Steel or concrete is time versus money

There are two main options available when it comes to building structures: concrete and steel frame. Traditionally buildings in the UAE have almost exclusively relied on reinforced concrete for their structural strength. Although concrete remains the number one method of supporting structures, steel frame is becoming increasingly common as more challenging projects with large open spaces are built. Construction Week learns why for some projects steel is the only viable option.

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By  Colin Foreman Published  June 6, 2004

Steel or concrete is time versus money|~|steelframebody.jpg|~|Emirates Towers' Steel frame could be seen clearly during construction|~|There are only a limited number of steel frame buildings currently standing in the UAE, but their ranks are swelling as the construction boom continues at pace and more ambitious projects are tackled. Projects like Dubai Airport Expansion; a number of high-rise towers such as the Burj Dubai and ADNOC headquarters; commercial buildings and shopping centres like the Mall of the Emirates; and various petrochemical facilities will all include steel frame as part of their structures. “The main project we are currently working on is phase II of the Dubai International Airport expansion. This includes the new concourse terminal and car park, and we have been working on it since January 2003. We are due to commence erection works within the month,” says John Cameron, estimating manager, Cleveland Bridge & Engineering Middle East. In the past steel frame has not been a viable option for most of the UAE’s tower blocks as they only really make economic sense once a tower exceeds 20 storeys. Once over this threshold height steel frame buildings are a serious option. “Steel frame buildings are not really used for high rise structures below 20 storeys. Steel becomes a better solution when buildings exceed that height because foundations can be reduced as the building is lighter than it would be if it were made of concrete,” says Cameron. material prices For pricing, it all depends on the relative material prices. With the current volatility of all building materials at the moment it is very difficult to ascertain the cost of both concrete and steel structures. “To be honest, with the price fluctuations in all building materials, I am not sure which method is the most costly right now. But up to 20 storeys concrete will be less expensive to use concrete, and above that it is not clearly defined. A lot depends on the engineering,” says Cameron. Although material prices are important other factors also need to be considered. One such consideration is time. “Steel frame buildings can be built faster than concrete buildings because the steel can be procured, detail drawing can be made, and actual fabrication can all be done while the foundations are being constructed,” says Cameron. This is in stark contrast to concrete where most structures rely on situ concrete, although precast is becoming more popular. “Obviously it depends on the size of the building, but in general you can do a floor every four days using steel. It might take twice as long if concrete is used,” says Cameron. Prefabrication also means that erection is a lot less labour intensive on site, which again is another potential saving. Rather like a meccano kit, the frame is made up of individual pieces that are normally transported to site on trailers in 10-16 m sections. “We fabricate all the components in kit form while the foundations are build. It is then bolted together on site,” says Cameron. Quality control is another issue that should be considered. Precast concrete like steel both benefit from being manufactured in a factory environment and can be fabricated with a higher degree of accuracy than can be achieved on site. Steel also has a much lower weight:strength ratio. This allows for open plan offices with fewer columns, which helps the owner as he only receives rent on the lettable area, not on lost space occupied by columns. “In a commercial building a span of 16 m would be normal, but for hangar-type buildings 100 m clear spans can be achieved,” says Cameron. Steel frames structures can be used for a number of applications. Although high-rise buildings are, in many cases, built using steel frame, they are not the biggest market. At present the market for the structures associated with airport projects is the biggest. The vast concourse buildings, hangars, technical centres, and car parks all require steel frames. “Airports are a big thing at the moment because have wide open spaces and require large spans,” says Cameron. With wholesale airport expansions underway in Bahrain, Qatar, Saudi Arabia and of course Dubai it is no surprise that there is currently an abundance of this type of work. Another market segment that looks set to continue growing is commercial buildings. These range from sports stadiums and shopping malls through to ski domes. A number of stadiums are already underway in the region, such as Khalifa Stadium as Qatar gears up for the Asian Games in 2006. Stadium work is also on the near horizon in Dubai as Dubailand will include three outdoor stadiums, one indoor stadium and an array of other sporting venues. Shopping centres will also be significant, as a number of large-scale projects will are expected to come on stream by 2010. Another area that will likely make use of structural frame is infrastructure development. Projects such as Dubai’s third bridge across the Creek, and the planned multibillion-dollar monorail will no doubt keep the industry busy. Although the benefits of using steel are clear, it must be used properly taking into account its limitations. The most important thing to consider is fire. Unlike concrete, which is naturally fire rated, steel will lose its structural integrity if exposed to high enough temperatures for long enough. This was dramatically demonstrated when the Twin Towers collapsed on 11th September, 2001. Although the debate as to what could have been done to prevent that tragedy continues, one thing is clear, if the steel does lose its integrity then the consequences can be deadly. “Because a concrete building is naturally fireproofed, you don’t need to fireproof it. But a steel building needs fireproofing,” says Peter Sneyd, sales and operations manager, Al Mazroui & Peiniger International. Fireproofing must prevent steel losing its structural strength by protecting it from getting too hot. The most obvious option is to use concrete, but this will increase the weight and column sizes and negate the whole point of using a steel frame. Therefore lightweight materials that don’t intrude on the space inside the building should be used. Although there are a number of options available, two main methods are available. First, there are thin film intumescents that follow the contour of the steel and add just a few millimetres, and second, cemititious sprays that again follow the contours of the steel but are less efficieint. Intumescents were developed for the bottom of the NASA space shuttles to prevent them from burning up on re-entry. Intumescents work by foaming up and expanding by up to 40 times once they reach a certain temperature. When applied in multiple layers the top layer foams, chars and burns away before the next layer expands and begins the process again, giving protection for a prolonged period. “Making sure that a certified fire proofing product is applied is absolutely crucial,” says Waleed Al Hamzi, managing director Middle East and Africa, Peiniger International. Unfortunately, fireproofing not often taken as seriously as it should, as there is a temptation to use substandard products to reduce the overall cost of a project. It may save money in the short-run, but may be disastrous in the long-run.||**||

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