Supporting a Staircase

Zach is a carpenter who partially gutted a house he bought. Before outfitting the house with a new hardwood floor and an oak staircase, he hired PERCH to analyze his framing and see if it was up to the task.

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Top of the new stairs.

Zach was smart to hire a structural engineer in this situation, because as it turned out the framing needed several improvements. The additional weight from the staircase was too great for the existing first floor joists to carry, so PERCH specified an additional 2×10 joist to pick up each of the stair stringers. The next step was to follow the load path and note that the joists are all supported by a triple-wide beam running across the middle of the basement ceiling. With increased weight on the triple-wide beam, it needed an extra support post too.

What about the rest of the first floor, where Zach would install new flooring? A finish floor is typically ¾” thick and doesn’t add a ton of weight. But some of the existing joists were notched to accommodate utilities like air ducts, and this weakened them beyond the minimum code requirement. PERCH added sister joists across the notches to bring these joists back to full strength.

Now it gets weird. The house has a partial second floor with a balcony overhanging a central loadbearing wall. (That loadbearing wall is directly above, you guessed it, the triple-wide beam in the basement.) The staircase ends in two winder stairs supported by the overhang. Anticipating the increased cantilver load, Zach had already added an extra joist where the staircase was supported. Analysis confirmed this extra joist was adequate for the load, provided he fastened it to the original joist over the entire length.

But again we need to follow the load path, and in this case it leads past the central loadbearing wall to the back wall. If there’s a big weight on the end of the cantilever, and no weight anywhere else, then the joist actually pushes UP on the back wall. (If you sit on one end of a seesaw, the other end goes up.) That means there needs to be a stud ABOVE the joist to carry the load up through the back wall, where it gets balanced out by downward forces from the roof. In reality the weights on the joist will be evenly distributed most of the time, and uplift will rarely occur. Still, a situation that causes uplift is plausible (like a crowded party on the balcony with nobody in the other rooms), so PERCH designed for it.

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Original second floor renovation plan in PERCH Engineering Report.

When Zach received the Engineering Report, he asked if there was any way to support the stairs without the last stud. Opening up the back wall would require a lot of extra labor on his part, refinishing a wall he had already completed. We pitched ideas together and settled on supporting the top half of the staircase with a post leading back down to the first floor. PERCH located and sized the post to leave room for a piano behind the stairs, and sent along the revised design. This exploration of options is called value engineering, and it always proves that good communication makes good projects.

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Mockup of the staircase support post location.

Mind the Gap

Jeff bought his home in the middle of summer several years ago. The first winter, a gap opened in the drywall between the gable-end wall and the vaulted ceiling. The gap seems to close every summer and open again every winter. Jeff hired PERCH to diagnose the problem and recommend solutions.

What on Earth was going on here? My initial investigation ruled out several possibilities. The gap is seasonal, not progressive (although Jeff does think it gets worse each year), so it doesn’t indicate a problem with the superstructure but something environmental. Frost heave seems unlikely, as Jeff has no uneven floors or major cracks elsewhere in the house.

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The gap between gable-end wall and ceiling.

For a little while I was wooed by the idea of truss lift. Roof trusses are known for expanding and contracting in cold climates: in winter, the bottom chord stays warm and damp because of its exposure to inside air, while the top chord and webbing gets cold and dry right below the roof. The top chord and webbing contract and pull up the bottom chord. But Jeff with his vaulted ceiling clearly doesn’t have roof trusses. They must be rafters, no deeper than 2×6, for which any differential contraction would be barely visible.

The best-fitting explanation was not frost heave, but a different kind of heave. Certain soils (typically soils with a lot of clay) are known as expansive soils because they collect groundwater and expand during wet seasons, then lose the groundwater and contract during dry seasons. If the gable end wall was built on an expansive soil, it would tend to drop in the winter and rise in the summer.

But wait. Wouldn’t the rest of the house move, too? I found evidence to the contrary during my site visit. On the first floor, a wall adjacent to the offending wall had a few minor cracks in the drywall. Directly below, in the basement, a crack ran across the plaster covering the concrete foundation. Aha – the walk-out side of the basement is framed by a stud wall, which is much lighter than the concrete walls on the other three sides. The soil under the stud wall hasn’t compacted as much as in the other locations, so it’s more susceptible to subsidence when the soil contracts.

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Cracking in the first-floor wall, about 6 feet from the corner.

Sometimes structural engineering is like solving a mystery. I search for clues and weigh possibilities against the evidence, and hitting upon the right answer is very rewarding.

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

Sir Arthur Conan Doyle

Barn Stormer

Geoff owns a barn. An old barn. A barn so old, the gravel road that once ran next to it has been successively filled and regraded, and now towers a good four feet above. All that extra weight, and the drainage pattern it creates, spell danger for the barn’s foundation wall.

Geoff hired PERCH to ascertain if the barn is safe to keep horses and store 16 tons of hay. He also asked for advice on any low-cost repairs he could make to stabilize the structure. My visual inspection revealed lots of band-aids placed by previous owners. Two tall reinforced concrete blocks prop up the fieldstone foundation wall that’s bowing from the surcharge of the raised-up road. A cable runs from one gable end to a ground anchor, apparently to resist prevailing wind. Extra posts have been added on the lower level to support sagging beams.

My first suggestion to Geoff was to add more posts to carry vertical load directly to the ground, relieving the burden on the mist critical foundation wall. In particular, I said, the hay should be supported as directly as possible. I also suggested a buried drain pipe between the barn and the road – hydrology is not my area of expertise, but anything to drain water away from that spot will be beneficial. (Geoff believes he already has a drain there, but it’s probably clogged.)

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Beam has migrated right and is no longer supported by the column.

Geoff was pleased to implement my suggestions. Engineers assign a smaller importance factor to a barn than a house because of the low risk to human life, but I want to keep his horses safe too!

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Inside the barn.