm structures. However, certain classes of buildings have flexible floor or roof diaphragms. The influence of diaphragm flexibility on the R-μ-T relationship has not been studied extensively. In this paper, diaphragm flexibility is considered in generating the R-μ-T relationship. A two-degree-of-freedom model was created by splitting the mass tributary to the lateral force resisting system and the mass tributary to the diaphragm. An elastic spring is used to connect the lateral force resisting system and the diaphragm, which representing the diaphragm flexibility. The R-μ-T relationships were then developed using nonlinear time history analyses for different diaphragm stiffness. Thirty ground motions recorded from historical far-field earthquakes on soil class D were used in the analysis. It has been found that the diaphragm flexibility tends to increase the structural ductility demand at a given R for short period buildings. For intermediate to long period buildings, the influence of diaphragm flexibility varies. The structural ductility demand can either increase or first increase and then decrease as the diaphragm flexibility increases, depending on the magnitude of R.