Manual on Seismic Evaluation & Retrofit of Multistory Rc Blds

Manual on

Seismic Evaluation and Retrofit of Multi-storeyed RC Buildings

March, 2005

Retrofit of soft-storeyed building

Sponsored by Department of Science and Technology

Government of India

Indian Institute of Technology Madras Chennai 600 036

Structural Engineering Research Centre Chennai 600 113

SHEARWALL CONCRETEJACKETINGSTEEL

BRACING

Prepared by

Structural Engineering Laboratory

Department of Civil Engineering

Indian Institute of Technology Madras

Chennai 600 036

In collaboration with

Structural Engineering Research Centre

Taramani, Chennai 600 113

Sponsored by

Department of Science and Technology

Government of India

i

PREFACE

The Manual of Seismic Evaluation and Retrofit of Multi-storeyed RC Buildings is the

outcome of a research project, jointly undertaken by IIT Madras and SERC Chennai, with

the sponsorship of the Department of Science and Technology, Government of India.

The purpose of this Manual is to provide a methodology to enable a structural engineer to

assess the seismic vulnerability of existing multi-storeyed buildings in India and to select

suitable methods of retrofit, wherever required and possible. Thus, the Manual has been

organised into two major parts: the first part dealing with seismic evaluation (data

collection, preliminary evaluation and detailed analysis) and the second dealing with

seismic retrofit (global and local retrofit strategies). Various options of seismic retrofit

are possible, and the designer is required to re-analyse the retrofitted structure to ensure

that the desired performance is achieved. Some explanatory examples demonstrating the

prescribed procedure are given in the chapter on case studies. Detailed references are

also cited in this Manual for users interested in further research. Seismic retrofit is still in

a nascent stage, and considerable research and experience with practical real-life

applications is called for.

ii

This Manual is intended primarily for use by the practising engineer, but is also useful for

academic purposes. Some background information on the basic theoretical concepts are

given, but for a full understanding, the user is expected to have a reasonable knowledge

of structural dynamics, earthquake engineering, reinforced concrete design and IS code

requirements. It is also assumed that the user has some exposure to the use of standard

finite element software packages (such as SAP 2000, STAAD Pro, etc.). As part of the

DST sponsored project, a software called SAVE (Seismic Analysis and Vulnerability

Evaluation), has also been developed (as an alternative to existing commercial packages)

and is now made freely available for users of this Manual. Details of SAVE (User

Manual and CD) are given separately, and are not included in the scope of this Manual.

This Manual in its present form represents a consolidation of several studies (theoretical

and experimental) and discussions undertaken by the coordinators of the DST-sponsored

project, which commenced in 2002. As part of the project, as many as 40 sample

buildings located in different parts of India (in Zones III, IV and V) were evaluated,

including the difficult process of data collection and field survey. It is observed from

these case studies that the majority of existing multi-storeyed buildings in India,

particularly residential apartment complexes, fail to meet the current code compliance

requirements and are in danger of damage (of varying degrees) in the event of a

earthquake of expected intensity.

Occupants of multi-storeyed apartment complexes were a worried lot in the aftermath of

the Gujarat earthquake in 2001, but this worry has gradually faded with time, and lessons

have not been learnt. It should not take another disastrous earthquake to make us act

proactively to avoid such disasters. Building owners have a responsibility of getting their

buildings properly evaluated and strengthened, before it is too late.

iii

Unfortunately, there are at present few structural engineers who have the expertise to

assess the seismic vulnerability and suggest appropriate retrofit measures. This Manual

is expected to enhance that number manifold. Workshops and training programmes

related to the use of this Manual are planned for this purpose.

Numerous persons have helped us in preparing this Manual. These include project

associates, Ph.D. and M.S. research scholars, M.Tech. and B.Tech. students, laboratory

technicians and secretarial staff. A list of all the major contributors is given in the

Acknowlegement page. We are also grateful to the Department of Science and

Technology for their funding and encouragement.

IITM SERC Project Team

March 12, 2005.

iv

IITM SERC Project Team

1 Dr. Devdas Menon

Principal Investigator 1 Mr. T S Krishnamoorthy

Principal Investigator 2 Dr. Amlan K Sengupta 2 Dr. N Lakshmanan 3 Dr. V Kalyanaraman 3 Mr. C V Vaidyanathan 4 Dr. A Meher Prasad 4 Dr. K Muthumani 5 Dr. S R Satish Kumar 5 Mr. K Balasubramanian 6 Dr. P Alagusundaramoorthy 6 Dr. K Balaji Rao 7 Mr. V T Badari Narayanan 7 Mr. R Ravichandran 8 Mr. Gnanasekharan 8 Mr. N Gopalakrishnan 9 Mr. Pradip Sarkar 9 Mr. M Manjuprasad 10 Ms S Prathibha 10 Mr. K Satish Kumar 11 Mr. Rajib Chowdhury 11 Dr. B H Bharatkumar 12 Mr. Robin Davis P 12 Ms. P. Kamatchi 13 Dr. S R Uma 13 Ms. R Sreekala 14 Mr. A. Asokan 14 Mr. D Dhiman Basu 15 Mr. G Ravi Kumar 15 Mr. S. Avinash 16 Ms. K N S Susmitha 16 Mr. S Gopalakrishnan 17 Mr. Anand Gupta 18 Mr. Biju Kumar Patir 19 Mr. Lakki Reddy 20 Ms. Praseetha Krishnan 21 Mr. Rajesh Lal 22 Mr. Ramaseshan 23 Mr. Ramesh Pativada 24 Mr. Ravi Chugh 25 Mr. Santosh K Barnwal 26 Mr. Sheshu Reddy 27 Mr. Shiv Shanker 28 Mr. Srinivas, B. 29 Mr. Srinivasulu Reddy


vii

CONTENTS

Preface i

1. INTRODUCTION 1

1.1 Background 1.2 Objective 1.3 Scope 1.4 Methodology

2. PRELIMINARY EVALUATION 7

2.1 Introduction 2.2 Data Collection and Condition Assessment of Building 2.3 Rapid Visual Screening

2.3.1 Scores for a building 2.3.2 Cut-off Score 2.3.3 Building Type Descriptions 2.3.4 Score Modifier

2.4 Quick Checks for Strength and Stiffness 2.4.1 Column Shear 2.4.2 Shear Stress in Shear Wall 2.4.3 Axial Stress in Column 2.4.4 Frame Drift 2.4.5 Strong Column Weak Beam Check

2.5 Evaluation Statements 2.6 Decision for Detailed Evaluation


viii

3. EVALUATION BASED ON LINEAR ANALYSIS 35

3.1 Introduction 3.2 Computational Model

3.2.1 Material properties 3.2.2 Structural element model

3.2.2.1 Beams and columns 3.2.2.2 Beam-column joints 3.2.2.3 Slabs 3.2.2.4 Appendages 3.2.2.5 Walls (structural and non structural)

3.2.3 Modelling of Column Ends at Foundation 3.2.4 Load Combinations

3.3 Linear Analysis Methods 3.3.1 Equivalent static method

3.3.1.1 Centre of mass 3.3.1.2 Centre of rigidity of storey 3.3.1.3 Effect of torsion 3.3.1.4 Seismic weight 3.3.1.5 Lumped mass 3.3.1.6 Calculation of lateral forces

3.3.2 Response spectrum analysis 3.4 Evaluation Results

4. EVALUATION BASED ON NONLINEAR PUSHOVER ANALYSIS 53

4.1 Introduction 4.2 Capacity Spectrum, Demand Spectrum & Performance Point 4.3 Pushover Analysis Procedure

4.3.1 Seismic Load Distribution 4.3.2 Load Deformation Behaviour of Elements

4.4 Performance Based Analysis 4.4.1 Performance Objective 4.4.2 Performance Levels of Structure and Elements 4.4.3 Seismic Hazard Levels 4.4.4 Selection of Performance Objective

4.5 Evaluation Results

5. SEISMIC RETROFIT 63

5.1 Introduction 5.2 Goals of Retrofit 5.3 Definitions


ix

5.4 Steps of Retrofit 5.5 Performance Objectives 5.6 Retrofit Strategies

5.6.1 Global Strategies 5.6.2 Local Strategies 5.6.3 Energy Dissipation and Base Isolation 5.6.4 Mitigating Geological Hazards

