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- oquestionador
- Aug 16 2009, 10:41 PM
Just an example... Do you know Portuguese? That's a report from a civil engineering magazine. Link
Translated into English via Google.
- Resistance at the limit.
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Experts differ on the causes of the collapse of the twin towers of the World Trade Center assigning greater or lesser importance to the fire that followed the impact. Attacks also warm discussions on the feasibility of high buildings
The conclusion of a good part of the engineers who studied the structural collapse of the towers of the World Trade Center in New York, is that a number of factors acted together to ensure that the buildings collapsed. This is also the opinion of the engineer Mário Franco structures, responsible for several projects of large buildings in São Paulo. For Franco, the impact of the planes caused the immediate destruction of a part of the pillars of the facade reached and the structure of the nucleus, causing a collapse of the first floor reached. "I believe that about 2 / 3 of the pillars in front were destroyed and the main beams of the structure in the region of collision." Added to the impact, the explosion of the fuel tank caused the destruction of part of the side walls and opposite. The fire happened that aggravated the situation of the remaining pillars of steel, only to sustain the upper portion of the towers, which have lost the resistance. The top collapsed on the bottom and caused the collapse of the towers. A number of factors, therefore, but not as simple as that.
In fact, so careful to conclude what happened in the structure of the towers, it is necessary to understand the architectural project and the process that allowed the executive construction of buildings 415 and 417 m in height. The program, presented by the Port Authority of New York and New Jersey to architects Minoru Yamasaki and Emery Roth, required the construction of about 1 million sq meters of floor area in an area of 65 thousand square meters. For this, the architects devised a commercial and business complex with five buildings and a shopping center in a strategic area of downtown Manhattan, near the Hudson River, connecting with the underground transport network metroviária (plant 1). The two twin towers, with 110 floors each, housing offices distributed in an area of 4 thousand meters per floor. The construction of the entire complex began in 1966 and was completed in 1977. The twin towers were completed in 1973.
Steel Skeleton
The structure of the towers, designed by the engineering firm of Worthington, Skilling, Helle and Jackson, was composed of 57 pillars of steel placed on each facade. Formed by folded sheet flow with voltage between 700 and 295 MPa and thickness ranging between 12.5 and 7.5 mm, the pillars had cross section of 35 x 35 cm with Wheelbase of about 1 m. Each pillar was covered with sheets of aluminum, resulting in a final section of 45 x 45 cm. The windows between the pillars had placed only 57 cm wide. These pillars are joined at three at the ground, forming in each front pillar 19 spaced every 3 m. Each corner of the towers had two pillars positioned diagonally on the front panels, which extended to the foundations, without transition. All pillars are interconnected by peripheral beams of 1.32 m high, made of sheet metal, which is joined to the pillars by means of copper, together with 12 screws on each side. Externally the structure based on this set of metal beams and pillars, solely responsible for the absorption of horizontal loads, which were not transmitted to the nucleus. "It was a tube of square section of wall thin, with mechanical properties change due to present discontinuous walls, perforated," said John Franco. "The external structural mesh was the only responsible for contraventamento of buildings," concludes.
Each tower, with 63 x 63 m across, had a core of elevators and stairs in plan with dimensions of 27 x 42 m, structured by pillars and beams of steel, designed to resist 60% of the vertical loads. Supported in peripheral pillars, trusses beams support the floor with about 0.85 m 2 m distance between them - a beam every two pillars - in vain due to 18.50 and 10.50 m to the nucleus, where it is supported on cushions of neoprene, so as to transmit only vertical loads. The support structure of the slabs was complemented by cross beams, which ran parallel to the ducts of air conditioning. The slabs, made by all steel deck and lightweight concrete layer of 10 cm in thickness, provided the lateral stability of the pillars for cover. The action of high winds on structures should be absorbed to prevent oscillations and discomfort to users of the higher floors. Thus, in each building were installed about ten thousand visco-elastic dampers under the uprights below the beams trusses in the regions where they are supported on the pillars peripherals.
The walls were fitted with modules composed of three pillars and three-plate beams (Figure 1). Each beam is a floor-plate, so that each panel covered the facade of three floors in a total of about 11.25 m high with a width of 3 m. The modules were connected together by four screws per pillar. The slabs were assembled in panels, with three beams and cross beams and trusses structure. The larger panels were rectangular form with 18.50 x 6 m, and covering the extension of two front panels, the lower panels measuring 10.50 x 4 m. The entire steel structure was passive protection against fire: a 3 mm layer of vermiculite.
Impact and structural failure
A Boeing 767-200 aircraft with 48 m in size, equivalent to 75% of the width of the building, shocked with the North Tower of the WTC. The plane had about 180 t net weight, which amounted up to 90 tonnes of fuel. Another plane hit the same type against the South Tower after 18 minutes. The shock of the planes against the towers caused the immediate destruction of a good part of the pillars of the facade of the perimeter beams, plate girders and trusses, and a part of the structure of nuclei. With the impact, the fuel tanks of the aircraft exploded, which destroyed part of the remaining structures, and cause damage to walls and opposite side. "The vertical loads of the upper portion of the towers is redistribute the remaining pillars, through the operation of the beam structure and the type Vierendel," says John Franco. The fire further weakened the pillars that have lost their strength and collapse. The volume above the region of impact was momentarily without support - which is called structural implosion of vacuum - and starts a downward movement. The shock of the two parts of the building is not absorbed by the structure below, which then enter into ruin.
