Chris asked me to design a construction hoist for a building in Boston. The building’s basement has a larger footprint than its ground floor, and nobody wants the hoist to fall through the basement ceiling. We could put the hoist base on the basement floor, but then we’d force the general contractor to leave out a large chunk of the ceiling. Instead, Chris suggested we install shoring – temporary support towers – to support the hoist below the first floor.
Our shoring system looks very much like scaffolding. It comes in 2-foot-wide frames of various heights. Two frames can be connected via cross-bracing to create a 2-foot-by-4-foot tower. Couplers allow us to stack the frames up to about 15 feet high, and adjustable screws allow us to fine-tune the overall height.
In this case, the hoist mast is about 2 feet square, and most loads on the hoist (the weight of the mast, the cabs, and the payload inside the cabs) go directly to the base of the mast. Therefore, I put one shoring tower directly under the mast.
In the very unlikely event a cab hits the bottom at speed, Tower of Terror style, it lands on buffer springs that protect the equipment and riders. To pick up the buffer springs I specified two more shoring towers, one on each side of the main tower. The result is six frames (3 towers x 2 frames per tower), with four directly under the mast and two picking up the outer buffer springs.
Each frame is rated for a load of 25,000 pounds – with a generous safety factor built in, meaning it would take a load exceeding 100,000 pounds to break. While every project is different, under most conditions the configuration of three shoring towers I just described can support a hoist up to 450 feet tall. The 200-foot hoist for this building is well within range. We’ll shore floor-to-ceiling, running timbers across the top of the legs to avoid any load concentration on the floor slabs.
I’m also investigating a different shoring system, PERI, which uses adjustable-length posts similar to Lally columns. Look for this system in a future hoist project!