Abstract
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 capacity of existing structures, commonly affected by greater vulnerabilities, it may become critical 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 maximum capacity of floors. Experimental and numerical analyses are than carried out to validate 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 capacity, 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 failure, which is commonly associated with the ultimate capacity at the tied-arch supports.
Highlights
About half of European reinforced concrete (RC) buildings were built in the aftermath of the WWII in the lack of seismic regulations
For the retrofit of RC structures, great attention has been paid to structural vulnerability assessment and many seismic retrofit solutions have been proposed and validated throughout the years; the issue of the in-plane capacity of existing floors has rarely been addressed
The floor in-plane capacity may become critical in two cases: (1) the retrofit intervention increases the total stiffness of the building and no additional damping is provided, leading to a reduction of the structure fundamental period and to an increase of the seismic actions transferred across the diaphragm; (2) the span of the resisting tied-arch increases up to the span between the elements of the new and stiffer lateral force resisting system, leading to higher internal actions and higher reaction forces at the tied-arch supports
Summary
About half of European RC buildings were built in the aftermath of the WWII in the lack of seismic regulations. These systems do not seem critical for the capacity of the existing structures, which are often affected by vulnerabilities associated with the LFRS vertical elements, they may become critical when an additional stiff LFRS is provided to the structure This issue is relevant when retrofit interventions are applied from the outside of the building by means of structural exoskeletons, suitable to avoid inhabitants’ relocation and building downtime (Marini et al 2017; Passoni et al 2020). Experimental results are implemented into a bidimensional nonlinear finite element model of the diaphragm, considering the floor in the as-is and in two different retrofit configurations (i.e. resulting in different diaphragm span to width ratios— adding 2 or 4 transversal shear walls) and subjected to a uniform distribution of inertial forces These analyses allow to validate the proposed diaphragm internal load paths and the associated diaphragm ultimate capacity for varying the mechanical properties of the system and for varying the retrofit configurations. The implications of such study for the design of seismic retrofit interventions, especially when carried out from outside, are discussed
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