Understanding the complexities of designing diaphragms in buildings for earthquakes

Abstract

The variety of layouts of lateral force resisting elements in structures, subjected to inelastic behaviour, make the design of diaphragms significantly more complex than the traditional "simple beam" approach typically employed. Traditionally held views that diaphragms are inherently robustness and hence do not require significant engineering input have been shown to be inappropriate by recent major earthquakes and recent laboratory studies. The simple beam method, at times, fails to recognise that the traditional load paths assumed are compromised by localised damage in the floors (diaphragms) due to incompatibility of deformation between the floors and the supporting structures (walls, beam and columns). "Strut and tie" methods are suggested as a means of tying these diaphragms into the lateral force resisting structures and as a way of dealing irregular floor plates and penetrations (stairs, lifts, atriums) through the floors. The focus of recent research in determining the seismic lateral forces into and through floor diaphragms has been on the magnitude of the floor inertias. However, it has been shown that primary structural elements interacting through the diaphragm, can cause stresses in the floors many more times than those of the inertia effects. These two sources oferees and stresses are interrelated. This relationship requires further study. Survivability of a building rests, in part, on the diaphragms tying the structure together. Suggestions for maintaining the integrity of the diaphragms, particularly in precast concrete types, are discussed.