6. BUILDING DEFICIENCIES 70 6.1 Introduction

6.2 Global Deficiencies

6.2.1 Plan Irregularities

6.2.2 Vertical Irregularities

6.3 Local Deficiencies 6.3.1 Columns

6.3.2 Beams and Beam-Column Joints

6.3.3 Slabs

6.3.4 Unreinforced Masonry Walls

6.3.5 Precast Elements

6.3.6 Deficient Construction

6.4 Miscellaneous Deficiencies 6.4.1 Deficiencies in Analysis

6.4.2 Lack of Integral Action

6.4.3 Failure of Stair Slab

6.4.4 Pounding of Buildings

6.4.5 Geotechnical Aspects

6.4.6 Inadequate detailing and documentation

7. GLOBAL RETROFIT STRATEGIES 84 7.1 Introduction

7.2 Structural Stiffening 7.2.1 Addition of Infill Walls

7.2.2 Addition of Shear Walls

7.2.3 Addition of Steel Braces

7.3 Reduction of Irregularities

7.4 Reduction of Mass


x

8. LOCAL RETROFIT STRATEGIES 90 8.1 Introduction

8.2 Column Strengthening 8.2.1 Concrete Jacketing

8.2.2 Steel Jacketing

8.2.3 Fibre Reinforced Polymer Wrapping

8.3 Beam Strengthening 8.3.1 Concrete Jacketing

8.3.2 Steel Plating

8.3.3 FRP Wrapping

8.3.4 Use of FRP Bars

8.3.5 External Prestressing

8.4 Beam-Column Joint Strengthening 8.4.1 Concrete Jacketing

8.4.2 Concrete Fillet

8.4.3 Steel Jacketing

8.4.4 Steel Plating

8.4.5 Fibre Reinforce Polymer (FRP) jacketing

8.5 Wall Strengthening

8.6 Footing Strengthening

9. CASE STUDY I 129

10. CASE STUDY II 173

11. CASE STUDY III 211

APPENDIX A: MAPPING OF SOIL TYPE A1

APPENDIX B: MODELLING OF INFILL MASONRY WALL B1

B.1 Modelling of Masonry Infill B.2 Effect of Openings


xi

B.3 Strength of Equivalent Strut B.3.1 Local Crushing Failure B.3.2 Shear Failure

APPENDIX C: MODELLING OF PLASTIC HINGES C1

C.1 Flexural Hinges for Beams and Columns C.1.1 Stress Strain Characteristics of Concrete C.1.2 Stress Strain Characteristics of Steel C.1.3 Moment-curvature Relationship C.1.4 Modelling of Moment-curvature in Confined RC Sections

C.1.4.1 Assumptions C.1.4.2 Numerical Algorithm for Moment-curvature for Beam Sections C.1.4.3 Numerical Algorithm for Moment-curvature for Column Sections

C.1.5 Moment Rotation Parameters C.2 Shear Hinges for Beams and Columns C.3 Axial Hinges for Equivalent Struts

APPENDIX D: VULNERABILITY INDEX D1

APPENDIX E: ADDITION OF STEEL BRACES E1

E.1 Types of Bracing E.2 Connection of Braces to RC Frame E.3 Analysis and Design of Braces E.4 Non-Buckling Braces


1

CHAPTER I

INTRODUCTION

1.1 BACKGROUND

Existing multi-storey buildings in earthquake prone regions of India are vulnerable

to severe damage under earthquakes, as revealed by the recent Gujarat earthquake.

There is urgent need for seismic evaluation and retrofit of deficient buildings.

There are experts in the country who can assist in the seismic evaluation and

retrofit of individual buildings on a case-to-case basis. The magnitude of the work,

however, is so large that it cannot be accomplished by limited number of experts,

and needs involvement of many structural engineers, who are properly trained.

Hence, there is a need to provide appropriate guidelines for seismic evaluation and

retrofit of existing buildings to the vast majority of structural engineers in our

country who lack the expertise. To address this problem, this manual has been

prepared to facilitate seismic evaluation and recommend strategies for retrofitting,

so that the risk of failure is minimised in the event of a future earthquake. This

manual addresses the seismic evaluation of existing RC multi-storey building. The

document is a part of a research project supported by Department of Science and

Technology (DST), Government of India.


2

Indian codes of practice for earthquake resistant design (IS 1893: 2002) and

detailing (IS 13920: 1993) give guidelines to construct new buildings which are

expected to perform adequate in terms of load and deformation capacities. The

existing buildings constructed as per older codes are likely to show inherent

deficiencies and may not meet the demands as estimated by the current codes.

Hence, the task of seismic evaluation involves correlation between the imposed

demand level of earthquake and the expected performance level of building. The

code refers to two levels of earthquakes such as Design Basis Earthquake (DBE)

and Maximum Considered Earthquake (MCE). The concept of seismic design

philosophy is to ensure life safety under DBE and prevent collapse of the building

under MCE. These are two performance objectives which are to be ascertained

with the existing buildings.

A systematic procedure is to be followed in assessing the vulnerability of existing

buildings. Firstly, a detailed survey of the building of interest should be

undertaken. The basic information would include a review of the building

configuration, soil profile and the period of construction. An evaluation is to be

performed based on the available documents, to ensure code compliance. This is

done with the help of quick checks and evaluation statements. The above tasks

form the essence of the preliminary evaluation procedure.

However, a detailed evaluation is necessary in order to identify the deficiencies

associated with the structural components with regard to the expected behaviour of

the building. The code compliance of the building can be ascertained only when

the available member capacities are compared with the respective demands due to

the earthquake. The demands in the structural members are determined for the

seismic forces estimated as per IS 1893-2002 through linear static analysis. The

member capacities are determined using the procedures prescribed in IS 456-2000.

The deficient members are identified when the Demand to Capacity Ratios (DCR)

exceed unity indicating the need for retrofitting in order to establish compliance

with prevailing codes.

Chapter I - Introduction

3

In the case of deficient buildings, a more enhanced and sophisticated analysis

procedure is recommended to determine the load versus deformation behaviour of

the building taking into account of the non-linear behaviour of its components.

Non-linear static pushover analysis provides a basis to determine whether the

building can meet the imposed displacement demand at expected performance

level. It also indicates the likely mode of failure and the spatial distribution of

plastic hinges. If the performance is unsatisfactory various retrofit strategies can

be tried to achieve satisfactory performance.

1.2 OBJECTIVES

The objective of the manual is to provide comprehensive guidelines for seismic

evaluation and retrofit based on the Indian code of practice. The followings are

the main objectives.

1. To give a well-defined procedure that enables a proper assessment of

the seismic vulnerability of a given (existing) multi-storeyed RC

building.

2. To propose various strategies for seismic retrofit that can be used for

buildings found to be deficient.

3. To develop software that facilitates Seismic Analysis and Vulnerability

Evaluation (SAVE) of RC buildings.

The work related to the first two objectives is covered in this manual. It may be

noted that any of the commercially available software can be used to carry out the

analysis. Details of the free software SAVE developed as part of this DST

sponsored project are given separately (user manual and CD), and are not included

in this manual.


4

1.3 SCOPE

This procedure aims at two seismic safety objectives, namely (i) life safety under

design basis earthquake and (ii) collapse prevention objective under maximum

considered earthquake. It does not address other performance objectives. The

buildings treated in this section are mid-rise (3 to 10 storeys) reinforced concrete

moment resisting framed buildings. The report deals only with structural aspects

of the building. Non-structural and geotechnical aspects lie outside the scope of

the report. Special attention should be taken for the evaluation of buildings

located in liquefiable soils.

1.4 METHODOLOGY

The evaluation process essentially consists of two phases, viz., preliminary

evaluation and detailed evaluation. Preliminary evaluation is a quick procedure to

identify potential risks in buildings due to earthquakes. If the building satisfies

the requirements of preliminary evaluation, detailed analysis may not be

necessary.

The following are the methods recommended for detailed analysis:

1. Linear static analysis Equivalent static analysis as per IS 1893: 2002

2. Linear dynamic analysis Response spectrum analysis as per IS 1893:

2002

3. Non-linear static analysis Push-over analysis

It is recommended that all the above methods be performed sequentially for a

proper assessment of the seismic vulnerability, as demonstrated in the case studies

given in Chapter XI. It may be noted that more rigorous analysis (nonlinear

dynamic time-history analysis) is possible, but this is not recommended as it is

more involved and time consuming and not recommended for normal building.

Figure 1.1 gives the flowchart explaining the evaluation and retrofit process.

Chapter I - Introduction

5

Preliminary evaluation

Deficiencies?

YES

NO

Detailed evaluation

Deficiencies?

YES

Retrofit not necessary

Development of retrofit scheme

Post-retrofit analysis

Deficiencies? Report preparation NO

YES

Development of different retrofit scheme

NO

Figure 1.1: Flowchart summarizing the evaluation and retrofit process

The steps to be undertaken in the seismic evaluation of existing building are as

follows,

1. Preliminary evaluation

i) Data collection and condition assessment of building.


