The Evolution of Building Design and Construction Methods - Research Paper Example

The Evolution of Building Design and Construction Methods for increased human safety Building s have been in use for over 100 years in the US and are based both on engineering knowledge and the findings from the investigation of building failures due to natural and manmade causes. The progressive collapse of the Ronan Towers in the UK and the Murrah Federal Building in Oklahoma City led to revisions in the codes for structural design for tall buildings. The 11 September 2001 attack on the World Trade Center in New York City has given impetus to far reaching reviews of the codes for structural design as well as fire safety and emergency evacuation of buildings. The changes to these codes is expected to make tall buildings safer around the world and better equipped to handle a disaster. Keywords: building codes, collapse, fire safety, egress Question 1 The types of building collapse that may occur and the changes made in building codes to prevent them Building codes have been in use in the US for over 100 years since the first model code was written by the National Board of Underwriters in 1905. These codes have been revised and updated based on new materials and construction methods as they became available. Building damage during natural events like hurricanes, tornadoes and earthquakes led to revisions and upgrades of the building codes. The codes are written to establish the minimum requirements but the construction industry has often treated these as the maximum requirements (Dehring, 2006, p10, 11). Two comparatively recent major building disasters have led to the coining of the term “progressive collapse” and have led to the modification of building codes to prevent these from happening. In 1968, a gas explosion in a kitchen on the 18th floor of the 22-story Ronan Towers Apartment Building in London, UK knocked out pre-cast concrete load bearing panels in a corner of the building and that loss of support caused an entire corner bay of the building to collapse. The Murrah Federal Office Building in Oklahoma City was destroyed in 1995 by a bomb in a truck in the basement of the building. The bomb explosion damaged or destroyed three columns which led to the failure of a transfer girder. This caused the columns supported by the transfer girder and the floor areas supported by those columns to collapse causing a general collapse (Nair, 2004, p 1, 2). In both cases, the structural collapse is considered disproportionate to the trigger and the engineering community and the codes have attempted to change design codes to prevent such disproportionate damage. In general, codes attempt to improve building safety using three approaches – increasing local resistance, creating redundancy or improving interconnection (Nair, 2004, p2, 3). Increasing local resistance is, as the name suggests, strengthening the structural members to withstand damage. The Public Buildings Services Standard issued by GSA in 2003 requires that “the failure of a beam, slab or column shall not result in the failure of the structural system above, below or in adjacent bays” (Nair, 2004, p2). This is what may be termed a “performance code” which does not prescribe how the building design or construction should achieve this objective. This makes such code difficult to interpret, both for the regulatory officials and the construction industry (Dehring, 2006, p10, 11). Increasing local resistance would obviously not have saved either Ronan Towers or the Murrah building from damage. The American Society of Civil Engineers ACSE 7 - 02 code goes a step further and makes the prescriptive suggestion that “the design should have sufficient continuity, redundancy or energy-dissipating capacity where alternative paths are available to carry the load when one component such as a column fails”. The American Concrete Institute ACI 318-02 specifications provide even greater prescriptive suggestions such as ensuring continuity of reinforcing steel and tying together of precast structural components (Nair, 2004, p2, 3). The Department of Homeland Security has published its own set of recommendations in June 2011 where to prevent a Murrah Building type of collapse, it has recommended that concrete columns be encased in steel or composite material and steel columns encased in concrete. The recommendation also includes backing masonry and concrete panels with liners and using laminated window glazing (BIPS 05, 2011, p 8-4). The problem with such prescriptive provisions to increase structural safety remains the issue of increased cost. It also tends to make it difficult for any new design or construction approaches to find acceptance both with code regulators and with building insurance companies (Dehring, 2006, p10). Question 2 Changes in US building construction methods based on fire safety and the impact of forensic analysis of the 11 September 2001 attack. Fire protection engineering applies science and engineering principles to protect people and their environment from destructive fire. After Rome burned in 64 AD, Emperor Nero required fire proof materials to be