Tim Wilkinson, Professor, School of Civil Engineering, University of Sydney, also believes that the impact of planes, followed by the explosion and fire, destroyed a large part of the structure in the affected region. "Despite being the only evidence of television images and photographs, the collapse of slab structures affected and the consequent loss of lateral stability may have caused a buckling of the pillars peripherals." According to the professor, so these structures will collapse, the fire intensified, the upper floors have lost support and collapsed on the bottom. The impact caused the top portion of the collapse of the entire building.
Mário Franco believes that the critical point of the steel structure was in the joints of pre-made modules. Both junctions of the top of the pillars and beams in the front-plate. "The examination of images of the debris shows clearly that there was no break in each module." Predominates in the perimeter beams by shear fracture of the screws with little tear sheets of the holes for connection.
World Trade Center
Exposure to fire
The fire that followed, fueled by thousands of gallons of jet fuel - burning jet fuel - and by the thermal load of the building, is one of the experts differ about the causes of the collapse. Many believe that the main cause of the collapse of the towers was the structural weakness caused by the fire. "The main and only cause of the fall of the towers was the occurrence of high temperatures for a long time because of the enormous quantity of fuel oil spilled into the structure," says the professor of the School of the Polytechnic University of São Paulo, Paulo Roberto do Lago Helene. For the teacher, to protect the existing towers would not be sufficient to withstand the additional heat load as well as the impact of aircraft may have broken a good part of that protection in various parts of the structure. As the steel is an efficient conductor of heat, the high temperature of the parts exposed and unprotected transmitted to other pillars, slabs and beams of the structure. "The temperatures reached more than 1.000oC and 500oC enough for that steel loses 80% of its strength at room temperature," says Paulo Helene. Thus, a vertical drop of the building, such as "implosion" can be explained by the uniform heating of several pillars, which have reached the limits of resistance to flow and simultaneous manner. This state has created a drop in upper third of the structure on the 2 / 3 lower than the mechanical impact resisted. "The structure has been gradually breaking up and down to reach the ground." Helene recalls the fall of 40 floors of the building near the towers, which has not suffered no loss of impact protection. "Collapse due to heating of the structure of the foundation by proximity to the fire of the two towers."
But the engineer Pignatti and Valdir Silva, also professor of Poli-USP and specialist in the design of structures of steel and its protection against fire, not attributed to high temperatures all responsibility for the towers fall. According Pignatti, material-based fuel oil, if oil spills, gas generates very hot, but temperatures well above the fires of cellulosic material. "The scientific nonsense to associate structural safety in time of exposure to fire can lead to false conclusions," he says. A piece of steel without thermal protection in a fire reaches of cellulosic material, called fire pattern around 1.100oC in 150 minutes. The curve of a fire resulting from burning of hydrocarbons shows that in 20 minutes he reaches 1.100oC. "If the steel is designed with thermal protection for a time required for fire (TRRF) of 150 minutes, you can not say that it will collapse in 20 minutes, because what matters is not the temperature and time of exposure to fire, "he says. If the structural element of fire follow the curve of oil, it can resist indefinitely. In the case of the World Trade Center, the steel was coated with material of thermal protection, which reduces considerably the temperature of the steel, although many parts of protection have been uprooted by the impact and explosion. Still, Pignatti believes the cooperation of the fire in the collapse of buildings was subsidiary (Figures 1 and 2). "We can not forget that the first tower that collapsed was that less time of exposure to fire," recalls. Pignatti says that the North American buildings are among the safest in the world, and a fire, however severe it was, not lead to the WTC collapse. "Admitting that, would declare the bankruptcy of fire safety engineering."
RETAINERS the subsoil are destabilized
The wall of restraint that allowed the implementation of the seven floors of the underground complex of the World Trade Center could be destroyed. Call for bathtub, bath tub that serves as a dam the water on the outside. Its walls were executed with concrete and steel structure with about 912 m in length, and contained the underground floors of the entire WTC complex. With the impact of the planes, the buildings collapsed on the floor and increased the pressure in the containment. The difference in pressure destabilized the structure that now needs reinforcements.
At the beginning of American colonization, the Hudson River was wider than it is today. Around the year 1615, Dutch settlers built houses along the reefs of the river and established the strong Amsterdam. After hundreds of years, garbage, shoes, gun and bullets from ships sunk by landfill served as the extension of the coastline. To support this site and pressure of the water of the Hudson, the engineers built the WTC bathtub, between 1967 and 1968.
To avoid the destabilization, the city of New York seeks to reinforce the wall around 19 m depth with the deployment of a line of tirante base. An excavation along the outside of the wall, to a depth of 24 to 30 m, will allow the setting of steel pins in the wall of containment where the SWR partirão setter that will be approximately 3 to 6 m in the direction of the bed river. Such bond will be connected to the wall every 3 m. S.S.