6

ii) Rapid Visual Screening (optional).

iii) Quick checks for strength and stiffness.

iv) Evaluation statements (structural checklist).

2. Detailed evaluation

i) Computational modelling.

ii) Perform linear static and dynamic analysis and check the code

compliance at critical section.

iii) Study DCR of structural components

iv) Perform non-linear (static) push-over analysis and assess the

performance.

v) Compare with performance objectives

i Code compliance i Desired failure mechanism i Drift capacity

The first two among these three performance objectives are mandatory

requirements to be satisfied whereas the third one is a desirable performance

objective.

3. Selection and design of retrofit strategies and subsequent verification of the

retrofit scheme.

Remodelling the structure according to the trial retrofit scheme and analysing the

building model. If the performance is not satisfactory different retrofit scheme is

to be selected.

4. Preparation of seismic evaluation and retrofit report.


7

CHAPTER II

PRELIMINARY EVALUATION 2.1 INTRODUCTION

The purpose of the preliminary evaluation is to identify the areas of seismic

deficiencies in a building under investigation. It is a non-detailed analysis

consisting of the following procedures

i) Data collection and condition assessment of building.

ii) Rapid Visual Screening (optional).

iii) Quick checks for strength and stiffness.

iv) Evaluation statements (structural checklist).

The collection of all available data pertaining to the building structure, especially

related to the construction, as well as an on-site inspection of the building form the

first step in the preliminary evaluation procedure. The Rapid Visual Screening

procedure, adapted from FEMA 154 gives some preliminary idea, based on a

scoring system, of the seismic vulnerability of the building. However, this

screening is optional and not mandatory, as FEMA guidelines are not directly

applicable to Indian conditions.

The RVS procedure was proposed by Applied Technology Council in the documents FEMA 154 and FEMA 155. In the present report, the data collection form shown in Table 2.1, is adapted from FEMA 154 published in 2002. The form was modified to include the seismic zones and soil types as per IS 1893: 2002 and to define the pre-code and post-benchmark criteria.


8

Quick checks are approximate checks for strength and stiffness of building

components. The evaluation statements are in the form of a simple questionnaire

that gives an overall idea of the building and identifies areas of potential weakness,

in terms of seismic performance. It also checks the conformity with seismic design

and detailing provisions.

2.2 DATA COLLECTION & CONDITION ASSESSMENT OF

BUILDING

In order to facilitate a proper assessment, it is necessary to collect as much relevant

data of the building as possible through drawings, enquiry, design calculations and

soil report (if available), etc. It may be noted that physical evaluation (condition

survey and walk through) of the building is essential.

Condition survey and walk through of the building gives a general description of

the building. It notes the available drawings and reports, identifies the basic

architectural features, material properties and their deterioration and several

helpful information. A suggested form of the building survey data sheet is given in

Table 2.1 and 2.2 is modified from the proposed amendment in town and country

planning legislations, Regulations for Land Use Zoning in Natural Hazards Zone

of India (Draft version, 2005).

Table 2.1: Building survey data sheet: General data

S.No. Description Information Notes

1 Address of the building

Name of the building Plot number Locality/Town ship District State

2 Name of owner

3 Name of builder

4 Name of Architect/Engineer

Chapter II Preliminary Evaluation

9

Table 2.1 (Contd.): Building survey data sheet: General data


5 Name of Structural Engineer

6 Use of building

7 Number of storeys above ground level

8 Number of basements below ground level

9 Type of structure

Load bearing wall RC frame RC frame and shear wall Steel frame

10 Soil data

Type of soil Design safe bearing capacity

IS 1893: 2002

IS 1904: 1986

11 Dead loads (unit weight adopted)

Earth Water Brick masonry Plain cement concrete Floor finish Other fill materials

IS 875: Part 1:

1987

12 Imposed (live) loads

Floor loads Roof loads

IS 875: Part 2:

1987

13 Cyclone/Wind

Speed Design pressure intensity

IS 875: Part 3:

1987

14 History of past earthquakes and

tremors

15 Seismic zone IS 1893: 2002

16 Importance factor, I IS 1893: 2002

17 Seismic zone factor, Z IS 1893: 2002

18 Response reduction factor, R IS 1893: 2002

19 Fundamental natural period, T IS 1893: 2002


10

Table 2.1 (Contd.): Building survey data sheet: General data


20 Design Horizontal acceleration

spectrum value (Ah)

IS 1893: 2002

21 Seismic design lateral force

22 Expansion/ Separation joints

Table 2.2: Building survey data sheet: Building Data (moment resisting frame)


1 Type of building Regular frames Regular frames with shear

wall Irregular frames Irregular frames with shear

wall Open ground storey

IS 1893: 2002

2 Number of basements

3 Number of floors

4 Horizontal floor system Beams and slabs Waffle slab Ribbed floor Flat slab with drops Flat plate without drops

5 Soil data Type of soil Recommended foundation

- Independent footings - Raft - Piles

Recommended bearing capacity Recommended type, length, diameter and load capacity of piles Depth of water table Chemical analysis of ground water Chemical analysis of soil

IS 1498: 1970


11

Table 2.2 (Contd.): Building survey data sheet: Building Data (MRF)


6 Foundations Depth below ground level Type

Independent Interconnected Raft Piles

7 System of interconnecting foundations

Plinth beams Foundation beams

IS 1893: 2002 Cl. 7.12.1

8 Grades of concrete used in different parts of building

9 Method of analysis 10 Computer software used 11 Torsion included IS 1893: 2002 12 Base shear

a) Based on approximate fundamental period

b) Based on dynamic analysis c) Ratio of a/b

IS 1893: 2002

13 Distribution of seismic forces along the height of building

IS 1893: 2002

14 The columns of soft ground storey specially designed

IS 1893: 2002

15 Clear minimum cover provided in Footing Column Beams Slabs Walls


12

Table 2.2 (Contd.): Building survey data sheet: Building Data (MRF)


16 Ductile detailing of RC frame Type of reinforcement used Minimum dimension of

beams Minimum dimension of

columns Minimum percentage of

reinforcement of beams at any cross section

Spacing of transverse reinforcement at any section of beam

Spacing of transverse reinforcement in 2d length of beam near the ends

Ratio of capacity of beams in shear to capacity of beams in flexure

Maximum percentage of reinforcement in column

Confining stirrups near ends of columns and in beam-column joints

Diameter Spacing

Ratio of shear capacity of columns to maximum seismic shear in the storey

Column bar splices location and spacing of hoops in the splice

Beam bar splices location and spacing of hoops in the splice

IS 456, Cl. 5.6 IS 13920, Cl. 6.1 IS 13920, Cl. 7.1.2 IS 456: 2000 Cl. 26.5.1.1(a) IS 13920: 1993 Cl. 6.2.1 (a) IS 13920: 1993 Cl. 6.3.5 IS 456: 2000 Cl. 26.5.3.1 IS 13920, Cl. 7.4 IS 13920, Cl. 7.2.1 IS 13920, Cl. 6.3.5

However, in many cases, such drawings may not be available (or at best, partially

available). Tables 2.3 to 2.6 summarize the data collection process, relating to the

availability of the drawings and level of evaluation. The various data to be

collected when the original construction drawings are available are indicated in These items are from Table 5.1 to Table 5.4 of ATC-40 (Volume 1): Seismic Evaluation and Retrofit of Concrete Buildings, Applied Technology Council, California.1996.


13

Tables 2.3 and 2.4. Tables 2.5 and 2.6 should be followed when construction

drawings are not available. It is suggested, as shown in tables that in addition to the

visual inspection, it is recommended to carry out non-destructive testing to assess

the strength of concrete.

Table 2.3: Information required for Preliminary evaluation when original construction drawings are available.

Required Item Yes No Comment

Structural calculations Helpful but not essential

Site seismicity and

geotechnical report

Helpful but updated report should

be done.

Foundation report Helpful but not essential

Prior seismic assessment

reports Helpful but not essential

Condition survey of building

Alteration and as built

assessment

Walk through dimensioning Unless required by undocumented

alterations

Non-structural walk through Identify falling hazards, weight

Core testing Unless concrete appears

substandard

Rebound hammer testing Unless concrete appears

substandard

Aggregate testing

Reinforcement testing

Reinforcement location

verification

Unless insufficient info. on

drawing

Non-structural exploration


14

Table 2.4: Information required for detailed seismic evaluation when original construction drawings are available.

Required Item Yes No Comment

Structural calculations Could be helpful

Site seismicity and

geotechnical report Helpful but not essential


Prior seismic assessment

reports Helpful but not essential


Alteration and as built

assessment

Walk through dimensioning Spot checking is appropriate

Non-structural walk through Identify falling hazards, weight

Core testing Minimum 2 per floor, 8 per

building

Rebound hammer testing Minimum 8 per floor, 16 per

building

Aggregate testing Each core

Reinforcement testing Optional

Reinforcement location

verification

Pachometer @ 10% of critical

location, Visual @ 2 locations.


