| Quake-resistant Construction A Mitigation Strategy Nikhill Philip Koshi
The loss of life and property are just the “tip of the iceberg”, so to speak, of the consequences that these disasters bring. Beneath the surface lie even greater challenges, challenges of reconstruction, rehabilitation, generating new livelihood opportunities and primarily the greatest test of picking up pieces, re-working life and moving on. Development Alternatives' work in these natural calamities has taught us many things. Disasters leave societies extremely vulnerable to hardships in all areas - physical, economic and social. And, our work is to put forward a response, a strategy for rebuilding these lives and creating livelihoods that work and are sustainable as well as strong. The Mechanisms of Response The four R’s in terms of the mechanisms of response are: Relief Reconstruction Rehabilitation Readiness What we see and learn from the ferocity of tragedies, both man-made and natural, is that they can be averted through planning and preparation. Negative impacts of disasters can be managed if social, ecological and economic consequences of our actions are considered and the decisions made accordingly. But while we can be as prepared as humanly possible, we really can not completely eliminate the impact that they have. The Geography of the Sub-Continent The sub-continent lies at the north-western portion of the Indo-Australian plate which encompasses India, its neighbours, Australia, a major portion of the Indian Ocean and other countries. This plate is colliding against the huge Eurasian plate and going under it; this process of one tectonic plate going under another is called subduction. When continents converge, a large amount of shortening and thickening takes place. The three main tectonic sub-regions of India are the Himalayas in the north, the gangetic planes and the peninsular region. Most quakes occur along the Himalayan plate boundary (called inter-plate earthquakes) but many have also occurred in the peninsular region (known as intra-plate earthquakes). The Home… The concept of a house, dwelling, home or whatever we call it, is our very own shelter and space on this planet. And, a horrifying aspect about these disasters is that these structures of security and comfort become the chief weapons against life. In reality, no building can be earthquake-proof but it can be made finitely resistant to resist quakes of particular magnitudes. The main objectives should be to check the building from collapsing as well as prevent the loss of life. This article takes lessons from the occurrences of the month to provide answers and explanation for the home, and how simple strategies can be adopted to make buildings resistant and strong to withstand the impact of tremors. Oscillations of Buildings During earthquakes, the ground shakes. This implies that the base of all buildings would move along with the ground and that the building would swing back and forth with the tremors. Therefore, if the building was rigid, every point on the building would move by the same amount as the ground. But, most buildings are flexible and that means different parts of the structure oscillate by varying amounts and not in consonance with the ground. The tremors during earthquakes contain a mixture of different types of sinusoidal waves, ranging between short to long waves (.03-33 sec.). The time it takes for the wave to complete a cycle of motion is called the period of earthquake wave. Yet, even within this range, some earthquake waves are stronger than others. But, the intensity of these waves at a particular location depends on many factors, including the magnitude of the quake, the epicentral distance, the type of ground the waves travel through and even the soil type and its thickness. Another extremely important factor governing quake resistance is the Fundamental Natural Period of the building. To understand this concept, imagine a fat rope tied at one end to the roof of a building and the other end fastened to a motor vehicle with huge torque say a tractor. Start the vehicle and pull the building (For a more flexible building, the movement is larger). Cut the rope and measure the time taken for the building to complete one back-and-forth horizontal oscillation. The time it takes to complete this motion is called the Fundamental Natural Period (or `T') of the building. The value of T is directly proportional to the flexibility and mass of the building. Thus, the greater the mass and flexibility of a structure, the longer is the T. Therefore, by combining these factors, it can be said that depending on the value of T and on the characteristics of the earthquake ground motion (period and amplitude of quake waves), some buildings will be shaken more than others. Flexible buildings undergo larger displacements, which result in damage to the building’s non-structural elements like glass windows and unsecured shelves which cannot take large lateral movements. These damages may not affect the safety of the building but cause economic loss and injuries. Impact upon Open Ground Storeys Recently, a new form of building has emerged in urban India. Reinforced concrete structures with a special feature - open ground storeys for parking. This means columns in the ground storey do not have any partition walls between them. These buildings have two distinct characteristics: n Relatively flexible ground storey, which means the horizontal displacement it undergoes in this storey is much larger than the