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.

IMG_1710
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.

Figure
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.

IMG_1727
Mockup of the staircase support post location.

About the Author Scott

When I decided as a teenager that I would become an engineer, what I really wanted to do was build houses. But then I went to college and got tricked into thinking I should work for a big company, design big structures, and make lots of money. With a professional license in my pocket, it's time to get back to following my dreams, and I hope my perspective can teach you something new.

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