Features
An integrated approach for buildings against natural disasters

by Asitha Jayawardena

(BSc Eng, MPhil, AMIESL, Associate Member of SSE-SL)

"Current popular practices in planning, design and construction of buildings in Sri Lanka have made the buildings we work and live today highly vulnerable to disasters – especially earthquakes," revealed Eng (Prof) M. T. R. Jayasinghe, Professor of Civil Engineering, University of Moratuwa, in his presentation on "Construction of buildings to resist natural forces including earthquakes" at the recent seminar on "Engineering aspects to be considered when reconstructing the tsunami devastated areas" organised by the Institution of Engineers Sri Lanka.

Among the undesirable practices he cited are double-storey entrance lobby of multi-storey buildings, slender columns adopted for floor space maximisation and aesthetics, car parks at basements, highly asymmetrical building plans, failure to coincide the building’s shear and geometrical centres, elongated building plans, slender (i.e., very tall) buildings, construction of adjacent multi-storey buildings touching or very close to each other, and inadequate tying of the building at foundation to ensure behaviour as one unit.

Design and construction of a building concentrating on one type of disaster can make the building highly vulnerable to another type of disaster, he warned, citing an example where exclusive focus on tsunami resistance severely undermines the building’s earthquake resistance. Therefore, he proposed an integrated approach for design and construction of buildings against all probable types of natural disasters. This article is based on his presentation.

Threats against buildings and their occupants

A building should be designed and constructed to provide safe shelter for its occupants. Therefore, it should perform satisfactorily both under normal and adverse conditions.

There are mainly two types of adverse forces that threaten buildings and their occupants, namely, natural and man-made. Natural forces include earthquakes, tsunami, cyclones, floods, landslides, floods, volcanic eruptions, lightening while man-made threats include bomb blasts, underground explosions and vehicle accidents. This article confines itself to natural forces only.

Earthquakes are the worst of the disasters because they are unpredictable. Earthquakes are of two types:

Inter-plate and intra-plate. Located away from well known tectonic plate boundaries, Sri Lanka is vulnerable only to intra-plate type earthquakes, which are due to local effects such as active faults.

The intra-plate type is less frequent, less severe but totally unpredictable while some level of prediction may be successful with inter-plate type.

Although earthquakes are not frequent in Sri Lanka, the country has experienced earthquakes. In Sri Lanka an earthquake on 14 April 1615 killed about 2000 people while the country experienced an earthquake measuring 4.7 on Richter scale in 1993. Moreover, many have felt earth tremors in December 2004. Even more disturbing are the recent reports of forming a new earthquake zone – 300 to 400km away from Sri Lanka. Therefore, there is a possibility of inter plate type earthquakes also being felt in Sri Lanka as tremors or minor earthquakes.

All buildings in Sri Lanka should be designed and constructed with a reasonable level of earthquake resistance because earthquakes can occur anywhere in the country. Although severe damage is acceptable in a massive earthquake, collapse of buildings should be avoided to prevent loss of lives.

Tsunamis are not frequent and affect only the coastal regions. They are predictable only if an early warning system and the people’s response to it are effective.

Cyclones are sudden but some level of prior warning can be obtained through weather reports. Countries located within 15 degrees of the equator are vulnerable to cyclones. In Sri Lanka, the Northern and Eastern provinces are the mainly vulnerable areas to cyclones. However, there is a possibility for a cyclone to divert its path and affect any part of the country.

Floods are not sudden and affect low-lying areas near rivers. Landslides, too, can give prior warning. Locations with steep slopes in wet zone are vulnerable to landslides. Volcanic eruptions are highly unlikely to occur in Sri Lanka. Lightning on the other hand is sudden and can hit any location.

An integrated strategy against natural forces

An integrated approach is essential for design and construction of buildings against natural forces. Suppose a building is designed and constructed to resist tsunamis where the ground floor is kept free of walls and the building is located in the upper floors. This will make it highly vulnerable to earthquakes since the ground floor supported on columns can suffer severe damage due to heavy load concentrated on the upper floors.

An integrated strategy against natural forces in summarised form can be presented as the following:

* Achieve a reasonable level of earthquake resistance for all buildings irrespective of location of construction

* Design for other forces the building is likely to encounter, namely, tsunami resistance for buildings coastal regions, cyclone resistance for buildings in cyclone-prone areas in North and East provinces, landslide resistance for buildings in steep slopes in wet zone, and flood resistance for buildings in low-lying areas near rivers.

* Threat of volcanic eruptions is highly unlikely and can therefore be totally ignored since earthquake resistant structures can have some degree of resistance against vibrations induced by volcanic eruptions, too, if ever volcanic eruptions occur.

* Lightning can strike anywhere. However, resistance against lightning can be obtained by simple means such as proper grounding, having enough number of trees around the building and therefore requires no special design.

A cost-effective approach for earthquake resistance

Wherever the building is located, it should possess a reasonable level of earthquake resistance due to several reasons. Firstly, earthquakes are unpredictable. Moreover, they can occur anywhere and can make the building collapse, causing loss of life.

Achieving earthquake resistance through earthquake resistant design for any possible earthquake is expensive. However, a dual design philosophy as can be found in Australian code is cost-effective.

