The San Francisco skyline is in the midst of a dramatic transformation, and we’ve played a role in the ongoing progress at one of the city’s most distinctive new structures, the 56-story, 802 foot-tall 181 Fremont Street supertower. We have worked with steel fabricator/erector The Herrick Corporation (THC) since the Fall of 2014, providing an array of steel erection engineering services to help ensure success during the steel erection, including temporary falsework, erection aids and lifting lugs, tower crane tie-ins, estimation of truss deflections and elastic column shortening, and analytical prediction of how the building behaves during the erection process.
Just above street-level, story-deep "mega-trusses" collect perimeter column loads from the third to the 37th floor and deliver it to the corner “mega-columns.” The trusses, each over 100 feet in length and each weighing over 100,000 lbs, could not be trucked or craned as a single piece and required falsework to facilitate erection. We designed a system of inclined kick-stands to give THC the support points they needed to piece the trusses together in-place.
The project is the first ever in the United States to use Wolffkran’s Wolff 700B luffing tower crane, a massive piece of equipment that can reach extreme heights within a relatively small footprint. We devised a system of receivers to support the tower crane at 10 separate locations up the height of the building. Our challenge was to find a way to develop significant loads into a steel-framed building that does not possess a concrete shear core. Our unique receivers use HSS tubes to splay loads to two floors above and below, and we leveraged the permanent steel beams and metal deck diaphragms to then transfer loads out to perimeter moment frames.
We built an analytical predictive tool called the “dashboard” that helps THC position columns in the field during erection. The dashboard accounts for elastic shortening, building lean, and building twist during erection, allowing for the erection crew to plan for future building behavior and attempt to counteract it ahead of time.
We assisted THC in the design of erection aids and lifting lugs to ensure safe crane picks and stable erection sequences for some of the project’s heaviest and tallest column elements. The corner mega-columns, three-foot square boxes with 2 to 3 inch-thick walls, weigh between 25,000 and 75,000 lbs per two-story “assembly” and are some of the first elements to get erected as the building climbs upward. In one extreme instance, our erection aids enabled THC to set a 50-foot-tall, free-standing mega column to maximize the use of a high-capacity mobile crane within a short window of time.
Construction has advanced smoothly and efficiently in part due to Degenkolb’s participation as the steel erection engineer. Through creative detailing and expedited responsiveness, we have played an integral role in the construction of this new landmark on the San Francisco skyline.