Verify anchorage and bracing

conditions for components sensitive

to building performance.

It is desirable to do core testing, when the condition of the concrete is suspect.

Any evidence of deterioration, cracking and corrosion of reinforcement should be

noted. Testing of reinforcement for yield/ ultimate strength and ductility is

desirable. It is also desirable to ascertain the nature of reinforcement detailing,

especially anchorage of bars and hooks, spacing of stirrups/ ties to the extent

possible using device such as rebar locator.


15

Table 2.5: Information required for Preliminary evaluation when original construction drawings are not available.

Required Item

Yes No Comment

Structural calculations Could minimize scope of site

work

Site seismicity and geotechnical

report

Could minimize scope of site

work

Foundation report Could minimize scope of site

work

Prior seismic assessment reports Could minimize scope of site

work


Alteration and as built assessment

Walk through dimensioning Sufficient to define primary

element

Non-structural walk through Identify falling hazards,

weight

Core testing (limited) Minimum 2 per floor, 8 per

building

Rebound hammer testing

Could be helpful, especially

if concrete appears

substandard

Aggregate testing Several cores

Reinforcement testing

Reinforcement location verification Could be helpful


Unless there is sufficient evidence to suggest that the ductile detailing provision of

IS 13920: 1993 have been followed, it is judicious to assume non-compliance with

the code. Based on an assessment of reliability of the data collected, an


16

approximate knowledge factor should be applied to the material properties for

detailed analysis (Table 3.2).

Table 2.6: Information required for detailed seismic evaluation when original

construction drawings are not available.

Required Item

Yes No Comment

Structural calculations Could be helpful

Site seismicity and geotechnical

report Helpful but not essential


Prior seismic assessment reports Helpful but not essential


Alteration and as built assessment

Walk through dimensioning

Must be done very

thoroughly, particularly if

structure will be retrofitted.

Non-structural walk through Identify falling hazards,

weight

Core testing (limited) Minimum 2 per floor, 8 per

building

Rebound hammer testing Minimum 8 per floor, 16 per

building

Aggregate testing Each core

Reinforcement testing 2 per type

Reinforcement location verification Pachometer for all critical

location, Visual on 25%.


Verify anchorage and

bracing conditions for

components sensitive to

building performance.


17

2.3 RAPID VISUAL SCREENING

The Rapid Visual Screening (RVS) was proposed by FEMA as a means of quickly

assessing, using a scoring system, the seismic vulnerability of buildings in a

locality, based only on visual inspection. Considerable research has gone into the

formulation of the RVS scoring system, and although the specific scores may not

be directly applicable to Indian conditions, the RVS does provide a rough guideline

for reference. Since the RVS is based on visual inspection, the results may vary

from that of a detailed analysis. In general, however, it is expected that the

building that passes the RVS cut-off score criterion, will be found to perform

adequately during an earthquake. If a large number of buildings need to be

evaluated, performing the RVS helps to minimise the number of buildings that

require a detailed analysis.

Table 2.7: Rapid Visual Screening data collection form

Region of Seismicity

High Seismicity (Zone V)

Moderate Seismicity (Zone IV)

Low Seismicity (Zone II and III)

Building Type MRF SW URM INF MRF SW URM INF MRF SW

URMINF

Basic Score 2.5 2.8 1.6 3.0 3.6 3.2 4.4 4.8 4.4

Mid rise +0.4 +0.4 +0.2 +0.2 +0.4 +0.2 +0.4 -0.2 -0.4

High rise +0.6 +0.8 +0.3 +0.5 +0.8 +0.4 +1.0 0.0 -0.4 Vertical

irregularity -1.5 -1.0 -1.0 -2.0 -2.0 -2.0 -1.5 -2.0 -2.0

Plan irregularity -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.8 -0.8 -0.8

Pre-code -1.2 -1.0 -0.2 -1.0 -0.4 -1.0 N/A N/A N/A Post-

benchmark +1.4 +2.4 N/A +1.2 +1.6 N/A +0.6 +0.4 N/A

Soil Type I -0.4 -0.4 -0.4 -0.6 -0.8 -0.6 -0.6 -0.4 -0.4

Soil Type II -0.6 -0.6 -0.4 -1.0 -1.2 -1.0 -1.4 -0.8 -0.8

Soil Type III -1.2 -0.8 -0.8 -1.6 -1.6 -1.6 -2.0 -2.0 -2.0

Final Score

Comments


18

In this procedure the building under consideration is compared with a benchmark

building through visual inspection. Table 2.7 represents the data collection form,

which quantifies the potential seismic hazard for any building based on the

seismicity level of the locality. The form addresses reinforced concrete (RC)

moment resisting frame buildings (MRF), concrete shear wall buildings (SW) and

concrete frame buildings with un-reinforced masonry infill walls (URM INF).

2.3.1 Scores for a Building

In the data collection form, for a particular type of building, the structural scoring

system consists of a basic structural hazard (BSH) score and a set of score

modifiers. The BSH score can be defined as negative logarithm of probability of

collapse of the benchmark building under maximum considered earthquake

(MCE). Thus a BSH score for moment resisting frame (MRF) in moderate

seismicity region of 3.0 implies that for every thousand (103) benchmark buildings

one building is likely to collapse.

Benchmark buildings are the representative building for which the structural

hazard scores (BSH score) were developed for different seismic regions. A

Benchmark building is a low rise, ordinary building (not detailed as per seismic

detailing code) located on an average rock strata (Soil Type B of UBC 1997) and it

has no plan and vertical irregularity. The building is assumed to be designed as per

the current seismic code.

2.3.2 Cut-off Score

FEMA 154 recommends that if the final score is less than the cut off score of 2, a

detailed analysis of the building is required. In selected cases, in order to have a

safer environment (at a correspondingly higher cost) a higher cut-off value can be

used.

The BSH scores are developed from fragility and capacity curves, generated by HAZUS (developed by National Institute of Building Sciences, USA) based on seismic hazard maps.


19

2.3.3 Building Type Descriptions

There are three different building types mentioned in Table 2.7. The definitions of

these buildings are as follows.

(a) Concrete Moment Resisting Frame Buildings (MRF): The buildings with

reinforced concrete frame as the only lateral load resisting system.

(b) Concrete Shear Wall Buildings (SW): Buildings with shear walls are

considered in this type. It also includes buildings having shear walls and frames,

but where the frames are either not designed to carry lateral load or do not fulfil

the requirements of dual system. These buildings generally perform better than

concrete frame buildings and this is reflected in the magnitude of BSH score.

(c) Concrete Frames with Un-reinforced Masonry Infill Walls (URM-INF): In

this type of buildings, un-reinforced masonry infill walls are also part of the lateral

load resisting system.

2.3.4 Score Modifier

BSH scores were calculated for a standard benchmark building. For a specific

building, which may have different characteristics due to higher number of storeys

or structural irregularities or different soil type, it is necessary to modify the BSH

scores using score modifiers (SM)**. So a specific building will arrive at a final

score (S) after modifying the BSH score. The final score S is an estimate of the

probability that the building will collapse if a ground motion equal to or exceeding

the MCE ground motion occurs. S = BSH SM. Definitions for the score modifiers used in Table 2.7 are discussed below.

High-rise and Mid-rise Buildings: 4 to 7 storey buildings are categorised as mid-

rise building whereas buildings with 8 or more storeys are as high-rise building.

** A positive modifier implies reduced probability of failure and vice versa. The following definitions of the score modifiers are from FEMA 154, changed suitably as per IS 1893: 2002 and IS 13920: 1993.


20

Plan irregularity and Vertical irregularity: This are defined in detail in Tables 2.9

and 2.10 in the section 2.5

Pre-code: Buildings designed for gravity loads only and not for lateral loads are

defined as pre-code buildings. In the absence of any mention of code in the

construction documents, it is difficult to judge pre-code. Then, if at the beam-

ends, the bottom steel is less than 50% of the top steel provided, the building can

be considered to be designed for gravity loads only. As the benchmark building is

assumed to be designed as per the current seismic code, pre-code buildings have a

negative score modifier.

Post-benchmark: Building designed and constructed as per the ductile detailing

requirements of IS 13920: 1993 are considered as post-benchmark buildings.

Values of the score modifier for post-benchmark buildings are positive as these

buildings perform better than the benchmark building under seismic loading.

Soil Type Definition: Score modifiers for three soil types are mentioned in the data collection form.

