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The dilapidated failure of the twin towers of the world trade center in USA due to the terrorist attack on September 11, 2001, have put immense pressure on designers of future high rise buildings to explicitly ensure specified levels of safety against progressive collapse: impact, blast, fire, etc. The particular system where progressive collapse is triggered by the floor system, disengaging from its support over all part of the building, as appeared to be the case for the world trade center.
This research abstract brought a method involving evolutionary and "Pareto" optimization to analyze the load-path safety of high-rise commercial office buildings against progressive collapse under abnormal loading. The study was motivated by the progressive-collapse failure of the twin towers of the World Trade Center in New York on 11 Sep, 2001; consider the same as the best exemplary for being provoked. The assessment of load-path safety against progressive collapse is based on the degree of force redundancy that the structural system of a building has, optimal tradeoff surface formed by a population of conceptual designs for a particular office building project is established in the three dimensional space of capital cost and operating cost and income revenue. The "gray scale" filtering of the cost-revenue trade-off surface is employed to highlight the relative safety of the different building designs, thus design of buildings to withstand or delay progressive collapse under abnormal loading.
Factors that Promotes or Hinders Safety Levels in High-Rise Buildings:
ƒ Cyclones, Tsunamis, Volcanoes
ƒ Power Failure
There are so many natural calamities such as Cyclones, Tsunamis, Volcanoes but Earthquakes and Fire hazards are more devastating then them.
Earthquakes and Fire calamities have already acclaimed thousands of life in last 10 years. The figures death toll of 19th century and 20th century are not that authentic.
An earthquake is a rapid shaking of the Earth caused by the breaking and progressive shifting of rock under the surface of Earth. For hundreds of millions of years, forces of tectonics plates have shaped the Earth as the huge plates that form the Earth's surface move slowly over, under and past each other. Sometimes the movement is sudden, at other times; the plates are locked together, fails to release the accumulating energy. When the accumulated energy grows strong enough: plates break free causing ground to shake. Most earthquakes occur at the boundaries where the plates meet; however, some earthquakes occur in the middle of plates. Ground shaking from earthquakes can collapse buildings and bridges, disrupt gas, electric, other services, and sometimes cause landslides, avalanches, flash floods, fires, and huge, destructive ocean waves, Buildings with foundations resting on un-consolidated landfill and other unstable soil. Trailers and homes not tied to their foundations are at risk because they can be shaken off their mountings during an earthquake. When an earthquake occurs in a populated area, it may cause deaths and injuries and extensive property damage.
The dynamic response of building to earthquake ground motion is the most important cause of damage to buildings. The damage that a building suffers primarily depends not upon its displacement, but upon acceleration. Whereas displacement is the actual distance the ground and building may move during an earthquake and acceleration is a measure of how quickly they change speed as they move. The conventional approach to earthquake resistant design of buildings depends basically upon providing the building with strength, stuffiness and less elastic deformation capacity which are great to with stand a given level of generated force by earthquake.
In simple words: "Earthquakes are simply ground oscillations of very large amplitude and rather low frequency. The predominant forces of nature trigger, mode of excitation is horizontal, not vertical as in normal ground borne noise."
Causes of Earthquakes
Earthquakes are caused by active faults, which are, caused by the sudden movement of the two sides of a fault with respect to another. The occurrence of tectonic earthquakes can be explained by the theory of elastic rebound. As it has been stated that ""The motion along the fault is accompanied by the sudden buildup of elastic strain energy within the rock along the fault, the rock stores this strain energy like a giant spring being slowly tightened.""
The stress along the fault exceeds the limit of the rocks at that point to store any additional strain. The fault then ruptures i.e., it suddenly moves a comparatively large distance comparatively short amount of time. The rocky masses which form the two sides of the fault then snap back into a new position. This snapping back into position, upon the release of strain, is the ""ELASTIC REBOUND"" theory. The rupture of fault results in sudden release of the strain energy that has been built up over the years, the most important form which this sudden release of energy takes is that of produced by seismic waves.
ƒ ACTIVE FAULTS: They are caused because of fault lines passing through the tectonic plates.
ƒ MOVEMENTS OF TECTONIC PLATES: They are caused because of tectonic plates are continuously floating on the mantle and thus they are set in motion.
ƒ VOLCANIC ERUPTIONS: They are caused due to internal pressure building up inside the Earth's crust.
