Heavy Lifting: A Housemover's Journal, continued
Placing the Beams
One of the many challenges to raising an existing building from its foundation is access. A steep or otherwise limited site increases the difficulty and therefore the cost of lifting. It's a definite plus to have clear, open space around the building to insert the long, 12x12-inch steel H-beams and wood cribbing that provide the main lifting support.
While the author swings a two-ton main beam into position with the backhoe, a worker makes sure the far end doesn't take out the neighbors' place across the street.
Happily, access was not a problem on this job. There was plenty of room to maneuver a backhoe and dig starter trenches and cribbing pits for the two main beams at the building's gable ends. We completed the trenching under the house by hand, digging deep enough to account for the combined depth of the main beams and the crossing "needle" beams, the tops of which had to plane out coincident with the underside of the chimney slab we'd just poured. This point was about 18 inches lower than the sill.
Ordinarily, lifting a house of this size takes a pair of main beams and four 8x8-inch needles — one on either side of center span and one under each outside bearing wall.
Two main beams and four needle beams supported the house for lifting. To avoid deflection from the masonry load on the main beams, the author placed the needles under the chimney mass first.
The needles either go in tight against the floor joists or, when a chimney lift is involved, are inserted below the hearth slab at a lower level than the sills and shimmed between their tops and the underside of the floor joists. The shims we use are various blocks of wood, from 8x8 off-cuts on down to cedar shingles. Compressible rigid foam board also comes in handy for cushioning the pressure points and filling irregular gaps.
Pre-loading the chimney. On this job, we inserted the center needles under the chimney mass first. We slid the first one in and jacked it snug against the modified chimney base, so that it began to shoulder, but not raise, some of the masonry load. Then we inserted the second needle and snugged it up, too, effectively transferring the full weight of the chimneys onto the needles.
After carefully removing every other stone supporting the three chimneys and pouring a unifying reinforced concrete slab between them, the author slipped two needle beams beneath the masonry, initially jacking them just enough to transfer the load.
Usually, you'd place the main beams, then set the needles; but then, this wasn't a usual case. Despite the overall bearing capacity of the H-beam needles, there is still considerable mid-span deflection under a masonry load, which moves independently of the wood structure. We accounted for that deflection by transferring the chimney weight first. Lifting the building was then a matter of placing 40-foot-long 12x12-inch main beams under the center needles, installing the outer needles, then shimming as needed.
We chute the beams in on rollers set on low cribbing stacked just outside and just beyond midway inside the foundation. My derrick usually does the heavy work — a 40-foot H-beam weighs 3,600 pounds — and also pivots the far end of the beam up or down to clear the floor by using the outside roller as a fulcrum. There's always a man under the house keeping an eye on the blind end of the beam and shouting directions.
Rollers facilitate beam placement under the structure.
The beam is chained to the backhoe bucket to keep it from launching itself forward like a battering ram.