Structural Collapse: Kinds, Causes and its Power to Change Building Codes Essay
There are five basic construction types used in the United States. Any building should belong to at least one of these. The types are identified by Roman Numerals and are arranged in a scale based on the amount of combustible material used in the construction (Dunn, n.d.).Type I buildings are referred to as fire-resistive because they were originally designed to contain fire within the building and in only one floor (Dunn, n.d.). Such buildings were built with relatively fire-resistive components such as masonry load-bearing walls, reinforced concrete or protective steel columns, and poured or precast concrete floors and roofs (“Introduction to Fire Science, Section 5”, 2002). Most high-rise buildings are classified under this category. You may have noticed that type I buildings are referred to as originally designed to be fire-resistive. This is because at present, fire can spread even in modern fire-resistive buildings, usually through two avenues – by central air-conditioning system and by auto-exposure, a term used to describe the vertical spread of the fire from window to window.Type II buildings are called non-combustible, referring to the materials used to construct the building. In lieu of fire-resistant materials, non-combustible materials such as steel beams, columns and masonry or metal walls are used (“Introduction to Fire Science, Section 5”, 2002). The roof however is combustible and provides an opportunity to spread fire. The roof covering of a type II building is usually a layer of asphalt water proofing, covered with combustible felt paper. When fire occurs inside a type II building, flames may rise to the underside of the steel roof deck, which in turn conducts (the transfer of heat through a solid) heat and cause the combustible material in the roof to ignite (Dunn, n.d.). An ordinary (type III) constructed building is also referred to as a brick-and-joist structure. Buildings under this category consist of masonry exterior load-bearing walls that are of noncombustible construction. The interior framing, floors and roofs are made ofStructural Collapse 3combustible materials such as wood. Typical examples of buildings under this category are the 19th and early 20th century apartment buildings, which are also known as “Main Street U.S.A.” (“Introduction to Fire Science, Section 5”, 2002). Buildings may also be classified as “protected ordinary construction” if the roof and the walls are made of materials that have one-hour fire resistance rating and all openings through the floors (such as staircases) also have one-hour fire resistance rating components.Type IV construction types are referred to as heavy-timber construction. Sometimes called “mill construction” because of its wide use in turn-of-the-century textile mills, type IV structures are characterized by masonry walls, heavy timber columns and beams, and heavy plank floors (“Introduction to Fire Science, Section 5”, 2002). Unlike ordinary construction, heavy-timber buildings do not contain plaster walls and ceilings covering the interior wood framework (Dunn, n.d.). Type IV buildings are not really immune to fire but the fact that timbers used are 14 cm and larger, flames may char the timber but usually may not consume it. When the timber does ignite however, it could result to conflagration which is difficult to extinguish.Wood frame (type V) constructions are the most combustible among the construction types. The interior framework as well as the exterior walls may be made of wood. Among the five types of construction, type V buildings are the only structures that have combustible exterior walls. At times, the exterior walls may be covered with brick veneer, stucco, metal clad or asphalt.Types and Causes of CollapseRegardless of the materials used in building construction (and that means regardless of the construction type), structural failures may occur which could result to structural collapse. Collapse may result from a major technical failure but may just as well be causedStructural Collapse 4by seemingly minor faults that create a chain of events that ultimately lead to structural collapse which can be classified into three categories:1) Localized Collapse – This refers to collapse which is limited to a small part of the building and does not spread to other portions of the building.2) Disproportionate Collapse – Disproportionate collapse occurs when one of its members is by mistake or accidentally removed. This happens when the load bearing elements such as walls, floors and roofs are not strong enough to resist the loads and stresses applied to them(Anumba, Egbu & Kashap, 2006).3) Progressive Collapse – This kind of collapse occurs from a structural member which is the first to collapse but initiates a string of events which ultimately brings the whole structure down. By definition then, progressive collapse is a disproportionate failure relative to the extent of the initiating damage (Smilowitz, 2002). This is the type of collapse that is believed to have brought the World Trade Center (WTC) down. It is said that there would be no need for an airliner for the WTC to collapse – a simple explosion would have done the job. The building is built in a manner that is prone to “house of cards” failure (Calven, 2002).4) Smilowitz (2002) also referred to a fourth kind of collapse – general or global collapse. In this kind of collapse, the applied loads simultaneously overwhelm multiple members of the structural system.Since the five construction types are categorized according the combustibility of the materials used, it is perhaps proper to discuss collapse in relation to fire. Most often, large fires create both progressive and disproportionate collapse. Among the construction types, type IV buildings are most susceptible to collapse. While it is