Study on Reliability Based Seismic Resistance Design of Open Ground Storey for Framed Structure

 

Thanniru Malyadri

Assistant Professor, Sree Dattha Institute of Engineering and Science, Hyderabad

*Corresponding Author Email: malyadri.137@gmail.com

 

ABSTRACT:

Open Ground Storey (OGS) framed buildings in which the ground  storey is kept open without providing any infill walls and mainly used for parking, are increasingly common in urban areas. Vulnerability of this type of buildings has been exposed in the past earthquakes.  OGS buildings are conventionally designed considering a bare frame analysis, ignoring the stiffness of the infill walls present in the upper storey's,  which under-estimates the inter-storey drift and the force demand  in the ground storey columns. To compensate this, a multiplication factor (MF) is introduced by various international codes while calculating the design forces (bending moments and shear forces) in the ground storey columns. Present study focuses on the evaluation of seismic performances of OGS buildings designed with alternative MFs through performance-based design approach using a probabilistic framework. The probabilistic seismic demand models and corresponding fragility curves for all the selected OGS buildings are developed for different performance levels. Reliability curves are developed for the OGS building frames against the seismic hazard associated with maximum seismic zone of India (Zone-V of IS 1893, 2002). Similar analyses are also carried out on bare frames and fully infilled frames for reference. It is found from the present study that the application of MF only in ground storey, as suggested by many literatures and design codes (including Indian standards), is not an appropriate solution for design of OGS buildings as it leads to vulnerable adjacent storey. This study proposes an effective scheme of MF for design of OGS buildings that yields acceptable levels of reliability index.

 

KEYWORDS:

 


INTRODUCTION:

Proper utilisation of space has become a major concern in developing countries like India due to rapid urbanisation and population growth. As a result, multi-storey residential buildings in urban areas are forced to have parking in the ground floor. In such framed buildings, the ground storey is generally built without any infill walls to allow easy movement of vehicles but the upper storeys are covered with infill walls. This type of framed building is referred as ‘open ground storey (OGS) building’ in this study. Fig. 1.1 presents a typical OGS building.

 

Fig 1 A typical OGS building                                                                                      Fig 2 OGS Effected Building

 

Although this type of OGS buildings has many functional advantages, they possess a potentially dangerous type of vertical irregularity. The sudden reduction in lateral stiffness and strength of the ground storey in OGS building results in large lateral displacements in ground storey level, which increases the curvature and force in the ground storey columns. The collapse of this type of buildings is predominantly due to the formation of soft-storey mechanism in the ground storey columns. Past earthquakes have demonstrated the vulnerability of OGS buildings. A number of OGS framed buildings have experienced severe damage during the 2001 Bhuj earthquake (Fig. 1.2).

 

OBJECTIVES:

Based on the literature review presented in Chapter 2, the main objective of the present study has been identified as to propose suitable schemes of MF for seismic design of OGS buildings considering a desired degree of reliability. To achieve this objective the problem is being divided into different parts with following sub-objectives

i) To establish limit state capacities of each storey of framed building for various performance levels.

ii) To develop probabilistic seismic demand model (PSDM) and fragility curves for benchmark OGS framed buildings designed with various schemes of MF.

iii) To develop reliability index for OGS framed buildings designed with various schemes of MF.

iv) To propose appropriate schemes of MF for the design of OGS buildings based on the reliability indices achieved by the benchmark frames.

 

SCOPE OF THE STUDY:

i) The present study is limited to framed buildings up to eight-storey designed as per prevailing BIS.

ii) The present study is limited to OGS reinforced concrete multi-storey frames that are regular in plan. Hence, representative plane frames are used in the present study. The plan asymmetry arising from possible irregular distribution of infill walls are not considered in the analysis.

iii) The infill walls are assumed to be non-integral with the surrounding frames.

iv) Out-of-plane action of masonry walls is not considered in the study.

v) Uncertainties in structural properties and loading are considered as applicable to Indian context.vi) The present study uses an equivalent single strut approach based on recent studies (Celarec et al., 2012) for modelling infill walls.

vii) Random variables considered in the present study (concrete strength, steel strength, infill strength and damping ratio) are assumed to be uncorrelated.

