ICWES15 - Rules for the Povision of Earthquake Resistance in Small Buildings in Ghana. Presented by Carlien D Bou-Chedid, Ghana

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  • 1. Rules for the Provision ofEarthquake Resistance inSmall Buildings in Ghana byIng. (Mrs) Carlien Bou-Chedid Presentation at ICWES15 - Australia 2011

2. Seismic Activity in GhanaSource: USGS National Earthquake Information Centre & International Seismological Centre Although earthquakes are rare in Ghana, they do occur andcan produce very damaging consequences. The largest onrecord was Ms 7.1 in 1862 (Similar to Haiti). The mostrecent damaging earthquake was Ms 6.5 in 1939.Presentation at ICWES15 - Australia 2011 3. Areas at Risk Areas most at risk include regions where the capital cityAccra, two major port cities and two other regional capitalsare situated. These are also some of the most denselypopulated areas of the country. Structures in Ghana must therefore be built to withstandstrong earthquakes without excessive deterioration. The majority of buildings in Ghana are one and two storeybuildings. Professional Engineers are often not involved intheir design or construction. Poorly trained craftsmen areusually responsible for construction.Presentation at ICWES15 - Australia 2011 4. Factors Affecting Building Performance A study of building practice currently used in theconstruction of many small buildings suggests that they willbe unable to respond favourably in event of an earthquake. Building performance in earthquakes is affected by Architectural forms Structural forms Methods of construction Materials usedPresentation at ICWES15 - Australia 2011 5. Architectural Forms 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 ofthe building. Several decorative features such as balustrades, fin wall etcare also used. Presentation at ICWES15 - Australia 2011 6. Structural Forms, Materials &Methods Two basic forms are common. Unreinforced masonryusually tied at lintel level with a beam, and reinforcedconcrete. Materials are ordinary portland cement, mild steel,unprocessed aggregates and large masonry blocks (125mmX 200mm X 400mm) made from cement and sand. Material proportions for concrete may be determined byeye. Foundations can be shallow (750mm). Columns willoften be built to match wall thickness (125mm x 125mm).Stirrups and links are anchored in 90o bends. Presentation at ICWES15 - Australia 2011 7. Presentation at ICWES15 - Australia 2011 8. Vulnerability of CurrentBuilding Stock Most areas in the south of Ghana lie within a seismic zoneassigned a peak ground acceleration of 0.35g. Other zonesare 0.25g, 0.15g. These correspond to VIII, VII and VI on aModified Mercalli Scale.% of Buildings Reaching Each Damage State Damage State (Modified Mercalli Intensity) Sandcrete Block Reinf. Concrete VI VII VIII IX VI VII VIII IX0 None (No Damage) 15 0 0095 9050 201 Minor( Very little non-structural damage)55 5015 551025 252 - Moderate (Widespread non-structural damage)25 2525 15 -- 25 553 - Severe (Extensive non-structural & some structural damage) 52555 55 -- --4 Very Severe (Completely destroyed) -- 525 -- --Presentation at ICWES15 - Australia 2011 9. Comments Structural forms used in Ghana are a mixture of reinforcedconcrete construction and confined masonry construction. Reinforced concrete construction for earthquake resistanceis 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 specialskills are required. Masonry block walls confined by tiebeams and tie columns form the main lateral load resistingsystems. The masonry block unit must be load bearing.Presentation at ICWES15 - Australia 2011 10. Recommended Guidelines Engineering rules are to be presented in two separateguidelines. Guidelines for construction of single storey buildings are tobe based on confined masonry construction and may beused by craftsmen. Guidelines for construction of buildings up to two storiesmay be based on reinforced concrete design and must beimplemented by technologists. Both guidelines must give direction on good constructionpractice. Especially bending of stirrups and links to 135o . Presentation at ICWES15 - Australia 2011 11. RC buildings up to Two Stories These can be based on existing rulesGuidelines for Earthquake Resistant Non-Engineered Construction (Arya et al, 2010) Cover to steel in Columns and beamsshould be 30mm rather 25mm to reflect thevery aggressive atmosphere experienced inGhana. Fixing details will be required for theprestressed precast concrete systems incurrent use. Presentation at ICWES15 - Australia 2011 12. Rules for Single Story buildings Minimum wall thickness 125mm Tie columns and tie beams toflush with walls Teeth like indentation at the pointof connection with tie columns orsteel dowel bars Sandcrete blocks to be loadbearing. Minimum wall density of 5% toreflect uncertainty associatedwith sandcrete block Presentation at ICWES15 - Australia 2011 13. Conclusion and Recommendations Buildings in Ghana must be constructed towithstand earthquakes Guidelines for construction of buildings up totwo stories should be produced using reinforcedconcrete design as a basis. Technologists can beresponsible for construction. Confined masonry is recommended in Ghana forsingle storey construction as it is less demanding.Craftsmen can be responsible construction.