The building at 110 N. Wacker is a $700-plus million, Class A, 57-story office tower with more than 1.5 million square feet of leasable space. It’s the tallest all-commercial building built in the city in the last 30 years. It stands on the last remaining Wacker Drive riverfront address, which is a one-acre parcel of land wedged between the Chicago River on the west and the double decker Upper and Lower Wacker Drive on the east. To the north and south are W. Randolph and W. Washington Streets, both effectively river bridge approaches, complete with bridge houses.

Prior to construction, the site was completely occupied by the historically significant 60-year-old former headquarters of the Morton Salt Company. Working on a tightly constrained, occupied site would be a challenge. Adding to the project’s complexity was the spectre of the then-emerging and subsequently ongoing COVID-19 pandemic. To say development of the site under all these conditions was a challenge is an understatement.

A few years ago, Riverside faced development challenges on a site just across the river at 150 N. Riverside (see “Tower of Power: Strong Steel Overcomes Constraints at a Tight Site,” Informed Infrastructure, April 2016). Although every site presents its own unique challenges, one of the means by which these challenges are successfully solved is universal: working with a unified, collaborative team.

Cognizant of the looming challenges, Riverside tapped many of the same players that made the 150 N. Riverside project so successful. This included architecture firm Goettsch Partners LLP; general contractors Clark Construction; geotechnical engineers GEI; and steel erectors Chicago Steel. Rounding out the core team were structural engineers Thornton Tomasetti; steel fabricators Cives Steel Company; MEP and fire-protection engineers Environmental System Design Inc.; and demolition contractors Heneghan Wrecking Company.

This team of professionals, whom Chris Payne, vice president with Riverside, calls the “best in the business,” not only solved the challenges, but turned them into assets, finding the silver linings that a lesser team may have missed.

History Removed … and Preserved

The first hurdle the team faced was obtaining project approval. To do so, they needed to address issues related to the site’s existing structure. Built in 1958, the five-story Morton Salt Headquarters Building stood for almost 60 years, serving as the global headquarters until the company relocated in 1991. It then was occupied by General Growth Properties from 1997 until 2018. The building’s stainless-steel cladding added to its iconic look and status as part of Chicago’s architectural history. In fact, the Illinois State Historic Preservation Office determined the building was eligible for inclusion in the National Register of Historic Places, and groups such as Preservation Chicago and Docomomo Chicago advocated for preserving the building for its historic significance. Ultimately, the City of Chicago’s Commission on Chicago Landmarks did not landmark the building, and the decision was made to move ahead with redevelopment.

However, advocacy for preservation led to positive public discourse and increased public engagement around the project as well as other positive outcomes. Riverside and Hughes agreed to incorporate 25 of the stainless-steel cladding panels into the new building in a permanent exhibit on the riverfront, along with informational plaques explaining the history of the Morton Salt Company. The developer also sponsored an educational seminar on mid-century modern architecture in Chicago, which was open to the public, and commissioned a study of the architecture of the building, which was donated to the Art Institute of Chicago.

After clearing the hurdles associated with the entitlement process, the next set of challenges the team faced were related to the site itself. How do you remove a five-story building and replace it with a 57-story tower when there’s no room to work? The team would ultimately rely on the unique existing infrastructure to answer this question and many others to follow.

Occupied and Constrained Site

The existing building occupied nearly the entire 1-acre site, leaving little room onsite for demolition or construction staging and material storage, and preventing any predesign subsurface investigation.

“As the site is confined on three sides by congested roads and a river on the fourth, there was no room offsite to accommodate any of these activities,” notes Joseph Sadowski, project executive with general contractor Clark Construction Group, Chicago.

Featured prominently in the movies “The Blues Brothers” and, more recently, “The Dark Knight,” Wacker Drive is a double-decker road, referred to as Upper and Lower Wacker Drive. Upper Wacker looks like any other major road in a major city. In fact, standing on Upper Wacker, you would never know that down at river level there was another thoroughfare, complete with intersections and stop lights, coursing beneath your feet.

 

The fact that Lower Wacker is “hidden” underground helped solve the demolition problem: the team devised a plan to demolish the building from the inside out. They started by creating a hole in the center of the building, working outward to dismantle the building, and remove debris through the basement and out the loading docks on Lower Wacker.

