159 A NEW FORMULATION TO ASSESS THE SEISMIC DEMAND OF MASONRY STRUCTURES BY MEANS OF INPUT ENERGY MEZA J. MIGUEL * , PEÑA FERNANDO † * Instituto de Ingeniería (IdeI) Universidad Nacional Autónoma de México Circuito Escolar, s/n, Ciudad Universitaria, 04510 México D.F., México Email: jmeza.mendez@gmail.com , www.iingen.unam.mx † Instituto de Ingeniería (IdeI) Universidad Nacional Autónoma de México Circuito Escolar, s/n, Ciudad Universitaria, 04510 México D.F., México Email: fpenam@iingen.unam.mx , www.iingen.unam.mx Key words: Input Energy, Spectrum, Seismic demand, Unreinforced masonry. Abstract. The main objective of this paper is to evaluate the elastic input energy of unreinforced masonry structures by means of the input energy spectrum. The energy is a novel approach which allows evaluating in a global and easily way the performance of the masonry structures. Structures modeled with non frame elements require of a great number of 2D or 3D elements, thereby making the calculation of the input energy a complicated issue. In this context, a new formulation that calculates the input energy using an input energy spectrum and the balance of energy is proposed. Two examples of application of unreinforced masonry structures were considered to evaluate the input energy and compare it with the proposed formula. The formula proposed shows interesting results that allowed identify the key features of the accelerograms that influence the input energy into structures. 1 INTRODUCTION There are several methods proposed in the literature to quantify the severity, intensity or earthquake damage potential. Currently, all terms are intended to measure the same property of the ground motion: its effect on the structure. The only observed effect of ground motion on the structure is the permanent damage, so this measure is usually assessed by the degree of correlation with the observed damage. This can lead to a mistake, because the amount of damage depends on the qualities in building on the site. For this reason, macro-seismic scale of Modified Mercalli Intensity is not a reliable measurement of destructiveness potential (Orosco and Alfaro, 2008), though it is very used to describe the distribution of damage to the affected area. A rational correlation between a measure of intensity and observed structural damage could be established only if in the affected site buildings were uniformly designed in accordance with standard building code. Unreinforced masonry buildings are structures with a particularly complex structural behavior. This complexity is given by the mechanical properties of the masonry, specially, by XI International Conference on Computational Plasticity. Fundamentals and Applications COMPLAS XI E. Oñate, D.R.J. Owen, D. Peric and B. Suárez (Eds)