Loft Support using Ledger - Stability Question 2

[PYDC402040]

I have a project where a client is converting an RV garage into an ADU. There is a 10 ft by 10 ft section of loft that will be supported on one side by a ledger beam. The existing wall has 2x6 studs at 16 in o.c that are 13 ft tall, it is a load bearing wall for the roof rafters above (5:12 roof pitch). I'd like to tie in using a ledger beam to conserve as much room as possible instead of building additional posts and beams, but there's no real driving force not to other than the little space that would be taken up by the extra posts.

Thinking about stability, there is approximately 88lb*ft moment on each stud if using a 4" moment arm to the seat of the attaching joist (it has approx. 267 lb load per joist reaction). I would imagine the primary failure modes would be bending of the stud and the stud bottom sliding outward.

Is installation of a ledger beam into 2x6 studs a difficult or unusual task to ask of a contractor? The contractor that looked at the plans said he thought it would collapse and wanted to add a couple posts and a beam under the joists. The inside of the existing wall will be removed during construction as they are putting in a new kitchen under the new loft. I thought I would have them add brackets or anchors at the stud to sill plate connections to help secure the studs.

Just looking for general thoughts on ensuring the side-loading is adequate before I either double down on the ledger beam or change my layout. The wall should be open, so there won't be any guesswork. In total the wall length is approx 10 ft, with about 8 studs/connection points, so the load does accumulate at about 705 lb *ft moment in total (88 per joist * 8).

 

[Greenalleycat]

You're speaking Americanese so some of the words went over my head(ledger beam? think it's what we call a ribbon plate)
Based off your sketch, this seems like a pretty standard form of construction so I'm surprised that they are worried - we do this type of thing all the time
10' (~3m) is not a particularly large span so this seems reasonable

If they're worried, could you run the joists over the ribbon plate and fix them in to the side of the studs with a couple of screws?
That way you can claim no eccentricity and could just use the ribbon plate as basically a construction aid and make it smaller

 

[Eng16080]

First off, is the 40 psf load on the floor the total load (dead load + live load) or is this just live load? Normally, the live load should be 40 psf and I would expect a dead load somewhere between 10 and 20 psf, meaning that the total load would be more like 50 to 60 psf. The exception to this is for sleeping areas, where a 30 psf live load is allowed. You should first verify that your loads are correct. If the loads are higher than what you noted, you'll probably find that the 2x6 floor joists don't meet code.

Thinking about stability, there is approximately 88lb*ft moment on each stud if using a 4" moment arm to the seat of the attaching joist (it has approx. 267 lb load per joist reaction). I would imagine the primary failure modes would be bending of the stud and the stud bottom sliding outward.

Based on the gravity loading discussed here, the primary failure mode of the stud is likely the combined effect of the compression forces from the floor and roof plus the bending due to the eccentricity from the ledger. In analyzing the studs, you need to account for the load from the roof as well as any other loads (like wind) acting on them. The outward force at the bottom of the stud (and the top) is nearly insignificant, unless I'm misunderstanding something. Summing moments about either the top or bottom of the stud, I get an outward force of 88 ft-lb / 13 ft = 6.8 lbs (which is nothing).

Is installation of a ledger beam into 2x6 studs a difficult or unusual task to ask of a contractor? The contractor that looked at the plans said he thought it would collapse and wanted to add a couple posts and a beam under the joists.

Any competent contractor should be able to install a ledger. With that said, if the contractor thinks that adding the ledger could cause a collapse, I would not take that lightly. Perhaps the contractor sees something that you're aren't aware of. I can't emphasize this enough! Contractors will whine all day long about how everything engineers design is way overdone, so when they have a concern like this, you need to listen!

Just looking for general thoughts on ensuring the side-loading is adequate before I either double down on the ledger beam or change my layout.

In terms of the ledger connection, I would first check that you can get a connection to work between the ledger and each stud. Depending on the specific fastener you use, you're probably looking at 3 or 4 nails/screws to support the 267 lb load, assuming this load is correct (see above). Now, if you're following the recommended minimum spacing for nails/wood screws found in the NDS commentary, I think you'll find it difficult to fit that many fasteners, especially if the ledger is only a 2x6. (The spacings are only recommended, but I think it's a good idea to follow them.) Also, before you call out a connection using 16d common nails or similar, check to see what kind of nails your contractor is actually using. Most nail guns seem to take 0.120" diameter nails these days, which are a fair amount weaker than 16d commons.

If this was my project, I would consider the alternative connection of sistering each joist to the side of each wall stud and adding a new 7 ft stud directly below each joist, sistering that to the existing stud. I would also add fire blocking at the new floor level between adjacent studs, which is likely required by code anyway.

 

[PYDC402040]

Thank you very much to both of you for the advice, I'm going to do just that, extend the joists to meet the studs and connect from the side. It sounds like keeping the ribbon plate is a good idea even if we add studs. I'll talk to the contractor again once the detail is worked out and listen for his concerns.

Cheers!