Vulnerability models cannot be applied to every building in a region as the structural and other details for each of them can vary significantly and may not be easily available. The vulnerability assessment thus should be done for a building representative of a building stock that has been classified and characterized according to some homogenous behavior. A common terminology or taxonomy is critical to document variations in building design and construction practices around the world. However, extrapolation of the results obtained on a single building to an entire building stock has the drawback of not being able to exactly quantify the variability associated with the response of each building in a stock due to uncertainty of building parameters.
2.1 Structural Inventory for Seismic Vulnerability Assessment
Exposed building stock classification and its damage description is essential for risk analysis. In seismic risk assessment, estimation of earthquake hazard, structural vulnerability and exposure of building stock are the three equally important components, out of which, the development of inventory databases is the most difficult aspect of damage prediction. Vulnerability of a building mainly depends on the form and quality of construction of the main vertical load-bearing elements. For instance, a building with unreinforced masonry walls can be expected to be much more vulnerable than a timber frame building. Therefore, preparation of a building typology is necessary for seismic vulnerability assessment. Usually the following parameters are considered for classifying the buildings-structural material, structural system, number of stories, age, function, etc.
However, since in most cases the entire set of necessary data may not be available, reference is made to both the surveyed data by rapid visual screening (RVS) and to the data available from literature about the building stocks. Hundreds of building typologies from different countries are documented in the literature. Comprehensive classification of buildings is available in: MSK-64 Scale (Medvedev and Sponheuer 1969), ATC 13 (1985), European Macroseismic Scale (Grünthal 1998), FEMA 154 (2002), HAZUS (2013), Spence et al. (1992), Cardona and Yamin (1997), Eleftheriadou and Karabinis (2011), PAGER global inventory (Jaiswal et al. 2010), and Global Earthquake Model (Brzev et al.
2013), FEMA P-58 (2012).
The existing building stock of Indian cities is a rich mix of several different building types and construction technologies. In India, there exists a considerable unreinforced masonry building stock that has to be evaluated in terms of seismic safety considerations. In addition, there are also a vast number of masonry infilled RC buildings in seismically active urban and semi-urban areas. Consideration of masonry infill walls in RC buildings as non-structural elements has led to informal use and treatment of masonry walls in such buildings. This, in addition to several functional reasons, has resulted in uneven placement of masonry infill walls in RC buildings leading to creation of irregular buildings. For example, open ground storey (OGS) buildings in which masonry walls are provided in all the stories except the ground storey. Such buildings perform poorly during earthquakes, and therefore, it is essential to assess the seismic vulnerability of irregular reinforced concrete buildings with masonry infill walls. Seismic behavior and performance of such buildings in the past earthquakes is reviewed in this section. Building inventory data for other types of buildings is not reviewed in the thesis.
Masonry infill reinforced concrete buildings are commonly constructed in many other countries also. Although infills contribute large lateral strength and stiffness to the building, their influence depends greatly on their distribution in the building frame.
Providing infills only in the upper stories of a building (commonly known as open ground storey building), renders the ground storey relatively more flexible and weaker compared to the upper stories. Generally, open ground storey columns lack adequate ductility capacity, stiffness, and strength needed to resist the high demand of storey shear. This generally leads to an undesirable column-sway failure mechanism in OGS buildings subjected to strong earthquake excitations in which plastic hinges are mainly formed in the columns of the open ground storey. In contrast, the infills restrain the deformation of the upper storey, and thus, little damage is incurred in the upper stories (Murty et al.
2006, Kaushik and Jain 2007, Kaushik and Dasgupta 2013, Yuen and Kuang 2015).
Considering this fact, past researchers (Moghaddam and Dowling 1987; Dolšek and Fajfar 2001; Chintanapakdee and Chopra 2004; Kaushik et al. 2006, Kaushik et al.
2009; Özhendekci and Özhendekci 2012; Al-Nimry et al. 2015; Rai et al. 2017) have reported the significant ‘negative’ influence of OGS on the overall seismic performance of such buildings. Previous experiences from earthquakes all over the world also reveal the fact that such buildings performed quite poorly even during moderate shaking, and in some cases, complete collapse of the ground storey columns was observed (Kaushik and Jain 2007; Dutta et al. 2015). Fig. 2.2 shows two such OGS buildings collapsed during 2004 Sumatra and 2011 Sikkim earthquake, respectively; both buildings had big openings in infill walls of upper stories but still both collapsed. Davis et al (2010) studied the seismic behavior of OGS frames and found them to be more vulnerable in comparison to fully infilled (FI) frames; in general, fragility increases with increase in number of stories and decrease with increasing number of bays. Chintanapakdee and Chopra (2004) and Mondal and Tesfamariam (2014) observed that stiffness irregularities in ground storey has significant effect on the seismic response (base shear and inter storey drift ratio) of RC frames. On several platforms (for example, NICEE Workshop 2014), various stakeholders have expressed that it is a general assumption in design practice that infills with openings are less effective in imparting lateral strength and stiffness to the OGS frames, and hence, such frames can be analyzed and designed as bare frames. This leads to a common perception that presence of openings in infill walls of OGS frames reduce the seismic fragility of such frames. However, the above mentioned assumption is valid