Will new faces deliver landmarks?

Early in 2009, Chicago’s architecture community was a buzz about the opportunity to design spaces for the 2016 Olympics. They looked to showcase the city’s rich architectural history and implement new modern forms being explored by a new generation of architects. Adina Balasu was one such enterprising designer completing her graduate degree. As part of her studies, she devised a landmark bridge to allow pedestrians access Olympic venues on Northerly Island, just across the marina from Soldier Field.

The concept, christened Chameleon, was to create a functional bridge that would be a destination in itself. Two levels of walkway would be suspended within a futuristic space frame shell. The large interior space might also be used as a multi-purpose venue for entertainment, retail and relaxation. After the Olympics left, the structure would be a necessary link to further the planned development of the little-used island park. An inspirational form and engineering feat, visitors would make a visit to the Chameleon part of their itinerary, expanding the traditional Chicago tourist district several blocks south.

I was introduced to the project at a meeting of the AIA Young Architects Forum. Following Ms. Balasu’s presentation, we had the opportunity to discuss the details of the project. I was intrigued by the structural challenge and impressed with her desire to express the structural form in the bridge’s appearance to reflect the technology of the times.

I delayed in my review for several months before picking up the concept with a fresh perspective. Unfortunately, in that time, Chicago was passed over for the Olympic bid. Despite this missed opportunity, I started wondering who deliver the trend-setting designs of the future. The current recession seems to have stalled several major projects, and missing out on the Olympics further deflated the local architecture community. When the economy turns around again, who will be at the forefront. I suspect that many of the innovative architects that I met at the YAF will lead the charge.

The Chameleon appeals to me as one of those great next-generation architectural concepts. Over the next few blogs I will outline my thought process and presents some potential strategies for making the Chameleon stand.

What do you think, when will we escape the current economic downturn? Will their be a new generation of architects leading the way at that time? Where should enterprising structural engineers look to network with these future partners? Please comment below.

Will it stand on a parking garage?

Constructing a new building on top of existing structure can sometimes feel like trying to fit a square peg into a round hole. That’s the problem that designers faced in the construction of a 25-story residential tower on top of an existing 8-story concrete garage. In an effort to respond to market changes, the developer abandoned the plans for a square hotel tower with a large light core. Instead a football-shaped condo building that maximized window space was desired.

The immediate problem faced by the design engineers was how to support the new tower on the existing base. The tower columns did not align with the existing grid, and the structure itself was centered over columns and foundations that were not designed for such loading. In addition, the podium’s lateral system was primarily located outside of the envelope of the planned tower.

Remembering their approach to a similar problem in which a new building planned to reuse existing drilled-pier foundations, the engineers envisioned a 6’-0” deep transfer mat on top of the parking garage. In addition a 16’-0” deep transfer girder was introduced along the center of the mat to further alleviate load from the smaller interior columns. Computer analysis helped determine the required size and reinforcing of the structural members. About 500 tons of steel reinforcing were required for the transfer mat.

However, in order to make the transfer mat and girder system economical, the residential tower needed to be as light as possible. A steel framing system was found to be the best option. A wind tunnel test revealed that the local wind environment funneled winds off of Lake Michigan and could excite the tower in a twisting motion. Although the building was sufficiently strong to resist these winds, the particular motion had the potential to upset the occupants.

The design engineers immediately began discussing options with the architect and developers. Stiffness of the tower played the biggest role in reducing the uncomfortable wind-induced accelerations. The engineers proposed changing the lateral system to a stiff cast-in-place concrete shear wall system. Large openings in the wall were required to accommodate the condominium units’ floor plans. And for even greater stiffness in the east-west direction, a hat truss was added at the 31st floor to link the cores located at either end of the tower.

The construction team likewise faced challenges with the site. The first major construction challenge was figuring out how to support six feet of wet concrete from a series of lightweight post-tensioned parking decks. The supermarket on the 1st floor would have to remain open throughout construction, so it was not possible to shore down to grade. Moreover, the existing parking floors could not support all six feet of wet concrete. Instead, the transfer mat was constructed in two lifts. The first 3’-6” lift included additional shear transfer reinforcement and was designed to support the full wet weight of the remaining mat layer. In this way, the parking decks were actually re-opened ahead of schedule.

With some engineering, the square peg can be fit into the round hole. In this case, a 25-story residential tower was engineered to fit on an existing podium. Creative solutions for distributing gravity load and resisting lateral forces enabled the developers to create a building that could succeed in the current economic environment – even if that meant a drastic departure from the originally designed program.

In what other ways might the design team have addressed these challenges? Could the need to pump so much wet concrete 8-stories above ground have been avoided? Comment below or contribute to the willitstand wiki.

Acknowledgements:
Destefano & Partners, 330 North Wabash, Suite 3200, Chicago, Illinois
Linn –Mathes Inc., 309 South Green Street, Chicago, Illinois
Thornton Tomasetti, 330 North Wabash, Suite 1500, Chicago, Illinois