As real estate and land prices continue to soar, high rises in urban areas like Chicago and Toronto are being forced into tighter footprints in dense downtown locations. To optimize the number of units per floor and the number of floors per building, many top engineers are going with inventive, new design specifications.
The Chicago office of structural engineering firm Magnusson Klemencik and Associates (MKA) favors the use of high-elastic modulus concrete (high-E concrete) in new tower designs, making it possible for thinner columns, beams and shear walls to carry the same loads as traditional designs.
Over the past three years, MKA's Chicago team has moved to a new standard practice calling for modulus specs on all structures above 45 stories. Engineers rely on experts at Prairie Material to develop and refine the high-E mixes needed for the city's most prestigious tower projects.
Why high-E works
"The elastic modulus of concrete is essentially the measure of the concrete's stiffness - how much will it deform or flex under loading," explains Nick Beristain, Prairie Material's Lab Services Manager.
150 N. Riverside construction - Courtesy of Curbed Chicago
High-E also allows engineers to design smaller column cross-sections, giving them a new way to meet the small-footprint-with-more-floor-space challenge.
"From the analysis done on one project, we learned that for each increase of 10% in elastic modulus, there was a corresponding potential 10% decrease in the size of the columns," Beristain says. "Of course, there's a point where the cost and placement characteristics of high-E become less practical, usually above 7,500 ksi."
First use with Trump Tower
Prairie first developed high-E mixes in 2005 for use at Trump Tower in Chicago. Due to its height and small footprint, project architects at Skidmore Owings and Merrill designed a series of setbacks at the 15th, 28th and 50th floors to resist wind and weather pressures at the site. SOM engineers specified an E average of 6,200 ksi for the 12,000-psi concrete at the 15th floor and 7,200 ksi for the 16,000-psi mix used at the upper levels.
Setbacks at Trump Tower, Chicago
Prairie's mix design experts worked closely with Skidmore's engineers to create and test a number of mix designs before settling on designs that would provide the required E and compressive strength in a self-consolidating, pumpable form that would not be prohibitively expensive.
New specs drive mix innovation
Traditional specifications use a calculated algorithm from the American Concrete Institute that estimates the elastic modulus based on the compressive strength requirement. Responding to MKA's project needs, Prairie developed new high-E mixes that eliminate concerns about reaching specific strengths.
"We started our testing with high-strength mixes," says Beristain. "Even though higher psi is generally indicative of higher modulus, we found that other things, particularly the density of the stone, has a greater impact on modulus values."
With the quality and availability of local aggregate playing a crucial part, special steps were needed to assure the mix would work in production. All test cylinders and beams were batched with locally available materials at plants that supply the projects and brought to the lab for testing, Beristain reports.
Notable high-E projects rising in Chicago
At 200 North Michigan Avenue, a prime location on Chicago's Magnificent Mile, Pepper Construction recently completed a 42-story residential tower with high-E concrete of 6,755 ksi from Prairie Material.
Designed by bKL Architects and developed by the John Buck Company, the property - known as MILA for its location at Michigan Avenue and Lake Street - offers 402 luxury rental units.
Courtesy of CTBUH
"Teaming up with Prairie to get higher e-modulus numbers allows us to give the customer exactly what they need and help them to achieve a better bottom line," says Paul Treacy, Concrete Superintendent for James McHugh Construction, concrete contractor for MILA.
Just a few blocks west at 150 North Riverside Avenue, a newly completed tower with an extremely slender base - 162 feet long and just 47 feet wide - rises to a total height of 742 feet, flaring to its maximum width at the 7th floor.
150 N. Riverside - Courtesy of Riverside Investment & Development Company
The structure, designed by Goettsch Partners for owner Riverside Investment & Development Co. of Chicago, and built by Clark Construction, calls for several different high-E specifications at various levels due to the unusual footprint, Beristain notes.
Above the foundation and through level 9, the design called for 12,000-psi concrete with 6,600 ksi modulus of elasticity. The mix for floors 10 through 22 delivered 10,000 psi and 6,000 ksi E, with all successive levels at 8,000 psi and 5,400 ksi E.
Courtesy of Studio Gang Architects
Prestigious new projects now breaking ground in Chicago also call for high-E concrete. The 93-story Vista tower at Lakeshore East, designed by architect Jeanne Gang, has E specifications of 5,400 ksi, 6,000 ksi and 7,000 ksi at various levels for the hotel/condo project. 1000M, the 73-story residential tower in the city's South Loop district designed by Helmut Jahn, is also specifying high-E concrete.
A different story in Canada
Although high-rise construction in the Greater Toronto Area has been booming for the past decade, specifications for modulus are not yet part of the picture, says Stephen Parkes, CBM's Leaside Laboratory Manager.
"At present, modulus is seen as supporting information here," Parkes notes. Instead, engineers rely on high-strength concrete to achieve the results they want, he says.
"That said, we know that the specification of modulus is coming to our market in the future, so it is on our radar," Parkes says. "Once we see the first actual need we will work closely with Prairie Material to bring it into the mainstream here."
Advanced lab capabilities for ideal mixes
At Prairie Material's concrete labs in Bridgeview, Illinois, experts use a sophisticated set of tools to make direct measurements of elastic modulus in test mixes. Special software plots a stress-strain curve with precise accuracy. The testing setup can accommodate both standard 4-inch x 8-inch and 6-inch x 12-inch cylinders.
Precision testing equipment measures high-E concrete.
Using these tools, Prairie's distribution team has simplified its catalog of high-E mixes. Each formula is optimized to perform in a specific setting, so specifiers can choose the right mix, whether it's a 6,600-ksi modulus mix or a 12,000 - 14,000 psi mix, depending on the pour requirements of the day.
"We've made a long-term investment," says Beristain, "because we believe the trend toward modulus specification will continue in our market and we're committed to developing and delivering the right mixes for each job."