“It was a very collaborative effort between Clark, the city and Heneghan (the demolition contractor),” says Payne. From start to finish, the demolition took only three months to complete.

Lower Wacker would later be utilized for material deliveries throughout the project. For example, all concrete deliveries were made on Lower Wacker, where the concrete pump was located.

 

 

As the building came down, geotechnical and structural engineers as well as foundation contractors could begin the subsurface investigation. Up until this point, the team was in many ways “designing blind.” From incomplete paper records and scuba divers’ visual inspections, they knew there were many subsurface obstacles, they just didn’t know where. This wasn’t a trivial matter when the design called for 120-foot-deep rock-socketed caissons.

“We had design drawings from the original building, but there are always surprises,” says David Weihing, P.E., S.E., senior principal with Thornton Tomasetti. “One of the biggest surprises was the river-wall-stabilizing rakers and battered piles.”

As it snakes through the heart of downtown Chicago and passes the project site, the Chicago River is kept in its banks by sheet piling supported by battered piles. The piles extend several feet onto the site, creating potential critical conflicts with the proposed foundation.

The project schedule anticipated surprises and provided time accommodations. Working from the available information, the team overlaid the predicted locations of the existing subsurface elements on the actual locations as they were discovered. Where possible, the design was modified to work around the obstructions. Where the proposed foundations couldn’t be altered, the team moved or modified the existing rakers and piles to maintain the integrity of the river wall. “Working with the river” would be a recurring theme throughout the project.

Due to limited onsite space and the tight schedule, the caisson outer steel casing and rebar were fabricated offsite, working from preliminary design locations. The final configuration and location of the caissons was determined by the real-time process of identifying test drill locations for samplings; testing the samples offsite to verify strength; obtaining city approval; modifying the casing and rebar; and then drilling to their final locations. Transitioning from demolition to deep foundations with no delays and no cost overruns was a huge challenge.

“The city played a big, positive role in ensuring the proper process was followed, approvals were given and information distributed,” notes Payne. He credits the team’s collaboration and dedication to a shared goal to overcome this unique situation. “We couldn’t have been successful without the team’s diligence and strong effort.”

While making creative use of Lower Wacker, the river and limited use of Upper Wacker, more work space was required for a project of this magnitude. With none to be found, the team created the space it needed. One of many “outside-the-box” solutions, the plaza-level slab (the first floor on level with Upper Wacker) was thickened to serve as a construction platform.

“We worked hand-in-hand with the structural engineer, Thornton Tomasetti, to design a structural solution that would allow the building’s plaza level to be constructed prior to pouring the cast-in-place slab for the lower level (Lower Wacker Drive level),” explains Clark’s Sadowski. To support the additional construction loads on the slab, a two-story A-truss system was constructed parallel to Wacker Drive on the east side of the site, and it’s now embedded in the east foundation wall. “The truss system utilized triangular segments that sat on the caisson caps and avoided the originally planned foundation wall openings at Lower Wacker Drive,” adds Sadowski.

Squares and Rectangles Preferred, Not Trapezoids

When planning a multimillion-dollar office tower, the amount of leasable space is a key driver behind the building’s design and final form. As such, square and rectangular footprints are the most-efficient response to market demands, and this typically is achievable on most sites. The site at 110 N. Wacker is not like most sites.

“The site determines what the building will look like,” notes Erik Harris, AIA, NCARB, associate principal with Goettsch. “Normally, we want [the building footprint to be] a rectangle where the ratio of the side lengths are close to the golden ratio (ф = a/b = (a+b)/a = 1.618). Here we have a site that is a trapezoid.”

The site’s unusual shape forced the architects and engineers to deviate from that desired shape for a design that fit the lot and still met market demands. In what has become a recurring theme, the team turned this seemingly negative attribute into an undeniable benefit.

 

The site gets narrower as you move north along the river, forcing the building to do the same. “The east side of the building along Wacker was easy,” notes Harris. “It was straight and formed 90-degree angles with the north and south faces. The river side was the challenge.”

 

The solution for the face of the building along the river is a series of 10 sawtooth steps, each about 30 feet wide and 5 feet deep, narrowing the building footprint in “step” with the narrowing lot. Although this stepped geometry increased design complexity, it resulted in 10 additional “corner offices” on each floor, each with a spectacular view looking up and down the river. “Instead of the typical four corner offices per floor, 110 N. Wacker has 14,” adds Harris.

