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Hat Truss-Supported Office Tower Tops Off in Downtown Salt Lake City

Photo © City Creek Reserve, Inc. (left); Photo © SOM (right)

111 Main has become the newest addition to the Salt Lake City skyline. Currently under construction in the heart of the downtown City Center neighborhood, the roof hat truss structure of the 25-story, Class A office tower was topped off in January, with its loads successfully transferred from a temporary shoring support system to the permanent structural system during a one-day 12-hour period. Designed by architect and structural engineer of record Skidmore, Owings & Merrill LLP (SOM) with VCBO Architecture and Dunn Associates Inc., the building’s structural engineering features an innovative and integrated solution to a complex site challenge—how to suspend a portion of the tower over an adjacent building.

An air rights agreement with the neighboring property owner defines the project site’s south property line. 111 Main is on a contiguous parcel with the new Salt Lake County Center for the Arts’ George S. and Dolores Doré Eccles Theater, which overlaps on the lower four stories and basement level of the tower footprint. To accommodate the Eccles Theater under the southern portion of 111 Main’s tower, a structural system was required that did not extend columns below the fifth level of the tower on the south side.

SOM designed the penthouse roof level of the 387-foot tall building to be comprised of a balanced two-way steel hat truss system that supports the office tower’s 18 perimeter columns in an integrated load-balanced structure. The central reinforced concrete core walls provide the tower's only connection to its foundation and resists all gravity loads, as well as wind and seismic vertical and lateral loads. Conventional long-span, composite-steel floor framing construction connects the central core walls to the perimeter steel frame and suspended columns, providing open office spaces free of interior columns and a completely column-free lobby at the tower’s base.

Located in a region of high seismicity in close proximity to the active Salt Lake Segment of the Wasatch Fault Zone, 111 Main was designed to withstand maximum considered earthquake hazards and performance-based seismic design procedures. Six articulating spherical steel structural bearings are provided at the top of the ductile reinforced concrete core wall superstructure to transfer compressive gravity and lateral loads from the penthouse roof hat trusses. The core wall loads are transferred to a deep foundation steel H-pile system extending to depths of 100 feet and greater below grade.

To meet a demanding construction schedule, the construction and engineering design assist team—which included SOM, general contractor Okland Construction Company, Inc., SME Steel Contractors, Inc., Hassett Engineering, Inc., and exterior wall subcontractor Steel Encounters, Inc.—developed a sequential, bottom-up staged construction scheme. The staged temporary shoring included a saddle cable system anchored through the core walls with additional steel bracing to allow for the temporary support of construction loads at the south side of the building over the new performing arts center, which was also being constructed simultaneously. Upon completion of the roof hat truss assembly, a jacking process transferred the compressive loads from the temporary shoring columns to the hat truss system, reversing the stress in the perimeter columns from compression to tension. 111 Main’s anticipated completion date is in August 2016.

About Skidmore, Owings & Merrill LLP

Skidmore, Owings & Merrill LLP (SOM) is one of the leading architecture, interior design, engineering, and urban planning firms in the world, with an 80-year reputation for design excellence and a portfolio that includes some of the most important architectural accomplishments of the 20th and 21st centuries. Since its inception, SOM has been a leader in the research and development of specialized technologies, new processes and innovative ideas, many of which have had a palpable and lasting impact on the design profession and the physical environment. The firm’s longstanding leadership in design and building technology has been honored with more than 1,700 awards for quality, innovation, and management. The American Institute of Architects has recognized SOM twice with its highest honor, the Architecture Firm Award—in 1962 and again in 1996. The firm maintains offices in New York, Chicago, San Francisco, Los Angeles, Washington, D.C., London, Hong Kong, Shanghai, Mumbai, and Abu Dhabi.