ƒ SURFACE AND SUBSURFACE EXPLOSIONS: They are caused due to man made explosions such as blasts, tunneling and such man made stress.
SEISMIC WAVES AND ITS TYPES
When fault ruptures which cause earthquakes, the sudden breakage. Movement along the fault can release tremendous amount of energy, some of this energy is used up in cracking and pulverizing the rock as the two blocks of rock separated by the fault grind past each other. Part of the energy, however, speeds through the rock as seismic waves can travel for and cause damage at great distances, these waves continue through the earth until their energy is used up.
There are two basic types of seismic waves, and they travel at different speeds through earth. The faster p waves and the slower s waves.
Primary, push waves "P waves":
These are longitudinal in nature like sound waves. The velocity of P waves is highest about 5.4 km/s and depends on the density of the rock and resistance to compression. P waves can pass through liquids also.
Secondary or shake waves or S waves:
These are transverse in nature like light waves. The velocity of S waves is about 3.3 km/s, the velocity of S waves depends upon density of the rock and resistance to distortion of S waves cannot pass through liquids.
L waves or Raleigh Waves: - These are also transverse in nature like S waves. The velocity of S waves is about 3.0 km/s, L waves are formed due to dashing of P and S waves against the solid crust of the earth. These are the waves which we feel in the form of earthquake and are responsible for the destruction of the life and property. The height of L waves is about 30cm and distance between two successive crests is about 10 m and increase in their amplitude beyond only 1/16 of an inch is capable of causing lot of destruction. Thus high velocity of these waves cause the civil engineering structures vibrate and a typical sound due to passage of energy is heard.
ƒ Ground Motion: - The most destructive of all earthquake hazards is caused by seismic waves reaching the ground surface at places where human-built structures, such as buildings and bridges, are located. When seismic waves reach the surface of the earth at such places, they give rise to what is known as strong ground motion.
ƒ Ground Sliding: - These damages to the soil and ground can take a variety of forms such as cracking and fissuring and weakening, sinking, settlement and surface displacement.
ƒ Ground Tilting: - Sometimes due to earthquake, there is tilting action in the ground. This causes plain land to tilt causing excessive stresses on buildings, resulting in damage to buildings.
ƒ Differential Settlement: - If a structure is built upon soil which is not homogeneous, then there is differential settlement with some part of the structure sinking more than other. This induces excessive stresses and causes cracking.
ƒ Liquefaction: - Compaction of the soil may result in the development of excess hydrostatic pore water pressures of sufficient magnitude to cause liquefaction of the soil, resulting in settlement, tilting and rupture of structures
For Earthquake Resistant Buildings: -
The first thing is to make the highest bit, the roof, as light as possible. This is best done with profiled steel cladding on light gauge steel poopers with a roof slope between 3 and 15 degrees. If it is required to have a 'flat' roof, this could be made with a galvanized steel decking and solid insulation boards, and topped with a special membrane. Even a 'flat' roof should have a slope of about 2 degrees. Then an "RC slab" can be poured over the roof, the slab will only be say 110mm, 4 ½", and will weigh only about 180 kg/sqm. Such a slab will be completely bonded to the frame and will not be able to slip off, or collapse. If the building is a normal single storey one, then any normal portal frame or other steel framed building, if competently designed and built, will be able to resist Earthquake loads, more needs to be done to ensure its survival in an earthquake. With the roof, the floors should be made as light as possible. The first way is to use traditional timber joists and timber or chipboard or plywood flooring, If this is done it is vital that the timber joists are firmly through bolted on the frames to avoid them slipping or being torn off. The frame needs them for stability and the floor must never fall down. A better alternative is to substitute light gauge steel joints for the timber joists. These can span further and are easier to bolt firmly to the framework. Then, floor-boards or tongue-and-groove chipboard can easily be screwed to the joints, however in Motels, Apartment buildings, Offices, concrete floors may be needed. In such cases we should reduce the spans to the spanning capacity of composite decking flooring, and pour reinforced concrete slabs onto our decking. The decking is fixed to the joints, the joints into the main beams, the main beams into the columns and the concrete is poured around all the columns. There is simply no way that such floors can fall off the frame.
