K..Amonoo-Neizer

BSc(Sc.), BSc(Eng.), MPhil., PhD.

Former Dep. Director, BRRI

Former Head, Structures, Building Design and Planning Div., BRRI

1. Background1.1 CSIR – BRRI

WABRI(1952) BRRI(1960)

CSIR – BRRI Vision:

“ Commercial- oreinted research and

development organization in the construction industry”

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1.2 BRRI Divisions

Administration Commercial and Information Geotechnical Traffic and Transportation Structures, Building Design and Planning Construction Material Development

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1.3 Pursuance of mandate

Structures, Building Design and Planning Division (SBDP)

Building Regulations Development

WABRI(1960 ) – Revision of 1940’s Colonial Regulations; including Part 3: “Loads” of draft Code for design of

civil and engineering structures.

BRRI/SBDP(1970’s) – Re-examination of Building Regulations

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Regulations, by 1980’s, unrealistic

so far as conditions in Ghana were

concerned.

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2. Ghana Building Code1988 – Draft Building Code by BRRI

2.1 Sections

Part 1 – Administration

Part 2 – Use and Occupancy

Part 3 - Structural Loads and Procedures

Part 4 – Foundations

Part 5 – Housing and Small Buildings

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Part 3 – Structural Loads and Procedures

Design Loads and EffectsDead LoadsLive Loads due to Use and OccupancyDynamic LoadingEffects of WindEffects of Earthquake and Method of Analysis

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Method of Analysis Originally based on Uniform Building Code

(UBC) of the Structural Engineering Association of California (SEAOC)

However,

Superseded by new developments in earthquake analysis and design

Designs to be Performance-based

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Performance-based design

Behaviour of structure during earthquake should be as assumed in the analysis and design

Hence, need for revision of 1988 rules

BRRI(1990) - Code for Seismic Design of Reinforced Concrete Structures

A performance –based design Code2/21/2011 seismic code 9

3.0 - 2006 Seismic Code

A Revision of 1990 Code

Also performance-based, using Eurocode model, with necessary inputs dictated by local conditions

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Scope of 2006 Code

Publication – The Seismic Design of Reinforced Concrete Structures

– gives minimum design requirements

when earthquake action is considered critical,

-in conjunction with other permanent or variable loads(dead and imposed)

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Arrangement of Publication:

i. Seismic (Earthquake) Code

ii. Design Guide to use of Code made up of:

- Regular buildings – by Equivalent Static analysis

Frame system Wall system Dual system - Irregular buildings – by Dynamic analysis Frame system

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Analysis/ design considerations of 2006 Code

1. Seismic Actions

Seismic activity of a country is described

by means of:

Seismic Risk Zone Map

Design Seismic Action

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i Seismic Risk Zone Maps

Maps based on:

- historical records and/or geological and seismotectonic data

Ghana map based on :

- historical data and (for areas in Accra) also on geological data

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Seismic Zones Risk Map of Ghana

Ghana is divided into a number of zones according to seismic intensities. In this case, the intensity is the normalized ground acceleration, which has been given a constant value, within each zone.

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Seismic Zone

Assigned Horizontal Design

Ground Acceleration, A

( g unit of gravity )

0 0

1 0.15

2 0.25

3 0.35

Seismic Risk Map of Ghana

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ii. Design Seismic Action

Normalized Elastic Response Spectrum

Defined by the shape of an elastic response spectrum, normalized to a unit peak ground acceleration for rocky or firm soil conditions

A 5% damping has been applied to the response spectrum

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Normalized Elastic Response Spectrum

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Site Effects on elastic spectrum

Effect of site conditions on building response based on 3 soil profile types, known by their site coefficients, S

S1 – Rock S2 – Deep cohesionless or stiff clay S3 - Soft to medium stiff clays and sands

Site coefficients, S, are used to modify the standard/normalized elastic response spectrum to account for site conditions.

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Site dependent Normalized Elastic Spectra

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Site Coeff.

Soil Profile Type

S S1 S2 S3

1.0 1.2 1.5

2. Design Actions

a. Reliability Differentiation/ Importance Factor Target reliabilities are established based on the

consequences of failure of the building, considering both safety and serviceability.

Consequences of failure( both monetary and non-monetary) depend on:

use of the building their contents importance of their function both during and after the earthquake.

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Reliability / Importance Levels

Code recognizes two reliability levels.Class I : Buildings required to remain

functional and with reduced damage after strong earthquake

Class II : Buildings not included in the above class

The reliability levels to each Class is obtained by amplifying the design action with a factor, I, called importance factor

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Importance Factor

Class Importance Factor

I 1.4

II 1.O

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b. Ductility Levels

Ductility is the capacity of a member to undergo large inelastic deformations without significant loss of strength or stiffness

The Code allows structures to be designed to possess 3 different “ductility” levels according to these classifications:

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Ductility Level I(DL I) Associated with relatively large design forces so that

little inelastic response should occur; suitable for small buildings.

Ductility Level II( DL II) Structures that enter the inelastic range of response

under repeated reverse loading, while avoiding brittle type failure.

Ductility Level III(DL III) Associated with the development of selected stable

mechanisms as a result of large energy dissipation capacities. DL III structures should be preferred whenever large uncertainties exist.

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3. Structural Analysis

General form of analysis:

Dynamic method is applied in analysis.

Under this may be methods for;

1. Planar models

2. Non – Planar models

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If a building is regular, analysis can be simplified by restricting the analysis to the first mode of vibration of the building only; also known as “equivalent static analysis”

limits for applicability of equivalent static analysis;oHeight not more than 80moFundamental ( lowest) period shorter

than 2sec.

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4. Design Procedure

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THANK YOU

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