Andrew Lehrer, Practice Leader, High Performance Buildings, Environmental Systems Design, points out the subtle accommodations made for ventilating multiple F&B outlets at the base of the tower, which experiences strong stack effect.

The dome enclosing the atrium of the Catalog podium spans 23 meters and is composed of flat glass panes.

The rooftop terrace offers unique views of other Chicago landmarks, such as the Board of Trade.

A detail view of the fireproofing and acoustical backing exposed on a wall that will eventually face the public terrace on the podium roof.

The tour group visited the rooftop of the "Catalog" podium extension of the Willis Tower, where a public terrace will provide fresh air and views.

Chicago Chapter of the CTBUH Special Events Manager Ed Curley, Director of Architecture, Epstein, explains the intricate work required to remodel the lobby of Willis Tower while keeping the building open for 30,000 daily occupants.

The interior of the Catalog atrium is nearly complete, the result of intensive engineering and negotiations with the city regarding fire codes.

August 28, 2019

CHICAGO – The CTBUH Chicago Chapter led an illuminating tour of the wholesale renovation of the base of Willis Tower, the largest such construction project to take place since the building’s completion in 1974 as the Sears Tower.

The operation is both an enhancement to, and correction of, prior retrofits that had never quite accomplished the level of street-level engagement that was intended, said Stephen Katz, Technical Director, North Central Region for Gensler. The original building, sitting on a full city block with a 7.5-foot (2.3-meter) grade change and conceived in an era when companies and people were leaving American cities in droves, always suffered from a foreboding street presence, with several sides faced in windswept, blank granite. As part of the new “Catalog” project, the prior interventions, including the barrel-shaped “lunchbox” atrium on Wacker Drive, the Skydeck entrance canopy on Jackson Boulevard, and a main entrance sunken below street level, are all being demolished and replaced.

The “Catalog” will bring 200,000 square feet (18,581 square meters) of retail, meeting and food and beverage space to a new podium structure built around the base of the original tower. It will incorporate a new entrance for the Skydeck observatory and feature two substantial skylights, drawing the eye upwards to the tower while admitting light into lower-level areas. The rooftop will feature a public garden and seating area. The “Catalog” name is a reference to the Sears, Roebuck & Company’s origins as a mail-order catalog company.

Attendees of the tour were able to get close-up views of, and insights into the new Wacker Drive tenant lobby, Jackson Boulevard retail entrance, the roof deck and the bulbous skylight atop the podium. It is perhaps unsurprising that adding a new podium to an existing skyscraper that accommodates 30,000 people per day was a construction-sequencing challenge.

Eric Wheeler, Senior Associate, Thornton Tomasetti, explained how the team had to “really sharpen our pencils” to figure out how to reinforce existing columns in the lobby while knocking out a floor slab, and how to match the column layout of the new structure onto the existing one, without penetrating parking ramps.

An engineering associate from Thornton Tomasetti covered the intensive engineering behind the new glass work around the perimeter envelope, and the engineering of the dome over the retail core, which spans 75 feet (23 meters) and required terminations into large plate girders, and extra rebar in the receiving slabs, in order to transfer the combined horizontal and vertical loads downwards. The team went through several iterations of the dome design, all of which were deemed too expensive, and finally settled upon a system of flat-plate parallelogram-shaped panels, which could be stitched together to form a complex double curve in the aggregate.

Among the greatest complexities to be worked out was how to compensate for the immense stack effect presented by the combination of the piston-like action of more than 100 elevators in constant motion, combined with Chicago’s harsh winter and summer conditions. Andrew Lehrer, Practice Leader, High Performance Buildings, Environmental Systems Design, explained that creating the airy feeling of the atrium within fire-code requirements required extraordinary adjustments.

“We worked out a design agreement with the city,” he said. “We had a lot of concerns with making a code-required BOCA (Building Officials and Code Administrators International)-90 atrium work with 2.5 psi (17.2 kPa) of negative stack pressure in the winter, which changes to +0.75 psi (5.1 kPa) in the summer. We did not think that was an effective way to approach the problem. We designed a system of fire curtains, advanced sprinklers, advanced smoke detection, and an advanced warning system, instead of a typical smoke evacuation system, because we were convinced it was never going to perform properly.”

This meant a great deal of training and advance work with the individual food & beverage tenants, which had to link into the centralized ventilation system for kitchen exhaust, as well. But the project is moving at a rapid clip, with individual F&B outlets set to open within months, according to Lynda Leigh, Senior Project Manager, Turner / Clayco.

The tour concluded on the rooftop, where, with the help of the design team, attendees could easily imagine the garden in a completed state, taking in views of a warm summer evening with food and beverages in hand.