“All-steel” buckling-restrained braces for seismic upgrading of existing reinforced concrete buildings

Abstract

Buckling-Restrained Braces (BRBs) are a relatively recent development in the field of seismic resistant steel structures. Respect to classic concentric braces (CCBs), BRBs can be considered a much more efficient structural system for resisting lateral forces due to earthquakes because (i) they provide complete truss action, (ii) they exhibit symmetric load-deformation behaviour and large energy absorption capacity. Results of an experimental test on a real two-story reinforced concrete (RC) building equipped with BRBs and CCBs are presented and discussed. The tested BRBs are special ‘all-steel’ devices, which have been specifically designed for seismic upgrading of RC buildings, without interference with functions and aesthetics of the building. Indeed, the main characteristic of the braces consists in the possibility to hide them within the space between the facing and the backing of masonry infill walls commonly used for claddings of RC buildings. possibility to close the building (or part of it) for the duration of the retrofit work, or having to heavily reinforce existing framing due to the increased seismic demands the retrofit strategy may place on it. The use of BRBs in such structural typology may be a rational, cheap and safe option. Experimental tests on a real RC building equipped with BRBs are shown in this paper. The building has been subjected to a cyclic loading history which resembles the one typically used to experimentally validate BRBs in the laboratory. In order to highlight the benefit induced by BRBs the RC building has been contemporaneously equipped with the novel BRBs and CCBs. 2 EXPERIMENTAL ACTIVITY This study started from the exceptional opportunity to carry out experimental tests in the inelastic range of response on a real RC building (Figure 1), which is located in Bagnoli (Naples, Italy), within the area of dismantled Italian steel mill ILVA, and was destined to demolition by competent Authority. Figure 1. The tested RC structure in its original condition. The building was built at the beginning of 1980s. It is rectangular in plan (18.50 × 12.00 m), on two floors, with first and second floor heights equal to 4.60 m and 8.95 m (Figure 2). This building has initially been tested two times, in the original conditions and after some repairing. In particular, it was tested by lateral loading in the Y-direction (Fig. 2a) up to severe damage of both structural frame members and infill walls. Lateral loads have been applied according to an inverted triangular distribution. Both tests showed the formation of a weak story at the first floor. Detailed information about the first two tests will be found in Della Corte et al., 2008. After these tests, the structure has been partially repaired and two BRB systems have been designed. In particular, BRBs were designed to be hidden in the inner hole of facing walls as shown in Figure 2a where the braces (indicated with the dashed lines) are inserted into the two perimetric bays on the short building side. Figure 2b shows the brace configuration, with the BRBs mounted only at the first floor. Y X 18,50 4,00 4,25 4,20 4,55 0,30 4,30 4,55 4,50 4,85 18,50 4,00 4,30 3,45 3,75 2,00 2,30 2,70 3,00 4,55 4,85