Structural ComplexitiesOriginal building designs called for a cast-in-place concrete structure. Boston-based Suffolk Construction recommended changing the structural system to staggered steel trusses with concrete plank floors when they were brought onto the project. “The system was more economical using 25- to 33-percent less steel than a conventional structure,” Becker says. “Also less concrete is used since it has voids in the floor system.”
The redesign allowed for faster construction, which was critical to meet the aggressive schedule. Becker wanted the building ready for leasing in summer 2010, when Yale graduate students are looking for apartments.
Switching to a staggered steel truss system enabled Suffolk to construct a floor every four days. “In Connecticut poured-in-place concrete is not as widely used and you are lucky to get a floor up every week,” Becker explains.
“The trusses and the hollow core plank could also be fabricated offsite so there was less labor onsite for forming and other activities,” explains Rick Pielli, senior project manager with Suffolk Construction.
Cast-in-place concrete construction was retained for the podium, Downing says. “Construction could begin sooner with concrete than if we switched to steel, and cast-in-place concrete is a good system for a parking garage because of its durability.”
Podium construction began while the tower’s structural design was refined and completed. “We overlapped the design with the construction process,” Becker says. “It’s not something you always want to do but we realized if we didn’t get the project started when we did we might not have been able to get financing given the state of the financial markets.”
Structural DesignThe tower is supported by a mat foundation 36- to 64-inches thick. “The thickness varies for efficiency and because of the groundwater table,” DeSimone’s Downing explains.
The basement at the northwest end of the building is 20-ft. below grade, 8-ft below the water table. The mat at this location is designed for up to 10-feet of hydrostatic pressure.
Only half of the site has a basement. From the west to the east end of the tower the mat takes three major steps up with the last step just 5-ft. below grade.
“This was purely economical,” Downing explains. “It minimized the amount of sheeting and jet grouting required alongside the tunnel for underpinning and minimized the amount of excavation needed at the site.”
The podium between Chapel Street and the tower is supported by pressure-injected footings (PIFs). PIFs could be installed quickly, avoided excavation and were relatively inexpensive compared to drilled piles, Downing explains.
The podium is constructed with precast double-t and plank systems for the Chapel Street section and post-tension concrete for the podium section atop the tunnel.
