Industry workhorses keep the lead
Advances in how structural steel and concrete are used continue to push the envelope of building performance. Whether its new concrete formulations that provide added strength or steel frame designs that minimize floor heights, Building Teams are relying on these two industry workhorses to meet market demands.
A steel-framed high-rise residential tower in Chicago and a massive concrete cathedral in Los Angeles are indicative of how steel and concrete are being used in innovative ways by Building Teams.
X marks the spot
The steel X-bracing of the $40 million Erie on the Park tower in Chicago resembles several steel-framed buildings in that city, most notably Skidmore, Owings & Merrill's John Hancock Center. The 27-story structure is unique for being the city's first high-rise residential tower to utilize a steel-frame system since Mies van der Rohe's 860-880 Lake Shore Drive went up in 1952.
"What we did was to go against a trend by using steel for cladding and the structure," says Lucien Lagrange, principal with design architect Lucien Lagrange Architects, Chicago. Lagrange says using a steel frame enabled longer column spans, smaller columns, and more flexibility in the distribution of M/E/P systems.
"Concrete has a span of about 20 ft., so there's columns everywhere, including inside the apartments," he says. "Steel spans are up to 37 ft., which gave us flexibility with designing the space."
Lagrange says additional flexibility was attained by utilizing a bar joist floor system, which provided about 18 in. of plenum space between the floors. "Compared to concrete, where the plumbing and conduit are buried in the floor slab, steel provided this flexible plenum space that allowed us to move the spaces around."
Using steel also meant having "a lot less structure, so we end up with more glass, including floor-to-ceiling windows," he adds. "It also gives a lighter expression to the building."
If steel works so well in high-rise residential, then why does concrete dominate the market? It's a matter of economics, says Joe Burns, principal with Thornton-Tomasetti Engineers, New York, structural engineer on the project.
Steel chevron braces highlight the new Erie on the Park residential tower in Chicago. At 27 stories, it is the city’s first high-rise residential building in 40 years to utilize a steel frame.
"One huge advantage concrete has over steel in the high-rise residential market is that it's economical, because the concrete is used for the cladding, as well," says Burns. "As soon as the owner wants to clad it in something else, be it masonry or glass, then steel become competitive."
The beam and bar joist system employed at Erie on the Park is one of several steel frames used in high-rise residential work, a growing market for the steel industry. (See sidebar, page 39.)
Rock of ages
While frame designs take steel to new markets, the latest concrete formulations are literally taking concrete structures to new heights.
"Nearly every physical attribute that you can think of can be addressed in concrete mixtures today," says David Bilow, director of engineered structures with the Portland Cement Association, Chicago. "The incorporation of admixtures, such as fly ash, into concrete mixes is becoming more commonplace."
By mixing these additives with cement, Building Teams can create concrete with a variety of traits: high strength, stiffness, or density; corrosion resistance; fast- or slow-setting rates; shrinkage reduction; even color.
"Where concrete is exposed to salt, a mixture can be made that resists penetration of chlorides from salt," says Bilow. For high-rise buildings, concrete that can hold up to 20,000 pounds per sq. in. can be achieved using pozzolans, which include fly ash, blast furnace slag, and silica fume.
A case in point is the Cathedral of Our Lady of the Angels in Los Angeles, which opened to critical and popular acclaim in September.
The $195 million structure was constructed with 59,000 cubic yards of specially engineered adobe-colored concrete designed to hold up for 300 years. It is one of the largest colored-concrete structures in the world.
"To ensure the durability, our task was to make the concrete as dense as possible," says Nicholas Roberts, project architect with executive architect Leo A. Daly, Omaha, Neb. The firm worked with general contractor Morley Construction Co., Santa Monica, Calif., and several concrete consultants to devise a special mix that had to be dense, resistant to shrinkage and thermal cracking, and uniform in color so that it would mix well with the color pigment.
With 25,000 cubic yards of exposed concrete architectural concrete, the new Cathedral of Our Lady of the Angels in Los Angeles is one of the largest colored-concrete structures in the world.
Fly ash — "a very fine material, almost like talcum powder" — was chosen to provide a concrete mix that would "fill all of the holes between the sand and cement particles," says Roberts. The fly ash is mixed with water-reducing agents, local aggregates, and a white portland cement called Lehigh Aalborg, imported from Denmark.
Roberts says while the special cement is expensive to ship, it was specified because of its low heat of hydration. "Because these concrete walls are so massive — 60 in. thick in some places — a great amount of heat is generated when the cement and water interact during the curing process," he says.
If the concrete gets above 165° F, it loses its structural integrity and thermal cracking occurs, which can allow corrosive chemicals to penetrate the structure. "We started off using a type-3 cement, but it just got too hot," Roberts recalls. "This material eliminated that problem."
Complicating the mixing process was the use of color pigment to give the cathedral its adobe look. Supplied by Davis Color, Los Angeles, nearly 100 tons of pigment was delivered to concrete supplier Catalina Pacific, Glendora, Calif. There, exactly 8 pounds of pigment were mixed for every cubic yard of concrete.
"The whole process required very precise control to ensure consistent color," says Roberts. "The contractor literally had stop watches timing the truck leaving the batch plant, and if it got to the site too late, it was rejected."
He says the contractor also had to closely monitor the water level in the mix, the amount of time the concrete was vibrated after it was poured into the forms, and the amount of time the forms stayed on.
PCA's Bilow says high-range water-reduction additives, known as superplasticizers, are being used to create self-consolidating concrete — concrete that flows under its own weight.
He explains: "Normally, after concrete is poured into the forms, it has to be vibrated so that it flows around steel reinforcement bars and properly fills out the form," he says. "With superplasticizers, concrete can be made more viscous, so that it flows much easier, eliminating the need to vibrate the concrete into place."
Another concrete technology that has made its way into the mainstream is a lithium-based additive designed to reduce alkali-silica reaction in concrete, a major cause of decay. According to Michael Calderone, principal engineer with Skokie, Ill.-based engineer and materials testing firm Construction Technology Laboratories, over time, alkali-silica penetrates concrete and reacts with the aggregates, forming a water-thirsty gel that expands and cracks the concrete.
Lithium reacts with the aggregate, too, but the gel produced does not take on water and expand, says Calderone.
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Taking concrete to new heights
|Source: Portland Cement Association
|Self-consolidating||Superplasticizers||Increases viscosity to allow concrete to flow under its own weight, thereby eliminating the need to vibrate the concrete once it's poured into the form. Also suitable for applications where concrete must flow through forms that are heavily congested with steel rebar.|
|High strength||Fly ash; slag; silica fume; calcined clay; Metakaolin; calcined shale||Increases the compressive strength of concrete up to 20,000 pounds per sq. in., versus 4,000 psi for traditional concrete. This allows concrete-framed structures to reach heights of more than 1,500 ft. It also allows columns be smaller than the size of members cast with standard concrete.|
|Freeze/thaw resistance||Air-entraining admixtures||Improves the durability of concrete that is exposed to freeze-thaw, deicers, sulfates, and alkali-reactive environments.|
|Corrosion resistance||Corrosion inhibitors||Raises the chloride threshold level at which corrosion starts and reduces the rate of corrosion after it begins. Especially suitable for structures, such as parking garages, exposed to chloride salts.|
|Quick-setting||Accelerating admixtures||Accelerates the rate of setting and provides early strength gain.|
|Delay-setting or hardening||Retarding admixtures, such as water-reducers||For placing concrete in hot weather, in difficult places, or with special finishing processes where setting or hardening time must be delayed.|