The need to balance this unusual footprint forced by site geometry with market realities left little flexibility from a structural design point of view. One stark example of this is the design for wind-load resistance. The core of highrises serve to transfer the lateral forces on the building to the foundation and are normally about 40 feet wide. For this project, the core is only 25 feet wide. To offset this core narrowness, engineers designed a belt and outrigger system at levels 23 and 24. The steel of the outrigger is embedded into the concrete core and together they resist the overturning moment forces.

The belt and outrigger system actually serves a dual purpose. In addition to lateral stabilization, the system also dampens the tower’s natural swaying, solving two problems with a single solution. Sway mitigation often is accomplished with a tuned-mass damper, which requires substantial floor space. As with many solutions developed by the 110 N. Wacker team, the solution to the slender-core challenge ultimately increased the value of the final product, creating more leasable space while simultaneously solving engineering problems.

Images show the installation of the belt and outrigger truss connection (computer rendering on bottom).

 

Another consequence of the smaller core is a reduced number of elevators to service the building. 110 N. Wacker has only 24 passenger elevators. Traditionally, a building of this size might require three to six more standard elevators to provide the same quality of service and wait times for occupants.

To compensate for the fewer passenger cars, the elevator contractor, KONE, designed the most-advanced destination dispatch system in Chicago. Riders enter their destination floor, and the system assigns riders to cars, creating the most-efficient routes. As a further benefit toward energy efficiency and LEED certification, the cars are suspended by “UltraRope” carbon-fiber ribbons instead of typical braided-steel cables. The ribbon is much lighter than steel, uses less energy and requires less maintenance. The result is a very efficient solution for full-height travel cars.

Down by the River

Real estate along the Chicago River has always been a coveted, high-value asset in limited supply. Unfortunately, for more than a century the river served as the city’s sewage and industrial-waste receptacle. In fact, it was so polluted it actually caught fire on more than one occasion in the late 19th century.

 

Starting in the 1990s, the city embarked on an ambitious effort to reclaim the river as a usable natural resource to be enjoyed by the city’s residents. A 2002 zoning ordinance requires all new developments along the river to provide a minimum 30-foot setback from the river’s edge, “open to the sky.” Adherence to the literal letter of the ordinance would make development of the parcel at 110 N. Wacker economically unfeasible, and lesser teams moved on to other sites. This team saw this as yet another challenge to overcome and again found the silver lining in a unique solution.

Working in close coordination with the city, the design teams struck upon a configuration that not only met the spirit of the 30-foot setback requirement, it provided a setback 45 feet wide, one-and-a-half times required, and created 22,000 square feet of public open space, half the size of the parcel itself. To accomplish this, the building was designed to overhang the public space, 66 feet above the ground level.

3 to 1

To provide an open, unobstructed area between the building and the river at ground level, the load-bearing columns from the upper floors were transferred to the foundation with three massive tridents. As with many aspects of this project, the trident solution to the open-space challenge resulted in additional benefits. Namely, fewer columns along the river meant less chances for conflicts between the proposed foundation and the existing subsurface river-wall battered piles and rakers.

The design of the tridents and the nodes where the three columns converge was one of the project’s main focal points. “The unbalanced loading on the outer columns of the trident made the design tricky,” notes Thornton Tomasseti’s Weihing. To resist the rotation of the tridents caused by the imbalance, the floor slab connected to the trident is stiffened with a horizontal truss. During construction, the movements of the tridents were closely monitored and checked against predicted values.

Fabrication of the nodes also was a challenge. Early design concepts called for internal stiffeners that would’ve been difficult, if not impossible, to fabricate. Instead, the designers at steel fabricators Cives Steel Company worked with the structural engineers to develop an alternate solution. The final nodes are massive laminate-plate assemblies with six to eight layers of 4-inch steel plates welded to form a monolithic node. At its base, each node transfers up to 24 million pounds to the foundation.

Although this laminate approach solved the fabrication problem, it introduced more complexity to the procurement of the steel itself. “Purchasing steel plate is different from wide-flange,” says Jacob Payne, Civies Mid-West Division senior project manager. “Plate requires a minimum purchase with longer lead times. It took a lot of time to get through design, so we knew what to purchase. Luckily, we had a very good team—including the developer, general contractor and structural engineer—and everyone worked together great. It made working on challenges easier.” The innovative trident node design was showcased at AISC’s 2019 Steel Day.