used for external walls in rebuilding the city. The Great London Fire of 1666 destroyed over 80 % of the city and led to building regulations requiring the use of stone and brick for house construction. The Iroquois Theater fire in the US of 1903 killed 602 people and brought attention to the ignition and flame spread properties of curtains, drapery and wooden partitions. The January 1967 fire at the McCormick Place exhibition hall in Chicago caused the steel truss roof 37 feet above the floor to collapse in 30 minutes. This fire led to the requirement of automated sprinklers and established fire suppression criteria for exhibition halls. The February 1971 fire at One New York Plaza highlighted the problems of fire protection in high rise buildings. This led to the development of the National Fire Protection Agency (NFPA) “Fire Safety Concepts Tree” which was later applied to buildings like the Sears Towers (Cote, 2008). The 11 September 2001 attack on the twin towers of the World Trade Center in New York City was by Boeing 767 planes flying at high speed. The impact caused structural damage to core columns, floors and perimeter columns. The towers would have remained standing except for the intense fires that melted the core steel columns in WTC 1. These columns became exposed to the fire due to their fire insulation being dislodged due to the impact. In WTC 2, the core was severely damaged in the southeast corner. The fires on the east side of the building caused the floors there to sag, causing the east perimeter columns to buckle. If fire insulation had not been dislodged in the aircraft impact, both towers may have remained standing (NIST WTC Report, 2011). A set of 23 changes to model building and fire codes have been accepted by the International Code Council based on the findings of the investigation into the World Trade Center attack. The definition of the primary structural frame of the building which includes columns, girders, beams, trusses and spandrels directly connected to columns has been broadened to include the bracing members such as floors and cross bracing, whether or not they carry gravity loads. All members of the structural frame are required to have the fire resistance rating so far used for columns. The fire resistance rating has been increased by one-hour for buildings more than 420 feet high. An initial proposal requiring buildings more than 420 feet high to survive a building contents fire to total burnout without major structural damage has been deferred for further evaluation (NIST WTC Report, 2011). The bond strength for fireproofing has been increased to three times current requirements for buildings below 420 feet and seven times for taller buildings. The fireproofing needs to be tested after installation to demonstrate adhesion and inspected to ensure that there are no cracks or voids or exposure of the substrate (NIST WTC Report, 2011). Major changes have also been made in respect of the people evacuation process. A third exit stairway is required for buildings above 420 feet and stairway widths are increased by 50 %. The stairway entrances are required to be separated by minimum distances depending on the floor area. The enclosures of stairways and passageways and elevator shafts need to be hardened to prevent jeopardy to human life. An important new suggestion is to permit the use of elevators to evacuate people. These elevators will meet specific criteria. A minimum of one fire service elevator must be provided in all buildings over 120 feet in height. The requirements for luminous (glow in the dark) path markings have been increased and clarified. An emergency radio responder system has been mandated with specific levels of signal strength, system design, installation and maintenance (NIST WTC Report, 2011). The changes to the codes are widely expected to prevent total collapse in future skyscrapers. Many of the changes were overdue. The 44 inch width of stairwells had not changed since the 1900s and do not take into account increased human body size and obesity in the last 100 years. Advances in lightweight construction materials have made it possible to build super tall structures larger than 100 stories with large floor spaces unobstructed by support columns. Each floor of the World Trade center had nearly 60 tons of paper, furniture and other flammable material. In such situations, firefighters have no choice but to let the fire burn itself out while they concentrate on getting the people out safely (Mangels, 2011). Question 3 Egress design, means of egress and fire code administration The International Building Code (IBC) for “Accessible means of Egress” defines a means of egress as the path available for a person to leave a building, structure or space. A means of egress consists of three parts: exit access, exit and exit discharge. Exit access is the path from any point in the building to the exit. An exit is typically the door leading to the outside or in the case of a multi-story building, an enclosed exit stairway. Exit discharge is the path from the exit to a public space such as street or alley (ICC Egress, 2010). The IBC requires at least