Guido Magenes

In-plane seismic response of brick masonry walls

The paper addresses the problems of evaluation of strength, deformability, and energy dissipation capacity of unreinforced brick masonry walls, within the context of seismic assessment of existing buildings. Possible approaches to simplified strength evaluation are discussed on the basis of experimental and numerical data, and formulae for assessment are presented. The role of the shear ratio in the shear failure mechanisms is put in evidence and shear strength formulae are proposed accordingly. The most significative parameters regarding deformability under cyclic loading are highlighted and energy dissipation due to hysteretic behaviour is quantified for possible use in dynamic models. Experimental results show how ultimate drift seems to be a parameter with high regularity for walls failing in shear. Based on such result, a possible approach for seismic assessment is outlined.

EVALUATION OF OUT-OF-PLANE STABILITY OF UNREINFORCED MASONRY WALLS SUBJECTED TO SEISMIC EXCITATION

The paper presents a systematic assessment of a simplified procedure to evaluate the response of unreinforced masonry walls subjected to out-of-plane seismic excitation. The nonlinear force-displacement response of a wall is idealised by means of a suitable tri-linear curve. The meaningful parameters characterising the walls and different ground motions were combined for a total of 1248 case studies. For each combination of para-meters, a nonlinear SDOF dynamic time-history analysis was performed, and the results were taken as the reference for a simplified “equivalent stiffness” approach. It is shown how a suitably accurate prediction of collapse can be made by using appropriate stiffness values and elastic response spectra. Among the most relevant results for applications, it appears that initial stiffness (and therefore initial period) is not crucial in determining the occurrence of collapse. Instead, collapse depends primarily on the second and third branches of the tri-linear force-displacement relationship, i.e. on maximum strength and ultimate displacement capacity. It is shown how these latter parameters axe only mode-rately sensitive to material mechanical parameters which are usually affected by strong uncertainty when assessing an existing building, namely the elastic modulus E and the compressive strength of masonry.

Relevance of beam-column joint damage and collapse in RC frame assessment

The role of joint damage and collapse in the seismic response assessment of existing reinforced concrete frame buildings is herein investigated. Based on recent results from experimental investigations on frame system and subassemblies designed for gravity-load-only, considerations on structural performance based on hybrid local and global failure mechanisms related to joint damage are provided, with particular attention to displacement demand, interstorey drift and damage distribution. Effects of bond deterioration and slip of reinforcing bars passing through an interior joint are discussed in terms of local hierarchy of strength and sequence of events. A simple analytical model for joint response is proposed and adopted for preliminary investigations on frame systems with substandard structural details. The occurrence of a “shear hinge” in the joint might protect to some extent soft-storey mechanisms, reducing the interstorey drift demand, with no significant effects on the global displacement demand. On the other hand, typical inadequacies of structural details (i.e. end-hook anchorage in beam bars combined with use of smooth bars) might cause severe strength degradation leading to particularly brittle failure mechanism.

Testing of masonry structures for seismic assessment

The experimental evaluation of strength, deformability, and energy dissipation capacity of unreinforced masonry buildings subjected to seismic loadings presents unique and complex problems, both for laboratory and field evaluations. The paper addresses these problems, focusing on the relative merits and roles of several experimental techniques, including quasistatic, dynamic, and pseudodynamic loadings at full and reduced scale.

The demise of the URM building stock in Christchurch during the 2010-2011 Canterbury earthquake sequence

The progressive damage and subsequent demolition of unreinforced masonry (URM) buildings arising from the Canterbury earthquake sequence is reported. A dataset was compiled of all URM buildings located within the Christchurch CBD, including information on location, building characteristics, and damage levels after each major earthquake in this sequence. A general description of the overall damage and the hazard to both building occupants and to nearby pedestrians due to debris falling from URM buildings is presented with several case study buildings used to describe the accumulation of damage over the earthquake sequence. The benefit of seismic improvement techniques that had been installed to URM buildings is shown by the reduced damage ratios reported for increased levels of retrofit. Demolition statistics for URM buildings in the Christchurch CBD are also reported and discussed.

