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Special report: Buildings kill, not tremors
On Oct 8, as the rest of Islamabad stood firm, a section of the elite residential buildings known as the Margalla Towers slumped to the ground like a pack of cards, becoming a tomb for its unsuspecting residents who considered it their “safe abode”. Within seconds, an earthquake had mercilessly brought down what was a luxurious and expensive building situated right in the heart of the most modern city in Pakistan, developed and looked after by the much talked about Capital Development Authority (CDA).
As blame for the disaster shifts towards the builders, and an FIR is lodged, one wonders what could have caused such a catastrophic failure. What made Margalla Towers, and all other buildings that couldn’t withstand the recent quake, so vulnerable?
In an effort to answer this, and other related questions, Sci-tech World has analysed for you the components of buildings that ensure the stability of their structures and also studied the causes of their failure. We talked to a number of local and international experts who gave us their views on building safety and stability during quakes.
The basics of any building begin with its foundations. With the increasing height of a building, the importance of stronger foundations becomes increasingly important. A foundation is basically a structure that transmits loads from the building to the ground. The foundations extend under ground and for larger buildings they even go into the bedrock, the hard surface of the ground.
Some types of foundations include walls that go beneath the surface. Others include piling (long stakes of concrete that go deep into the ground for support), slab and beam foundations. Some of the problems and dangers for the foundation include movement — such as oscillations through earthquakes and wind shears in the case of tall buildings — changes in water table, weakening soil or changes in soil moisture due to temperature. This is why a survey of the soil conditions is vital and buildings situated in areas having unconsolidated soils and old waterways are quite vulnerable.
Amongst other important structures in the building are beams, columns and joints. Beams are horizontal members that support loads. A column is a vertical load member or pillar that supports loads in vertical loading or in compression. Both beams and columns are made of concrete reinforced with steel rods for strength.
The strength of individual beams and columns is determined by the type of concrete mixture used and the reinforcement applied, all of which jointly determine the bending and load-bearing capacity of the structure. If you observe closely, a highrise is usually raised as a structure of beams, columns and roofs. The walls come later, to fill the voids between beams and columns. Joints basically join the beams and columns, producing a single structure.
Concrete is a composite building material made by combining aggregate with cement. A common form of concrete may contain gravel, sand, cement and water. The hydration and hardening of concrete develops in it physical and chemical properties like strength, low permeability and chemical stability. The compressive strength of concrete is quite high as compared to the tensile strength which is quite low.
Due to this, concrete usually fails in case substantial amounts of tensile stress are applied. However, strength is added to it by reinforcing it with steel bars. Reinforcement allows concrete to take additional tensile loads, adding to its overall strength.
The overall strength of the structure is hence highly dependent on the type and quality of the concrete mixture used (cement, gravel, sand and water, and their ratios), the reinforcement, and the type and number of the steel bars employed to bring in the required tensile strength. If quality is compromised to save a few bucks, the safety and strength of the whole structure gets compromised. The same applies to the concrete mixture used in foundations as quality has even greater implications there.
Joints are of extreme importance too. In other words, the way the foundation is connected to the building, the columns to the beams and the beams and columns to the roof is vital. If the joints are weak, the whole structure becomes a pack of cards which can ultimately come down.
What happens during quakes
The behaviour of buildings during earthquakes explains why they collapse more easily in tremors as compared to hurricanes. According to the Multidisciplinary Centre for Earthquake Engineering Research (MCEER), State University of New York at Buffalo, buildings fail because of their dynamic response to the ground motion. Failure of ground soil beneath buildings is also a major cause for failure. When the seismic waves of the quake reach the ground level, they cause “earthquake ground motion” which when occurs underneath the building triggers a complex motion starting right from the foundation to the top. The duration, amplitude of displacement, velocity and acceleration of the seismic waveform and frequency of the ground motion have the greatest impact on buildings.
When seismic waves spread through the earth’s various media, a number of waves of different frequencies begin to interact with each other. This distribution of frequencies in ground motion is called frequency content. The building that is now vibrating itself, possesses its own natural frequency. The shorter the building the higher its natural frequency and the taller the building the lower its natural frequency. When the ground vibrates with the same frequency as the natural frequency of the building, resonance occurs. This amplifies the magnitude of the whole vibration waveform. When in resonance or close to it, an object absorbs more energy and vibrates more easily. This is the reason why during an earthquake one building may collapse as nothing happens to the adjacent buildings. Put differently, this is so because the ground reaches the resonant frequency of one building and not the others. This is also why the response of a building to various frequencies needs to be analysed.
Buildings with higher natural frequencies, and a short natural period, tend to suffer higher accelerations but smaller displacement. Conversely, buildings with lower natural frequencies, and a long natural period, will experience lower accelerations but larger displacements.
MCEER further says that it is not the displacement of the building that causes the real damage but the acceleration of the displacing motion. It is associated with the example of a rug being pulled out from underneath a person. If it is done very suddenly the person would fall immediately. If, however, the rug is pulled slowly and at an increasing speed, you would get pulled much farther before going off balance. Inertia or the building’s resistance to motion also plays a role in creating the strain on its structures. A free-standing structure would overcome its inertia and move in the opposite direction when subjected to ground vibrations, whereas a fixed structure needs to absorb this energy within itself.
Flexibility and ductility of the buildings play a major role in earthquake resistance. The taller a building the more flexible it is as compared to a stiff shorter building. However, ductility determines how much deformation a structure can take. If a structure is rigid, it breaks under lesser forces as compared to a ductile structure that can bend but does not rupture. Earthquake resistant designs need to have enough ductility to withstand the deformations caused due to ground vibration.
