Top trumps

While it will not eclipse Chicago's famous Sears Tower in terms of total height, the city's new Trump Tower is likely to set a new record in terms of construction technique.

When completed, the new 92 storey Trump Tower will contain almost 138000 m³ of concrete and be 415 m tall, just 27 m short of claiming the height record from than the Sears Tower. However, it will be the tallest reinforced concrete building in the US, and possibly even the world, depending on whether you count the concrete core of the Burj Dubai (see IC, September 2005) as its structure.

Construction of this massive US$ 600 million scheme, designed by US architect Skidmore, Owings & Merrill, is being project managed by Bovis Lend Lease and started in March 2005. The tower is due to be completed in 2009 and will have over 241000 m² of floor space occupied by 472 apartments, a 286 room hotel, shops and 1000 parking spaces.

While most tall buildings of this nature, such as the Sears Tower and Chicago's Hancock Building, are steel-reinforced, the Trump Tower will be a concrete structure, mainly because of limitations of the riverside site. More than US$ 130 million of the building's construction budget is earmarked for the concrete elements and is being undertaken by Chicago-based contractor T J McHugh.

But before the main construction work could get under way the former headquarters of the Chicago newspaper The Sun-Tribune had to be cleared from the site. A disused freight tunnel, which passes below part of the site, also had to be sealed and, due to the riverfront location, old dock piles also had to be removed.

The tower's foundations are formed by steel reinforced concrete raft supported by over 240 caissons sunk up to 33.5 m through the underlying clay and into the limestone bedrock below. McHugh senior vice president Dale Hendrix said, “We built the foundation raft, which measures 61 m in length, 20 m in width and 3 m in depth, in autumn last year using more than 3800 m³ of self consolidating concrete. The raft effectively anchors the building to the foundations below.”

Orchestrating what Mr Hendrix calls 'The Big Pour' was a monumental task. Thirty concrete trucks worked around the clock for almost 24 hours to complete the work in a single pour.

Prairie Material Sales provided the concrete and the trucks, making 600 trips between the construction site and its concrete distribution site. Chemists were on hand during the continuous pour to ensure the special formula of concrete was to exact standards. Designed specifically for the job, the concrete needed to be able to support the 327000 tonne load of the building.

According to Prairie's vice president of operations Paul Blatner, the compressive strength of conventional concrete is 48 N/mm²but the mix used for the Trump Tower foundation raft is designed to reach 69 N/mm². Special additives were used in the mix to ensure the concrete met the strict design, which included keeping the concrete's temperature during the placement below 26.7 °C and the temperature during curing below 76.7 °C.

Construction of the foundation raft, basement levels and mechanical facilities were completed in December 2005, clearing the way for McHugh to start bringing the structure “out of the hole,” as Mr Hendrix described it.

To take the structure skyward, McHugh has leased two Liebherr tower cranes. The first, a 420 ECH with a final height under hook of 393 m, went up in late November last year while the second, a Liebherr 420ECK, went up in mid-December. The second tower crane will have a final under hook height of 376 m. Both were leased from Morrow Equipment.

“We chose the Liebherrs' because they were the only ones we could get that were high enough for this building,” said Mr Hendrix.

Jim Huels, district service manager for Morrow Equipment, which leased the tower cranes to McHugh, said that the cranes will be on site for a couple of years. “Once the structure is taller than the cranes, the cranes will be moved from their foundations and climb through the building,” said Mr Huels.

Once the building is topped out the first tower crane will dismantle and take down the second tower crane, and then a derrick crane will be brought in to take down the final tower. “But that's a long time away,” said Mr Hendrix.

The two hydraulic climbing towers will provide most of the lifting needed on the site, although Hendrix says he will lease a variety of cranes from crane houses throughout Chicago to meet specialized lifting needs. To this point, two to three mobile cranes have been at the site daily since work started on the building.

The main construction focus over the next few years will be to build the concrete structure, floor by floor. Each floor will be cast in-situ using a self climbing formwork system supplied by Peri. The walls on the structure will be formed using a mix of Domino, Trio and Rundflex panels and the Multiflex and Skydeck systems will be used for the slabs.

Chicago is known as the windy city and, according to McHugh's senior vice president of concrete construction Dave Alexander, wind and weather will pose a real challenge on this project. “The concrete teams may face a temperature difference of up to 17 °C between street level and the upper floors as well as high winds,” he said.

To overcome the potential problems of high winds, and to boost safety on the scheme, McHugh with use a hydraulically jacked, three story high wind screen around the perimeter of the working floors. The Climbing Perimeter Protection (CPP) system has been developed by Peri for use on high rise projects, like the Trump Tower.

McHugh is currently using a crane lifted version of CPP on construction of the 44 storey Regatta Tower, which is also being constructed on Chicago's waterfront. “Use of CPP has boosted both safety and productivity for all trades working on the Regatta Tower,” said Peri CEO Tom Ameel.

“Using CPP means that work is less affected by adverse weather conditions and workers feel like they're on the ground floor. The addition of a hydraulic climbing system for the Trump Tower construction means that the protection can be raised as construction progresses without the need for crane time.”

Pumping the concrete from the ground up to over 400 m to reach the upper construction levels also presents a significant challenge. McHugh has imported a concrete pump from Germany, which was custom built for the Trump Tower scheme. The pump has a capacity to move up to 175 m³ of concrete per hour at ground level but can still pump up to 76.5 m³ of concrete per hour to a height of over 518 m.

“The pump has the capacity to deliver concrete to a height more than 100 m higher than the final height of the Trump Tower but the delivery lines for the project will feature several horizontal loops in order to keep the 16 tonnes of material flowing,” said Mr Alexander.

The Trump Tower may find itself in the record books for being the tallest reinforced structure in the world, but it is not a record which will be obvious once the building is complete. The design for the structure includes a stainless steel and glass curtain wall exterior which will hide the building's concrete claim to fame when work is completed in three years time.