Rapid Visual Screening method for vertically irregular buildings based on Seismic Vulnerability Indicator

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Abstract

Seismic vulnerability assessment of a large group of buildings can prove to be extremely difficult as it requires huge amount of time and resources. Rapid Visual Screening (RVS) alleviates this problem by classifying buildings into different classes of vulnerability thereby limiting the need for detailed analysis only to a few selected vulnerability classes. RVS survey of five G+1 houses were conducted using Arya et al. method, Sinha et al. method, Jain et al. method, FEMA Level 1 and FEMA Level 2 method. The lack of details regarding irregularities resulted in the same RVS score for buildings with significantly different configurations. This renders the RVS procedure difficult to be used for comparison. This paper provides a qualitative comparison of seismic vulnerability among vertically irregular buildings in terms of mass, geometric and stiffness irregularity through Rapid Visual Screening. 3D Response spectrum analysis of reinforced concrete buildings have been done using STAAD Pro software and Seismic Vulnerability Indicator (SVI) values are calculated for comparison of seismic vulnerabilities with respect to vertical irregularities. Mass irregularity near the mid height of building or stiffness irregularity below the mid height of building is found to be more critical among the possible locations of irregularity. Elevation asymmetry above mid height is found to decrease the seismic vulnerability of a building. Presence of stiffness irregularity is found to be significantly more critical than mass or geometric irregularity.

Introduction

Several earthquakes have caused widespread damage in the mainland of India over the centuries. The major ones among them in recorded history are Great Assam earthquake (1897), Kangra earthquake (1905), Bihar-Nepal earthquake (1934), Assam earthquake (1950), Bihar Nepal earthquake (1988), Uttarkashi earthquake (1991), Killari earthquake (1993), Jabalpur earthquake (1997), Chamoli earthquake (1999) and Bhuj earthquake (2001) [1]. The frequent occurrence of earthquakes and the damage data indicate the need for proper seismic vulnerability assessment to safeguard the structures from damage by suitable retrofitting and other mitigation measures. This ensures the safety of occupants and minimizes the economic losses incurred by both individuals and the government. Many of the old buildings across India even if constructed as per the relevant standards at the time do not follow the specifications in the new updates of the codes. Even today in several parts of the country, people do not give proper attention to the requirement of seismic design and are unaware of its consequences. This is clearly evident from the aftermath of Bhuj earthquake (2001). The newly constructed concrete buildings in Ahmedabad could not stand the tremor even though it was located around 200 km from the epicentre. Even after 14 years when Nepal earthquake hit, the situation was no different from earlier. The constructions in the Patna town of Bihar suffered severe damage even though the epicentre was 283 km away.

Hence it is of utmost interest to predict the performance of a building in the likely event of an earthquake. Seismic vulnerability assessment methods play a vital role in this regard. It can be used to understand the vulnerability profile of existing constructions [2]. This would help the authorities to take up retrofitting measures to ensure the safety of structures against damage in case of minor earthquakes and collapse in case of major earthquakes [3]. The main hurdle in conducting seismic vulnerability assessment is the huge number of building stock present across the country. The most accurate method would be to gather all the details of the building and perform structural analysis. But this would be difficult because the as built drawings of most of the existing buildings will not be available and even if available, it requires skilled professionals and is also time consuming to do the analysis. The modelling of buildings, analysis considering the load combinations and interpretation of results require a huge amount of time and resources rendering such a seismic analysis impossible. Therefore it is required to classify the buildings into broad categories which require analysis and do not require analysis. This would limit the seismic analysis procedures to be conducted on a select few buildings and would lead to considerable savings both in terms of time and resources.

Rapid Visual Screening (RVS) is a method which assists in classifying the buildings into those that require detailed investigation and those that do not [4]. It helps us in evaluating the resistance of a building against earthquake without performing any structural calculations. In this method, a visual inspection of the buildings is done and the structural features are assessed within around 30 min and an accompanying form is filled out [4]. This form assigns a positive score or negative score to different features present in the building according to whether that particular feature contributes to increase or decrease in seismic resistance. The final score is then compared against a cut-off score which marks the distinction between buildings which require analysis and those that do not.

This paper proposes suitable suggestions to assist in Rapid Visual Screening (RVS) method to study the seismic vulnerability of vertically irregular buildings. The current RVS methods in India does not provide for a comparison within vertically irregular buildings [2,3,5]. All vertically irregular buildings are treated alike. Vertical irregularities consist of mass irregularity, geometric irregularity, stiffness irregularity, strength irregularity and irregularity due to in-plane discontinuity. The contribution to seismic vulnerability depends upon the nature and extent of irregularity. Thus the effect of variation of vertical irregularity on the seismic vulnerability of the building need to be considered.

