Penny Pritzker stood atop a 50-story Loop high-rise on a sunny but bitterly cold late autumn afternoon 18 months ago for a simple reason.

It was all about glass.

Tom Scheckelhoff, senior vice president and project manager of the Chicago architecture firm A. Epstein & Sons, said Pritzker was helping determine the exterior glass that would be used in Hyatt Center at 71 South Wacker, the Loop skyscraper her family's company would soon be building.

"Everyone wanted as light a glass as possible," said Michael Damore, senior executive vice president and director of architecture and planning with A. Epstein & Sons, the architect of record on the Hyatt Center project. "Reflective buildings are not as effective aesthetically. In addition, they really give a darker view and at night become a mirror from the inside. We wanted to avoid that."

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Fortunately, advancements in glass technology allowed alternatives to be considered.

"Modern glass has improved a lot in the last few years, and they use coatings on the glass and gas between glass layers to ensure today's glass approaches the energy efficiency of tinted or reflective glass a few years ago," Scheckelhoff said. "That allows us to use more glass, larger pieces and more transparent glass than just a few years ago."

From a list of 50 possible glass alternatives, the design team narrowed the search to about a dozen, then to a short list of three. A light box - a 3-ft. cube designers slid different glass panes into - was constructed to compare options. The objective of the box was to demonstrate how the glass would look on the side of the building.

"You can move it around and pick up the sunlight," Scheckelhoff said. "That was where the decision was eventually made."

It was at this point that the Pritzker family made a decision on a brutally cold day in late 2002. "Imagine Penny Pritzker standing up on a roof of a 50-story building a block away from 71 South Wacker on Nov. 18, 2002," making a determination as to which glass should be used, Scheckelhoff said. "It was almost a comedic scene."
Owatonna, Minn.-based Viracon was chosen as the glass supplier.

Has 1.75 Million Sq. Ft.

Rising on a half block site bordered by Wacker Drive, Franklin and Monroe streets and an alley one-half block north of Monroe, the 1.75-million-sq.-ft. building will soar 49 stories.

Through the sixth floor, Hyatt Center's floor plates measure 50,000 sq. ft., with each of the floors above six encompassing about 34,000 sq. ft.

A look at a few numbers demonstrates the sheer scope of the building project, said Jeff Riemer, executive vice president of Bovis Lend Lease, the construction manager in Chicago.

Covering 1.5 acres, Hyatt Center will use 12,000 tons of structural steel and feature about 65,000 cu. yds. of concrete. The exterior of the building will be made up of about 7,000 unitized curtain wall panels measuring 5 ft. wide by one story - or 14 ft., 2 in. - tall.

Excavation began in July 2002, with construction beginning four months later. First occupancies of the structure are slated for late this year.

The site had been home to three buildings. One was the 13-story Hart Shaffner & Marx manufacturing plant, which covered the east half of the site. Two other buildings stood on the northwest and southwest corners of the parcel, the latter serving as the home of the Kent College of Law.

The buildings all fell to the wrecking ball in the 1980s, and the parcel has been used as a parking lot in recent years.

Dealing with Bell Caissons

Excavation work extended down to the second basement level of the previous buildings. "There was a sizable excavation because a foundation for the previous building was still in the ground," said Jim Swanson, principal of Chicago's Halvorson Kaye Structural Engineers.

"What we found down there were wood piles in about half the site, and bell caissons in the other half."

The wood piles weren't difficult to remove and if they interfered with the new building's caissons, they were simply pulled out of the ground.

But the bell caissons proved more problematic.

Swanson said a bell caisson is created by drilling a hole in the ground and carving out a conical shape at the bottom of the shaft to create a subterranean space larger at the bottom than the top.

The bell caissons were enough of an obstruction that in some instances, two new caissons had to be added to straddle the old bell caissons, with a transfer beam bridging the top.

No transfer beams were needed above ground because the columns extend all the way from the building's roof to its foundation, Swanson said.

"All the lateral loads are resisted by the concrete core walls in the center of the building," he added. "The profile of those core walls really mimics the perimeter of the building. It's a football-shaped building, and it comes to a bit of a point at each end.
The distance between the perimeter columns and the core is a consistent dimension.
That allowed all the steel beams to be of the same length - an issue of economy."

The core is about 50 ft. at the center, and the building height is about 700 ft., Swanson said. The resultant aspect ratio of the height to the core is a fairly narrow 14 to 1, which compelled designers to provide the building with a stiff core designed to resist lateral loads.

"[That] really means concrete becomes the best solution," he added. "The core is concrete and the structure outside the core structural steel.

"It's really classified as a composite building."

Materials Issues

That composite nature allows the builders to take advantage of the best characteristics of each material. Concrete is not only stiffer and therefore a better material for the core, but is also heavier, which helps the building resist overturning in heavy winds, Swanson said.

The steel, on the other hand, allows the gravity-supported system to produce a much lighter-weight structure. Lightweight concrete slab on metal deck is supported by the steel beams emerging from the core. The building will feature unobstructed clear spans of 42 ft., meaning steel beams were the only conceivable choice.

Both the concrete and steel systems support the gravity loads from the floors. But due to the different properties of the two elements, the building features differential shortening between the stiff concrete at the core and the lighter steel columns at the perimeter, Swanson added. To compensate, the floor framing is being installed out of plumb, because some settling will occur.

"One of the trickier things the contractor had to do was ensure as the building is erected, it's not being erected flat like you'd normally want, but erected with pitched floors," he said. "And that pitch varied from floor to floor."

'Doable but Fast' Schedule

Riemer called the schedule "doable but fast" and said the team from the beginning "mobilized and planned" for that tight timetable.

"When we first planned how to do the structural erection, we worked out that the project could be done with one crane, and still make the schedule," Riemer added.
"But we had this gnawing worry of what happens if we have a severe winter or high winds.

"So we made the decision we would add another crane, to add to our capacity to erect steel. And that was a good decision. That helped us maintain schedule."

The delivery and staging of materials benefited from the same careful planning. Most of the materials were manufactured and stored at factories producing them until they were needed onsite.

Steel, for example, was manufactured in Wolcott, Ind., about a two-hour drive from Chicago. When needed, the steel would be loaded on trucks and arrive onsite two hours later. Curtain walls were manufactured in Connecticut and inventoried there until needed. They were brought to the site on trucks and stored on the floors where they would be used.

Similarly, mechanical equipment was stored at the manufacturing facilities producing it until contractors were ready to install the machinery. "We had a lot of cooperation from our suppliers," Riemer said. "We've had a great team, and that's been a key factor in the success we've been able to achieve."