Austin, Texas, saw one of its largest concrete pours ever, as 8,650 cubic yards were placed late last month for the Block 185 office tower’s mat foundation. Sixty trucks worked in three shifts to deliver 866 loads over 27 hours. Thornton Tomasetti’s Austin, Dallas and Los Angeles offices are providing structural engineering services to developer Trammell Crow Company for the 1.5-million-square-foot, 37-story concrete tower, designed by Pelli Clarke Pelli and STG Design. When completed in 2022, it will be the capital city’s fourth tallest building at 575 feet.
Located in the Second Street District in Downtown Austin, Block 185 is being constructed on 1.26 acres in the last undeveloped section of the former Green Water Treatment Plant. The tower will provide 745,000 square feet of office space, 10 levels of above-grade parking with an additional six levels below ground, and ground-level retail and dining. The project sits on the bank of Shoal Creek, just 300 feet from the Colorado River, and will provide easy access to nearby hiking and biking trails.
The distinct sail shape on the western façade and a stepped and terraced southern façade posed some unique structural challenges. The shape is achieved through sloped structural concrete columns on the tower’s perimeter, which required careful coordination between Thornton Tomasetti, the architects and the curtain wall design-build contractor, Permasteelisa. With a series of balconies at every fifth floor, the setbacks on the southern façade are made possible by strategically placed columns, the majority of which avoid transfer beams. A concrete core provides the tower’s lateral stability and houses elevators, stairs and MEP back-of-the house functions. The office-level structural floor framing consists of an 8-inch post-tensioned slab spanning between wide-shallow (4-foot-wide by 22-inch-deep) post-tensioned concrete beams.
Significant planning went into the column grid and layouts to accommodate the office and above and below-grade parking programs. The chosen system entails a generous 45-foot span in the east-west direction, and 30-foot span in north-south direction. The resulting leasing bays allow for open office interiors with an efficient desk planning layout and provide for ample space for the many amenity areas requested by the tenant and owner. Large areas of the slabs have also been prepared without post-tensioning to accommodate future modifications, such as for communicating interior stairs or slab openings. The below-grade parking system was optimized by adding a column to each bay, such that the typical bay (30 feet by 45 feet) was reduced to 30 feet by 30 feet. This efficient solution allowed for a flat mild reinforced slab system to be used below grade, which minimized the vertical floor-to-floor dimension since no beams were required. It also reduced the overall excavation depth.