Alessandro Galasco

Shaking Table Test of a Strengthened Full-Scale Stone Masonry Building with Flexible Diaphragms

An extensive experimental campaign has been conducted at EUCENTRE to understand the dynamic behavior of historic stone masonry structures and evaluate the seismic performance of selected strengthening strategies, aimed at improving wall-to-floor connections and in-plane diaphragm stiffness. Shaking table tests were performed of full-scale two-storey buildings in undressed double-leaf stone masonry with timber floor and roof. A first prototype (Building 1), representing a vulnerable building without antiseismic detailing and devices, was tested showing a response characterized by in-plane distortion of the flexible diaphragms and local out-of-plane failure mechanisms. In Building 2 the wall-to-diaphragm connections were improved, providing only a moderate in-plane stiffening of the wooden diaphragms. When subjected to shake-table testing, the strengthened building showed a global type of structural response without the occurrence of out-of-plane mechanisms. In the present paper the strengthening interventions on Building 2 are described, and the results obtained during the dynamic tests are illustrated, focusing on the response of the structure, the evolution of damage mechanisms during the tests, in comparison to the seismic performance of the first unstrengthened reference prototype response. The improvement of the connections proved to be very effective, increasing significantly the seismic capacity of Building 2 with respect to Building 1.

The Effect of Stiffened Floor and Roof Diaphragms on the Experimental Seismic Response of a Full-Scale Unreinforced Stone Masonry Building

An extensive experimental program was carried out at EUCENTRE, within a research project on the evaluation and reduction of the seismic vulnerability of stone masonry structures. The main part of the experimental program has been devoted to the shaking table tests on three full-scale, two-story, single-room prototype buildings made of undressed double-leaf stone masonry. The first building tested was representative of existing unreinforced stone masonry structures with flexible wooden diaphragms, without any specific anti-seismic design nor detailing. In the second and third buildings, strengthening interventions were simulated on structures theoretically identical to the first one, improving wall-to-floor and wall-to-roof connections and increasing diaphragm stiffness. In particular, in the third specimen, steel and r.c. ring beams were used to improve the diaphragm connection to the walls and collaborating r.c. slab and multi-layer plywood panels were used to stiffen floor and roof diaphragms, respectively. This article describes the strengthening interventions applied to the third building prototype and presents the experimental results obtained during the shaking table tests. The results obtained permitted the calibration of a macroelement model representative of the nonlinear behavior of the structure.

Numerical Simulation of Shaking Table Tests on Full-Scale Stone Masonry Buildings

ABSTRACT Based on the outcomes of a recent experimental project addressing unreinforced stone masonry undertaken at the European Centre for Training and Research in Earthquake Engineering, the seismic response exhibited by two full-scale building prototypes during shaking table tests was simulated according to an existing equivalent frame modeling approach involving nonlinear macroelements. Given the use of different strengthening solutions, the two building specimens strongly differed in the in-plane stiffness of their timber floors and roof diaphragms. This article addresses several issues concerning numerical modeling of the seismic response of this type of masonry construction, particularly its effect upon assessing the global response of the discretization and geometry of the equivalent frame model and upon definition of model parameters based on tests of material characteristics and lateral response of structural members. Even in the case of flexible diaphragms, the results of pushover analysis of the calibrated models provided a fair approximation in terms of both envelope curve and damage pattern. The results of time history analysis accounting for cumulative damage indicate good simulation in terms of hysteretic response as well.