true that they are quite resistant to fires in a sense that large timbers usually only get charred instead of catching flames, in they event that they do ignite, conflagration could result (Dunn, n.d.). In this case,Structural Collapse 5disproportionate collapse may occur as the floors are usually the first to collapse followed by the walls being pushed outward.The existence of trusses in a construction can also make the structure quite vulnerable to collapse in the event of a fire. When steel truss assembly is exposed to heat, the steel begins to weaken and what is usually considered normal load becomes excessive load. As the truss slowly fails, the other members will be forced to take on the extra load eventually causing their collapse. Wooden truss assemblies also share the same fate. The gusset plates that hold the wooden truss assembly will fall apart once the wood decomposes from the heat. This causes the eventual collapse of the floor or the roof assembly.In his analysis of the World Trade Center collapse, Calven (2002) classified the causes of collapse under the following general heading:· Bad Design & Errors in Construction – This does not only refer to wrong computations, but the failure to take into consideration the loads that the structure is expected to support as well as erroneous theories, insufficient knowledge, and improper choice of building materials or the misunderstanding of their properties. Faulty construction is also one of the major causes of structural failure. Unacceptable construction practices such as the use of salty sand to make concrete, the substitution of an inferior kind of steel to the one specified and excessive use of the drift pin to make the holes line up will most likely weaken the structure and increases its risk of collapse. These kinds of errors are caused by individuals – designers, manufacturers, engineers and other authorities responsible for checking a design or inspecting a construction. Usually, not only one but a number of errors cause a collapse and several individuals can be held liable since checking and communication are necessary parts of design and construction (Allen, 1972). Structural Collapse 6· Foundation Failure – Even excellently-designed and constructed structures fail because the earth it stands on cannot support it. The Tower of Pisa is a perfect example of foundation failure.· Excessive loads during catastrophic events – Decades ago, it was thought that nothing could be done to avoid the effects of catastrophes like hurricanes, earthquakes, tornadoes or even repeated heavy snowfalls. Now, it has been known that both lives and structures can be saved by proper design and construction (Allen, 1972). A building that is intended to last for many years must be able to withstand such catastrophes. As earlier mentioned, design and construction must account for different disasters – a flimsy, flexible structure for example will be able to withstand an earthquake while a solid masonry building will most probably be destroyed.· Unexpected Failure Modes – This is probably the most complex reason for a structural failure/collapse. A new type of structure is vulnerable to unexpected failure until its properties have been well understood. Suspension bridges were initially thought to be the answer to bridging large gaps as everything was supported by tension, a reliable and understood member. It wasn’t until the collapse of the Tacoma Bridge in the 1940’s that it was realized that the bridge deck is capable of swaying and twisting without restraint from the supporting cables.· Combination of Causes – Some structural failures may not be caused a single reason but a combination of causes just mentioned.Revision of Building Codes Resulting from Knowledge of Structural FailuresThe World Trade Center collapse during the September 11 tragedy fueled debates about how the building performed during the disaster and started an intense study on why the buildings collapsed. Barely a year after the tragedy, a hearing located in the United StatesStructural Collapse 7Custom House in Lower Manhattan gathered together structural engineers, firefighters and city building officials, most of whom argued for a need to revise building codes (Lipton, 2004). While the speakers were in disagreement as to how well the twin towers performed – others argued that the structure exhibited an admirable sturdiness while some suggested that the building’s design was inadequate – they agreed that there is a long list of legislated improvements that could be made as to how buildings should be put up. The structural engineers were the first to propose revisions in design codes to prevent an isolated failure to destroy a whole building, perhaps in part by looking into the impact of fire on the structure.After four and a half years of intensive study regarding the WTC collapse, the National Institute of Standards and Technology (NIST) released a report offering 30 recommendations on ways that tall buildings can be made safer. In March 2006, the first proposed changes to building codes based upon the NIST recommendations were submitted to the International Code Council (ICC). The proposed changes addresses issues such as increased resistance to building collapse by fire and other incidents, performance of fire protection systems (such as automatic sprinklers), elevators for use by those responding to the emergency and those who are evacuating the building, the number and location of stairwells, signs indicating exit path, and fuel oil storage/piping (NIST, 2007). In May of this year, ICC adopted the first set of these building code changes.Although it is rather unfortunate that a tragedy should strike before building codes are reviewed and revised, this scenario illustrates that further knowledge of abnormal loads (such as airline impact) can lead to better and more reliable building codes.