In-plane capacity of existing post-WWII beam-and-clay block floor systems

A growing attention has been paid to the deep renovation of RC buildings, particularly focusing on their structural vulnerability and on the development of retrofit strategies; however, the issue of the in-plane diaphragm action and the capacity of existing floors has rarely been addressed. Although floor capacity does not seem critical for the seismic capac- ity of existing structures, commonly affected by greater vulnerabilities, it may become crit- ical when an additional lateral force resisting system is introduced. This paper investigates the in-plane capacity of beam and hollow-clay-block floor system, typical of the European post-WWII RC buildings. Considering the diaphragm action as associated with an in-plane tied-arch mechanism developing within the floor thickness, the main failure mechanisms are discussed, and some simplified equations are provided to preliminary estimate the max- imum capacity of floors. Experimental and numerical analyses are than carried out to vali- date the simplified analytical model. The relevant influence of possible staircase openings on the in-plane load paths and on diaphragm flexibility and capacity are also considered. Finally, the influence of the floor capacity on the seismic vulnerability assessment and in the conceptual design of a seismic retrofit intervention is discussed. This preliminary study shows that only some of the beam-and-block floor systems have a reliable in-plane capac- ity, while other typologies cannot serve as floor diaphragms. When the diaphragm action can be relied upon, the diaphragms often exhibit a fairly stiff behaviour up to a brittle fail- ure, which is commonly associated with the ultimate capacity at the tied-arch supports