Planners questioned the viability of megatorres
The building is to incorporate high quality and economic benefits inherent in the multiplicative factor of the concentration of opportunities. It is believed that the architect Edison Musa, former President of Asbea (Brazilian Association of the Office of Architecture). It also, according to the architect, an important symbolic component of economic and technological statement. "Thus was born in the post-depression the Empire State in the post-war Sears Towers, and the post-depression New York the Twin Towers." The phenomenon of extreme verticalisation happening today in developing countries, seeking a statement of technology, such as the Twin Towers in Malaysia and China, with Kohn Pedersen Fox tower for Musa, if the aggregate of the symbolic project of overcoming the economic difficulties achievement, a tall building is fully feasible. "This type of architecture is part of the imaginary city, and even Lucio Costa has already proposed a building of 100 floors to Barra da Tijuca.
The tall buildings always accompanied man through history. In ancient Egypt, the pyramids of Cairo showed the power of the pharaohs, the Gothic cathedrals reach sought the power of God and in the modern age, to the skyscrapers emerged as a symbol of status and power. However, the Paulo Afonso Rheingantz professor, School of Architecture, Federal University of Rio de Janeiro, there is no practical advantage to justify the construction of these buildings. "Its existence is directly related to vanity, power, wealth and technological challenge." The operating system of a tall building is more complex and costly for a building with up to four floors, which dispenses the use of lifts and complex devices for maintenance and cleaning of walls. A building of 300 m height is technologically feasible, but may not be aligned with the new models of sustainable society, economy and security. This model verticalisation not be justified, according Rheingantz, or to urban centers. "Perhaps only in the cities of Japan, a country with high population density and has territory composed of volcanic rocks," he said. If, however, a high tower is located, are essential to design appropriate networks of urban infrastructure and the study of the influence of the building in his immediate surroundings. The buildings of large influence on a direct traffic, sunshine and the wind regime, and can create shadows and turbulence often disregarded at the time of the project. "The neglect of these impacts with the tall buildings rise two effects: privatize profits and socialize a number of problems." It is common that the resources of large cities in the poorest countries are used to rebuild networks already consolidated urban infrastructure rather than expansion of networks in the suburbs.
Peter Marcuse, professor of urban planning, Faculty of Architecture of Columbia University, believes that the tall buildings are only built for two reasons: the high price of land, which requires an optimization of the use, and the expression of power and status of the owner the building. "Outside these two reasons, the skyscrapers are a much more expensive to produce spaces for office buildings than low." Marcuse cites investments in elevators, operating system and components of the structure. According to the teacher, the desire for status will monitor the interest in the property market in the centers of large cities, strengthened by the ease of access, high demand and economic agglomeration. "It is clear that a pause is happening in this market due to fear of terrorism and the problems of congestion in urban centers," says the professor. "But if the authorities continue to invest in infrastructure in urban centers, the housing market, so logic will use these facilities and insist on the release of large buildings, which can allow compensation in the high value of the land. "
For the English architect Norman Foster, the trend continues. "In the world, buildings that concentrate a large number of people are being designed so as to reduce to the maximum of safety and operational problems," says the responsibility for projects of high buildings and the Commerzbank in Frankfurt, Germany. Nevertheless, Foster believes that the consequences of the WTC disaster will affect all who are involved with projects for tall buildings. S.S.
Concrete versus steel
The disaster shows that the WTC, one way or another, the structures must be well designed and protected against fire and rekindle the issues that face structural steel and concrete. Paulo Helene says that the concrete structures are more resistant than those of steel, and cites examples of fires in concrete structures that resisted, such as buildings and Joelma Andraus in Sao Paulo, and several bridges and viaducts that have shares of fires in slums. "Recently, the viaduct of high metallic Red Line collapsed by a single fire in slums," says Helene. Thus, we recommend protection to all steel structures, which can be coated with cement mortar with thermal insulating, such as expanded clay or fiber of asbestos, plaster or mortar. This protection must be sized for one or even three hours of fire pattern.
Pignatti, however, shows that any structure poorly scaled, and that includes the concrete, is subject to collapse by the action of fire. The concrete loses strength from 80oC, and reaches 80 and 50% of their initial resistance to 350oC and 570oC respectively, with the aggravating factor of a phenomenon occurs called spalling, which is the chipping of the surface of the structural element due to internal pressure of the water which vaporizes to 100oC. "In high-strength concrete, the low porosity, the phenomenon is more relevant," says Pignatti. The structural elements are constructed of concrete with higher volume due to low mechanical strength of the material. Due to this characteristic, the temperature inside the material amount is way slower. "This gives greater security to the actual fire, but since the other materials compared not meet the design and protection required," he adds. Indeed, for the correct sizing of structures for fire situations, should be the degeneration of physical and chemical characteristics of structural materials such as steel and concrete, causing considerable reduction in strength and rigidity. "The engineering design of each structure adequately." S.S.
url=http://www.revistatechne.com.br/engenharia-civil/57/artigo32423-1.asp
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1:18 AM Nov 24