Steel would fix the main beams to the outer columns with full capacity joints. This will almost always mean crosslink the connections. Great care would be taken to consider the shear within the column at these connections. The connections should be equally strong in both up or down directions, and the bolt arrangement should never fail before the beam or the column. In extreme earthquake sway, the beams should always be able to form hinges. In this way the frame can deflect, the plastic hinges can absorb energy and the resonant frequency of the structure is altered. All without collapse or major loss of strength. All this takes a little time until the tremor passes. Then, the floors are fitted, low weight or conventional cladding is fitted to the frames. Light-weight or thin concrete roofs are fitted as described. if we have a building that will behave very well in an earthquake, but nothing can be guaranteed to be able to resist any possible earthquake, but buildings like the ones proposed here by Steel would have the best possible chance of survival and would save many lives, providing greater safety from an earthquake.
Fire Calamities: -
The construction of tall and even super-tall buildings, particularly in the Gulf and Far East regions, raises new fire safety concerns for those responsible, Fires on construction sites have, for many years, been recognized as a serious problem for construction companies and insurers, but less of a public safety issue since there are relatively few fire deaths in such incidents. However, the trend for ever-taller buildings, particularly in the Gulf and Far East, has resulted in a number of significant fires during construction which have caused deaths and demonstrated the difficulties facing fire brigades when tackling such incidents. On several occasions, helicopters have had to be used for fire suppression and rescue.
Some of the taller buildings of Far East are:-
ƒ Al Burj/Nahkeel Tower 1400m high
ƒ Murjan Tower, 1022m high
ƒ Tower of Russia, 612m high
Hazard and Risk Assessment
One issue that has taken some time to be fully appreciated by the construction industry is how vulnerable high-rise buildings can be. Timber shuttering burns extremely well; membranes used in damp proofing and their adhesives which are combustible; and materials are delivered to site in disposable packaging that is also combustible. It is a unique challenge because of the difficulties of risk, taking into account the changing nature of the site activity and the ever-increasing financial exposure that increases as a project approaches completion.
Fire Safety Problems
The causes of fires during tall building do not differ radically from, conventional single floor buildings. different is the scale and extent of the problems and complications regarding egress for the workforce, fire brigade civil defense access and water supplies and firefighting.
Causes can involve electrical risks, smoking, rubbish burning, overheating equipment and escapes of gases or flammable liquids, all of which are well recognized and should be amenable to normal fire safety measures, as such specific risk areas in tall buildings include:
ƒ Slip and jump forming equipment, and other systems with large hydraulic fluid requirements
ƒ Storage of building materials within the structure
ƒ Temporary buildings within the structure
ƒ Diesel- and petrol-powered generators, compressors, welding machines and tools
ƒ Part-occupation, particularly of basements, for car parking, storage, workshops and offices
ƒ Discarded timber storage.
Risk Management: -
ƒ Storage of Materials: - While off site storage space and transport can be expensive, this is always to be preferred to on-site storage, Although much of the material used for construction may not be easily combustible, the packaging that protects it usually is. In addition to cardboard and plastic sheet, the contribution to the fuel load of palettes and cable drums should not be overlooked. In another location, 350 electrical control cubicles were protected from transit damage by timber frames, thick plastic sheet and plastic tarpaulins. Not only would this packaging have provided fuel for a 5MW fire and the plastic materials would have generated huge volumes of acidic and toxic smoke.
Removal of Rubbish: - Removing rubbish is one of the most commonly encountered hazards and disaster caused. Perhaps the only reasonable excuse for non-compliance is in very tall buildings where hoist access may be restricted. One solution is to permit the limited accumulation of waste in certain specified areas on specified floors, and for an administrator to be tasked to remove the waste when hoists are less in demand, usually at night.
No-Smoking Policy: - Smoking should be banned in all, only where permitted by law, restricted to designated areas.
Flammable Gases: - Consideration should be given to restricting or even banning the use of oxyacetylene in favor of other equipment, such as oxy-propane. If it is essential it should be possible to set up a procedure to ensure that "acetylene" is brought to the building for the work only and removed at the end of every working day.
Implementation of Sprinklers: -
Types of Sprinkler
ƒ Non Sodden pipe: - These are the most common systems and are used in buildings where there is no risk of freezing. They are quick to react because water is always in the pipes above the sprinkler heads. Wet systems are required for multi-storey or high-rise buildings and for life safety.
ƒ Alternate: - As alternate systems can have their pipes full of water for the summer and be drained down and filled with air: under pressure for the winter. This is important for buildings that are not heated.
