Hoist With His Own Petard

Chris owns a construction equipment company in Massachusetts. He supplies hoists – temporary elevators – for multistory buildings under construction. If you see a residential or office development going up in Boston, odds are about 1 in 3 that Chris’s company is involved.

Chris needs a structural engineer to design each hoist, and he hired me. All the hoists in his inventory come from the same manufacturer, Alimak-Hek. The mast comes in 5-foot sections and can grow up to 500 feet. (An installation guide gives some ideas for how to build an even taller tower, like connecting three masts together in a triangle shape.) The cab rides along the mast via a rack and pinion system; pulleys and counterweight are built in. The first sections of mast typically get installed when the building is five or six stories tall – that is, tall enough to make stairs and ladders inconvenient. As the building grows, the hoist can grow with it.

When I take on a hoist project, I first consider the weight of the assembly – fixed loads like the mast, base enclosure, and motorpack, and moving loads like the elevator cab and its capacity. I might design a concrete slab for the base to spread and support this vertical load. Next I look at wind, which varies with geographic location, surrounding topography, and height above ground. Wind blows on the elevator cab and the mast itself. Since the operators don’t run the cab above a certain wind speed, I must check two load cases: a “moderate” wind speed in service (mast and cab), and a “high” wind speed out of service (mast only).

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Chris operates a hoist cab.

The wind load dictates how far apart I place tiebacks to the building walls. Attachment points transfer the wind load to the building; typically they’re two or three stories apart. The connection details are largely up to me, and may be quite complex depending on what the building offers for fastening.

 

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