On the site of the old Chicago Sun-Times building at the edge of the Chicago River sits the new Trump International Hotel and Tower. Presently workers are constructing “belt walls” on the 28th floor, making their way to topping out at 1134 feet above ground. When completed in 2008, it will stand as the tallest structurally reinforced concrete building in the United States. The height to the top of the spire will be 1362 feet. Dane Rankin, an associate at Skidmore, Owings & Merrill (SOM), Chicago, reports that the building's complexity pushes the capabilities of high strength concrete.
Jill Cremer, vice president of the Trump Organization, New York City, says that after developing numerous buildings in the New York area, they decided to build in Chicago when they learned that the Sun-Times location became available. “It was impossible to turn it down due to its location in the center of the city and the inherent developmental potential.” The Trump Organization is selling both individual units and hotel units—people also will be able to own hotel units and lease them back to the hotel.
Donald Trump favors the use of structurally reinforced concrete for his projects. It reduces “sway” in tall buildings and saves owners money. Robert Sinn, associate partner for SOM, says that much of the engineering for the structural concrete actually started in 1999 when SOM contracted to design “7 South Dearborn” in Chicago, which was planned to be the world's tallest structurally reinforced concrete building but was never built. The engineering and research for that building made it possible to use concrete for structural purposes in very tall building construction. Prairie Materials, Bridgeview, Ill., and CTL Group, Skokie, Ill., researched high-strength mixes with high modulus of elasticity (E) and their shrinkage characteristics. When SOM started work on the Trump building, this research proved useful.
Very tall buildings represent architecture and engineering at its best. The challenges are on the cutting edge. Wind effect, for instance, means little to a one-story building but becomes a crucial issue for this type of construction.
The concrete contractor James McHugh Construction, Chicago, is a general contractor but does all of his own concrete work, which accounts for about 40% of his business. Dave Alexander, senior vice president for McHugh, says that a major challenge for a project like this is wind and temperature. Conditions at ground level can be gentle but bitterly cold and windy 900 feet up. He adds that the 2007-2008 winter months will pose the greatest challenges for them.
Work on the Trump project started with caisson construction in the spring of 2005. Ten-foot-diameter holes encased in steel were drilled approximately 130 feet deep, 6 feet into bedrock, and filled with 10,000 psi concrete. In the core of the building, a 10-foot-thick, 4700-cubic-yard mat slab using 10,000 psi self-consolidating concrete (SCC) was placed in the fall of 2006. It was believed to be the largest single placement of SCC anywhere in North America. McHugh constructed reinforced concrete caps and grade beams over the caissons surrounding the mat slab to distribute the loads imposed by the building's columns. Column and floor construction followed, with particular attention given to transition floor levels where the footprint of the building decreases and complex engineering and construction transfers the dead loads and braces the building against wind forces.
Super tall buildings
Occupants of tall buildings don't like the feeling of buildings swaying back and forth due to high winds. One way to minimize this effect is to install dampening equipment at the top of the building to counter swaying motions. Building composite structures is another popular method to minimize swaying. This involves constructing a building core with heavily reinforced concrete to provide the stiffening and using structural steel to support the floors. An even better way, though, is to build structurally reinforced concrete buildings, which have the least sway of all, as well as other advantages. Stan Korista, a director at SOM, says that structurally reinforced concrete floors make “flat slab” construction possible, significantly reducing the height required between floors, saving owners significant money. “What has really made the difference for concrete construction has been the development of high-strength concrete mixes, the greatly increased efficiency of concrete pumps and placing booms in the past few years, and the development of forming systems that can be erected safely and quickly, and then moved to the next location,” he adds.
Sinn says that design and engineering work together as a project develops. A central concern is the forces generated by wind on tall structures. During the design phase, SOM conducted wind tunnel tests using models to see how the structure would perform. “Our goal is to find ways to break up a unified wind force, causing it to move in several directions so that there is less impact on the structure.” He refers to this process as “vortex shredding” or “confusing the wind.” Both architects and engineers usually are present during these wind tunnel tests. As the model design changes, testing continues until they achieve the least wind forces against the model, while maintaining good design features. In the case of the Trump building, wind shear is controlled by the rounded surfaces of the buildings shape and the floor setbacks as the building rises in height.
Challenges for this construction include the floors where setbacks occur, columns spaced at 30-foot intervals supporting flat-plate 9-inch-thick floors, and the slender north-south orientation of the building with an aspect ratio of 14 to 1. “Because of the setbacks, there isn't a single column line with a centerline running the length of the building,” Sinn says. “These setback locations are the most stressed and reinforced parts of the building.” They occur at the 16th, 29th, and 51st floors and at the top of the building. On each of these floors there is a system of shear walls, outriggers, and belt walls that transfer the load from columns above each setback to those below. Alexander says that on the three-story-tall 29th floor, workers are placing 3000 tons of rebar. SOM knew this would cause congestion making the consolidation of concrete during placement very difficult so SCC was specified—12,000 psi compressive strength for the 16th floor and 16,000 psi for the other locations. The high strength is needed to transfer the column loads and provide the building's shear walls, also located here, with the strength to resist wind shear.