icwes15 - rules for the povision of earthquake resistance in small buildings in ghana. presented by...

Rules for the Provision of Earthquake Resistance in Small Buildings in Ghana

by

Ing. (Mrs) Carlien Bou-Chedid

Presentation at ICWES15 - Australia 2011

Although earthquakes are rare in Ghana, they do occur and can produce very damaging consequences. The largest on record was Ms 7.1 in 1862 (Similar to Haiti). The most recent damaging earthquake was Ms 6.5 in 1939.

Seismic Activity in Ghana


Source: USGS National Earthquake Information Centre & International Seismological Centre

Areas most at risk include regions where the capital city Accra, two major port cities and two other regional capitals are situated. These are also some of the most densely populated areas of the country.

Structures in Ghana must therefore be built to withstand strong earthquakes without excessive deterioration.

The majority of buildings in Ghana are one and two storey buildings. Professional Engineers are often not involved in their design or construction. Poorly trained craftsmen are usually responsible for construction.

Areas at Risk


A study of building practice currently used in the construction of many small buildings suggests that they will be unable to respond favourably in event of an earthquake.

Building performance in earthquakes is affected by Architectural forms

Structural forms

Methods of construction

Materials used

Factors Affecting Building Performance


Buildings are individually designed. They are often irregular in plan. Water may be stored in water tanks on the roof. Gable walls are common. Large window and door openings are used. Open car ports and garages can often be an integral part of

the building. Several decorative features such as balustrades, fin wall etc

are also used.

Architectural Forms


Two basic forms are common. Unreinforced masonry usually tied at lintel level with a beam, and reinforced concrete.

Materials are ordinary portland cement, mild steel, unprocessed aggregates and large masonry blocks (125mm X 200mm X 400mm) made from cement and sand.

Material proportions for concrete may be determined by eye. Foundations can be shallow (750mm). Columns will often be built to match wall thickness (125mm x 125mm). Stirrups and links are anchored in 90o bends.

Structural Forms, Materials & Methods


Most areas in the south of Ghana lie within a seismic zone assigned a peak ground acceleration of 0.35g. Other zones are 0.25g, 0.15g. These correspond to VIII, VII and VI on a Modified Mercalli Scale.

Vulnerability of Current Building Stock


Damage State

% of Buildings Reaching Each Damage State

(Modified Mercalli Intensity) Sandcrete Block Reinf. Concrete VI VII VIII IX VI VII VIII IX

0 – None (No Damage) 15 0 0 0 95 90 50 20

1 – Minor( Very little non-structural damage) 55 50 15 5 5 10 25 25

2 - Moderate (Widespread non-structural damage) 25 25 25 15 - - 25 55

3 - Severe (Extensive non-structural & some structural damage) 5 25 55 55 - - - -

4 – Very Severe (Completely destroyed) - - 5 25 - - - -

Structural forms used in Ghana are a mixture of reinforced concrete construction and confined masonry construction.

Reinforced concrete construction for earthquake resistance is complex. RC frames form the main load resisting system. Special detailing is required and it requires special skill.

Confined masonry construction is much simpler no special skills are required. Masonry block walls confined by tie beams and tie columns form the main lateral load resisting systems. The masonry block unit must be load bearing.

Comments


Engineering rules are to be presented in two separate guidelines.

Guidelines for construction of single storey buildings are to be based on confined masonry construction and may be used by craftsmen.

Guidelines for construction of buildings up to two stories may be based on reinforced concrete design and must be implemented by technologists.

Both guidelines must give direction on good construction practice. Especially bending of stirrups and links to 135o .

Recommended Guidelines


RC buildings up to Two Stories These can be based on existing rules

“Guidelines for Earthquake Resistant Non-Engineered Construction” (Arya et al, 2010)

Cover to steel in Columns and beams should be 30mm rather 25mm to reflect the very aggressive atmosphere experienced in Ghana.

Fixing details will be required for the prestressed precast concrete systems in current use.


Rules for Single Story buildings Minimum wall thickness 125mm Tie columns and tie beams to

flush with walls Teeth like indentation at the point

of connection with tie columns or steel dowel bars

Sandcrete blocks to be load bearing.

Minimum wall density of 5% to reflect uncertainty associated with sandcrete block


Conclusion and Recommendations Buildings in Ghana must be constructed to

withstand earthquakes Guidelines for construction of buildings up to two

stories should be produced using reinforced concrete design as a basis. Technologists can be responsible for construction.

Confined masonry is recommended in Ghana for single storey construction as it is less demanding. Craftsmen can be responsible construction.

icwes15 - rules for the povision of earthquake resistance in small buildings in ghana. presented by...

Technology

icwes15 australia

construction of buildings

confined masonry construction

architectural forms

rc buildings

recommendations buildings

majority of buildings

masonry block walls