Construction Today Magazine - Issue 1, 2016

Page 47

Tower use and amenities The FMC Tower is situated at the gateway to the University of Pennsylvania, Drexel University and the entire University City neighborhood. The tower includes 229,000 square feet earmarked for 268 luxury apartments and executive suites and 10,000 square feet of retail space. Office space will be used for 622,000 square-feet of the tower. The University of Pennsylvania also will use 100,000 square-feet of the space. The tower is distinctive because it is composed of two separate structural systems. It is essentially two buildings stacked on top of each other — a 22-story concrete framed residential apartment building constructed on top of a 28-story steel-frame office building. This unique building with its separate structural systems posed design challenges. During the design process, there were three options for the structural systems: steel office and steel residential, concrete office and concrete residential, and steel office and concrete residential. There were pros and cons to each module, but ultimately, the steel office with concrete residential option was chosen. Choosing a structural system Steel residential over steel office option: The steel residential over steel office option would have been the quickest construction duration at 21.5 months until first occupancy, and would have been the lightest building and least expensive foundations with no transfer floor required. However, it would have been the most expensive choice. “There also would have been an additional cost of hard ceiling and other finishes in the residential portion — about $1.5 million,” Stone says. The structural envelope of 36 inches in that option would have resulted in a floor-to-floor height of 12 feet, 8 inches or additional building height of 36 feet, relative to the cast-in-place option — estimated at $1.5 million for the curtainwall only. “This doesn’t factor in lateral design costs,” Stone adds. “There were also inefficiencies in this system because a 30-foot grid was not efficient for a residential module. The 45-foot span at the sides of the core were also inefficient for residential.” Concrete residential over concrete office: The concrete residential over concrete office

presented itself as an attractive option in terms of cost. “It was the least expensive option,” Stone says. The transfer floor could be done with cast in place. There was also the possibility of using post-tensioned concrete to reduce column sizes at the base of the building and flexibility in column locations. “Although this was the most cost-effective option, this structural system had the longest schedule at 26 months so it wouldn’t meet tenant requirements,” Stone adds. “The proposed 45-foot span at the sides of the core also were inefficient for castin-place post-tensioned options in both the office and residential floors. In addition, the spans were not conducive to a flat slab system because the bays are rectangular. This option also had the largest column sizes at 4.5 feet and the largest foundations.” Concrete residential over steel office: This option came with its own set of pros and cons, but presented the best option from a structural and economic standpoint. “The cost for concrete over steel was just 3 percent more than the lowest cost,” points out Madison Concrete Construction President Jim Dolente, Jr. “A complex transfer is required and there is uncertainty in cost related to simplicity of the transfer design. There is also heavy built-up shapes at the base of the building. Despite the cons, the pros outweighed them making the concrete residential over steel office the best structural system choice.” The 22-month schedule met the tenant requirements of 22.5 months, and is only one-half month longer than the quickest option. “The concrete over steel choice offered flexibility in column locations,” Dolente says. “Because residential areas only require smaller spans, they can have more columns due to their living uses whereas office spaces need larger areas. Using concrete also reduced the need for high load cranes to lift steel to the high floors.” This option also made structural envelope under 12 inches, resulting in the shortest building. “No ceiling treatment was required at residential floors, and this is the most efficient system for each occupancy,” Dolente says. Understanding the structural system The lower portion of the FMC tower is composed of office space. The structural system

Project at a glance Project Name: FMC Tower at Cira Centre Owner: Brandywine Realty Trust Project Location: 2930 Chestnut Street Philadelphia, Pa. Design Architect: BLT Architects General Contractor: Turner Construction Company Structural Engineer: Thornton Tomasetti MEP Engineer: BALA Consulting Engineers Contractor: Turner Construction Net Area: 830,000 sq. ft. Height: 735 ft., 7 in. (49 stories-28 floors office, 22 floors residential and three mechanical/utility levels) Start Date: Summer 2014 Completion Date Estimate: mid-2016 Budget: $341 Million Total Steel: 5,000 tons Core Concrete: 15,000 cubic yards Total Concrete: 40,000 cubic yards Other team members: ReSteel Supply Company, Bayshore Rebar, Action Supply (concrete supply), Forming Concepts, Madison Concrete Construction ISSUE 1 | 2016 47