The design philosophy we can adopt is as follows. Design for an earthquake that may occur during the life span of the building. For Sri Lanka, a magnitude of 5 to 6 Richter scale is sufficient. Then, ensure that the possibility of collapsing the building is very low in the event of a severe earthquake, which is a very rare occurrence.

For this, the building should be made robust and ductile and the strong elements (e.g. lift core, stair well) and columns should be prevented from collapse. In other words, in the event of a severe earthquake, severe damage is acceptable but not collapse, which will cause loss of life.

This approach is an integration of desirable structural forms, reinforcement detailing and planning practices together, and comprises the following:

* Tie the different parts of the building together so that the building does not fall apart when confronted by the horizontal forces of an earthquake acting on the building’s base.

* Enhance ductility of the building so that the building can effectively dissipate the energy transferred from the earthquake forces.

* Ensure that beams of the building develop plastic hinges before its columns. Make the chances of the columns failing as much remote as possible in order to prevent total collapse of the building. Do not optimise the columns as much as possible, but use columns of generous cross sections.

* Prevent adjacent buildings hitting each other.

Several measures for earthquake resistance for reinforced concrete buildings are:

* For plan of the building, adopt simple shapes (e.g. square, rectangle without much difference in width and length, circle) and avoid shapes like T, I or U.

* Arrange shear centre of the building to coincide with the geometric centre of the building’s plan (e.g., in a multi-storey building, arrange the lift core/ staircase as closest to the building plan’s geometric centre as possible)

* Avoid a building with elongated plan (e.g., avoid a rectangular plan with length "too long" and width "too short")

* Avoid slender and very tall buildings

* Avoid soft storeys (e.g., double height mezzanine floor for entrance lobby, car park at basement). If these are unavoidable, make columns of these soft storeys as bulkier and stronger as practically possible.

* Tie the building at the base itself

* Provide shear links for columns within beam depth as well

* Use full length reinforcement bars at bases, avoiding starter bars

* Do not construct multi-storey buildings touching or very close to each other

In brickwork buildings one-brick thick walls (220 to 230 mm thick) are sufficient for achieving a significant level of earthquake resistance. Several desirable measures for brickwork buildings are:

* Avoid many openings (i.e., doors and windows) on one particular wall

* Use tie beams of reinforced concrete round the entire building at the base of the building, plinth level, window sill level and lintel level.

* Support gable walls with return walls up to roof level (i.e., above ceiling level)

* Adopt reinforced concrete columns where large open spaces such as living room are required.

In both cases, the foundation should be properly designed taking the soil conditions of the site into consideration. Failure of the foundation during an earthquake will cause the building to collapse no matter how ever much the building possesses earthquake resistance.

An integrated solution against natural disasters

Earthquake resistance should be integrated with the other relevant resistances. For example, consider a three-storey brickwork building to be built in a coastal area. Besides earthquake resistance, the building should possess tsunami resistance.

For tsunami resistance, the building can be designed on stilts (i.e., columns only without much support from walls). However, this creates a soft storey at the ground floor level, undermining the building’s resistance to earthquakes.

Therefore, an integrated solution should be developed for this building:

* Arrange the building on stilts to achieve tsunami resistance.

* To strengthen the consequent "soft storey", make the stilts (i.e., columns of the open-plan ground floor) bulkier and stronger than necessary.

* Use the staircase as the strong element by arranging a brickwork wall round it. To coincide the geometric and shear centers, locate the staircase in the centre of the building

* To tie the building together, use reinforced concrete tie beams at plinth level and the window sill levels and lintel levels of the first and second storeys.

The building with both earthquake resistance and tsunami resistance is given in Figure.

Suppose the building will be located in coastal area in a cyclone-prone region. For cyclone resistance, a roof slab is more appropriate than a roof, which will "fly away" when confronted by a cyclone. However, the roof slab will adversely affect the indoor thermal comfort. For enhancement of indoor thermal comfort, roof insulation can be adopted. The building will thus possess earthquake-, tsunami-, cyclone resistance.

In summary, therefore, an integrated solution should be developed in design and construction of buildings in Sri Lanka. All buildings should possess a substantial level of earthquake resistance to prevent the building from collapsing in the event of a severe earthquake – severe damage is acceptable, but not collapse, which will kill people. Then the earthquake resistance should be blended with other relevant resistances, for example, tsunami resistance for coastal buildings.

Other presentations

At this seminar, Eng Dr (Ms) Premala Sivaprakasapillai spoke on "Design and construction of buildings to resist foreseeable wind forces", Eng Tudor Munasinghe on "Sewerage infrastructure associated with reconstruction", Eng Ananda Ranasinghe on "Appropriate contract documents to expedite construction", and Eng (Gp Capt) Mervyn Gunasekera on "Post disaster project management. Another article highlighting the important aspects of these presentations will be published later.

pic

Veteran musician Shelton Premaratne and his pupils performing at a musical workshop conducted for the benefit of the children in the tsunami affected areas in the Mount Lavinia area. The event was held recently at the Subodharama Vihara with the participation of Attorney Keerthi Udawatta, Provincial Council Member and Attorney Wasantha Ranasinghe.

(Pic by Bernard Chandrasekera, Dehiwala)

 

 

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