CRE N Illinois

Engineering a renaissance at Willis Tower

Part of the $500 million overhaul at Chicago’s iconic Willis Tower is the transformation of the building’s base, replacing what had been an unwelcoming environment to something open, airy and revenue generating. Part of the $500 million overhaul at Chicago’s iconic Willis Tower is the transformation of the building’s base, replacing what had been an unwelcoming environment to something open, airy and revenue generating.
The addition,dubbed “Catalog,” will include a food hall, entertainment and retail options, all covered by a curved, gridshell skylight that turns the entirety of the mixed-use space into a winter garden. The addition,dubbed “Catalog,” will include a food hall, entertainment and retail options, all covered by a curved, gridshell skylight that turns the entirety of the mixed-use space into a winter garden.
Thornton Tomasetti’s duties on the project are many, from structural engineering for the new three-story addition to exterior façade engineering and temporary bracing of the tower columns and slurry wall. Thornton Tomasetti’s duties on the project are many, from structural engineering for the new three-story addition to exterior façade engineering and temporary bracing of the tower columns and slurry wall.
A pair of 16-inch-deep steel I-beams temporarily brace the built-up 39-inch-deep tower columns to install column reinforcement. The existing slab, which braces the tower columns, was locally removed in front of the column to install the reinforcement. The existing columns impart a thrust on the beams, which they transfer to a portion of the remaining slab via anchors. A pair of 16-inch-deep steel I-beams temporarily brace the built-up 39-inch-deep tower columns to install column reinforcement. The existing slab, which braces the tower columns, was locally removed in front of the column to install the reinforcement. The existing columns impart a thrust on the beams, which they transfer to a portion of the remaining slab via anchors.

For close to two years, Chicago’s iconic Willis Tower has been undergoing a $500 million overhaul, including new elevators, lighting and amenities. But the most evident update is the transformation of the building’s base, tearing out the fortified battlement at street level and replacing it with a modern and airy superstructure.

The addition—dubbed “Catalog” in reference to the one-time mail order giant, Sears Roebuck and Company, that anchored the building—will include a food hall, entertainment and retail options, an accessible rooftop green space and more. The most conspicuous change is the addition of a curved, gridshell skylight that turns the entirety of the mixed-use space into a winter garden.

Like a covered porch on a country home, the 300,000-square-foot space will wrap around the base of the tower. The Chicago offices of Gensler designed Catalog, which is slated to open next summer, and Thornton Tomasetti has been tasked with engineering this fusion between the 1973-constructed supertall building and the new addition.

Catalog will span five stories, three above ground and two below. It’s Thornton Tomasetti’s duty to figure out the nuts and bolts—literally and figuratively—of aligning the architectural vision for the annex with the original structure.

And those duties are many: structural engineering for the new three-story addition; exterior façade engineering including curtain wall, metal panel and terra cotta rain screens; temporary bracing of the tower columns and slurry wall; implementing architecturally exposed structural steel (AESS) and much more. Though construction is only partially complete, there have already been numerous challenges.

Within the lower levels of the property the existing caissons come in four flavors: rock caissons for the majority of the tower itself, belled caissons in the southeast corner of the plaza, different belled caissons over the loading dock and in the southwest corner of the plaza, a distinctive layout that was designed to carry the load of an imagined 10-story hotel that never came to be.

“One of the biggest challenges that we had during design was really trying to align a desired column layout by the architect and overlay it and marry it to the existing caisson layout around the plaza,” said Eric Wheeler, senior associate, Thornton Tomasetti. “We really had to sharpen our pencils and come up with a unique layout that the architect would be happy with.”

This required a series of transfer beams and transfer columns to distribute the loads amongst all the existing caissons or foundations. Designs were dreamed up and discarded, such as the new foundations that had to be scrapped as they would have impacted the number of underground parking stalls.

The challenges only continued after the design phase was completed and boots hit the ground. Once the engineering crew had access to the site, they encountered aligning issues, which perhaps shouldn’t be surprising in a 46-year-old building that rises 1,450 feet out of the ground.

“One thing we require on our drawings, especially for something of this magnitude, is for the contractor to carefully survey the location of the existing columns,” Wheeler said. “Upon doing their survey they noticed several columns were actually located four inches or more away from the theoretical gridline location.”

The most dramatic feature of Catalog, the gridshell skylight, also required the most engineering brainpower. Shell structures impart a large amount of vertical and horizontal thrust on the supporting structure, so this element provided an interesting challenge for the engineers from the outset. It would prove challenging in other ways too.

“Gridshell forms are very frequently engineering-driven in their logic, with, of course, the influence of the architect’s design aesthetic,” said Diarmuid Kelleher, associate, Thornton Tomasetti. “The original gridshell was design-focused. Like all projects, there are budget considerations that need to be met and we found that in order to keep a gridshell in the project, which was very important for ownership, we needed to simplify and find economies while still maintaining this undulating form.”

As originally conceived, the ultra-free-form, undulating gridshell made use of curved glass and would traverse multiple floors, ending in a cantilever over the Jackson Street entrance. It was a design that in the end would prove too costly, largely due to the greater expense of curved glass.

The Thornton Tomasetti team had to reimagine the gridshell using planar glass inserts. As Kelleher sees it, the redesigned gridshell may be cheaper, but it was fortuitous for another reason as it actually helps the addition meld with the original tower much better.

“We were able to create something that looked very elegant but was also logically driven and came with the economies that arise from that logic,” Kelleher said. “We are pretty proud of that fact, of creating something that was able to be built within the project budget while also maintaining this really great aesthetic that Gensler wanted.”

The engineers also performed occupant comfort analytics including testing how interior glare and daylighting would impact user experience in the winter garden beneath the gridshell. They also tested exterior glare to ensure that the concave skylight wouldn’t drastically impact tenants of nearby buildings, going as far as modeling other structures along with Willis Tower.

Wheeler is currently working on another iconic Chicago building, The Macy’s on State Street, where he is engineering the addition of an elevator core, among other things. Comparing the two projects is instructive of just how important proper documentation is.

“That particular building is over 100 years old and we have scanned copies of the original drawings, but they are illegible,” said Wheeler.

Adding to a 46-year-old tower like the Willis is challenging enough, but working on a vintage building like Macy’s requires steps such as material testing of the steel to make sure a weld will stick, or consulting a geotechnical engineer to understand the bearing capacities of foundations that are over a century old.

“Comparing these projects you can see what you have to deal with in a building that’s about 50 years old versus a building that’s over 100 years old, where access to information really influences the ease of design,” Wheeler said. “It’s part of the challenge but it’s also part of the fun.”