The earthquake in tiny Haiti on Tuesday 12 Jan. was devastating, causing buildings to topple and leaving possibly 100,000 dead, according to early estimates, though it is too early to say what the final toll is likely to be. Millions more are grimly affected by this grim tragedy.
Media coverage shows that even the Presidential Palace and the United Nations buildings in the capital Port-au-Prince have collapsed. Bernice Robertson, a senior analyst in Haiti for the International Crisis Group, spoke of “major damage to several buildings, which crumbled along the Delmas Road, a major street in the Metropolitan area”.
The major reason for loss of life during earthquakes is the collapse of buildings. In the 1991 Uttarkashi earthquake, around 48,000 houses were damaged, killing hundreds in the process. In the 2001 Bhuj earthquake, around 400,000 houses were destroyed completely and a much larger number were damaged. The death toll was at least 20,000 and the number of injured more than 200,000.
The earth’s outer shell or crust is divided into seven major and some minor tectonic plates which are continuously pushing against or pulling away from each other. This movement of plates results in building up stress. Earthquakes occur near such faults or fractures, where at some point the stress overcomes the friction and rocks slip, releasing seismic energy in the form of earthquake.
Hispaniola, Puerto Rico and the US Virgin Islands sit on top of small crustal blocks that are sandwiched between the North American and Caribbean plates. Indian Standard Code, IS 1893 (Part 1) 2002, divides India into 4 seismic zones( Zone 2,3,4 and 5) with Zone 5 having highest seismicity and Zone 2 having lowest seismicity. Kashmir, Punjab, the western and central Himalayas, the North-East region and the Rann of Kutch fall in Very High Damage Zone (Zone 5).
Most buildings are designed to resist their own weight and any live loads on them, and to some extent even wind loads. But they are not designed for earthquake loads. Earthquake loads (1) are inertia forces resulting from ground movements and they impose certain demands on the structures related to strength, ductility and energy. The magnitudes of these demands are highly variable and are dependent on the seismicity of the region and the dynamic characteristics of the structure.
The dead and Live Loads are vertical loads, whereas wind loads are horizontal loads. But earthquake loads have both horizontal and vertical components. The horizontal component of earthquake loads is very hazardous to a structure, as vertical component is resisted by the weight of a structure. Hence every structure, especially ones in earthquake zones, must be designed to resist these lateral earthquake loads.
IS 1893 (Part1) 2002, assigns a zone factor of 0.36 for Zone 5 and a zone factor of 0.24 for Zone 4. The 0.36 and 0.24 refers to the peak horizontal ground acceleration which is equal to 36% and 24% of acceleration due to gravity respectively. This is very important with respect to design of structures in Earthquake zones. Modified Mercalli Intensity Scale (MMI) described 12 levels of Intensity, the effect of earthquake on earth’s surface from instrumental, and feeble to disastrous and catastrophic.
In order to make buildings earthquake-resistant, the super-structure as well as foundation must be made to resist the sideways loads. During an earthquake, the lower part of a building tends to vibrate as it is in direct contact with the ground, but the upper portions remain static. This conflict of forces leads to collapse. As the magnitude of these forces is directly proportional to the weight of the building, the heavier the structure, the greater the damage.
Hence the roofs, walls, floors should be made as lighter as possible. Walls must be made to take sideways loads and they must be tied in frame and properly reinforced. If diagonal bracing is used to resist the lateral loads, then it must go equally in both directions. In case of moment resisting frames, joints should be made stronger than beams. Roof can be made lighter with profiled steel cladding on light gauge steel Zed purlins. Traditionally timber or plywood flooring is used to make light floors. A single storey building, if competently designed and built, will be able to resist Earthquake loads.
The GOI-UNDP Guidelines (4) for Jammu and Kashmir Engineers suggests certain measures for achieving seismic safety based on IS 4326 specifications. They suggest control on length, height and the thickness of walls, control on size and location of openings and control on material strength and quality of construction. Additionally, seismic bands are provided at plinth level, door-window lintel level and ceiling levels of floors.
The dynamic response of a building against an earthquake vibration is an important structural aspect which directly affects the structural resistance and consequently the hazard level. In order to design an earthquake resistant steel building, different methods can be used. The structural components capable to withstand lateral loads like shear walls, concentrically or eccentrically braced frames, moment resisting frames, diaphragms, truss systems and other similar systems should be used.
Compression structures like domes, vaults and arches should be avoided. The system needs to be tensile and the material flexible (like timber, steel and bamboo). The structure should be constructed in a way that it vibrates as one unit and sways together. Traditionally Northeast people followed this principle. These older houses had timber roofs held together by timber tie-bands, with horizontal timber beams spanning across the entire building, connecting the entire structure. They suffered very little damage during earthquakes. Traditional structures like Kuda, Thaat, Pherol, Chaukhat and Sumer of Garhwal Himalayas are the best examples of this (2).
Significant progress has been made in designing earthquake resistant structures. Base-Isolated systems, Passive energy dissipation systems, Active control systems are three new earthquake resisting technologies.
Conventional earthquake-resistant structural systems are fixed-base systems that are ‘fixed’ to the ground. But in base-isolated systems (3), the superstructure is isolated from the foundation by certain devices, which reduce the ground motion transmitted to the structure. These devices help decouple the superstructure from damaging earthquake components and absorb seismic energy by adding significant damping. In comparison to fixed-base systems, this technique considerably reduces damage to structural as well as non-structural component.
Energy Dissipating Devices/EDD (4) are like ‘add-ons’ to conventional fixed-base system, to share the seismic demand along with primary structural members. It reduces the inelastic demand on primary structural members, leading to significant reduction in structural and non- structural damage. In contrast, the active systems control the seismic response through appropriate adjustments within the structure, as the seismic excitation changes. That is, the active control systems introduce elements of dynamism and adaptability into the structure, thereby increasing the capability to resist earthquake loads.
These techniques have been successfully employed in many projects across the world. Japan has been foremost in this direction. They are also being used in earthquake prone areas of California, Indonesia and other places. It is high time these techniques are employed in the earthquake zones of India to reduce the damages from any future earthquakes in the sub-continent.
References
1] Durgesh C Rai, “Future Trends in Earthquake Resistant Design of Structures”, Seismology, 2000.
2] Prof Anand S. Arya, National Seismic Advisor, “Guidelines for Earthquake Resistant Reconstruction and New Construction of Masonry Building in Jammu And Kashmir State”, Home Ministry, 2005.
3] D.P. Agrawal and Manikant Shah, “Earthquake Resistant Structures of Himalayas”, Infinity Foundation, 2001.
4] Proceedings, seminar and workshop on seismic isolation, passive energy dissipation and active control; ATC-17, Applied Technology Council (ATC), Redwood City, CA, 1986.
Nitin Sridhar
The author is a student of civil engineering
