Will installing a beam under my shed's floor framing eliminate the bounce?

Question

The previous owner of my property built a roughly 16x12 outbuilding on posts. Besides doing many things in slightly stupid ways, the building overall isn't awful. I've converted it into a manhut where I drink beers and butcher deers.

The floor bounces, and when I crawled underneath, I could have punched him. He used 2x6 floor joists, 19" on center. I dont have much working room under the building and am on a budget.

Here is my plan: cut holes in subflooring. Using auger or post hole diggers, bury 6x6 posts (3) in concrete, shoulder cut 6x6 and use 2x10s to create a beam in the middle of the joists. Think this will be enough to take out the bounce and be strong enough to have a couple of guys standing, along with lots of outdoors gear/equipment in there without risking breaking joists?

Grade of soil shown in diagram. In the footprint diagram, the lowest clearance is in the top right corner. The ground slopes from top right towards bottom left.

Answer 1

I've installed beams in situations like you describe. It's not fun at all, and will probably be a lot more sweating and swearing than you anticipate.

I would be sistering the joists with 2x10s. They don't need to be the full length of the existing joists. As long as they are attached to the existing joists within a short distance of the end, they'll provide enough stiffness. You can probably do most of the work above by cutting channels in the subfloor near each side wall.

1. Cut access channels in the subfloor fairly close to the side walls about 18 to 24 inches wide.
2. Slide each joist, precut to roughly the same length as the original joists, but slightly shorter, into position from outside the building.
3. From inside the building lift the joists into position alongside the original joists, working through the channels you cut in the subfloor. Use ratchet straps or an assistant to pull them up tight to the subfloor. Run a screw through the subfloor at each end to hold them in place.
4. Bolt each end of the new joist to the corresponding end of the old joist. Run 2-1/2" screws into the new joist through the subfloor.
5. Screw the strips of subfloor that you cut out earlier back into position. Float scrap lumber blocks underneath joints were appropriate to prevent flexing.

This will stiffen up the floor nicely, and you shouldn't have much in the way of squeaking. Certainly not so much that it'll be a problem for your man hut.

Answer 2

I like Isherwood ‘s idea about working from above, but not the “sistering” idea because you’ll need to excavate 5”-6” out from under the ENTIRE building AND requires a lot of lumber.

I like @JimStewart ‘s idea about adding a beam at mid-span of the existing joists.

I’m sure the existing floor is very springy, because 2x6’s at 16” o.c. spanning 12’ will support about 20 lbs. per square foot. Adding a single beam at mid-span will increase the strength of the floor to about 185 lbs. per square foot. (The code requires normal rooms to be 40 lbs. per square foot.)

Adding a beam at mid-beam will require a trench, but only at the beam not under the entire building. The trench “runs” with the slope, so it could easily be dug to drain.

The beam could be a 6x10 or 3-2x10’s. Or, it could be a 4x8 or 2-2x8’s if you installed a support at mid-spans and only had the beam span 8’ I like @JimStewart ‘s idea about anchoring the ends (which could also be installed from above). If the coal bin is in the way, I’d move the support inboard a foot or so and let the beam cantilever the last foot or so.

BTW, Isherwood ‘s notion of NOT extending the sistered joists to a bearing is not acceptable. This is why drilling holes through joists is unacceptable.

Answer 3

If footings must go below the frost line, then I'd dig them at the edge, one on each side of the building in the middle of the 16 ft long side, and fill with concrete. Then use the truncated pyramid blocks, or other blocks on top to support a beam. Attach the beam loosely on to the joists above, then put jacks under each end and raise the beam (and the shed) until you can slide supporting blocks under the ends of the beam.

The particular footing I use from Lowes or HD has slots at a right angle for 2x lumber and a square for a 4x4 in the center. Here I am using one to support a cedar post (6" x 3.5") so I laid in a piece of PT 2x4 with notches cut out on the corners.