Soil Type I (Rock or hard soil): well graded gravel and sand gravel mixtures with

or without clay binder, and clayey sands poorly graded or sand clay mixtures with

standard penetration count, N > 30.

Soil Type II (Medium soil): All soils with 10 N 30 poorly graded sands or

gravely sands with little or no fines with N > 15.

Soil Type III (Soft soil): All soils other than sands poorly graded with N < 10.

2.4 QUICK CHECKS FOR STRENGTH AND STIFFNESS

The quick checks involve a set of initial calculations that checks the average

shear stress in the columns, shear walls etc and average axial stresses in columns

The values of the score modifier for soil type were obtained by mapping the soil types given in UBC-1997 to soil Types I, II and III as given in IS 1893: 2002. The details of the mapping is discussed in Appendix-A.


21

in each storey, due to the design lateral force determined from IS 1893-2002. This

includes a drift check which is a measure of the stiffness of the building and also a

strong column-week beam check recommended by IS 13920: 1993. The details of

the checks are given below.

2.4.1 Column Shear

The base shear (VB) is to be calculated as per Clause 7.5.3 of IS 1893: 2002. The

calculation of the base shear is explained in Section 3.3.1.5. The shear at each

storey (Vj) is calculated from the base shear as follows: n

i ii

V Q= (2.1) where, Vi Storey shear at ith storey,

Qi Design lateral force at ith storey (Ref. Section 3.3.1.5),

n Total number of storeys above ground level, i Number of storey level under consideration, Wi Seismic weight of ith storey,

The average shear stress in the columns (assuming that nearly all the columns in

the frame have similar stiffness) is given by,

c iavg

c f c

n Vn n A

= (2.2)

Where, nc Total number of columns in that particular storey, nf Total number of frames in the direction of loading, Ac Summation of the cross sectional areas of columns in

the storey under consideration,

Vi shear at storey, i.


22

The term cc f

nn n

is based on the assumption that shear force carried by the

columns at the end of RC frames are typically half of those carried by interior

columns. However, this leads to a very conservative estimate of shear for one-bay

frame (twice of the correct value), but this discrepancy is not so serious for frames

which are typically more redundant.

If the average column shear stress (avg) is greater than 0.4 MPa, a more detailed

evaluation of the structure should be performed.

2.4.2 Shear Stress in Shear Wall

The average shear stress in the walls at a storey can be calculated as follows.

iavg

w

VA

= (2.3)

Where, Vi shear at the storey under consideration, Aw summations of the horizontal cross sectional area

of all shear walls in the direction of loading. The

wall area should be reduced by the area of openings.

If the average shear stress in shear walls (avg) is greater than 0.35 MPa or

0.074fck MPa, a more detailed evaluation of the structure should be performed.

2.4.3 Axial Stress in Column

The base shear VB is assumed to be distributed in a parabolic pattern, in accordance

with 1893: 2002. The overturning moment due to these forces develop axial forces

in the columns. This may be computed as

58

B

f

V hPn L

= (2.4)


23

Here, h is the total height of the building, L is the total length of a frame and nf is

the number of frames in the direction of lateral forces. The factor 5 8 accounts

for the height of the resultant lateral force above base level.

The axial stress calculated from the force should be less than 0.24 fck for

acceptance.

2.4.4 Frame Drift

The approximate storey drift ratio can be determined using the following equation.

It considers that the storey displacement is equal to the flexural displacement of a

representative column, including the effect of end rotation due to bending of a

representative beam.

D R12

b cc d

b c

k k h V Ck k E+= (2.5)

where, DR Inter storey displacement divided by the storey height, kb I/L for a representative beam, kc I/h for a representative column, L Effective length of the beam, h Storey height, I Moment of inertia, E Modulus of elasticity, Vc Shear in column, Cd Deflection amplification factor to include inelastic effect. For ordinary RC moment resisting frames, Cd = 2.

For the value of I, an equivalent cracked section moment of inertia equal to half of

the gross section can be used. The above equation can be applied to the ground

storey if the columns are fixed against rotation at the bottom (for pile and raft

foundations). If the columns are pinned at the bottom (for isolated footing), an

equivalent storey height equal to twice the storey height shall be used in

calculating the value of kc.

If the drift ratio exceeds the limiting drift ratio of 0.015, the structure needs to be

evaluated for full frame analysis using the design lateral forces.


24

2.4.5 Strong Column Weak Beam Check

At a beam-column junction, according to good design principle, failure of the

column should not precede that of the beam, in order to avoid catastrophe (Global

failure). As shown in Figure 2.1, a strong column-weak beam combination is able

to sustain higher lateral loads through development of large number of plastic

hinges at the beam-ends prior to formation of collapse mechanism. In contrast

under strong beam-weak column construction, plastic hinging at the top and

bottom locations of the columns in a storey can bring down the entire building at

low lateral loads.

(a) Strong Column-Weak Beam (b) Strong Beam-Weak Column

Figure 2.1: Failure mechanism in an RC frame

A quick check (in an overall sense) of ascertaining whether plastic hinges formed

first in the beam sections rather than the adjoining column sections is by checking

that the sum of the moment capacities of the columns shall be 20% greater than

that of the beams at frame joints.

i.e., Moment capacities of the columns > 1.2 Moment capacities of the beams


25

2.5 EVALUATION STATEMENTS

The evaluation statements seek clarification on a variety of structural seismic-

resistant features, which if non-compliant, suggest that detailed evaluation is

required. The evaluation statements depend on the type of lateral load resisting

systems. Here, only the statements relevant for concrete moment resisting frame

buildings, with or without shear walls, are listed. The evaluation statements are

listed in Tables 2.8 to 2.15. Each of the statements should be marked as

compliant (C), non-compliant (NC) or not applicable (NA). Compliant

statements identify issues that are acceptable as positive seismic resistant qualities,

while non-compliant statements identify issues that need further investigation.

Certain statements that may not apply to the building under consideration can be

marked as not applicable.

Table 2.8: Evaluation statements Building system

Statements C / NC / NA

Load path: The structure shall contain one complete load path for seismic force effects from any horizontal direction that serves to transfer the inertial forces from the mass to the foundation.

Adjacent buildings: An adjacent building shall not be located next to the structure being evaluated closer than 4% of the height.

Mezzanines: Interior mezzanine levels shall be braced independently from the main structure, or shall be anchored to the lateral-force-resisting elements of the main structure. (Clause 7.3.4 IS 13920: 1993).

No deterioration of concrete: There shall be no visible deterioration of concrete or reinforcing steel in any of the vertical- or lateral-force-resisting elements.

The evaluation statements are based on FEMA 310 and are modified to match the clauses of IS 1893: 2002 and IS 13920: 1993. The definitions of structural irregularities are as per IS 1893: 2002 and the detailing provisions are as per IS13920: 1993. The statements for the life safety performance level are selected. The statements which are solely for immediate occupancy performance level are disregarded.


26

Table 2.9: Evaluation statements Vertical irregularities

Statements (Figure 2.2 and Table 5 of IS 1893: 2002) C / NC / NA

No weak storey: The lateral strength of a storey shall not be less than 80% of the strength in the storey above.

No soft storey: The lateral stiffness of a storey shall not be less than 70% of that in the story above or less than 80% of the average lateral stiffness of the three storeys above.

No mass irregularity: There shall be no storey with seismic weight more than 200% of that of its adjacent storeys. The irregularity need not be considered in case of roofs.

No vertical geometric irregularity: There shall be no storey with the horizontal dimension of the lateral-force-resisting system more than 150% of that in its adjacent storey.

No vertical discontinuities: All vertical elements in the lateral-load-resisting system shall be continuous to the foundation.

Table 2.10: Evaluation statements Plan Irregularities

Statements (Figure 2.3 and Table 4 of IS 1893: 2002) C / NC / NA

No Torsion irregularity: The distance between the storey centre of rigidity and the storey centre of mass shall be less than 20% of the width of the structure in either plan dimension.

No diaphragm discontinuity: There shall be no diaphragm with abrupt discontinuity or variation in stiffness, including those having cut out or open areas greater than 50% of the gross enclosed diaphragm area. The diaphragms shall not be composed of split-level floors.

No re-entrant corners: Both projections of structure beyond the re-entrant corners shall not be greater than 15% of its plan dimension in the given direction.

No out of plane offsets: There shall be no discontinuity in a lateral-force-resisting path, such as out of plane offsets of vertical elements.

No non-parallel system: There shall be no vertical element resisting the lateral force, not parallel to or symmetric about major orthogonal axes of the lateral-force-resisting system.