rest of the building. n Weaker ground storeys also imply that the total horizontal earthquake force it can carry is much smaller than the rest of the structure. This type of architecture has consistently shown poor performance during the previous quakes all over the world, and a significant number have collapsed. Yet, a large number of these building have been built in India in the recent years. The presence of walls in the upper storeys makes them much stiffer than the open ground storey. Therefore, the upper portion moves as a single block and most of the horizontal displacement of the building occurs in the ground storey itself - just like a building on chopsticks. The problem with open ground storey buildings is that they are inherently poor systems that have a low stiffness and strength in the ground storey. Thus, such buildings swing back and forth during quakes and the columns at the ground are severely stressed. If the columns do not have the adequate strength and ductility, they may be damaged and this could even lead to a collapse of the entire structure. A way to avoid this problem would be ensuring a continuity of walls at the ground level and strengthening the existing ground storey, to prevent them from collapsing during strong earthquake shakes. The Short Column Effect Quake resistance is also greatly reduced in buildings that have columns of different heights within one storey. Past earthquakes have shown that these types of columns have suffered greater damage when the column is short as compared to being of regular size in the same storey. If short and tall columns exist within the same storey level, then the short column attracts several times larger horizontal force to move the column and suffers more damage compared to the taller ones. This is known as the short column effect. The short column effect occurs in columns that support mezzanine floors or loft slabs that are added between two regular floors. Another special situation in buildings where short columns effect occurs, is when adjacent columns behave as short columns due to the presence of walls of partial height (usually to accommodate a window in the remaining space). In many cases, other columns in the same storey are of regular height as there are no walls adjoining them. Thus, when the floor slab moves horizontally during an earthquake, the upper ends of these columns undergo the same displacement. However, stiff walls restrict this movement and the column deforms by the full amount over the short height adjacent to the window, whereas regular columns deform over the full height. Since the effective height over which a short column can freely bend is smaller, it offers more resistance to the horizontal motion and therefore, attracts a larger force as compared to the regular column and as a result, short columns sustain more damage. The solution to this issue of column height can be addressed by retrofitting solutions to avoid the damage. Where walls of partial height are present, the simplest way out is by: n Closing the openings by building a wall of full height - this will eliminate the short column effect. n If that is not possible, we need to strengthen the short columns, using one of the well established retrofit techniques. Shear walls in Seismic Regions Reinforced concrete buildings often have vertical plate like structures called ‘Shear Walls’ in addition to slabs, beams and columns. These walls start at the foundation level and are continuous throughout the building height. Their thickness ranges from 150 mm to 400 mm, depending on the structure it is used for. Shear walls are like vertical wide beams that are used to carry earthquake loads downwards to the foundation. Properly designed buildings with shear walls have shown very good performances in past earthquakes. These walls in seismic regions require special detailing. However, in past earthquakes, even buildings with sufficient amount of walls that were not specifically detailed for shear walls were saved from collapse. These walls are easy to construct, because reinforcement detailing of shear walls is pretty straight-forward and can therefore be easily implemented. They are also effective both in terms of cost and effectiveness in minimizing quake damage, both in the structural and non- structural elements of the building. Since shear walls carry large horizontal earthquake forces, the overturning effects on them are large. Thus, their design requires special attention. These walls should be provided along both the length and the breadth of the building but, however, if they are only provided in one direction, a proper set of beams and columns in the vertical plane must be provided along the other direction to resist strong earthquake effects. In Conclusion Though this article tried to give you a few tips to quake-resistance, I recently came across something which I found interesting to close this article: `The team which includes A. S. Arya, Chief Advisor of the Union Home Ministry’s Disaster Management Cell and D. K. Paul of IIT Roorkee, found that houses built in Bajji-Diwari system had survived the quake. The system, with wooden frames and special nogging (placement of bricks), “proved to be quake-resistant”, Arya said. “Though Uri is in ruins because of indiscriminate and excessive use of mortar and stones for walls, in Baramulla we found that buildings constructed in the Bajji-Diwari style had suffered the least damage. Some of them, even old ones, developed only a few cracks,” he added.q | ||
