On the Development of Seismic Design Forces for Flexible Floor Diaphragms in Reinforced Concrete Wall Buildings

In the analysis and design of buildings, concrete floors are generally considered to be infinitely rigid in their plane. This idealisation affords the simplification of computer models used for analysis, as well as the overall seismic design of the vertical lateral force resisting elements. However, the poor performance of a multitude of buildings in recent earthquakes revealed that the rigid diaphragm assumption may not be appropriate for the design of certain floor geometries and floor systems. In fact, the damage and collapse of several buildings was largely attributed to deficient diaphragm designs. Precast concrete floor systems were observed to be quite flexible; and measured floor accelerations were several times larger than those prescribed by building codes for the design of diaphragms.
These findings led to the development of improved floor acceleration estimation methods by researchers. However, these methods are only applicable to either elastically responding structures, or those modelled with rigid diaphragms. Noting that all diaphragms are flexible to some degree, and that this flexibility can lead to “unconventional” force and displacement patterns in a building during seismic excitation, there still exists a need to thoroughly characterise the seismic response of buildings with very flexible diaphragms for their proper design.
A comprehensive study on the seismic response of flexible diaphragm RC wall buildings was undertaken. An extensive number of non-linear dynamic response simulations were performed on buildings of various heights that incorporated diaphragms with differing degrees of in-plane flexibility. The intensity level of the seismic input was incrementally increased, resulting in clear trends between the wall displacement ductility demands and the recorded diaphragm inertia forces.
The results show that a definitive relationship exists between the level of ductility demand in the lateral system and the peak floor accelerations. It was found that wall ductility demand plays a significant role in the recorded accelerations for the upper floor levels of a building, but not those of the lower levels. Furthermore, inertia force patterns in flexible diaphragm buildings differ greatly from those found in an analogous rigid diaphragm structure. These findings were used to develop a diaphragm inertia force estimation method that accounts for the dynamic properties of the building and diaphragms, as well as the ductility demand in the structure. The method was evaluated through application to arbitrarily designed RC wall buildings and was found to provide excellent design force envelopes for the design of flexible floor diaphragms.