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Concrete Superstructures

Source: | Updated: Feb 18, 2016

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 

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.

Designers occasionally use concrete columns in buildings with structural steel beams supporting the floors. This reduces cost by about one fifth over steel columns, though concrete columns are bulkier and add weight to a structure. From an architectural perspective, concrete is more adaptable because it can be molded into interesting designs.

With sustainability an increasing issue for all building construction, concrete becomes an obvious choice. High-performance concrete (HPC) mixes typically replace some portland cement with fly ash, slag, or silica fume (or all three) to use what would normally become a waste product. These mixes also enhance the performance of the final concrete and reduce greenhouse gasses associated with cement production. But the long life and energy efficiency of a concrete structure are the keys to its sustainability. Concrete structures reduce the cost of heating and cooling—especially when insulated concrete panels are used for curtain walls—making them more energy efficient.

Technical innovations

A number of the technology improvements have made high-rise concrete construction more competitive over the years, ranging from construction methods to enhancements to concrete mixes. Each has helped to increase concrete's advantages as the preferred building material for high-rise construction.

Flat-plate floors. Lawrence Novak, director of building structures for the Portland Cement Association, says that the development of flat-plate construction technology has had an impact on high-rise construction—especially for residential buildings. Without supporting beams, the height between floors is minimized and construction costs reduced, allowing the addition of floors without changing the height of the building. Novak adds that the introduction of headed shear stud assemblies that extend further out into floor plate areas from columns reduces punching shear. These devices can increase the allowable distance between columns to a degree and eliminate the cost of forming drop panels or capitals around the tops of columns.

Forming systems. The development of drop-head forming systems is a perfect fit with flat-plate construction. Form manufacturers have each developed systems that allow workers to quickly set floor forms from below—creating a safer, faster, and more efficient work environment. They also can be easily removed after concrete placement. Form tables by contrast are moved by crane and therefore are controlled more by crane schedules and wind conditions.

Today, it's common practice to construct building cores with cast-in-place concrete. Jump forms or self-jacking forming systems have become popular for core construction. After walls are cast, workers free the forms from the concrete and use hydraulic jacks built into the system to move interior forms to the next level, while exterior forms are lifted by crane to the next position.

Concrete pumping. Concrete can be pumped great distances through vertical lines by single pumps—more than 2000 feet in the case of the Burj Dubai, the world's tallest building. Pumps allow tower cranes, which deliver concrete more slowly, to concentrate on other work. Key elements of the system include the pump, steel pipe slick lines that are anchored onto or cast into the structure for the duration of the project, and placing booms mounted on masts and attached to decks or on top of building core forms. When they are mounted on self-jacking core forms, they move upward with the forms and don't require crane assistance.

Concrete mixes. HPC and self-consolidating concrete (SSC) mixes meet the engineering requirements for tall building construction. SCC mixes solve placement challenges where there is congested steel reinforcement. On the Trump Tower in Chicago, 16,000-psi SCC mixes were used to ensure good consolidation at the transition levels of the building that contained as much as 1000 tons of rebar. HPC mixes are frequently used in column construction. Baker adds that specifying mixes with high modulus of elasticity is as important as specifying concrete compressive strength.

Reinforcement. The software used to design structural steel buildings is still more advanced than for structural concrete. But this is changing as software for designing steel reinforcement improves. There have been improvements in rebar too. Upgrading Grade 60 rebar to Grade 75 or even to Grade 100 can help reduce congestion. Using headed reinforcement—rebars with buttons or washers welded on the ends—eliminates the need for hooks. This makes placement easier in tight areas and reduces congestion. Threaded rebar splices also helps to reduce congestion.

Advancements in rebar fabrication makes concrete construction more efficient. Neal Anderson, vice president of engineering for the Concrete Reinforced Steel Institute, Schaumburg, Ill., says that rebar can be made on automatic table fabricators, increasing the accuracy of each piece. Often reinforcement is assembled on jigs on the ground, away from the focus of construction. Assemblies are moved by crane into location as they are needed and speeds up construction. In California, instead of tying bar, resistance welding is used to assemble cages and mats.

Post-tension reinforcement, which places concrete under high compression, can be advantageous for high-rise construction. The distance between columns can be increased, floors are thinner, and structures can be lightened.

Advantages of steel structures

Depending on the design elements of a particular project, steel construction may be preferable over concrete.

  • Some office building owners prefer steel. One reason is that space is retro-fitted more easily for new tenants.

  • Concrete is least expensive when forms are used repetitively. When designs don't permit this, costs increase.

  • If a general contractor can't self perform the concrete, they rely on subcontractors. Costs can be higher and GCs have less control over their schedules.

  • If no concrete producers in the area make high-strength concrete or aggregate supplies are poor, steel construction can be the better choice.

  • Structural steel excels when you need a lighter weight structure due to soil conditions.

  • When designers want long spans between columns, steel is often preferable.

  • Market conditions drive prices. Price changes between steel and concrete influence the method of construction chosen. The backlog of a contractor's work also affects prices, although backlogs are currently not a problem.

Building with concrete

Novak refers to the modern construction of structural concrete high-rise buildings as a “ballet in the sky.” General contractors must be very organized and understand how to bring all the elements together in a timely fashion. Trades must work closely together to ensure that all penetration blockouts are in place and properly located. When you are on a three-day cycle, there isn't much time available to get everything ready.

The price of concrete, including placement, is a bargain. Baker says when jobs are cost accounted, purchasing and installing rebar is the most expensive element. The second is formwork, with concrete being the least expensive of all.

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