27

Table 2.11: Evaluation statements Moment resisting frames

Statements (Figure 2.4 and Figure 2.5) C / NC / NA

Redundancy: The number of lines of moment frames in each principal direction shall be greater than or equal to 2. The number of bays of moment frames in each line shall be greater than or equal to 2.

No interfering wall: All infill walls placed in moment frames shall be isolated from structural elements.

Shearing stress check: The building satisfies the quick check of the shear stress in the frame columns. (Section 2.4.1)

Axial stress check: The building satisfies the quick check of the axial stress in the frame columns. (Section 2.4.3)

Drift check: The building satisfies the quick check of storey drift. (Section 2.4.4.)

Short captive columns: There shall be no columns at a level with height/depth ratios less than 50% of the nominal height/depth ratio of the typical columns at that level. (Clause 7.4.5, IS 13920: 1993)

No shear failures: The shear capacity (VuR) of a frame column shall be greater than the shear demand which occurs when the column attains the probable moment capacity (Mpr). i.e., VuR 2Mpr/L. Consider Mpr = 1.4 MuR, where MuR is the moment of resistance in absence of axial load. (Clause 7.3.4, IS 13920: 1993)

Strong column-weak beam: The building satisfies the quick check of strong column weak beam. (Section 2.4.5).

Column bar splices: All column bar splices shall be provided only in the central half of the member length and hoops provided at spacing not exceeding 150 mm centre to centre. (Clause 7.2.1, IS 13920: 1993)

Column tie spacing: Frame columns shall have ties spaced at or less than b/2 throughout their length and at or less than b/4 or 100 mm at all potential plastic hinge locations. (Clause 7.4.6, IS 13920: 1993)

Beam bars: At least two longitudinal top and two longitudinal bottom bars shall extend continuously throughout the length of each frame beam. At least 25% of the longitudinal bars provided at the joints for either positive or negative moment shall be continuous throughout the length of the members.


28

(d) Vertical geometric irregularity

(a) Weak storey

Storey Strength (lateral)

0.8 1F Fi i< +

F1

F2

Fn

Fn-1

F3

Fn-2

(b) Soft storey

Storey Stiffness (lateral)

K1

K2

Kn

Kn-1

K3

Kn-2

1

1 2 3

0.7

0.83

i

i i i i

kk k k k

+

+ + +

< + +

(c) Mass irregularity

Storey weight

W1

W2

Wn

Wn-1

W3

Wn-2

1 12.0 (or,2.0 )i i iW W W+ >

A

A A

A A

L

L L

A/L > 0.25

A/L > 0.15 A/L > 0.1

Figure 2.2: Different types of vertical irregularity


29

(a) Torsional Irregularity

12

1 22

1.2( )2

+ >

EQ

L A

L

A

A

A/L > 0.15

(b) Re-entrant Corner

X

Y

(c) Non-parallel System

Opening Area, A2

Total floor area, A

2 0.5A A>

(d) Diaphragm Discontinuity

Lateral load resisting system

Figure 2.3: Different types of plan irregularity


30

Table 2.11 (contd.): Evaluation statements Moment resisting frames

Statements (Figure 2.4 and Figure 2.5) C / NC / NA

Beam bar splices: The lap splices for the longitudinal

reinforcement shall not be located within 2d from the joint face and

within L/4 from the location of potential plastic hinges. (Clause

6.3.5, IS 13920: 1993)

Stirrup spacing: All beams shall have stirrups spaced at or less than

d/2 throughout their length. At potential hinge location, stirrups

shall be spaced at or less than the minimum of 8db or d/4. (Clause

6.3.5, IS 13920: 1993)

Bent-up bars: Bent-up longitudinal steel shall not be used for shear

reinforcement. (Clause 6.3.4, IS 13920: 1993)

Joint reinforcing: Column ties shall be extended at their typical

spacing through all beam column joints. (Clause 8.1, IS 13920:

1993)

Deflection compatibility: Secondary components shall have the

shear capacity to develop the flexural strength of the elements.

No flat slab frames: The lateral-force-resisting system shall not be

a frame consisting of columns and a flat slab/plate without beams.

Prestressed frame elements: The lateral-load-resisting frames shall

not include any prestressed elements.

Diaphragm reinforcement: There shall be tensile capacity to

develop the strength of the diaphragm at re-entrant corners or other

locations of irregularities. There shall be reinforcement around all

diaphragm openings larger than 50% of the gross enclosed

diaphragm area. (Table 4, IS 1893: 2002)

Anchorage: Stirrups should have 135 degree hook* with 10-

diameter extension (but not less than 75 mm) at each end,

embedded in the confined core

* It is noted that unless the bend angle is mentioned as 135 degree and there is adequate extension beyond the bend, the hook will be considered as non-compliant.


31

Table 2.12: Evaluation statements Shear walls


Shearing stress check: The building satisfies the quick check of

shearing stress in the shear walls. (Section 2.4.2)

Reinforcing steel: The area of reinforcing steel for concrete walls

shall be greater than 0. 25% of the gross area of the wall along both

the longitudinal and transverse axes and the maximum spacing of

bars shall not exceed lw/5, 3tw and 450 mm. (Clauses 9.1.4 and

9.1.7, IS 13920: 1993)

Coupling beams: The stirrups shall be spaced at or less than 100

mm and shall be anchored into the core with 135 hooks. (Clause

9.5.2, IS 13920: 1993)

Diaphragm openings at shear walls: Diaphragm openings

immediately adjacent to the shear walls shall be less than 25% of

the wall length.

Table 2.13: Evaluation statements Connections


Column connection: All column reinforcement shall be dowelled

into the foundation. (Clause 7.4.2, IS 13920: 1993)

Wall connection: Wall reinforcement shall be dowelled into the

foundation.

Transfer to shear walls: Diaphragms shall be reinforced and

connected for transfer of loads to the shear walls.

Lateral load at pile caps: Pile caps shall have top reinforcement

and piles shall be anchored to the pile caps.


32

Table 2.14: Evaluation statements Geological site hazards


Liquefaction: Liquefaction susceptible, saturated, loose granular

soils that could jeopardise the buildings seismic performance shall

not exist in the foundation soils at depths within 15 m under the

building.

Slope failure: The building site shall be sufficiently remote from

potential earthquake induced slope failures or rock falls to be

unaffected by such failures or shall be capable of accommodating

any predicted movements without failure.

Surface fault rupture: Surface fault rupture and surface

displacement at the building site is not anticipated.

Table 2.15: Evaluation statements Foundations


Foundation performance: There shall be no evidence of excessive

foundation movement such as settlement or heave that would affect

the integrity or strength of the structure.

Deterioration: There shall not be evidence that foundation elements

have deteriorated due to corrosion, sulphate attack, material

breakdown, or other reasons in a manner that would affect the

integrity or strength of the structure.

Overturning: The ratio of the effective horizontal dimension, at the

foundation level of the lateral-force-resisting system, to the

building height (base/height) shall be greater than 0.6 Sa/g.

Ties between foundation elements: The foundation shall have ties

adequate to resist seismic forces where footings, piles, piers are not

restrained by beams, slabs, or soils classified as Type I.


33

Spacing B/4 or 100mm 75mm

Lapping in middle half of the column

Spacing 150mm

Figure 2.4: Reinforcement detailing for column as per IS 13920: 1993

Lapping prohibited in regions where longitudinal bars can yield in tension

At least 2 bars at top and 2 bars at bottom should go full length of the beam. Spacing d/2

Spacing 8db or d/4

2d 2d

Figure 2.5: Reinforcement detailing for beam as per IS 13920: 1993


34

2.6 DECISION FOR DETAILED EVALUATION

In this chapter the steps to be taken in order to carry out a preliminary evaluation

of seismic vulnerability of a given building have been outlined. At the end of the

preliminary evaluation a decision has to be taken whether to probe further and

carry out more rigorous detailed evaluation (described in Chapters III and IV).

Strictly, if the given building passes all the quick checks and satisfies all the

evaluation statements, detailed evaluation is not called for. Nevertheless it is good

practice to go ahead with the detailed evaluation, if an absolute confirmation

regarding safety and code compliance is desired. It may be noted that almost

every building out of 40 buildings randomly chosen for study under DST project

was found to be deficient in some manner or other during the stage of preliminary

evaluation. It is possible, as seen in some instances of the case studies carried out,

that a building found deficient in preliminary evaluation performs satisfactory

(without need for any retrofit) in the detailed evaluation. Thus, the preliminary

evaluation serves as a useful screening test for seismic evaluation and its outcome

is generally conservative.


35

CHAPTER III

EVALUATION BASED ON LINEAR ANALYSIS 3.1 INTRODUCTION

When a building fails to comply with the preliminary evaluation criterion, a

detailed structural analysis of the building should be carried out. Detailed analysis

includes developing a computational model on which linear / non-linear, static /

dynamic analysis is performed. Because of the difficulties and uncertainties in

non-linear dynamic analysis, this is not recommended in normal design practice.