two means of egress from all spaces within the building and from the building itself. Buildings or spaces with more than 500 occupants require three means of egress and those with more than 1,000 occupants require four means of egress. Only when there are fewer than 50 occupants, a single means of egress is permitted subject to the condition that no occupant would need to travel more than 75 feet to the exit (ICC Egress, 2010). After some major fires in US public places in early 1900s, the National Fire Protection Association formed a committee to frame the first set of model recommendations. The first proposal in 1914 related to the exit and stairways and the recommendation was that all the population on a single floor should be able to take refuge in a single flight of stairs. The 1935 National Bureau of Standards laid down various criteria for exits design including the 44 inch stair width which has largely remained the US code except for improvements made from time to time based on experience from fire accidents (Averill, 2011). In the 1970s, a proposal to permit the use of elevators during fire evacuation was considered by the NFPA but was not accepted its members. The various investigations and reports into the World Trade Center attack has led to engineers coming up with innovative solutions to the problems of egress and evacuation from tall buildings. For example in the building 7WTC to replace one of the destroyed buildings, the New York architects Skidmore, Owings & Merrill have proposed a hardened core to the building encased in reinforced cast-in-place shear walls. This core will contain two stairwells of 72 inch width with pressurized air to prevent smoke ingress and power supply from three sources, normal, emergency and battery back-up. Each floor of the building will also include a smoke-resistant refuge area. Phosphorescent paint strips throughout the core will improve visibility (Siddens, 2008). The emergency service elevators designed for WTC 1 include a second entrance for use by firefighters. These elevators will have a pressurized hoist well isolated from other elevators by fire resistant walls. Evacuation by elevators is still in discussion and has not yet been accepted in the codes (Siddens, 2008). Building permits for new construction or major modification are given by municipal or county authorities and are based on a review of the construction plans and drawings. The concerned building official will verify compliance with building codes and issue an approval. At various stages during the construction process, the building official will inspect the construction process to ensure that the codes are being met and that construction quality is maintained. The architect or builder can file an appeal for deviations from code where they find that they cannot comply with the letter of the code or if they can achieve the objective of the code by an alternative approach. Such appeals will be examined by the building official or by a committee of experts constituted for the purpose. The actual process varies from state to state and between counties but is broadly as described above. * * * * References: 1. Averill, J, D., (2011). “After the alarm sounds: Historical, Present and Future Perspectives”, Fire Protection Engineering, 1 April 2011. Retrieved from http://magazine.sfpe.org/ occupants-and egress/after-alarm-sounds-historical-present-and-future-perspectives. 2. BIPS 05, (2011). “Preventing Structures from Collapsing”, Homeland Security, June 2011. Retrieved from http://www.wbdg.org/ccb/DHS/bips_o5.pdf. 3. Cote, A.E., (2008). “History of Fire Protection Engineering”, Fire Protection Engineering, 1 Oct 2008. Retrieved from http://magzine. sfpe.org/professional-practice/history-fire-protction-engineering. 4. Dehring, C.A., (2006). “The Value of Building Codes”, Regulation, summer 2006. Retrieved from http://www.cato.org/sites/cato.org/files/serials/files/...../2006/..../v29n1-2.pdf 5. ICC Egress, (2010). “Accessible means of Egress”, International Code Council, 2010. Retrieved from http://www2.iccsafe.org/safety/Documents/MeansofEgressBroch.pdf 6. Mangels, J., (2011). “Decade after 9/11 World Trade Center attacks, skyscraper safety improving”, The Plain Dealer, 14 Aug 2011. Retrieved from http”//www. cleveland.com. 7. Nair, R.S., (2004). “Progressive Collapse Basics”, Modern Steel Construction, March 2004. Retrieved from http://www.modernsteel.com. 8. Newman, M.E., (2008). “Safer buildings are goal of new Code Changes based on recommendations from NIST World Trade Center Investigation”, 1 October 2008. Retrieved from http”//www.nist.gov/el/wtc_100108.cfm 9. NIST, (2006). “Best Practices for reducing the potential for Progressive Collapse in Buildings”, National Institute of Standards and Technology, August 2006. Retrieved from http:// www.bfrl.nist.gov/861/861pubs/collapse/NISTIR_7396.pdf 10. NIST WTC Report, (2011). “About the NIST World Trade Center Investigation”, 30 June 2011. Retrieved from http:// www.nist.gov/el/disasterstudies/wtc/ 11. Siddens, S., (2008). “Rethinking high rise egress, top to bottom”, Everglow, 1 Jan 2008. Retrieved from http://www.everglow.us. Read More