 

METHODOLOGY:

The methodology worked out to achieve the above-mentioned objectives is shown in Fig.1.3 through a flowchart. Step by step methodology is presented as follows:

i) To review the existing literature and different international design code provision on the design of OGS buildings.

ii) To select benchmark building frames ranging from 2-8 storey and design them considering different schemes of MFs.

iii) To develop computational model of selected frames to perform Pushover analysis (POA) and Nonlinear time history analysis (NTHA).

iv) Estimate the limit state capacities of different storeys of selected frames at each performance levels

v) Develop Probabilistic Seismic Demand Models (PSDMs), fragility curves and reliability indices for the selected frames

vi) Select the appropriate scheme of MF for design of OGS buildings that yields acceptable levels of reliability index.

 

Table 1 Characterization of soft-storey building as per international design codes

 

Table 2. characterization of weak-storey building as per international design codes

 

Indian Standard IS 1893 has been revised in 2002 to include new recommendations for the design of OGS buildings.

 

Table 3. MFs recommended by international codes/literatures

 

METHOD FOLLOWED EARTHQUAKE RISK ASSESSMENT:

The methodology reported by Ellingwood (2001) for estimation of seismic risk involves three parts. First part is the identification of the seismic hazard, P [A = a], described by the annual probability of occurrence of specific levels of earthquake motion. The seismic hazard at a site is usually represented through a seismic hazard curve, GA(x) which is a plot of P [A = a] versus the level peak earthquake acceleration (a) expressed in terms of gravitational acceleration (g). Second part is the analysis of global response of the structural system subjected to different levels of earthquake motions. The response analyses of the structure are carried out by conducting NTHA for different earthquakes, and the response is expressed in terms of maximum inter- storey drift at any storey. Third part is the calculation of limit state probabilities of attaining a series of (increasingly

severe) limit states, LSi, through the Eq. (3.1).

 

A point estimate of the limit state probability of state ‘i’ can be obtained by convolving the fragility FR(x) with the derivative of the seismic hazard curve, GA(x), thus removing the conditioning on acceleration as per above equation.

 

This shows fragility-hazard interface, identifying dominant contribution to the risk. The parameters of the fragility-hazard interface must be dimensionally consistent for the probability estimate to be meaningful.

 

Fig 3 Fragility-hazard interface, identifying dominant contribution to the risk (Elingwood, 2001)

 

Fig. 4 Reliability curve generation using NTHA

 

CONCLUSION:

1) The main objective of the present study has been identified as to propose suitable scheme

of MF for seismic design of OGS buildings considering possible uncertainties. The sub-objectives are divided into the following parts:

i) To establish limit state capacities of each storey of framed building for various performance levels.

ii) To develop probabilistic seismic demand model (PSDM) and fragility curves for benchmark OGS framed buildings designed with various schemes of MF.

iii) To develop reliability index for OGS framed buildings designed with various schemes of MF.

iv) To propose appropriate schemes of MF for design of OGS buildings based on the reliability indices achieved by the benchmark frames.

To achieve the above objectives, an extensive literature review is carried out on following three areas:

(i)    existing design methodologies for OGS buildings as per various international codes and literatures,

(ii)  fragility curves and reliability analysis on RC framed buildings and

(iii) macro-models available in literature for modelling infill walls.