In the current study, a reference regular building is modelled and vertical irregularities are introduced in it to obtain a set of irregular models. 3D Response spectrum analyses are conducted using STAAD Pro software to study the seismic response behaviour of the irregular models. Seismic Vulnerability Indicator (SVI) as proposed by Bhosale et al. [6] is calculated which is used to quantify the degree of seismic vulnerability of the building due to vertical irregularity. A chart for qualitative comparison of effect of different types of vertical irregularity on the seismic vulnerability and their most critical location has been developed based on the SVI values.

The scopes of this paper are to conduct RVS survey in Kharagpur Municipality to observe the limitations of current RVS methods, to study the effect of vertical irregularities on the seismic vulnerability of buildings and to propose qualitative comparison of seismic vulnerability of vertically irregular buildings to assist in Rapid Visual Screening (RVS) survey.

Section snippets

Methodology

Fig. 1 describes the research methodology. RVS survey was conducted to study the limitations of existing RVS methodologies. A comparison of different types of vertical irregularities is not found. Seismic Vulnerability Indicator (SVI) is found to be a suitable index to compare different vertical irregularities from the literature [6]. Study is conducted in terms of mass, geometric and stiffness irregularities and suggestions are proposed to assist in the Rapid Visual Screening of buildings.

Rapid Visual Screening

RVS method was first developed by Applied Technology Council (ATC) in late 1980s. Later Federal Emergency Management Agency (FEMA), an agency of the US Department of Homeland Security released its RVS methodology in 1988 which was later updated in 2002 and 2015 [4]. Various RVS forms have been developed by researchers, institutes and building codes considering variations in the construction materials and construction technology adopted in different parts of the world. In India, RVS forms

RVS survey

Kharagpur is a city in Paschim Medinipur district of West Bengal, India located at coordinates 22° 19′ 48.86″ N and 87° 19′ 25.15″ E. It is governed by Municipal Authority and consists of 35 administrative wards covering a total area of about 127 km2 with a population of 207,604 as per 2011 census data. It is the fifth most populated and the fourth largest city of West Bengal. G+1 houses made of reinforced concrete are common in the newly developed regions in the inner parts of the city. Along

Preliminary analysis

A 4 storey symmetric building with storey height of 3 m is selected as the reference regular building with 3 bays of 3 m each in both length and width direction (Fig. 3a). The details of loading, material and member dimensions are summarised in Table 4. For dynamic analysis, only 25% of live load is considered as per Table 10, IS 1893 (Part 1): 2016 [10].

Irregularities are introduced in the reference regular building and responses are obtained. The three dimensional building is simplified to a

Seismic Vulnerability Indicator

Vertical irregularity indices help in comparing the irregularity of one building with another. Various vertical irregularity indices have been developed by researchers over the years. Karavasilis et al. (2008) [11] proposed irregularity indices for storey-wise and bay-wise stepping in steel buildings. Sarkar et al. (2010) [12] developed regularity index (η) based on first mode participation factor to quantify the irregularity of stepped buildings. The irregularity index proposed by Varadharajan

Detailed analysis

The reference regular buildings considered are reinforced concrete buildings designed as per IS 456 and IS 1893 (Part 1): 2016 considering the extreme seismic zone of India. Irregularities are introduced in the reference regular buildings to obtain the irregular models. Seismic design has not been done for the irregular models since the paper aims to study the adverse effect of irregularity. Three dimensional dynamic analysis is conducted in the linear elastic range using STAAD Pro software by

Discussions

The current RVS methods are found to be unable to draw a comparison between vertically irregular buildings. The preliminary analyses clearly pointed out that the variation in type and amount of vertical irregularities affects the dynamic response of a building. Thus the details regarding the nature of vertical irregularities have to be considered in the RVS methods. As per the RVS methodology by A.S. Arya, all the buildings surveyed got a score of G1. As per the other methodologies considered,

Conclusions

The RVS survey conducted in Kharagpur Municipality highlighted the issue of same score being obtained for significantly different configurations of buildings. The existing RVS methods in India only consider the presence or absence of irregularities and does not address the issue of comparison of vulnerability among different types of irregularities. The current study develops a standalone qualitative classification of buildings with respect to vertical irregularities. This provides the screener

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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