Nonlinear Behavior of a Masonry Subassemblage Before and After Strengthening with Inorganic Matrix-Grid Composites F. Parisi 1 ; G. P. Lignola 2 ; N. Augenti 3 ; A. Prota 4 ; and G. Manfredi 5 Abstract: Past experimental tests on a full-scale masonry wall with an opening evidenced the key role of the spandrel panel in the in-plane nonlinear response of the system. Recent seismic codes do not provide specific criteria to assess and to strengthen existing masonry spandrel panels with inorganic matrix-grid (IMG) composites. Numerical finite-element (FE) analyses are used to deepen the knowledge about the nonlinear response of masonry walls and the role of the IMG strengthening system. The comparison of experimental and numerical results contributes to the development of a simplified analytical model to assess the influence of the external reinforcement system on the in-plane seismic response of masonry wall systems. Some hints about the strengthening design that could change the failure mode from brittle shear to ductile flexure are given. Finally, a further enhancement of the IMG strengthening system is proposed to avoid the undesirable splitting phenomena attributable to compression forces and to exploit the full compressive strength of masonry against bending moments. DOI: 10.1061/(ASCE)CC.1943-5614.0000203. © 2011 American Society of Civil Engineers. CE Database subject headings: Masonry; Walls; Rehabilitation; Composite materials; Finite element method; Nonlinear analysis; Seismic design; Full-scale tests. Author keywords: Masonry; Walls; Rehabilitation; Composite materials; Finite element method; Nonlinear analysis; Seismic design; Full-scale tests. Introduction Masonry constructions are widespread in the building stock of earthquake-prone regions with medium to high hazard levels. Nevertheless, in many cases, they have been designed for gravity loads only without any engineering measure for earthquake resis- tance. For this reason, seismic risk reduction programs have been promoted, at both regional and global levels, to evaluate and reduce the vulnerability of masonry constructions. Particular emphasis has been paid to masonry buildings placed within historic urban cen- ters, whose structural system is typically composed of unreinforced masonry (URM) load-bearing walls and flexible diaphragms. The latter consist of masonry vaults at the lower floors, and wooden, steel, or mixed floor systems at the upper floors. Because existing masonry buildings frequently do not have good connections among walls (e.g., RC ring beams, or steel/wood ties) or rigid floor diaphragms, a compact ‘box-type’ global re- sponse cannot be developed. As a result, earthquake resistance primarily depends on the seismic response of individual walls to in-plane and out-of-plane excitations. The nonlinear behavior of masonry walls under in-plane hori- zontal actions is typically evaluated by simplified macroelement methods in which each wall with openings is meshed into the fol- lowing types of two-dimensional structural members, for example, by extending the contour lines of the openings (Magenes and Della Fontana 1998; Brencich et al. 1998; Parisi 2010): (1) “pier panels, ” which are the vertical components; (2) “spandrel panels, ” which are the horizontal components; and (3) “joint panels, ” which link piers and spandrels together. A set of pier and joint panels along the same vertical alignment is currently referred to as a “pier, ” whereas a group of joint and spandrel panels along the same horizontal align- ment is referred to as a “spandrel. ” Strength and displacement capacities of piers have been investigated thoroughly both exper- imentally and theoretically (among others: Magenes and Calvi 1997; Calderini et al. 2009) and also in the presence of strengthen- ing systems composed of composite materials (Valluzzi et al. 2002; Lignola et al. 2009). Conversely, knowledge about the nonlinear behavior of spandrels is unavailable. From a general viewpoint, any masonry panel can fail in sliding shear, diagonal cracking, or flexure, depending on its material prop- erties, height/width ratio, and boundary conditions (Tomaževič 2000). Sliding shear may occur along a masonry bed joint, or along a diagonal of the panel, involving both masonry units (i.e., bricks, blocks, or stones) and joints. Diagonal cracking takes place if the principal tensile stress at the centroid of the panel reaches the 1 Ph.D. Candidate in Seismic Risk, Dept. of Structural Engineering, Univ. of Naples Federico II, Via Claudio 21, P.O. Box I-80125, Naples, Italy (corresponding author). E-mail: fulvio.parisi@unina.it 2 Assistant Professor of Structural Engineering, Dept. of Structural Engineering, Univ. of Naples Federico II, Via Claudio 21, P.O. Box I-80125, Naples, Italy. E-mail: glignola@unina.it 3 Associate Professor of Structural Engineering, Dept. of Structural Engineering, Univ. of Naples Federico II, Via Claudio 21, P.O. Box I-80125, Naples, Italy. E-mail: augenti@unina.it 4 Assistant Professor of Structural Engineering, Dept. of Structural Engineering, Univ. of Naples Federico II, Via Claudio 21, P.O. Box I-80125, Naples, Italy. E-mail: aprota@unina.it 5 Full Professor of Structural Engineering, Dept. of Structural Engineer- ing, Univ. of Naples Federico II, Via Claudio 21, P.O. Box I-80125, Naples, Italy. E-mail: gamanfre@unina.it Note. This manuscript was submitted on September 27, 2010; approved on January 7, 2011; published online on January 10, 2011. Discussion per- iod open until March 1, 2012; separate discussions must be submitted for individual papers. This paper is part of the Journal of Composites for Con- struction, Vol. 15, No. 5, October 1, 2011. ©ASCE, ISSN 1090-0268/ 2011/5-821–832/$25.00. JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / SEPTEMBER/OCTOBER 2011 / 821 Downloaded 02 Nov 2011 to 143.225.144.89. Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org