This manual is confined to the other types of analysis. This chapter briefly

explains the linear static and linear dynamic analyses as recommended in the code

(IS 1893: 2002). The main purpose of these analyses, from the seismic evaluation

perspective, is to check the demand-to-capacity ratios of the building components

and thereby ascertain code compliance. The non-linear static analysis (pushover

analysis) is explained in the next chapter. Some of the important modelling issues

will also be discussed in this chapter.


36

3.2 COMPUTATIONAL MODEL

Modelling a building involves the modelling and assemblage of its various load-

carrying elements. A model must ideally represent the complete three dimensional

(3D) characteristics of the building, including its mass distribution, strength,

stiffness and deformability. Modelling of the material properties and structural

elements is discussed below.

3.2.1 Material properties

The material properties of concrete include mass, unit weight, modulus of

elasticity, Poissons ratio, shear modulus and coefficient of thermal expansion.

The short-term modulus of elasticity (Ec) of concrete, as per IS 456: 2000, is given

by

5000c ckE f= (3.1) where ckf characteristic compressive strength of concrete at 28-days in MPa. For the steel rebar, the properties required are yield stress (fy) and modulus of

elasticity (Es).

For assigning the material properties, the procedure outlined in section 2.2 shall be

followed. As the characteristic strength is a 5 percentile value of the actual

strength, the strength in analysis may be increased by the factors suggested in

Table 3.1 for seismic evaluation purpose. This is done to estimate the expected

capacities of the members.

Table 3.1: Factors to estimate the expected strength

Material property Factor

Concrete compressive strength (fck) 1.50

Steel yield stress (fy) 1.00

Chapter III Evaluation based on Linear Analysis

37

However, the expected values need to be further modified to for the uncertainty

regarding the present condition of the material. A knowledge factor (mk) is used

to account for this uncertainty. Proposed values of the knowledge factor are

shown in Table 3.2.

Table 3.2: Knowledge factors

No Description of available information mk 1 Original construction documents, including material testing

report

1.0

2 Documentation as in (1) but no material testing undertaken 0.9

3 Documentation as in (2) and minor deteriorations of

original condition

0.8

4 Incomplete but usable original construction documents 0.7

5 Documentation as in (4) and limited inspection and material

test results with large variation.

0.6

6 Little knowledge about the details of components 0.5

3.2.2 Structural element model

3.2.2.1 Beams and columns

Beams and columns should be modelled by 3D frame elements. While modelling

the beams and columns, the important properties to be assigned are cross sectional

dimensions, reinforcement details and the types of material used. Plinth beams

should also be modelled as frame elements. The moment of inertia of a section

should be modelled properly to account for the effect of cracking and the

contribution of the flanges for T- or L- beam. The suggested effective moment of

inertia (Ieff) for the beams including the effect of cracking and flanges are listed in

Table 3.3

The table is adopted from IITK-GSDMA guidelines for seismic evaluation and strengthening of buildings prepared by Indian Institute of Technology Kanpur.


38

Table 3.3: Effective moment of inertia for the beam sections

Beam Sections Ieff

Rectangular 0.5 Ig

T - section 0.7 Ig

L - section 0.6 Ig

Here, the gross section moment of inertia (Ig) should be calculated considering the

rectangular area only as shown in Figure 3.1. In the case of columns, the

reduction in stiffness due to cracking is reduced by the presence of axial

compression. The suggested moment of inertia for column is: Ieff 0.7 Ig

T-Beam L-Beam

Figure 3.1: Rectangular area for the calculation of Ig

Total Length

Clear Length

End Offsets

Beam

Column

Figure 3.2: Use of end offsets at beam-column joint

Factors recommended here are adapted from Paulay and Priestley (1991)


39

3.2.2.2 Beam-column joints

The beam-column joints should be modelled by giving end-offsets to the frame

elements, to obtain the moments and forces at the beam and column faces. The

beam-column joints can be assumed to be rigid (Figure 3.2).

3.2.2.3 Slabs

The slabs need not be modelled by plate elements to simplify modelling. The

structural effect of slabs due to their in-plane stiffness can be taken into account

by assigning diaphragm action at each floor level. The weight of a slab can be

modelled separately as triangular and trapezoidal loads on the supporting beams.

In case of large openings or projections in slabs, different portions of the floor

may have differential translations, and in such cases, diaphragm action should be

assigned separately to the different sections.

3.2.2.4 Appendages

The effects of all significant appendages (for example, water tanks, stairways,

cantilever slabs) should be included in the model. Stairway slabs can be modelled

as inclined equivalent frame elements, with hinges at the ends. For water tanks

and cantilever slabs, the masses are lumped on the supporting elements.

3.2.2.5 Walls (structural and non structural)

Structural walls such as shear walls and walls in building core, which are

integrally connected to the floor slabs, can be modelled using equivalent wide

column elements. The master node of the column element can be at the centre of

gravity of the shear wall or core and it should be connected to the slave nodes of

the adjacent beams by rigid links (Figure 3.3). Non-structural walls such as infill

walls have weight and in-plane stiffness. They influence the behaviour of the

building under lateral load. The weight of an infill wall should be incorporated


40

separately as a uniform load on the supporting beam. The stiffness contribution of

an infill wall can be modelled using a simplified equivalent strut approach.

Calculation of the properties of the equivalent strut is explained in Appendix B.

When the stiffness contribution of the infill walls is included, the natural period of

the building is reduced and the base shear increases. But, the moments in the

beams and columns may reduce due to the truss action of the equivalent struts.

During an earthquake, the infill walls may fail due to out-of-plane bending. This

will increase the moments in the beams and columns. To calculate the demands in

the beams and columns, two extreme cases can be modelled. In the first model,

the lateral stiffness due to the significant infill walls is modelled by the equivalent

struts. In the second model, the stiffness is ignored. However, the weight of the

infill walls on the supporting beams should be considered in both the models.

Master Node

Beam

Slave Node

Rigid Links

(b) Core Wall

(a) Shear Wall

Figure 3.3: Modelling of shear wall and core wall


41

3.2.3 Modelling of Column Ends at foundation

The column end at foundation can be modelled by considering the degree of fixity

provided by the foundation. Depending on the type of footing the end condition

may be modelled as follows:

i) Isolated footing: A hinge is to be provided at the column end at the bottom

of the foundation. However, when it is founded on hard rock, the column

end may be modelled as fixed, with the level of fixity at the top of the

footing.

ii) Raft foundation: The column ends are to be modelled as fixed at the top of

the raft.

iii) Combined footing: Engineering judgement must be exercised in modelling

the fixity provided by the combined footings. If the footings are

adequately restrained by tie beams, the column ends can be modelled as

fixed.

iv) Single pile: Fixity of column is recommended at a depth of five to ten

times the diameter of pile, depending upon the type of soil, from the top of

pile cap.

v) Multiple piles: Assume fixity of column at top of the pile cap.

3.2.4 Load Combinations

The analysis results are to be for the following load combinations (IS 1893: 2002):

COMB1 = 1.5(DL+IL)

COMB2 = 1.2(DL+IL+EL)

COMB3 = 1.2(DL+IL EL) COMB4 = 1.5(DL+EL)

COMB5 = 1.5(DL EL) COMB6 = 0.9DL+1.5EL

COMB7 = 0.9DL 1.5EL


42

Here, DL Dead load, IL Live load, and EL Earthquake Load. The dead load and the live load are taken as per IS 875, 1987. When the lateral load resisting

elements are not orthogonally oriented, the design forces along two horizontal

orthogonal directions (X- and Y-) should be considered. One method to consider

this is the following.

(a) 100% of the design forces in X-direction and 30% of the design forces in Y-

direction.

(b) 100% of the design forces in Y-direction and 30% of the design forces in X-

direction.

An alternative method to consider the effect of the forces along X- and Y-

directions is the square root of the sum of the squares (SRSS) basis.

2 2x yEL EL EL= + (3.2)

The vertical component is considered only for special elements like horizontal

cantilevers in Zones IV and V. The maximum value of a response quantity from

the above load combinations gives the demand.

3.3 LINEAR ANALYSIS METHODS

The two different linear analysis methods recommended in IS 1893: 2002 are

explained in this Section. Any one of these methods can be used to calculate the

expected seismic demands on the lateral load resisting elements.

3.3.1 Equivalent static method

In the equivalent static method, the lateral force equivalent to the design basis

earthquake is applied statically. The equivalent lateral forces at each storey level

are applied at the design centre of mass locations. It is located at the design

eccentricity from the calculated centre of rigidity (or stiffness).