A MACRO-ELEMENT MODEL FOR THE NONLINEAR ANALYSIS OF MASONRY MEMBERS INCLUDING SECOND ORDER EFFECTS

Abstract. The second order effects are often neglected in the evaluation of the global seismic response of masonry buildings. Pier in-plane response is often governed by shear failure modes, particularly in case of rather squat panels and relatively high compression. Shear failure is usually associated with a limited deformation capacity which makes acceptable an approach based on the undeformed geometry of the structure. In case of structural members with a prevailing bending behaviour, the displacement capacity can be significantly increased and second order effects may no longer be negligible. Indeed, experimental tests on panels with a marked rocking behaviour show a gradual reduction of lateral strength for increasing lateral displacements, also in the absence of significant toe-crushing effects. The 2-node macro-element model implemented in the TREMURI computer program concentrates in the two extremities the coupled axial and flexural behaviour allowing for a rigorous representation of flexural cracking and toe-crushing effects. The shear response is concentrated in the central body of the element and the interaction between shear and bending-rocking behaviours is allowed by two internal degrees of freedom. The kinematic formulation of the macroelement and in particular the presence of the additional internal degrees of freedom easily allowed the introduction of P-delta effects in the nonlinear element response. Numerical results obtained with the upgraded macro-element model showed a very good agreement with experimental results. The new feature implemented in the macro-element model makes its use in the equivalent frame modelling further reliable and opens new perspectives and applications in other fields such as the analysis of rigid or quasi-rigid block systems.

RINTC PROJECT: NONLINEAR DYNAMIC ANALYSES OF ITALIAN CODE-CONFORMING URM BUILDINGS FOR COLLAPSE RISK ASSESSMENT

Abstract. This paper deals with the computation of the collapse risk of new masonry buildings designed according to the Italian Building Code. Companion papers describe the overall EUCENTRE-ReLUIS joint research project, funded by the Italian Department of Civil Protection (DPC), which considers different building types (r.c., steel buildings, etc) and uses multi-stripe nonlinear dynamic analyses by properly selected ground motion records. 2and 3-storey unreinforced masonry buildings have been designed in cities with increasing seismic hazard, considering two different soil conditions at each site. First, the paper describes geometry, material characteristics (clay block masonry) and main structural details of the buildings, discussing the effect of different design methods (rules for simple buildings, linear and nonlinear static analysis) and models (cantilever or equivalent-frame models). The models used for the assessment by nonlinear dynamic analyses are equivalent-frame models made by masonry piers and spandrels, as well as reinforced concrete members. Two alternative macroelement models are used for the in-plane response of masonry members. Out-of-plane failure modes are assumed to be prevented by the presence of ring beams and limited slenderness of masonry walls. Pushover analyses are used to esti-mate the EDP (maximum interstorey drift ratio) threshold for the collapse limit state. Finally, the results of the multi-stripe analyses are presented for 10 different earthquake’s return periods.

Towards the Use of Time-History Analysis for the Seismic Assessment of Masonry Structures

Despite being recognized as the most accurate analysis technique for the design and assessment of masonry structures, nonlinear dynamic analysis is not commonly used in the everyday engineering practice. Reasons for this can be found in the difficulties in the selection of appropriate input ground motion records, in the limited availability of computer programs allowing the performance of time history analysis, especially for the case of masonry structures, and in the issues related with interpretation of the results in terms of performance limits. Real records are well known to be a preferable choice with respect to artificial or synthetic ground motions, but the limited availability of real records often requires scaling them, with all the concerns associated with this operation. Also, a proper selection of seismic input requires some level of expertise, which is not so common in the professional field. Regarding numerical modelling of masonry buildings, an analysis tool capable of reproducing both global seismic response and local mechanisms would be the preferable option. Existing equivalent frame models including suitable nonlinear macro-elements representative of the behaviour of structural members allow performing time-history analyses of the global response of complete 3D building models. A modified macro-element model accounting for second order effects can be suitably adopted for the analysis of local failure modes, which are mainly associated with bending-rocking behaviour and out-of-plane wall response.

Seismic Vulnerability of Old Italian Railway Stations

Most of Italian railway stations date back to the second half of the 19th century, when the railway network was created. Although originally belonging to different companies and in spite of some modifications occurred over the years, most of these stations are masonry buildings conceived according to similar standards, with typical dimensions and morphology mostly depending on the expected traffic of passengers and goods. The common structural features of these buildings allow grouping them into building typologies, characterized by a similar structural response. This is relevant for the assessment of the seismic vulnerability of these structures, whose damage or collapse could lead to critical consequences, due to the potentially high number of victims and to the risk of interruption of critical lines. Nonlinear global analyses by equivalent-frame macroelement modelling and linear kinematic analyses of possible local failure modes have been used to assess the seismic response of building prototypes representative of the most common architectural typologies. A logic tree approach was used to consider uncertainties in mechanical properties, geometry and construction details for the derivation of fragility curves for different limit states.