Locations of historic elements and existing structure left a very narrow footprint available for the new tower structure within the existing building. Once the tower clears the 2nd floor roof and extends into the light court, the upper floors cantilever over the building and allow a slightly larger floor plate. Here, another unique solution was developed to deal with the construction conditions. On the upper floors, the back span is approximately the length of the cantilever. With the asymmetrical loading of the tower elements, the steel structure was erected out of plumb so that once it was fully assembled and loaded the tower would right itself and become level. Dead loads were recalculated multiple times as the design team worked through various options for the exterior envelope materials to precisely engineer the system. Why is a building in Michigan built to seismic design criteria? Code analysis determined it needed to be designed to meet seismic design criteria per the American Society of Civil Engineers (ASCE). The soil conditions, footprint of the tower, wind loads, and materials all figured into the seismic drift calculations. Because the tower is not square, the maximum drift in the north-south direction is different from the east-west direction. With no appreciable movement at its base, the top of the tower is calculated to move up to 5-inches in each direction (which translates to a 10-inch seismic joint). Joint sizes were regularized throughout the building for uniformity and ease of installation. Instead of changing sizes for each floor, nominal 4-inch, 6-inch, and 10-inch joints were specified to minimize the number of products purchased. The tower exterior has a large 10-inch exterior metal joint cover integrated into the exterior façade. The joint cover is a hinged door with magnets that hold the door in place under normal conditions but release during a seismic event. The infill panel was color matched to the adjacent metal panel allowing the joint to completely blend in with the exterior façade. Most of the interior joints were concealed between two walls so exposed cover plates could be minimized. At doors and
other isolated areas, a narrow joint cover with an aluminum or plate was used to blend in with historic metals.
How were materials brought to the site and installed if the building takes up a full city block and is occupied? A critical requirement of the project was to allow the building to remain operational during construction. Page worked with GSA and teams from The Christman Company as the CMc, Jacobs as the CMa and the US Courts to execute a phased renovation project. Impacted tenants and infrastructure had to be cleared out in the footprint of the tower early on to install the structure while the rest of the floor remained operational, including utilities, services, and emergency egress. Most tower elements needed to be lifted from the street over the top of the building and into the project site, including 528 tons of steel (over 1,400 individual pieces). A 275-ton crane with a 340-foot boom was used to hoist materials from street level, 11 stories over the building and into the courtyard. The crane operator communicated with the steelworkers by radio due to lack of visibility into the project site. This work needed to be precise, at times the steel would need to be lowered through a 40 x 60-foot roof opening to be installed inside the existing building. To make it more complex, work with the crane was done primarily at night and off schedule with building operations. While the overall building modernization was done in phases, the tower construction occurred throughout the entire 5 year, 7 phase construction process. It was critical to work closely with manufacturers to specify materials and systems that could be installed with these constraints in mind. Early on limestone or precast were ruled out as the cladding for the tower due to the weight and size of the material. Ultimately, structural steel, insulated metal panels, and a unitized curtain wall system were selected as the most lightweight and practical. Allowing as many items as possible to be fabricated in a shop off-site improved the quality of the overall product. Even with multiple high-wind days that stopped work, these elements saved time and positively impacted the schedule.
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NOVEMBER 2023 csengineermag.com
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