Shoring

Shore Thing

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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.

Shoring.jpg

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.

Shoring frames in a stack.
Base of the hoist.

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.

Shoring2

Three shoring towers centered under the hoist.

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!

Angular Momentum

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From destruction to construction. Today, Mark and I built the first new walls in Bob’s attic, defining the walk-in closet and the expanded bathroom. With a sloping roof overhead we had to manage some tricky angles, but we figured out each puzzle as it came and we picked up some nice momentum by day’s end.

Things might have been easier if the roof was actually 12-on-12, which would mean 45-degree angles. Instead, it varied – of course – with an average angle more like 35 degrees, corresponding to something weird like an 8½-on-12 pitch. Since we couldn’t count on field conditions matching any pen-and-paper (pencil-and-lumber?) calculation, we basically built the walls in place, installing bottom and top plates first and then measuring every stud to squeeze between.

For the longer west-east wall, we nailed the top plate directly to the new rafters, tilting it with respect to the floor. Every stud therefore needed a miter cut at the top, so Mark measured to the long point and I cut the angles using Hans’s excellent chop saw. We installed the studs flush with the top plate on the bedroom side of the wall. Thanks to the angle, the studs stick out beyond the top plate on the closet side, but there’s enough contact for transfer of loads. (Remember, we designed this wall to hang the ceiling joists from the roof.) The closet side will get finished with plywood, providing further strength and load distribution.

Mark measures stud lengths for the west-east wall.

Mark measures stud lengths for the west-east wall.

The north-south wall runs orthogonal to the axis of the ridge beam, resulting in a different sort of angle: the wall grows from almost nothing at the knee-space to around 8 feet tall near the peak. These studs also got a miter cut at the top, but the other way. We nailed the top plate to lateral blocks we spaced regularly between rafters. The walls intersect atop a ceiling joist, and the north-south wall is completely supported by that joist.

Intersection of the two walls.

Intersection of the two walls.

Later we pulled out more of the old subfloor, and we had a tête-à-tête with Bob about wall configuration at the top of the stairs. The bathroom entry will run diagonally (even more angles!), with a sliding “barn door” hanging from a 4×4 beam. We’ll need to build two more short walls to make this work, as well as a couple of posts to support the hanging beam and frame the doorway. But first, we intend to replace the stairs.

Jacked Up

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Back at Bob’s house, the sagginess of the original timber-framed structure made us uncomfortable. So we decided to do something about it.

CollarTie

I suspect the first-floor ceiling joists originally served as collar ties to keep the gable roof from collapsing outward. The horizontal component of the rafter load put the joists in tension and kept them taut, like rubber bands. When a full-length dormer was added to the north side in the 1970s, the joists were relieved of this tension and began to sag… and after we pulled up the floorboards this week, we noticed a definite bounce in some of them.

Mark and Bob pulling up floorboards.

Mark and Bob pulling up floorboards.

Bob’s temporary fix was to jack up the joists at midspan using some Lally columns he had hanging around. He set them on a couple plates on the living room floor to distribute the load, ensuring that he pushed the ceiling up rather than pushing the floor down. For a permanent solution, Bob wants to hang these joists from the rafters above via a stud wall in the bedroom. I haven’t decided yet if this is a good idea. The rafters are overbuilt now so an extra load won’t compromise them too badly.

We shored the ridge beam the same way as the ceiling joists.

We shored the ridge beam the same way as the ceiling joists.

The roof needed to be jacked up as well, since the ridge beam sagged in the middle. Until this week, a beefy timber post in the middle of the attic held up the ridge beam near midspan… but it didn’t do much good, because the base of the post was hanging half off one of the ceiling joists with no other support below! Now we have two lolly columns to shore up the ridge beam, and when we replace the timber post we’ll be sure to carry the load path straight down to the foundation.

Hans and Mark finished installing new rafters, and I helped Paté pull up the living room carpet, working around a lifetime’s worth of furniture. Bob’s visions include an open stairwell, a hearth and wood stove in the living room, and rustic barn doors to close off the master bedroom. Believe me, by the time we finish remodeling the inside of this house, you’ll hardly recognize it.

The tool room, neatly organized.

The tool room, neatly organized.