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

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.

The LCT Challenge: Defining New Design Objectives to Increase the Sustainability of Building Retrofit Interventions

“…A rotational spring (k θ = 320,000 kNm) was introduced at the base of the walls with a view to represent the existing foundation system flexibility. Finally, the floors were modelled as rigid diaphragms, but considering a finite ultimate capacity, which was calculated assuming a tied arch resistant mechanism according to Marini et al [71] (in this case, the maximum allowed in plane floor action is f = 108.4 kN/m, corresponding to the development of a brittle shear failure at the joist/block interface in correspondence to the arch supports).…”

“…In addition, to ensure a proper seismic resistance function of the exoskeleton, diaphragms play a critical role. The maximum in-plane capacity of the existing floors in the retrofitted configuration was thus calculated to assess the potential need for their strengthening [71]; the two building units A and B were analyzed separately, given that their slabs are set at different heights. Strengthening of the floors proved to be necessary for the diaphragm action to be ensured; to this end, perimetral steel chords and additional ties in correspondence to the staircase walls are needed.…”

Design of diagrid exoskeletons for the retrofit of existing RC buildings

“…The ultimate shear resistance (VMax; ξMax, Figure 11b), and the characteristic points of the flexural curvecracking (Mcr; φcr), yielding (My; φy), ultimate (Mu; φu), and residual (Mres; φres) (Figure 11a) -were calculated based on the formulations suggested from European [18] building codes. The building floors were assumed to withstand horizontal loads by developing an in-plane tied-arch (or strut and tie, [30]) resistant mechanism up to their ultimate capacity [31,32]. The maximum actions in the diaphragm were assessed to be smaller than the maximum capacity of the existing floors.…”

“…where 𝜏 "#$%& = 1.74𝑀𝑃𝑎 is the ultimate shear resistance of the brick/joists system, which was determined in an experimental campaign [23], l=9.35m is the height of the floor and t=40mm is the height of the RC slab [31].…”

Pin-supported walls as seismic retrofit for existing RC frames: performance improvement through link removal

“…This allows reaching a higher serviceability level: in fact, a global elastic behaviour may be obtained for higher intensity earthquakes compared to the as-is condition, therefore leading to a damage reduction. It should be noted that an overall strength increase may also lead to higher loads on the floor diaphragms which need to be carefully assessed (Bull 2004;Rodriguez et al 2007;Tena-Colunga et al 2015;Marini et al 2022). However, the authors preliminarily investigated not only the beneficial effects, but also the weaknesses of PS wall solutions when applied in the retrofit of existing RC frames (Casprini et al 2019).…”