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 Concrete vs. Steel

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تاريخ التسجيل : 25/03/2008

مُساهمةموضوع: Concrete vs. Steel   الأحد يوليو 13, 2008 1:47 pm

Concrete vs. Steel

Which is the better building material? You be the judge.


Concrete can be placed efficiently in buildings over 1,000 feet without using a tower crane.


Before modern engineering and the ability to manipulate concrete and steel, the world of architecture consisted of wood, adobe, thatch, and cave dwellings. We've come a long way. Today's cities reveal skies punctuated by buildings so tall and austere, even architectural tour guides experience the occasional pain in the neck. So, which material reigns supreme in the world of development today - concrete or steel? Both provide numerous benefits. As for whether or not one is better, Buildings lets you determine which side you're on.



SAFETY

Concrete: The recent announcement by Ground Zero Developer Larry Silverstein regarding safety measures at the new 7 World Trade Center (WTC) building echoes what the concrete industry has been saying for years: Concrete is safer. The building's core (where elevators, stairs, and power systems are located) will be encased in 2-foot-thick concrete for protection in the event of a fire or terrorist attack. "Cast-in-place reinforced concrete offers outstanding resistance to explosion and/or impact. Moreover, it can endure very high temperatures from fire for a long time without loss of structural integrity," says Alfred G. Gerosa, president, Concrete Alliance Inc., New York City .

Concrete requires no additional fireproofing treatments to meet stringent fire codes, and performs well during both natural and manmade disasters. Because of concrete's inherent heaviness, mass, and strength, buildings constructed with cast-in-place reinforced concrete can resist winds of more than 200 miles per hour and perform well even under the impact of flying debris.

With proper design, engineering, and construction, the seemingly rigid structures built with concrete can exhibit increased ductility - a must in areas prone to seismic activity. However, according to the Skokie , IL-based Portland Cement Association (PCA), the performance of any building during an earthquake is largely a function of design rather than the material used in construction.

Steel: While recent reports issued by the National Institute of Standards and Technology blame the reduced structural integrity of steel for the collapse of the WTC towers, the jet-fueled fires are to blame. Experts acknowledge that steel can soften and melt with exposure to extremely high temperatures. However, with the addition of passive fire protection, such as spray-on fireproofing, buildings built of structural steel can sustain greater temperatures and, therefore, provide additional safety.

Don't base your opinions about steel's performance on the events of 9/11. In the October 2003 Modern Steel Construction article "Blast Resistant Design with Structural Steel," authors Anatol Longinow and Farid Alfawakhiri recall the 1993 WTC attack. The article indicates that the inherent redundancy of the steel frames prevented the structure's collapse. "We're seeing a lot of structures constructed taking into account progressive collapse for blast conditions that are designed very effectively and very economically in steel," explains John P. Cross, vice president, marketing, American Institute of Steel Construction, Chicago.

Steel's strength and ductility, combined with solid engineering and design, make it a safe choice in seismic zones. "Steel framing does very well under high [wind] loads because it is ductile, which means it has the ability to bend without breaking and can absorb that kind of energy," says Larry Williams, president of the Washington, D.C.-based Steel Framing Alliance, of cold-formed steel.


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COST

Concrete: It's true: Prices of construction materials have spiked. However, the cost of ready-mix concrete remains relatively stable, and according to Ed Alsamsam, PCA's manager of buildings and special structures, even the increase in steel has had a minimal effect on reinforced concrete building projects. "Concrete prices remain very steady despite the fluctuating and substantial increases in other building material prices," he says.

While cast-in-place concrete construction can be pricier on the front end, the return on investment achieved can lessen the cost differential. According to Gerosa, "Insurance companies recognize the benefits of a cast-in-place reinforced concrete office building because the aforementioned benefits - increased safety and structural integrity - reduce liability on their part. Insurance companies also report that owners and developers of a Class-A, cast-in-place reinforced concrete-framed office tower with a concrete core and wider egress stairs will save nearly 25 percent annually on the cost of property insurance."

Steel: The big news in development is the price of steel. And while structural steel has experienced a 50-percent increase over mill prices since November 2003, experts like Cross stress that structural steel represents less than 20 percent of all the steel used in building construction. "If you look at the overall impact on project costs, in the past year we've seen project costs go up by about 10 percent as a result of increases in all types of construction materials. The increase in cost of the structural framing system represents less than 2 percent of the 10-percent increase in project costs," Cross explains.

And despite what some concrete experts say, if you thought you'd escape the rising steel prices by opting for reinforced concrete, think again, Cross says. "The costs for a concrete framing system have gone up pretty much equivalently to the costs of a steel framing system," he adds. To get an accurate reading on which material is most cost effective, analyze current steel and concrete framing prices on a project-by-project basis. And remember, according to Williams, "2004 was not a good year for any building material."


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MATERIAL AVAILABILITY

Concrete: As recently as the fall of 2004, many states were reporting a shortage of cement, the primary binding ingredient used in concrete. The hurricanes in Florida and atypical levels of winter construction activity resulted in greater demand and shorter supply. According to the PCA, other factors contributing to the shortage are shipping rates and the limited availability of transport ships. With imported cement supplementing domestic supplies, skyrocketing shipping rates and limited freight transport have resulted in higher costs and unreliable supply.

