High-rise buildings dominate the skylines of our cities, and in the race to have the highest or the most modern, the boundaries of architectural engineering are pushed even further.
The materials that make this possible – concrete and steel, each have their strengths and weaknesses. So, which is the better material?
Safety Considerations
Safety Considerations
The concrete industry has always maintained that concrete is safer than steel. It requires no additional fireproofing to meet fire codes and has performed well during natural disasters.
Buildings made of cast-in-place reinforced concrete can withstand winds of more than 200 miles an hour and can withstand flying debris.
Cast-in-place concrete provides very good resistance to impacts and explosions.
It can resist extreme temperatures from fires without losing its structural integrity.
Reports issued recently by the National Institute of Standards and Technology show the reduced structural integrity of steel was to blame for the collapse of the twin towers in NYC. The fierceness of the jet-fuelled fires contributed to the collapse. With measures such as spray-on fireproofing, buildings made of structural steel are capable of withstanding greater temperatures.
The strength of steel, and its ductility, along with design and engineering, make it a good choice in areas of seismic activity. It can bend without breaking in high winds.
You can create and shape anything out of concrete, making advanced design possibilities a reality.
Concrete has the advantage of offering extra space possibilities. Cast-in-place reinforced concrete can give more rentable space, due to the lower floor-to-floor heights. Donald Trump’s architects switched from steel to concrete so they could add two extra stories to the new building at the former Chicago Sun-Times site.
Steel remains the popular choice for office and multifamily developers. Use of girder slab, castellated beam construction, and staggered truss enables much lower floor-to-floor heights than expected in structural steel buildings.
Steel has the highest strength-to-weight ratio of any construction material.
Cost Considerations
Prices of construction material have increased, but the cost of ready-mix concrete has remained stable.
Insurance companies look favorably on cast-in-place reinforced concrete buildings because the safety and structural integrity reduce liability on their part. Owners and developers of such buildings can save nearly 25% annually on the costs of property insurance.
Structural steel represents less than 20% of all the steel used in building construction. The increase in costs of the structural framing system represents less than 2% of the 10% increase in project costs.
Costs for concrete framing systems have gone up equivalently to the costs of a steel framing system, despite what concrete experts say, according to John P. Cross, vice president of the American Institute of Steel Construction in Chicago.
Environmental Impact
Concrete is often locally sourced and needs less energy to transport to construction sites.
Rebar for concrete is often made from recycled steel.
At the end of its life, concrete can be crushed and recycled, but it can’t be used for new building concrete, whereas steel can.
85% of steel is recycled, according to the British publication Building.
New steel made from scrap uses about one-third of the energy necessary to make steel from virgin materials.
Steel fabrication is often completed at a significant distance from the construction site, which increases the use of energy needed to transport it.
Construction Scheduling
Buildings with concrete can almost always be constructed faster – sometimes twice as fast, according to Alfred G. Gerosa, president of the Concrete Alliance Inc, in New York City.
On a 2-day cycle, workers can pour up to 20,000 square feet of floor space every two days.
While steel can’t beat concrete’s 2-day turnaround; it does provide benefits of its own. John P. Cross says he believes structural steel framing systems are the way of the future, and they result in a faster construction schedule.
CAD programs can pass their information straight through a database as a 3-D model, and send it to detailing and shop floor fabrication programs. These productivity increases help to ensure the future of steel as a viable construction material.