Damage Control for Clay Masonry Infills in the Design of RC Frame Structures

The damage to non structural masonry infills in RC frame structures induced by earthquake ground motions represents a considerable source of economic losses and a serious threat to human lives. Although measures for the prevention of infill damage are to some extent included in modern seismic design codes, an effective design procedure has not yet been achieved. Hence, the objective of this research is to identify, through numerical investigations and a review of previous experimental findings, the performance of masonry infills in RC frame structures designed following European code provisions, with reference to different design parameters, including building height, level of design seismic loading, ductility class, masonry typology, and infill density. The introduction of possible improvements to the current design procedure is envisaged, with the aim of achieving enhanced infill damage control through the limitation of inter-story drifts. In particular, as the first part of a wider research program, the study is focused on the behavior of traditional unreinforced clay masonry infill typologies, with masonry panels constructed in complete contact with the surrounding RC frame, since such construction techniques are still being widely used in European seismic regions.

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.

Seismic Performance of Autoclaved Aerated Concrete (AAC) Masonry: From Experimental Testing of the In-Plane Capacity of Walls to Building Response Simulation

The need to assess the seismic performance of autoclaved aerated concrete (AAC) masonry arose in different countries in the last years. The use of AAC for load-bearing walls is quite common in low seismicity areas in Central and Northern Europe, where its thermal insulation properties, together with lightness and workability, are particularly appreciated. Increasing attention to energy-efficient buildings is now supporting the adoption of a material with such characteristics also in higher seismicity regions. Hence, in order to correctly study the seismic performance of this constructive system, the in-plane response of unreinforced AAC masonry panels has been assessed through an experimental test campaign aiming at obtaining a reliable description of the lateral cyclic behavior. The experimental results are summarized in the article and the derived essential seismic design parameters are presented. The test results allowed the calibration of a macro-element model representative of the nonlinear response of single piers, simulating their cyclic experimental behavior. Three-dimensional models of unreinforced AAC masonry buildings were then obtained using the TREMURI program. Their seismic performance assessment has been carried out through both a nonlinear static (pushover) procedure and nonlinear dynamic time history analyses. Nevertheless, the obtained results allow for some preliminary considerations on the global response of this type of construction and its potential for application in moderate and high seismicity countries.

Dry Stone Masonry Walls in Bending—Part I: Static Tests

A series of static 1:5-scale tests on dry stone masonry walls were performed with the main objective to verify existing analytical expressions for computation of resistance to horizontal loading proportional to the self-weight of masonry. The resistance is expressed by means of a collapse load multiplier, and the experiments were conceived to reproduce various out-of-plane bending failure mechanisms using an inclined plane machine built with steel profiles and with an aluminum platform. The collapse multiplier was calculated from the inclination angle of the platform that caused collapse. Since the walls were constructed without mortar, the shear strength along the joints was given purely by friction. The “bricks” were cut from marble—the selection criteria being the accuracy for the cutting, the hardness and durability of the material, and an appropriate friction coefficient. In total, 42 configurations were tested, varying the length of the walls, the presence and position of openings, the staggering ratio, the quality of the connection between walls, the existence of vertical overburden loads in the walls, and the number of stories. The test results are presented in terms of the measured collapse load multipliers and detailed descriptions of the associated collapse mechanisms.

Review of Out-of-Plane Seismic Assessment Techniques Applied To Existing Masonry Buildings

ABSTRACT Observations after strong earthquakes show that out-of-plane failure of unreinforced masonry elements probably constitutes the most serious life-safety hazard for this type of construction. Existing unreinforced masonry buildings tend to be more vulnerable than new buildings, not only because they have been designed to little or no seismic loading requirements, but also because connections among load-bearing walls and with horizontal structures are not always adequate. Consequently, several types of mechanisms can be activated due to separation from the rest of the construction. Even when connections are effective, out-of-plane failure can be induced by excessive vertical and/or horizontal slenderness of walls (length/thickness ratio). The awareness of such vulnerability has encouraged research in the field, which is summarized in this article. An outline of past research on force-based and displacement-based assessment is given and their translation into international codes is summarized. Strong and weak points of codified assessment procedures are presented through a comparison with parametric nonlinear dynamic analyses of three recurring out-of-plane mechanisms. The assessment strategies are marked by substantial scatter, which can be reduced through an energy-based assessment.