Abstract
A large part of the building stock in seismic-prone areas worldwide are masonry structures that have been designed without seismic design considerations. Proper seismic assessment of such structures is quite a challenge, particularly so if their response well into the inelastic range, up to local or global failure, has to be predicted, as typically required in fragility analysis. A critical issue in this respect is the absence of rigid diaphragm action (due to the presence of relatively flexible floors), which renders particularly cumbersome the application of popular and convenient nonlinear analysis methods like the static pushover analysis. These issues are addressed in this paper that focusses on a masonry building representative of Southern European practice, which is analysed in both its pristine condition and after applying retrofitting schemes typical of those implemented in pre-earthquake strengthening programmes. Nonlinear behaviour is evaluated using dynamic response-history analysis, which is found to be more effective and even easier to apply in this type of building wherein critical modes are of a local nature, due to the absence of diaphragm action. Fragility curves are then derived for both the initial and the strengthened building, exploring alternative definitions of seismic damage states, including some proposals originating from recent international research programmes.
Highlights
Analysis of unreinforced masonry (URM) buildings for seismic assessment purposes is typically carried out using the nonlinear static method involving planar (2D) and, less often, 3D models
Fig. (9) shows typical hysteresis loops recorded at different earthquake intensities; the responses shown in the figure correspond to damage states 1 and 2, whereas an example of DS4 was shown in Fig. (3)
This should not be interpreted only as an indication of the inefficiency of this technique, and as a result of the limitation of the nonlinear model used that does not account for failure mechanisms such as splitting at the intersection of orthogonal walls, at the corners of the building, which are sometimes observed in older URM buildings
Summary
Analysis of unreinforced masonry (URM) buildings for seismic assessment purposes is typically carried out using the nonlinear static (pushover) method involving planar (2D) and, less often, 3D models. The high computational cost and high analytical skills required for implementing continuum or discrete finite element models, make equivalent frame models the most popular choice for practical nonlinear analysis of URM structures. An evaluation of equivalent frame models for linear and nonlinear analysis of URM buildings and some comparisons with continuum models can be found in [1]. This method has sometimes been supplemented by limit analysis involving simplified models of critical components of the structure, aiming at the estimation of the ultimate load that can be sustained for a postulated collapse mechanism. Nonlinear dynamic response history analysis has been a far less popular tool in the case of URM structures, due to the absence of appropriate models, software, and expertise, not so much in carrying out the analysis but in interpreting its results, which are both voluminous and difficult to translate into global damage description
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