43

3.3.1.1 Centre of mass

The centre of mass is the point where the total mass of the floor level is assumed

to be lumped. The centre of mass can be calculated for each floor by taking

moments of the axial forces (from gravity load analysis of that floor only) in the

columns about an assumed reference axis.

CMx = i ii

W xW

; CMy =

i i

i

W yW

(3.3)

where

CMx coordinate of the centre of mass along x-direction CMy coordinate of the centre of mass along y-direction

iW sum of the weights of all components i iW x sum of the moments of weights about an assumed reference axis along

X- direction

i iW y sum of the moments of weights about an assumed reference axis along Y-direction

3.3.1.2 Centre of rigidity of storey

The centre of rigidity is the point through which the resultant of the restoring

forces in a storey acts. The centre of rigidity for each storey should be found out

separately. There are different procedures to calculate the centre of rigidity. One

of the procedures is explained below.

The columns of the storey are assumed to be fixed at the bottom. A unit force

along X-direction and a unit moment about Z- axis (vertical axis) are applied at a

certain test point in the top of the storey and the corresponding rotations are noted

down. The distance of the centre of rigidity from the test point, along Y- direction,

is calculated from the ratio of the two rotations. Similarly the distance along X-

direction is found out by applying a unit force along Y- direction and a unit

moment.


44

Let the co-ordinates of the test point be (x, y). Let (z)x, (z)y and (z)z be the

rotations about the Z-axis for the unit loads along X- and Y- directions and unit

moment about Z-axis, respectively. The co-ordinates of the centre of rigidity is

given as CRx,= x+x1, CRy = y+y1, where

x1 = -(z)x/(z)z (3.4a)

y1 = (z)x/(z)z (3.4b)

The static eccentricity of the centre of mass with respect of centre of rigidity is

given as follows.

esix = CMxCRx (3.5a) esiy = CMyCRy (3.5b)

3.3.1.3 Effect of torsion

The design eccentricity of the centre of mass (edix, ediy) is calculated considering a

dynamic amplification factor and an additional eccentricity of 5% of the

dimension of the building perpendicular to the direction of the seismic force. For

either of X- or Y- directions,

edi = 1.5esi + 0.05bi (3.6a) or,

edi = esi 0.05bi (3.6b) There can be four possible locations of the design centre of mass. To reduce

computation, only two diagonal locations can be considered.

3.3.1.4 Seismic weight

The seismic weight of each floor of the structure includes the dead load and

fraction of the live load (as per Table 8 of IS 1893: 2002) acting on the floor. The

weight of the columns and walls (up to the tributary height) are to be included. The

tributary height is between the centreline of the storey above and centre line of the

storey below.


45

3.3.1.5 Lumped mass

The lumped mass is the total mass of each floor that is lumped at the design centre

of mass of the respective floor. The total mass of a floor is obtained from the

seismic weight of that floor.

3.3.1.6 Calculation of lateral forces

The base shear (V = VB) is calculated as per Clause 7.5.3 of IS 1893: 2002.

B hV A W= (3.7)

2a

hSZ IA

R g = (3.8)

where W seismic weight of the building, Z zone factor, I importance factor, R response reduction factor, Sa /g spectral acceleration coefficient determined from Figure 3.4, corresponding to an approximate time period (Ta) which is given

by 0.750.075aT h= for RC moment resisting frame without masonry infill (3.9a)

0.09a

hTd

= for RC moment resisting frame with masonry infill (3.9b)

The base dimension of the building at the plinth level along the direction of lateral

forces is represented as d (in metres) and height of the building from the support is

represented as h (in metres). The response spectra functions can be calculated as

follows:

For Type I soil (rock or hard soil sites): 1 15 0.00 0.102.50 0.10 0.401 0.40 4.00

a

T TS Tg

TT

+ =

For Type II soil (medium soil): 1 15 0.00 0.102.50 0.10 0.551.36 0.55 4.00

a

T TS Tg

TT

+ =


46

For Type III soil (soft soil): 1 15 0.00 0.102.50 0.10 0.671.67 0.67 4.00

a

T TS Tg

TT

+ =

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 1.0 2.0 3.0 4.0

Period (s)

Spct

ral A

ccel

erai

on C

oeffi

cien

t (S

a/g) Type I (Rock,or Hard Soil)

Type II (Medium Soil)

Type III (Soft Soil)

Figure 3.4: Response spectra for 5 percent damping (IS 1893: 2002)

W1

W2

W3

h1

h2

h3

Figure 3.5: Building model under seismic load


47

The design base shear is to be distributed along the height of building as per

Clause 7.7.1 of IS 1893: 2002.

The design lateral force at floor i is given as follows 2

2

1

i ii B n

i ij

W hQ VW h

=

=

(3.10)

Here iW Seismic weight of floor i, ih Height of floor measured from base,

n Number of storeys in the building equal to the number of levels at which masses is located (Figure 3.5).

3.3.2 Response spectrum analysis

The equations of motion associated with the response of a structure to ground

motion are given by:

( ) ( ) ( ) ( ) ( ) ( )gx gy gzt t t u t u t u t+ + = + + x x xMu Cu Ku m m m (3.11) Here, M is the diagonal mass matrix, C is the proportional damping matrix, K is

the stiffness matrix, u , u and u are the relative (with respect to the ground) acceleration, velocity and displacement vectors, respectively, mx, my, and mz are

the unit acceleration loads and gxu , gyu and gzu are the components of uniform

ground acceleration.

The objective of response spectrum analysis is to obtain the likely maximum

response from these equations. The earthquake ground acceleration in each

direction is given as a response spectrum curve*. According to IS 1893: 2002,

high rise and irregular buildings must be analysed by the response spectrum

method. However, this method of linear dynamic analysis is also recommended

for regular buildings.

* The response spectrum is a plot of the maximum response (maximum displacement, velocity, acceleration or any other quantity of interest) to a specified load function for all possible single degree-of-freedom systems. The abscissa of the spectrum is the natural period (or frequency) of the system and the ordinate is the maximum response. It is also a function of damping. Figure 3.3 shows the design response spectra given in IS 1893: 2002 for a 5% damped system.


48

Response spectrum analysis is performed using mode superposition, where free

vibration modes are computed using eigenvalue analysis. The maximum modal

response (k) of a quantity (considering the mass participation factor) is obtained for each mode of all the modes considered. Sufficient modes (r) to capture at least

90% of the participating mass of the building (in each of the orthogonal horizontal

directions), have to be considered in the analysis. The modal responses of all the

individual modes are then combined together using either the square root of the

sum of the squares (SRSS) method or complete quadratic combination (CQC)

method. The SRSS method is based on probability theory and is expressed as

follows.

2

1( )

r

kk=

= (3.12) If the building has very closely spaced modes then the CQC method is preferable.

The base shear is calculated for response spectrum analysis in the following

manner. The Sa/g value corresponding to each period of all the considered modes

is first calculated from Figure 3.4. The base shear corresponding to a mode is then

calculated as per Section 3.3.1.5. Each base shear is multiplied with the

corresponding mass participation factor and then combined as per the selected

mode combination method, to get the total base shear of the building.

If the base shear calculated from the response spectrum analysis ( )BV is less than

the design base shear ( )BV calculated from Equation 3.7, then as per IS 1893:

2002, all the response quantities (member forces, displacements, storey shears and

base reactions) have to be scaled up by the factor /B BV V .

3.4 EVALUATION RESULTS

The demands (moments, shears and axial forces) obtained at the critical sections

from the linear analyses are compared with the capacities of the individual


49

elements. The capacities of RC members are to be calculated as per IS 456: 2000,

incorporating the appropriate knowledge factors (Table 3.2). The demand-to-

capacity ratio (DCR) for each element should be less than 1.0 for code

compliance.

Muy

Pu

Mux A

B C

DCR = AB/AC

Figure 3.6: Demand to capacity ratio for column flexure

For a beam, positive and negative bending moment demands at the face of the

supports and the positive moment demands at the span need to be compared with

the corresponding capacities. For a column, the moment demand due to bi-axial

bending under axial compression must be checked using the P-Mx-My surface

(interaction surface), generated according to IS 456: 2000. The demand point is to

be located in the P-Mx-My space and a straight line is drawn joining the demand

point to the origin. This line (extended, if necessary) will intersect the interaction

surface at the capacity point. The ratio of the distance of the demand point (from

the origin) to the distance of the capacity point (from the origin) is termed as the

DCR for the column (Figure 3.6).

Seismic Evaluation and Retrofit of Multi-storeyed RC B

Documents

Manual on Seismic Evaluation & Retrofit of Multistory Rc Blds