Despite tight supplies of cement in some regions of the United States , the impacts have been greatest on smaller companies, builders, or contractors. "Developers are using concrete frame buildings in every building sector, and count on concrete for stable pricing and availability regionally," says Alsamsam. Cement companies are rapidly expanding, and domestic capacity is expected to increase by 2008.





A pump boom on a mast enables concrete construction on high-rise buildings.
Photo courtesy of PCA


Steel: Steel availability has been the subject of more than a few conversations lately, with the blame placed on the ever-expanding construction activity in Asian countries like China . However, experts from the steel industry are eager to dispel the myth that there simply isn't enough steel to go around. "Over the past year, there's been a lot of misperception out there in terms of availability of material," says Cross. "There is no shortage. The United States ' structural steel industry has the capacity to produce 6 million tons of structural steel per year. In 2004, our usage was about 4 million tons of wide-flange structural product, which means there is certainly adequate capacity to meet any growth in the foreseeable future. Structural steel is readily available."


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CONSTRUCTION SCHEDULING

Concrete: The old adage "time is money" was never more true than when talking about construction schedules. According to Gerosa, "Buildings with concrete can almost always be built faster. When compared to structural steel, sometimes twice as fast. It is not uncommon for cast-in-place reinforced concrete buildings to rise one floor every other day. Developers can finish jobs faster, earn a profit, recoup capital, and move on to the next project." The popular construction process Gerosa refers to is known as the 2-day cycle. Robert A. Ledwith, business manager and financial secretary-treasurer for the New York City-based Local #46 Metallic Lathers Union and Reinforcing Iron Workers, explains: "A 2-day cycle is a very labor-intensive operation. On a 2-day cycle, we can [pour] up to 20,000 square feet of floor space every 2 days."

Completing a building faster because of the 2-day cycle offers significant benefits. "That is something that is absolutely unique for concrete construction. They have mastered it in New York City , and the cities of Chicago and Los Angeles are striving to match that, as their accessibility to downtown [with] construction cranes, equipment, and staging areas become ever smaller and more limited," says Alsamsam. "When those ready-mix trucks pop up on-site, they need to come in, dump, and leave very quickly." The faster the building is completed, the sooner the owner can allow occupancy and begin collecting income from tenants.



The Chestnut Hill condominium complex in Newton , MA , is steel-framed.
Photo courtesy of AISC


Steel: While concrete's 2-day cycle may seem to give it an unarguable advantage, steel provides many construction benefits of its own. "We believe structural steel framing systems are the way of the future," says Cross. "We believe that they result in an accelerated schedule. We also believe that quality is enhanced because of off-site fabrication, and that the productivity opportunities that exist in construction can be best addressed in off-site fabrication with a reduction of actual on-site time and on-site construction. "

Advancements in building information modeling have integrated the design, detailing, and fabrication of steel, which have resulted in an accelerated process. Design programs and CAD packages can pass information through a CIS/2 neutral database as a 3-D model to detailing and shop floor fabrication programs. "This is literally compressing the steel portion of the schedule of projects by 40 or 50 percent," says Cross of the advances in interoperability. These productivity increases position steel as a viable construction material both now and in the future.


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DESIGN POSSIBILITIES

Concrete: Concrete buildings are taking shape - many different shapes - everywhere. "Remember, concrete seeks the form," says Gerosa. "You can shape anything out of concrete; that's why the Guggenheim was built the way it was."

In addition to the unique aesthetics achieved with concrete construction, these buildings offer some very real space advantages. "Public and private developers should also realize that using cast-in-place reinforced concrete to frame a high-rise office building would yield more rentable space because of lower floor-to-floor heights," Gerosa explains. This is no secret to Donald Trump, the New York City developer currently building at the former Chicago Sun-Times site. Architects of the Trump Intl. Hotel & Tower recently switched from structural steel to concrete so that two additional stories could be added to the 1,125-foot building.

With proper engineering, concrete building can also offer uninterrupted floorplates. "A great example is the newest office building in New York City , utilizing 45-foot spans and awesome views of the park," says Alsamsam of 505 5th Ave.

Steel: "Steel has the highest strength-to- weight ratio of any construction material," says Williams. And with new construction methods, steel buildings remain a popular choice for office and multifamily developers. Use of girder slab, staggered truss, and castellated beam construction enables lower floor-to-floor heights than typically expected in structural steel buildings.

Looking for long spans of column-free space? Steel delivers. "Steel can accomplish extremely long spans in structures [and] very open-bay footprints without intermediate columns. It's a very flexible material in terms of different ways to address design requirements, " says Cross.

If you haven't decided which side you're on, don't worry. Your education on steel and concrete buildings has only just begun. To find out more, contact both local and national industry associations. Offering free information and "concrete" advice, their knowledge is a real "steel."

(Jana J. Madsen, managing editor of Buildings magazine)
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