ƒ Dry pipe: -These pipes are filled with air under pressure at all times and the water is held back by the control valve, When a sprinkler head opens, the drop in air pressure opens the valve and water flows into the pipe work and onto the fire. Dry pipe systems are used where wet or alternate systems cannot be used.
ƒ Pre: - Like dry pipe systems the pipes are filled with air but water is only let into the pipes when the detector operates like smoke detectors. Pre systems are used where it is not acceptable to have the pipes full of water unless, fire.
Other Natural Calamities: -
Cyclones: - A cyclone is a rotating storm that can be a hundreds of kilometers which can be very destructive and it looks like a huge spinning doughnut of clouds. The combination of wind, rain and waves knock down trees, flatten houses and wash out roads and could even block the exit stairways of a tall building.
Volcanoes: - Volcanoes are the large openings on the top of a mountain and sometimes on sides, through which melted rocks and gases escape with great forces from Earth's crust could create a possible disaster scenario if a tall building is near by with less exit openings.
Tsunami: - A tsunami is a very long-wavelength wave of water which is generated by a sudden displacement of sea floor or disruption of any body of standing water. Tsunamis occur suddenly and often they are extremely dangerous to costal tall buildings and even for city centers buildings since tsunamis generally have such a huge range of wipe outs.
Power Failure Management: -
Standby power generating system conforming shall be provided. The buildings shall be equipped with suitable means for automatically starting the generator set upon failure of the normal electrical supply systems and for automatic transfer of all functions required by the at full power within 60 seconds of such normal service failure. Building supervisions and administration should be able to manual start and transfer features shall be provided at the central control station.
An on-premise fuel supply sufficient for not less than two hours full demand operation of the system shall be provided, thus standby system shall have a capacity and rating that would supply all equipment required for to be operational at the same time including fire pumps required to maintain pressure. Fire pumps are required to have their own transfer switch. If in case generator is not sized to run fire pump or pumps depending on whether pumps are connected in series or parallel, then a diesel engine fire pump will be required for the backup pump if backup pump is required.
High rise building: -
ƒ For the purposes of this Chapter high rise building means a building having more than four floors and or 15 metres of height from ground level.
ƒ Every high rise building shall have at least two staircases.
ƒ The height of the handrail in the staircase shall not be less than 90 cms. and if balusters are provided no gap in the balusters shall be more than 10 cms wide.
Guard rails or parapets: -
ƒ Every slab or balcony overlooking any exterior or interior open space which are 2 metres or more below shall be provided with par-walls and guard rails of height not less than 1.20 metres and such guard rails shall be firmly fixed to the walls and slabs and may also be of blank walls, metal grills or a combination of both.
Provided that if metal grills are used they shall not be made of continuous horizontal members to prevent climbing on them:
Fire Escape Stairway: -
ƒ Every high rise building should be provided with a fire escape stairway.
ƒ Fire escape stairway shall be directly provided with public or common areas on all floors and shall lead directly to the ground.
ƒ At least one side of the stairway shall be an external wall either with large openings or with break open glass to facilitate rescue operations during an emergency.
ƒ External fire escape stairway shall have straight flight not less than 75cm wide, with 20cm treads and risers are not more than 19cm, the number of risers shall be limited to 16 per flight.
ƒ The height of the handrails shall not be less than the 100cm and not more than the 120cm.
ƒ Debarring the use of spiral staircase in high rise buildings.
Open space for fire fighting: -
ƒ Every high rise building, if it does not abut on two or more motorable roads, shall be provided with a minimum of 5 metres wide open space on any one of its sides contiguous to the road abutting it to facilitate fire fighting"
Lift for residential apartments: -
ƒ Every high rise apartment building having more than 16 dwelling units shall be provided with at least one lift capable of carrying a stretcher:
ƒ Provided that if only one lift is required for the building as per the rule 48, that lift shall be one capable of carrying a stretcher.
Parapets to terrace floor: -
ƒ Where access is provided over the terrace floor or to the terrace floor, the edges of the terrace floor shall be provided with parapet walls made of stable materials to a height of not less than 120cms.
Structural design: -
ƒ Application for construction or reconstruction or addition or alteration of any high rise building shall be accompanied by one set of structural design
ƒ including that regarding seismic forces as per the provisions contained in the National Building Code of India as amended from time to time and drawings and a structural stability certificate prepared and issued by a registered engineer.