Floor joists in non traditional orientation
First time poster with a question I can’t seem to find an answer for:
I’m framing a 30’ wide by 45’ deep shop. This shop will have an apartment over the top.
The ridge obviously runs in the 45’ dimension. Due to load bearing wall configurations – it would be better if my engineered floor joists run unconventional parallel to the ridge instead of perpendicular.
Here is my question:
Will floor joists and a fully decked floor still provide adequate resistance to wall spreading in this opposite configuration? It seems the floor becomes a monolithic wood slab once decked that would still serve this purpose but I can’t find information on this exact question. I certainly understand that without the decking the joists would need to go perpendicular to the ridge to perform this rafter tie function
Assume for this educational purpose I’m not using a structural ridge beam but just a ridge board.
Any help someone can provide (or a link to similar discussion) would be very much appreciated.
Replies
If this is a full floor, and not just a partial loft, there is no reason not to do this.
Thanks for the comment mikemahan3
If the second floor is actually at the level of the top of the load bearing exterior walls, then you would want to provide for adequate bracing (not the floor deck)
Might want an engineer to specify alternatives.
I am guessing that at the end of the day, providing headers on the long wall to take the load and running more traditional floor joists will end up more cost effective.
UncleMike42
Just to make sure I understand your comment… I have load bearing walls/beams running across the 30’ dimension that makes my I joist spans shorter running them the long dimension. Does this info change your opinion or not?
that does make a difference. I was not thinking of beams, but thought you had large open doors to work around on the long side.
what is the span and spacing of the beams?
The details matter, slope and material for the roof. required snow and wind load design, as well as the spans and sizes of all the elements.
Is part of the structure existing, or is this all still a paper design?
Without perpendicular ceiling joists the center of the exterior wall between the beams would be prone to the greatest spread. You would need some form of triangular bracing tying the center of the wall back to the beam to hold it from spreading. You could use floor boards on a diagonal or plywood bonded to the joists. You are likely doing that already so there is no extra costs other than making sure the plywood is bonded to the floor joists. I would also add blocking between the joists to keep them from twisting. Strapping for a lower ceiling will also help. Ultimately, I would gather some ideas and consult with an engineer.
Thank you all for the help. I think a structural ridge beam would make this all negligible which I will use but I wanted to understand the issue as I never see mention of the decking acting to tie the system together. Everything I can find to read seems to assume that ceiling joists are being used without the decking Using blocking and glue makes perfect sense on the floor system as VictorianCarpenter adds.
Why not make this thing 32 x 48? It will save you a lot of waste.
It’s actually quite a bit bigger than just the center section in discussion. The waste will be small percentage and the size is to fit the space. Good thought though.
If I understand the layout, the spreading forces will only occur at the top of the second story walls. and only then w/ a non-structural ridge board and collar ties instead of ceiling joists. End view (along the ridge) is a triangle on top of two rectangles.
The rectangles are floors/ceilings and walls. The triangle is either trusses or rafters and ceiling joists. In either case, if the engineer/architect/carpenter did good work, the triangle is rigid and only imposes vertical loads. The walls transmit the forces straight downward, ending at the foundation.
My strong recommendation is to use trusses- it will be cheaper, just as strong and easier to build than either a structural ridge beam or rafters and joists. There are lots of reasons almost all pitched roofs use trusses these days.
It is important to have a strong connection between the trusses or joists and the top plates of the second story walls. I prefer the SDWC Truss Screw but the Strong-Tie H1 will work. 3rd choice is the H2.5A. Similarly, the 2nd story floor joists (first story ceiling joists) must have strong shear connections to the first story top plate and the second story bottom plate.
The 2nd story bottom plate will be nailed to the floor sheathing (3 16d nails per stud bay) and the sheathing should be installed in a staggered pattern, so there will be a continuous load path across the 30' direction at the level of the floor. You also need to create a load path at the 1st floor ceiling level to restrain the top of the first story walls. Orienting the joists parallel to the walls complicates that- I'm not aware of any ready made solutions. My best idea is blocking in the first joist bay, perpendicular to the joists, attached to the top plate, the floor sheathing and the joists. I'd add something like the Strong-Tie LTB20 or 40 in pairs in the second and third joist bays, installed before the floor sheathing. If you're concerned about a really flat floor/ceiling, I'd recess the LTBs into the joists, which will slightly weaken them. The LTBs might work in the first bay too.