It is observed that the existing design codes and the literature have not adequately addressed the problem of earthquake-resistant design of OGS buildings. Major international design codes (ASCE/SEI-7, 2010; NZS 1170.5, 2004and ICC IBC, 2012) prohibit the construction of such buildings. However, the developing countries like India cannot avoid such type of building due to the scarcity of land in the urban areas. Other international codes (IS 1893, Eurocode 8, SI, Bulgarian code, etc.) allows this building category with a magnification of design forces (MF) in the ground storey columns. There is a wide disparity among these codes on the value of the MF. From the fragility curves and achieved reliability indices of the benchmark frames developed in this study the following generalised conclusions can be drawn:

i) OGS frames designed without any MF always found to have maximum probability of exceedance indicating vulnerability of these frames.

ii) In case of two storey frames, the application of MF only in ground storey columns improves the building performance. However, for building with more than two storeys, application of MF only in the ground storey makes the adjacent storey vulnerable. This shows that the scheme of MF applying in ground storey alone recommended by most of the international codes is not an effective solution.

iii) In general, an MF of magnitude less than 2.0 does not meet the acceptable degree of reliability.

iv) It is found that the application of MF in the increasing order does not necessarily improve the performance of the buildings beyond certain limits.

v) It is found that in case of two storey buildings MF applied only in ground storey meets the target reliability, similarly MF in ground and first storey for four storey buildings, MF in ground, first and second for six storey buildings and MF in ground, first, second and third storey for eight storey buildings meets the target reliability.

vi) It is found that the scheme of uniform value and the scheme of different values of MF in the different storeys results in similar performance.

vii) Based on the discussions presented in Section 6.8, following schemes of MF are proposed for design of OGS Building.

 

SCOPE OF FUTURE WORK:

The present study is limited to reinforced concrete multi-storey framed buildings that are regular in plan. Irregular distribution of infill walls in the upper storeys of OGS building can lead to plan irregularity. This study can be extended to such buildings considering the torsional effects arising out of the plan irregularity. Also, similar studies can be carried out on steel framed buildings. The present study can be extended to OGS buildings with basement, shear walls and plinth beams.

 

Soil-structure interaction effects are ignored in the present study. It will be interesting to study the response of the OGS buildings considering the soil-structure interaction. The floor slabs are considered in the present study as rigid diaphragms. This study can be extended for buildings with flexible diaphragms. Full scale shake table tests can be conducted for further clarity on the responses of OGS buildings subjected to lateral loading.

 

REFERENCES:

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2.     ASCE/SEI 41-06 (2007). Seismic Rehabilitation of Existing Buildings, American Society of Civil Engineers, USA.

3.     ASCE/SEI 7 (2010). Minimum Design Loads for Buildings and Other Structures, American Society of Civil engineers, Reston, VA.

4.     Bracci, J. M., Reinhorn, A. M. and Mander, J. B. (1995). Seismic resistance of reinforced concrete frame structures designed for gravity loads: Performance of structural system, ACI Structural Journal 92(5):597–609.

5.     Bulgarian Seismic Code (1987). Code for design of buildings and structures in seismic regions, Bulgarian Academy of Science Committee of Territorial and Town System at the Council of Ministers, Sofia, Bulgaria.

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10.   ICC IBC (2012). International Building Code, International Code Consortium, USA.

11.   IS 13920 (1993). Ductile detailing of reinforced concrete structures subjected to seismic forces - code of practice, Bureau of Indian Standards, New Delhi.

12.   IS 1893 Part I (2002). Indian standard criteria for earthquake resistant design of structures, Part – 1: General provisions and buildings, Fifth Revision, Bureau of Indian Standards, New Delhi.

13.   IS 456 (2000). Plain and reinforced concrete - code of practice, Fourth Revision,

14.   Janardhana, M. (2010). Cyclic behaviour of glass fiber reinforced gypsum wall panels, Ph.D. Thesis, Department of Civil Engineering, IIT Madras, Chennai, India. JCSS, (2001). JCSS Probabilistic Model Code, Zurich, Joint Committee on Structural Safety.

 

 

 

Received on 14.11.2015            Accepted on 20.12.2015           

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Int. J. Tech. 5(2): July-Dec., 2015; Page 235-239

DOI: 10.5958/2231-3915.2015.00026.7