Dampers play an important role in absorbing vibrations and reducing shock. Consider the shock absorbers in your car, which absorb the ground vibrations and provide a smooth ride internally. Similarly, buildings need to have dampers — a technique which is also known as Base Isolation.
Lead rubber bearings are amongst the frequently used base isolation devices. They consist of layers of rubber sandwiched between steel layers and have a lead plug in the middle. Swaying does not occur in the complete building in a base isolated structure. It is the rubber that takes the lateral deformation and the building remains safe. Since rubber is quite flexible, it does not get damaged. Lead, on the other hand, absorbs the kinetic energy and converts it to heat and with a reduction in energy, the bearing acts as a damper.
What experts say
Explaining why certain types of buildings fail during quakes, Dr Taiki Saito, chief researcher for the International Institute of Seismology and Earthquake Engineering, Japan, says: “In Pakistan as well as other developing countries, buildings are mainly designed for supporting their own weight (acting in the vertical direction) and not designed well for earthquake load (acting in the horizontal direction). Therefore, brick walls or slender columns easily fall down under horizontal loading in the case of earthquakes. Lack of seismic design code and low discipline of engineers are other factors for low seismic resistance of buildings.” He suggests that concrete walls arranged from the bottom to the top may be the best solution for existing buildings. The design of public infrastructure in Japan is governed by the Building Standard Law to ward off potential failure during quakes. The law is implemented strictly, he added.
Sci-tech World asked William T. Holmes of the Rutherford and Chekene Consulting Engineers —a member of the Earthquake Engineering Research Institute and an expert on structural engineering — about building codes. He says, “The building codes and practices differ from country to country. However, due to worldwide communication and international education opportunities, most countries today possess adequate knowledge about earthquake engineering. Similarly, most countries have adequate building codes. The real problem is implementation.
Only a few countries enjoy adequate control of construction practices, particularly when small buildings and individual residences are involved. The construction of self-built shelters from un-reinforced stone or masonry goes unchecked and the buildings are seldom tied together sufficiently for earthquake protection. As a result, when even one wall falls over, the roof or floor collapses.
Similarly, in many countries there are no controls ensuring that buildings are designed in accordance with the code. And even fewer controls exist that deal with the quality of construction. In addition, the location of buildings or entire villages generally remains uncontrolled. Consequently they are often located in areas in striking distance of landslides or mudslides.
Linking the lack of earthquake safety practices to economic factors, Holmes says: “The risk from earthquakes worldwide is generally not a technical problem but (rather) an economic and social one. The expertise exists worldwide to design and build structures that will not collapse —from a family hut to A highrise. Eventually, the safety of the built environment comes down to cost.
“How safe is safe enough? Building for rare disasters has a cost and since they only occur occasionally, many governments cannot maintain a strong programme over a long period of time. It is often more politically palatable to get a road open than to fix it for the next earthquake, or to get people sheltered quickly rather than getting them a shelter that will not collapse the next time.”
Holmes adds, “Making buildings earthquake safe is complex because of the many kinds of buildings in the world. First, the building elements — roof, floors, columns and walls — have to be well tied together so that shaking will not make them fall apart. Secondly, there has to be a structural system that resists lateral or side-to-side forces which are created when the ground shakes. This lateral system can be solid walls, diagonal braced systems, or beam and column frame systems. The design of these lateral force systems are strictly controlled by seismic codes to make them tough and resistant to shaking.
“Lastly, the lateral system of the building must have a proper configuration. It can’t be located at one end of the building, which will force the building to twist during an earthquake. Also, it can’t be very stiff and strong in the upper floors with a single flexible and weak level at the bottom. This is called a soft or weak storey and could cause a disaster.
“This is particularly true in countries that use masonry walls in concrete frame buildings, because the masonry in upper floors makes the frame stiff and the first floor is flexible because it doesn’t have the same wall layout. Lateral forces (parallel to the ground) act on the structure due to shaking. The inertia of the structural mass tends to prevent the top of the building from moving as fast as the ground and the structure is thus distorted. This sideways movement between floors tends to distort walls, windows and columns and if the movement called drift is sufficient, damage will occur. If a wall, rectangular in elevation, is forced into a parallelogram by drift of the floor above, cracks will develop.”
Prof Sarosh H. Lodi, head of the civil engineering department, NED University of Engineering and Technology, reveals that the country lacks appropriate laws. “Unfortunately there is no law to enforce building safety. Civic agencies do have their rules or norms, but the important thing is that we don’t have a building code. First we have to enforce legislation and then develop a building code, material specifications and load estimation specifications.”
To be sure, a majority of the houses and buildings that were razed to the ground in the quake were not designed properly, for obvious reasons. But the failure of Margalla Towers can simply not be justified. Newspaper reports say that some residents of the ill-fated building had notified the CDA about its many shortcomings and faults many months ago. But the complaints fell on deaf ears. Perhaps the authorities felt that quakes with high intensity were not possible in the country, hence the criminal neglect.
With highrise buildings gaining popularity in Karachi, Lahore and Islamabad, it is time that we got out of our slumber and did something about the structures of our buildings. It is surprising that the government has yet to order inspection of the existing buildings for safety.
Many of our major cities are within striking distance of the seismic faults zones. If even now we don’t do anything worthwhile about building safety we will be courting disaster.
The writer, an engineer, is a freelance journalist. Email: umer.asif@gmail.com
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