2018 IDEAS2 Awards Brochure - 20

111 Main brings a half-million sq. ft of Class A office space to
downtown Salt Lake City-topped with a rather impressive hat.
The 387-ft-tall building consists of 24 stories above grade, a
penthouse level and one basement level. But unlike a typical office
tower, it also had to account for the new five-story Eccles Theater
next door-next door meaning partially underneath. 111 Main's
southern property line was defined by an air rights agreement with
the property owner of the theater that allows the tower to extend
above the theater starting at Level 5. To accommodate the theater,
111 Main required a structural system that did not extend columns
below the fifth level on the south side, allowing design development and construction of the two projects to be undertaken concurrently, which satisfied the objectives of both.
Structural engineer and architect Skidmore, Owings and Merrill
(SOM) designed the penthouse level to be comprised of a balanced
two-way 3D steel hat truss system that supports the office tower's 18
perimeter columns in an integrated load-balanced structure. Measuring 28 ft, 1.5 in. deep and weighing 1,870 tons, the hat truss extends 40 ft to 45 ft from the core walls. The building is tuned with
longer clear spans for the 20 suspended south-side levels to balance
the load of the shorter spans in the 23 suspended north-side levels.
The central reinforced concrete core walls provide the tower's
only connection to its complex foundation and resist 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. The truss solution also allowed the project teams for 111
Main and the Eccles Theater to work independently and achieve
their unified architectural goals, and minimal structural and schedule coordination between the two project teams was required during the design and construction stages of both buildings.
Located in a region of high seismicity (near the active Wasatch
Fault Zone) the building was designed to withstand not only ground
motions based on actual earthquakes that might occur on the site, but
also an extreme earthquake that could happen once every 2,500 years.
Few if any buildings in Utah have been designed to withstand this
level of rigor. This is where six structural steel bearings, supplied by

Earthquake Protection Systems in Vallejo, Calif.-each consisting of
an articulated slider, a steel concave plate and a steel housing plate-
came in. A sliding spherical surface in each bearing is intended to serve
as a "pinned" connection at the interface of the steel hat truss and
the concrete core wall below to minimize any unfavorable twisting
effects caused by seismic and gravity deflection, as well as changes in
temperature, of the exposed hat truss. To provide sufficient horizontal constraint to the truss and avoid excessive sliding of the structural
bearings, seven W14×730 shear keys extend down from the bottom
truss chords to pockets at the top of the core walls. Complete joint
penetration (CJP) moment connections were provided between the
truss bottom chords and the shear keys. Below the core walls, a deep
foundation system comprised of steel H-piles extends more than 100
ft below grade. A total of 373 HP14×89 and HP14×117 piles were used.
To meet the demanding construction schedule, the construction and engineering design-assist team-which included SOM,
general contractor Okland Construction Company, SME Steel
Contractors, Hassett Engineering and exterior wall subcontractor
Steel Encounters-developed a sequential, bottom-up staged construction scheme. This scheme comprised staged temporary shoring involving a saddle cable system that was anchored through the
core walls north-to-south at Level 5, temporary hydraulic jacks and
additional steel bracing. This temporary shoring system supported
the seven interrupted columns in the south side of the building
over the Eccles Theater while both projects were constructed simultaneously. The remaining 11 columns were shored by temporary hydraulic jacks at Level 1. 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 during a single 12-hour period, reversing the stress in the perimeter
columns from compression to tension while three-quarters of the
building's exterior was in place. The perimeter columns and tips
of the hat truss were carefully positioned up during the temporary
shoring. After the load was transferred, all columns and truss tips
moved down as intended into the flat, permanent design condition.
For more on 111 Main, see "Hanging Out In Salt Lake City" at
www.modernsteel.com/111main.

A heroic effort to levitate the office tower was expressed beautifully
in the illuminated crown and boundless lobby.
-Grant Gustafson
20

http://www.modernsteel.com/111main

2018 IDEAS2 Awards Brochure

Table of Contents for the Digital Edition of 2018 IDEAS2 Awards Brochure