Term Paper: Firefighters Structural Analysis and Design of High Rise Buildings

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Firefighters Structural Analysis and Design of High Rise Buildings

Twin Towers of New York

Building Construction Trends

Truss Construction

Fire Fighters Structural Analysis

Firefighters Structural Analysis and Design of High Rise Buildings

This report attempts to examine why fire fighters have voiced concerns regarding the ever increasing size of skyscrapers. When the World Trade Centre Twin Towers, which were at the time two of the world's tallest buildings, were completely destroyed by a terrorist attack on September 11, 2001, the event should have raised enough doubt and questions about the future of high-rise safety to put an end to the construction of skyscrapers. Yet, after September 11th, the world's demand for newer and taller structures continued to dramatically increase.

It is apparent that the global population likes to live and work hundreds of feet above the ground. The other obvious fact is that the catastrophic events of September 11th did not have a very long lasting effect compared to the effect created by economic indicators such as the current historically low interest rates or consumer demand when it comes to the construction of skyscrapers. Designers and construction companies did have to address new safety concerns such as a building being hit by a bigger plane such as a 767. But, the response from the architectural world was one that said that the World Trade Centres' collapse was merely a fluke and therefore could not happen again.

Designers, architects and builders are confident that the only effect that a plane crash in the newer and higher buildings will be to generate lots of broken glass but limited structural damage only around the point of the impact. With this new found sense of confidence in the design and construction of ever taller buildings, the world demand for high-rises is stronger than ever.

But, from a firefighter's perspective, the new found confidence may not be warranted. The fire service is sure that newer and higher buildings are no more prepared for a next surprise attack than the Twin Towers of New York were. (Graham-Rowe, 2003)

Twin Towers

The results, presented in June 2002 at a meeting in New York organized by NIST, have now been published in Fire Safety Journal (vol. 37, p 707). The team calculated that the south and north towers would collapse after 75 and 115 minutes respectively. In fact, they fell after 56 and 103 minutes." (Graham-Rowe, 2003)

New York's World Trade Center Twin Towers were built in the 1970's. They were designed to withstand normal fires and even hurricane-force winds. The designers had considered what would happen if a plane, a Boeing 707, impact either of the buildings. The consensus was that none of these events could bring either tower down. In hindsight, additional testing was obviously needed. The towers were no match for today's faster and bigger planes. But why did the towers fall?

The Twin Towers were constructed by combining lightweight steel and glass and then supporting them to the external columns. Stairs and elevators were supported by the internal columns which formed the main core of each tower. The Twin Towers differed from previous high-rise construction in several ways which created a scenario for speeding up construction time and also helping builders raise the building to new heights.

First, the primary support for each of the towers came from an external sheathing. The idea was to open up the inside of the buildings for additional office space. Another unique concept used was based around supporting concrete-slab floors to the building frame by using steel trusses and other special plates that reduce the effects of strong winds.

Upon initial impact of the terrorist planes, the two towers looked as though they had indeed withstood the shock and aftermath of the explosions and fire. However, as the jet fuel that came out of the jets burned, it has been estimated that temperatures inside the towers may have risen to as high as 2,000 degrees Fahrenheit. To make matters worse, the buildings were actually undergoing increased insulation treatments at the time of the attack. The existing level of insulation was not thick enough to withstand these high levels of heat. "Six months before the attacks, the Port Authority received a copy of a report it commissioned from British consulting engineers Buro Happold to see if there was a more cost-effective alternative to applying thicker insulation." (Graham-Rowe, 2003)

The steel trusses that supported the flooring either melted or simply buckled which in turn released each upper floor down on the next below it and eventually falling completely to the ground. In other words, as the trusses failed, the concrete-slab floors literally plunged downward. When the two jets struck the buildings, the fact that they hit near the towers' tops, the weight of the floors became a force the buildings could not withstand. Although the towers went up easily utilizing these techniques, they were also the eventual cause of the buildings quick demise. The Twin Towers' radically different structural design created a situation where the load-bearing walls simply could not support the weight of the floors.

Building Construction Trends

The building industry has undergone change. Engineers today use the available technology to break barriers once thought of as impossible limits to building size. "The computer has allowed engineers to reduce the mass of a structure by its ability to more accurately determine the load-bearing capability of structural framework. Years ago, before the computer, builders were not sure of a structural element's load-bearing capability, so they overbuilt by using a so-called "safety factor." This built-in safety factor could result in a structure twice the required load-bearing strength." (Dunn, 2003)

In other words, buildings will keep getting taller. The height is measured from the sidewalk to the structural top and those tops are now higher than ever. A "building" is considered to be a structure that is designed for residential, business, or manufacturing purposes. (Infoplease, 2004) Towers may be in the name of the buildings, but these structures will always be occupied.

Take into consideration that the once tallest building in the world when built in 1931 will not even be in the top ten by the end of the year 2005. The Empire State Building is making a steady decline in height ranking as the construction teams throughout the world inch up higher and higher with each new edifice. (See Appendix A & B)

As of 2004, the new tallest building in the world, Taipei 101 in Taipei, Taiwan, technically has one less floor for occupancy than the now 9th ranked Empire State Building, but the difference in height between the two massive structures is more than four hundred feet. Throughout Asia there are several new and on-going projects that will soon easily dwarf Taipei 101. During the building of Taipei 101, work was stopped because of an earthquake that measured over 5 on the Richter scale. Even though several workers died during construction, the work did not stop long. The new construction trends are obvious, buildings will continue to escalate up into the stratosphere ignoring any hazards or signs of danger. Groups made up of builders, developers, financial people, and bureaucrats all concur -- up is good.

The same cannot be said for those in the fire service and the academic world of fire science engineering. Safety concerns and the 9/11 fundamental flaws in building construction bring about considerable concern with these newer and higher buildings. The majority of representatives from the fire science, fire engineering, fire service communities and experts in the collapse of burning buildings believe that the collapse of the Twin Towers should not have happened and that the builders of new skyscrapers, like their Twin Tower predecessors, will never be able to take into consideration all of the possibilities to guarantee complete safety.

The confidence of the building community may have some merit that future buildings may be better prepared for an airplane attack because how buildings are built does make a significant difference. It is more than obvious that some buildings are built better than others. For example, the trend in truss construction used for the Twin Towers was noted as a major reason for the World Trade Centers' collapse.

Had the terrorist struck the Empire State Building with one of the planes, some models of computer animation testing have demonstrated that the building may not have collapsed as the Twin Towers did because the Empire State Building was built with a classic column and beam construction strengthen by concrete masonry. The computer animations may be correct. The World Trade Center towers utilized what is known as tube construction with truss support made popular in the 1970's. In 1945, a B-25 bomber struck and left a huge hole in the Empire State Building and it continued to stand.

An airplane hitting one of the newer and higher buildings around the world may not be a cause of concern, but high-rise buildings have other types of alarming issues to consider. The most feared problems usually stem… [END OF PREVIEW]

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