At 80 stories the Aqua building in Chicago topped out in early March. Using flat-plate floor construction, they were able to complete concrete construction well ahead of a structural steel building being built nearby.
Credit: MICHAEL MARCOTTE
Traditionally, engineers have designed the structure for high-rise buildings using all structural steel or all structurally reinforced concrete. There are advantages to each building material. “Composite structures” combine the two using structural steel columns and beams for floor construction, concrete on metal deck floors, and a concrete core structure that contains elevators and emergency stairwells.
Not long ago, most high-rise structures were built with structural steel, such as the Sears Tower Chicago, which is the tallest building in the United States.
However, advancements in the concrete industry over the past few years, have made the current trend toward concrete possible. But regional and job-specific conditions can tilt a project toward or away from concrete. Bill Baker, a partner in charge of structural and civil engineering for SOM, says they try to avoid bias when they are designing and engineering a project. He adds that they often submit estimates for all concrete, all steel, and composite systems, leaving the decision to the owner—most decisions are made based on the best price.
Trends in building construction
Today, most high-rise residential towers, as well as hotel construction, use concrete construction systems (although a current trend for some hotels is structural steel). Geographical trends greatly influence building construction decisions. In the northeast, the trend is toward structural steel high-rise construction, except in New York City where virtually all residential high-rise construction is concrete. Chicago has a more balanced market, while in Las Vegas, nearly all casino hotel construction on the Strip has been concrete.
Design considerations also influence trends. Structurally reinforced concrete construction becomes very efficient when bay sizes between columns don't exceed 20x30 feet. This creates an ideal situation for contractors to use flat-plate floor construction methods (the bottom of a floor slab is flat). Flat-plate floors reduce forming time and increase productivity. Owners also can benefit from this form of construction because it's sometimes possible to get an extra floor without increasing the height of a building.
When column spaces are wide, Baker says it is necessary to add either concrete or steel beams, adding more time and expense to complete a floor level.
General contractors (GCs) who build tall structures usually self perform their concrete work but rarely self perform structural steel work, often hiring specialized subcontractors. Baker notices that when GCs control their own schedules and self perform work, they tend to be more inventive and find ways to be more productive.
SCC makes it possible to consolidate concrete around congested steel reinforcement, increasingly an issue for high-rise construction.
Credit: JOE NASVIK
Advantages in choosing concrete
There are many reasons why owners, designers, engineers, and contractors, choose to work with concrete. For end users, there is reduced noise between units, less building sway from wind forces (wind shear) because the mass and stiffness of concrete dampens movement, and increased fire safety.
For the design and construction community, concrete can be faster to install than structural steel. It's fairly common for contractors to construct floors on three-day cycles. For more than 50 years in New York City, many residential high-rise buildings have been built with two-day cycles.