Theoretical and experimental analysis of dissipative buckling restrained barces

Buckling restrained braces are passive energy dissipators used for seismic protection of building frames; such devices consist of slender steel bars connected usually to the frame to be protected either like conventional (concentric) diagonal braces or like chevron braces. Under horizontal seismic motions, the interstory drifts generate axial strains in the steel bars beyond their yielding points; such tension-compression cycles constitute the hysteresis loops. The buckling of the steel bars (core) is prevented by embedding them in a stockiest encasing; it consists usually of a steel tube filled with mortar. A crucial issue is to allow sliding between the core and the encasing to prevent relevant shear stress transfer. This work aims to contribute to a better understanding of the behavior of buckling restrained braces; the final objective is to foster its mass use in developing countries (in earthquake prone regions), particularly for reinforced concrete building frames. The research approach consists of designing, producing and testing (in Argentina) five reduced scale dissipators (about 400 mm long) and of taking profit of the gained experience to design, to produce and to test (in Spain) four full size (near 3000 mm long) prototype devices. All these tests are individual, i.e. no subassemblies (accounting for the building frames) are considered. The main conclusion is that it is possible to obtain a reasonably cheap (about 1000 US$ per unit; this amount corresponds to production in Spain (summer 2006) without optimizing the fabrication process), efficient, robust, low maintenance and durable prototype device requiring only a low-tech production process (suitable for developing countries). Moreover, the results show that the fatigue life of buckling restrained braces, even highly uncertain, can be significantly bigger than expected (according to some previously published results); it might allow extending the life of these devices after a number of strong seismic inputs. A numerical analysis of the buckling behavior of these devices is performed; it allows formulating some design recommendations. Further research needs are identified

Experiments on reinforced brick masonry vaulted light roofs

This paper describes structural tests of thin vaults made of reinforced brick masonry. The experiments consist of concentrated loading tests of 14 full-scale laboratory vaults. These vaults are designed to include common situations such as short- to midspan length, low-mid-high rise, rigid-flexible-sliding supports, instantaneous-sustained loading, low-high strength mortar, point-line loading, central-eccentric loading, point-line supports, hinged-clamped supports, symmetric-asymmetric shape, double layer versus single layer reinforcement, and uniaxial-biaxial bending, among others. The tests mainly aim to obtain the collapse loads and to characterize the pre- and post-peak response. The results show satisfactory structural performance, both in terms of ductility and strength. Moreover, it is possible to predict the structural response with numerical models developed specifically for this purpose. Flat specimens were also tested to determine the punching shear strength of the vaults. This work is part of a larger research project aimed at promoting innovative semi-prefabrication techniques for reinforced brick masonry vaulted light roofs

Numerical model of the hysteretic behavior of connections between waffle slabs and façade supports

Waffle-flat-plate structures have been widely used for residential and office buildings in the South of Europe between the seventies and nineties of the last century. These buildings were designed with old seismic codes and its vulnerability needs to be evaluated to determine whether they need to be seismic retrofitted or not. This article proposes a simple model that represents the moment transfer between plate and column, for existing exterior connections loaded in the direction normal to the building façade. The model takes into account singularities of existing waffle-flat-plate structures such as the concentration of reinforcement in the direction of the joints, and the existence of punching shear reinforcement. The model consists of frame elements for the columns and shell elements for the slabs, both connected with flexural and torsion hinges. The strength of the flexural hinges is obtained by estimating the portion of longitudinal reinforcement of the plate that transfers directly the unbalanced moment to the column by flexure. The yielding deformations of the flexural and torsion hinges are estimated with simple expressions. The ductility of the flexural hinges is taken from FEMA 356, while infinite ductility is adopted for the torsional hinges in accordance with previous experimental studies. It is shown that the proposed model provides a good approximation of the experimental response of the connection under monotonically increasing lateral displacements, in terms of stiffness, strength and ductility