Identification of material parameters for low bond strength masonry V. Sarhosis ⇑ , Y. Sheng School of Civil Engineering, University of Leeds, LS2 9JT, Leeds, UK article info Article history: Received 29 May 2013 Revised 3 December 2013 Accepted 11 December 2013 Keywords: Material parameter identification Brickwork masonry Discrete Element modeling Optimization abstract This paper investigates the parameter estimation problem for masonry constitutive models and describes the development of a computational model for low bond strength masonry. The conventional method of obtaining material parameters from the results of small sample tests was thought to be problematic. Instead, material parameters were obtained from the results from the load testing of large clay brick low bond strength masonry wall panels in the laboratory. Initially, the panels were modeled computa- tionally using UDEC software and an assumed set of material parameters. The differences between the results obtained experimentally and computationally were minimized using an optimization technique. The model was then used with the optimized parameters to predict the structural response of another wall panel that had been tested in the laboratory. Good correlation was obtained with the experimental results. The general procedure of the material parameter identification method and the numerical results are presented. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Many forms of masonry construction exhibit low bond strength characteristics. With the term low bond strength masonry, we de- fine masonry where the bond at the masonry unit/mortar joint interface is sufficiently low to have a dominant effect on the mechanical behavior such as the formation of cracks, re-distribu- tion of stresses after cracking and the formation of collapse mech- anisms. Modern mortars used for new construction tend to be cementitious with a relatively high cement content. As a result, the strength of the bond at the interface between the masonry units and the mortar joints tends to be much higher than is the case with other mortars. Low bond strength masonry may encoun- tered: (a) in historic constructions where lime mortar were mainly used; (b) masonry arch bridges, tunnels linings and earth retaining walls where unit/mortar joint bond has been disrupted by the ac- tion of water leeching through the masonry; and (c) in more recent examples of masonry constructed with low cement content mortar due to lack of quality control on site. Engineers often need the use of relatively sophisticated numerical or analytical methods in order to predict the in-service behavior and load carrying capacity of such structures and inform repair or strengthening decisions. However, any numerical or analytical model of analysis requires some forms of constitutive model to simulate the mechanical re- sponse of structures under various loading conditions. Constitutive models also require a number of input material parameters to be identified in order to characterize the behavior of the masonry. In the last few years, with the development of the sophisticated numerical models, the number of parameters for material models has increased significantly [1]. It is common practise to determine such material parameters from the results of various, relatively simple, small scale laboratory experiments. However, masonry is a highly variable, stress-state type dependant material which experiences non-uniform distributions of stress in real structures. Furthermore, the boundary conditions in small scale tests are un- likely to be representative of those exist in a larger structure [2,3]. These have cast doubts on the effectiveness of determining material parameters that are representative of masonry from small scale experiments. The aim of this study is to investigate the material parameter identification problem and develop a computational model for low bond strength masonry. Given the importance of the masonry unit-mortar interface on the structural behavior of low bond strength masonry [4–6], the micro-modeling approach based on the Discrete Element method of analysis have been used. The soft- ware used was the Universal Distinct Element Code (UDEC). The material parameters for the masonry constitutive model were ob- tained from the results of tests carried out in the laboratory on sin- gle leaf wall panels, each containing a large opening. Each panel was subjected to a gradually increasing vertical in-plane static point load until it collapsed. The wall panel tests were also mod- eled using UDEC. The load to cause first visible cracking, the ulti- mate load and the load versus mid-span displacement relationship obtained from the laboratory tests are compared with the computational predictions obtained from DEM modeling. An optimization procedure was then used to tune the parameters used 0141-0296/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engstruct.2013.12.013 ⇑ Corresponding author. Address: School of Engineering, Cardiff University, CF24 3AA Cardiff, Wales, UK. Tel.: +44 7725071212. E-mail address: SarhosisV@cardiff.ac.uk (V. Sarhosis). Engineering Structures 60 (2014) 100–110 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct