Concrete, Steel, Glass Shape New Soldier Field
Concrete forms foundation, steel supports bowl and skyboxes and glass frames suites
by Jeffrey Steele

The Soldier Field project will consume thousands of tons of steel, tens of thousands of cubic yards of concrete and enormous quantities of preglazed and glazed-on-site glass.

The flexibility to put so many different materials into place proved a major task for the construction teams working on the new stadium.

Curtain wall, for instance, is primarily used on the east side of the facility where the skyboxes and club lounges are located, said Mark Simonides, project executive with TBMK, the joint venture composed of Turner Construction Co., Barton Malow Co. and Kenny Construction Co. serving as general contractor. This portion of the building is heated and cooled and required pre-glazed "panelized cladding."

The lake-side curtain wall on the suite structure is made up of modular units, which were set in place by means of a hydro-crane over the east colonnade.

The field side of the suites is a different story. "It's a unique, stick-built-type system," Simonides said. "Some units came out in small modular units. The transoms above the viewing glass were preglazed modules. The main viewing glass is a low-iron, nonreflective glass, glazed at the site.
The active transoms open and close to allow noise and outside air into the suite."

Interior glass between the skyboxes was also glazed onsite, noted Joseph Burns, Chicago-based principal with Thornton-Tomasetti Engineers, the New York-based structural engineer on the project. In addition to the glass on the stadium's east side, a glazing shield is used on the outside of the concourse on the west side.

Issues Set in Steel, Concrete

Steel and concrete play fundamental roles in the structure.

Steel H-piles were driven about 100 ft. below grade to support the bowl, Burns said. "This was because there were existing obstructions, and also H-piles were quicker," he said. "There was the speed issue. After that came the cast-in-place pile caps and ground floor slabs."

Some 40,000 cu. yds. of concrete were required to complete the project, said Alice Hoffman, president of Hoffman Management Partners LLC, the Chicago-based developer's representative for the Chicago Bears.

Cast-in-place concrete was also used as the fill on deck, the slab on grade and the foundation walls, Simonides said. The seating bowl, which will feature some 61,500 seats, is constructed of precast concrete risers, which bear on steel raker beams.

While stadiums typically use a combination of cast-in-place concrete and structural steel in above-grade structures, structural steel was employed for the bowl and skyboxes in this stadium to accommodate the fast-track schedule and to minimize potential delays from inclement weather.

The project required 13,000 tons of steel, Hoffman said. The steel frame was stick built and erected piece by piece.

Coordination with teams handling glazing and seating risers, as well as the nonrectilinear dimensions of the bowl, were major issues.

Complex Bowl Geometry

"The geometric aspects of the bowl were the big challenge," Burns said. "That meant more three-dimensional analysis and design."

Agreeing that the geometry posed stiff challenges for the construction team is Ben Wood, design principal with the LW+Z Joint Venture. (The project is a joint venture of two architectural firms, Chicago-based Lohan Caprile Goettsch Associates, with primary responsibility for the master plan and North Burnham Park project, and Boston-based Wood & Zapata, with primary responsibility for the architectural design of the Soldier Field stadium.)

"It's all curvilinear," Wood said. "There are no square corners. We have one radius where the sidelines meet the end zone, one radius in the end zone and another radius for the sidelines.
Then you have transitions between those radii. It's in the transitions that most of the problems arise, and that's where the computer has to work overtime to resolve differences. It couldn't have been done without the massive, heroic efforts of the curtain wall contractor, the steel fabricator and even the precast contractor."

All had to work off a computer model overseen by the design team and structural engineers, he said.

Construction-industry professionals are accustomed to working with two-dimensional drawings of three-dimensional structures.

"And when you start issuing three-dimensional drawings of three-dimensional things, the learning curve is pretty steep for the people in the field," Wood said. "Steel can actually be fabricated right from the drawings. But you still have people in the field who are laying things out with chalk and crayon, the way we've always done it."

Wood explained that a machine was installed at precisely the middle of the stadium, on what will become the 50-yard line, that provided X, Y and Z coordinates for every point in the huge structure. Once the steel was erected, all the points had to be field-checked to ensure the steel had been erected with the predetermined allowances.

"Because after the erection of the steel, you had the erection of the curtain wall and the precast," Wood said. "You basically had everything hanging off that steel."

He also noted that due to the fast-track schedule, installation of the precast and curtain wall couldn't wait for all the steel to be erected. With two cranes operating, the one that erected steel all day long would erect precast that same night. The cranes were so large and required such enormous turning room that steel and precast couldn't be erected simultaneously, Wood said.

Sequencing Issues

Compounding the logistical woes was the sequencing. The farther a crane must reach, the less lifting capacity it offers. That meant that the lower bowl's seats were the last to be installed because cranes had to be positioned in those areas to lift seating into the stadium's highest reaches.

"Common sense would tell you that you start at the bottom and work toward the top," Wood said. "But in building these large stadiums, you start in the middle and work your way up to the top, then come back and do the bottom. You actually finish the very top of the stadium before you do the bottom. In other words, the last row of seats go in before the first."

All of these issues, as well as the task of demolishing a large portion of the existing bowl while retaining the colonnades and ensuring the concourses were self-supporting, made this a construction job to remember for everyone connected with the Soldier Field project.

"This is one of the most challenging projects I've ever worked on," Burns said.