A historic building in San Francisco will soon be preserved and incorporated into a 19-story tower that’s currently under construction in the financial district. The planning of this project began over 20 years ago, and since then, the construction team has worked vigilantly to convince city planners that they would maintain enough of San Francisco’s historic Mining Exchange in order to get the approval of wary preservationists. In the meantime, contractors and engineers had to figure out a way to rebuild a foundation underneath the old building without causing the new structure to crumble.
“This project was the most difficult planning approval I’ve ever done,” says Jeffrey Heller, president of Heller Manus Architects. “There isn’t anyone left in the city to get permission from. It was unbelievably complex and had to be dealt with very carefully.”
The first step in the shoring process began in early 2015. Shoring is classified into the following three classes either on the basis of their supporting characteristics or their position in the space: raking or inclined shores, flying or horizontal shores, and dead or vertical shores. This particular shoring process involved underpinning neighborhood buildings. But due to restricted access in such a highly packed neighborhood, the crews had to get creative and use shovels and buckets to excavate 34 pits by hand. The pits were four feet by 30 inches wide and had a range of depth between and 20 and 35 feet.
Federal OSHA limits the fall or arrest distance to six feet, and exceptions to this rule are rare. And considering the fact that in 2015, 4,836 workers were killed on the job, amounting to 13 deaths every day, extra precautions had to be taken due to the extreme depths. Air monitoring, ventilation, and a manual retrieval system had to be put in place to provide a safe working environment for the single worker at the bottom of the shaft. Keith Bizzack, district manager with shoring contractor Condon-Johnson and Associates Inc, says that it was “definitely a claustrophobic situation down there.” Once the shaft was completed, a reinforced concrete support pier was placed inside it.
The next step was to anchor a temporary welded frame of steel to provide support to the thin walls. Bizzack says that a system of 18 12-inch-diameter microphiles was designed and installed to act in both compression and tension to approximately 350 kips.
“We built essentially vertical trusses to support the walls both vertically and laterally with haunches to come out and pick up the vertical load and walers that went along the walls to support the walls laterally,” says Alan Robinson, vice president with Tuan and Robinson.
Despite all of the hard work done on this project, its construction was contingent on completing a five-story building at 500 Pine. This is because the new tower casts a tiny sliver of shadow onto a park in the vicinity, which violates San Francisco’s building regulations. The building at 500 Pine was completed by the same construction team and extends across the rooftops to compensate for the sliver of shadow.