The trident nodes were fabricated in a factory (top) and installed onsite (bottom).

Logistically Horrific

Overcoming the design and fabrication challenges of the steel components was only part of the battle. Delivering and erecting the steel was the other. “Logistically, the site constraints were horrific,” recalls Jon Hollowell, executive vice president with steel erector Chicago Steel. “Getting out of the ground is always a challenge, and we looked at a few options [for the crane location]. We considered setting the tower crane on Upper Wacker, but that would have required [providing] shoring down to Lower Wacker.”

The final solution was to use a barge-mounted crane, which resulted in more flexibility than possible with a traditional land-based crane. The barge crane would be moved north and south along the site, enabling the operator to unload materials from multiple locations. Working from the barge, the crane had a larger swing radius than a land-based crane and could pick and set heavier pieces. This had a direct impact on the design of the tridents. Early planning and tight coordination among all team members—general contractor, structural engineer, steel fabricator and steel erector—allowed the team to design and fabricate heavier trident nodes, which required less assembly onsite and resulted in a more-efficient design. Lemonade from lemons, yet again.

“We loved working with Riverside and Clark,” adds Hollowell. “They are not afraid to think outside the box and innovate. When you work with a team like that, it’s exciting.”

COVID-19, of Course

As with nearly everything else about 2020, the COVID-19 pandemic impacted the project at 110 N. Wacker. From team meetings and onsite visits limited by travel restrictions to considerations for improved tenant safety, the team had to overcome a series of hurdles no one expected.

The development team was a partnership between Chicago-based Riverside Investment and Development, and Dallas-based Howard Hughes Corporation. Travel restrictions prevented Howard Hughes team members from making regular site visits. And even for the local team members, walking the site was limited to reduce the risks of spreading the virus. To address this issue, the team employed web-based 360-degree photography to allow remote team members to see the site virtually. Not only did this work effectively for this project, but the positive reception of the technology will likely result in continued use, even after COVID-related restrictions are lifted.

Another challenge the team faced was keeping tenants safe and minimizing the potential exposure to COVID-19 and its variants. “The developer is very finely attuned to tenant safety,” says Tyler Jensen, studio leader with Environmental Systems Design Inc. As the pandemic spread across the globe, “Riverside was proactive in getting the full design team involved in designing retrofit upgrades to the HVAC and building control systems.”

The original HVAC design included outdoor ventilation rates that already exceed national standards (Chicago ventilation code is more stringent) and an airside economizer that enables 100-percent fresh-air operation when outside temperatures permit. Improvements catalyzed by the pandemic included an upgrade of the air filters from already superior commercial-grade MERV 13 filters to hospital-grade MERV 15 filters and the addition of an advanced bipolar ionization air-purification system to actively remove particles, VOCs, and pathogens from the circulating air and surfaces. The team also added an intelligent indoor air-quality monitoring system provided by Cohesion, with sensors in all common areas as well as the ability for tenants to connect to and extend the system to their leased space. Via the Cohesion system, tenants have a transparent view of the air quality in the building at all times via a mobile or desktop app. “Strong tenant engagement is the silver lining around the cloud brought on by COVID,” adds Jensen.

A bespoke building automation system from Siemens allows for touchless building access; transportation information; event listings; a portal to book onsite spa and fitness activities; and the ability to summon elevators from a smartphone or desktop app.

Best in the Business

As various team members were interviewed for this article, the same sentiment was repeated: the interactions, cooperation and collaboration among team members, the city and community—from concept to completion—is what made this project a success.

“Collaboration among the owners, architects, engineers and contractors was amazing,” notes Sadowski. “We were so successful because it truly is a team with everyone delivering on what they promised.”

“It was an honor to work on this,” adds Riverside’s Payne. “Not just the iconic building, but with the team. We wouldn’t be here without their hard work. They are the best in the business.” 

Video Interview

To learn more about the building and its development, Todd Danielson, Informed Infrastructure’s editorial director, interviewed Chris Payne, vice president at Riverside Investment & Development, via webcam. They discuss the structural engineering challenges at the project as well as what owners and developers look for when choosing its team. Watch the video at bit.ly/3tV667l