Low-rise Unreinforced Masonry Buildings Research Articles

Damage survey and seismic vulnerability assessment of unreinforced masonry structures in low-intensity Ambasa earthquake of northeast India

Unreinforced masonry (URM) buildings in many developing countries are susceptible to severe damage during earthquakes as these are designed and constructed without considering the earthquake risk of that region. Many low-rise URM buildings in Tripura, a northeast state of India, have suffered substantial damage during the recent low-intensity Ambasa earthquake (Mw 5.7). In this context, a damage survey was conducted at first to assess the cause behind the damaged URM buildings. Thereafter, a publicly funded three-storied URM building in Kumarghat, Tripura, has been chosen as a typical representative case for the vulnerability assessment. The Equivalent Frame Modelling approach has been used to model the building in SAP2000 with three different sets of material properties. The Incremental Dynamic Analysis is performed considering seven ground motions to obtain the damage measures for fragility analysis. The visual observations and numerical results match well for the damaged building. In fact, the modelling and analysis technique seems to predict the reasonably accurate damage scenario within a limited computational effect. This opened up a possibility for assessing seismic of masonry buildings on a regular basis which was otherwise a challenge. It has been found that the use of poor quality materials is the primary reason behind the damage to the buildings in such earthquakes. The derived fragility curves also show that with the deterioration of material quality, the probability of suffering moderate to extensive damage increased significantly at the PGA level of 0.052 g that was recorded during the earthquake. Hence, this paper may help the practising engineer to understand several cruxes of vulnerability involved in the design and construction of structures. Further, the method summarized in this study may be extended for the vulnerability assessment of all the building typologies of the earthquake-prone areas of developing countries.

Seismic analysis of in-plane loaded walls in unreinforced masonry buildings with flexible diaphragms

It is well recognised that the dynamic response of unreinforced masonry buildings with flexible timber diaphragms typically contains multiple dominant modes associated with the excitations of the diaphragms and the in-plane walls. Existing linear analysis methods for this type of structure commonly account for the multi-mode behaviour by assuming the independent vibrations of the in-plane loaded walls (in-plane walls) and the diaphragms. Specifically, the in-plane walls are considered to be rigid and the unmodified ground motion is assumed to be transmitted up the walls to the diaphragm ends. While this assumption may be appropriate for many low-rise unreinforced masonry buildings, neglecting the dynamic interaction between the diaphragms and the in-plane walls can lead to unreliable predictions of seismic demands. An alternative analysis approach is proposed in this paper, based on the mode properties of a system in which (1) the mass ratios between the diaphragms and the in-plane wall are the same at all levels, and (2) the periods of the diaphragms are the same at all levels. It is proposed that under these conditions, two modes are typically sufficient to obtain the peak seismic demands of the in-plane walls in elastically responding low-rise regular buildings. The applicability of the two-mode analysis approach is assessed for more general diaphragm configurations by sensitivity analysis, and the limitations are identified. The two-mode approach is then used to derive a response modification factor, which may be used in conjunction with a linear static procedure in the seismic assessment of buildings with flexible diaphragms.

A comparison of two regional seismic damage estimation methodologies

This paper presents a comparison of the two main regional damage estimation methodologies currently in use, namely the modified Mercalli intensity (MMI) based approach and the spectral parameter based approach. In the first methodology, expected damage is related to ground shaking intensity in terms of MMI through damage probability matrices. In the second methodology, the ground motion intensity is described in terms of spectral acceleration (SA), and building response in terms of spectral displacement (SD). Both methodologies were applied to buildings in Vancouver of three different construction types: single-family wood-frame houses, low-rise unreinforced masonry buildings, and high-rise concrete frame structures with concrete shear walls. The two methodologies predict damage that lies in the same general damage categories of light and moderate, which are defined by fairly broad ranges in mean damage factors. The specific mean damage factors predicted by the two methods for a given location are significantly different, however. The significant differences in mean damage factors imply significant differences in damage costs and hence in seismic risk.Key words: earthquake, damage, seismic risk, vulnerability, modified Mercalli intensity (MMI), spectral response, displacement, acceleration.