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Why did Margalla Towers collapse?

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Posted: 09 October 2005 at 9:24pm | IP Logged
 

Why did Margalla Towers collapse?

By Hammad Husain

A block of the Margalla Towers, the grey concrete building that greets the visitors approaching Sector F-10 from the Margalla Avenue, has collapsed, burying most of its occupants under rubble. Surprisingly, and luckily, all other buildings in the area — and in other areas of the city — managed to withstand the severe earthquake measuring a staggering 7.6 on the Richter scale that struck northern Pakistan on Saturday.

The fact that only one building fell victim to the earthquake raises some important questions. Before analysing the reasons, one needs to understand first why and how buildings collapse in the event of an earthquake.

Most earthquakes result from movement along the plane of faults inside the earth's crust. This sudden movement of the fault releases a great deal of energy, which travels through the earth in the form of seismic waves. At some time after their generation, these seismic waves reach the earth's surface, and set it in motion, which is referred to as "earthquake ground motion". When this earthquake ground motion occurs under a building and when it is strong enough, it sets the building in motion, starting with the building's foundation, and transfers the motion throughout the rest of the building in a complex way. These motions in turn create forces which produces damage.

The intensity and direction of the seismic wave, its frequency and amplitude (height), the type of surface through which it passes, the soil under the affected building are some of the factors that make it impossible for engineers to either predict the impact of an earthquake or to make "earthquake proof" buildings. At best, they make the buildings "earthquake resistant" where primary importance is given to protection of human life. In short, earthquake resistant building may suffer damage from a severe earthquake but they do not collapse the way Margalla Towers did.

If the earthquake ground motion is strong enough to shake up and damage buildings, then its effect, in most cases, is uniform on all the buildings in an area. Then why do some buildings collapse and others don't? The answer to this question lies in the structural design and construction of the building.

The world is broadly divided into 4 zones according to seismic activity risk. Islamabad, which for years was in the "moderate risk" category, was upgraded to "moderate-high" a few years back. However, most engineers in Islamabad failed to adapt to the change. I see three reasons for this (1) Making a building earthquake-resistant entails extra cost (2) The Capital Development Authority (CDA) has no strict building codes for seismic design, neither do they check or verify whether the design of a building is safe against an earthquake (3) The engineers do not usually bother to go into complex calculations as it requires extra effort and time - and with nobody to check, why care?

In mid-rise buildings, most Pakistani engineers only employ "shear walls" which are supposed to resist lateral forces and keep the building stiff. However, "shear walls" in regions with high seismic activity like Japan or Turkey (or Islamabad) are not enough to make a building earthquake-resistant. Since the devastating earthquakes of the recent past including Kobe (Japan) in 1995 and Turkey in 1999, engineers have devised various techniques for minimising risk of building collapse. These include base isolation, diagonal bracing, and passive damping. Base isolation works by separating the building from the moving ground. Teflon pads, large rollers, springs or other devices allow the ground to move under the foundation without transmitting all of that motion to the building (like pulling the tablecloth out from under the plate and having the plate stay still). Diagonal bracing helps resist shearing forces that come from the swaying motion of a shaken building.

The fact that only Margalla Tower collapsed while its neighbouring Al-Mustafa and Park Towers stood their ground indicates that the problem lies either in the structural design or the construction of the building and not in the seismic wave caused by the earthquake. In the larger context, the problem lies with the regulatory authorities like CDA that have been negligent in enforcing proper checks on design and construction of large-scale buildings. The Pakistan Engineering Council and the CDA need to review their building codes regulations and law enforcement and verification systems, especially for the high-rise towers and apartment blocks mushrooming all over the capital with alarming speed, and make sure they are structurally sound and earthquake resistant. This 'command and control' system should be made public so that greedy developers and callous engineers and contractors are not allowed to put precious human lives at risk.

The writer is a former chairman of the Institute of Architects, Pakistan (Islamabad Chapter)

http://www.jang.com.pk/thenews/index.html

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