4 story wood building studs/floor joists/trusses line up

[Engin1]

Hello everyone,

One question came up recently. For a 4-story wood building, the first two floor studs may need 2x6@12"OC and upper floors 2x6@16"OC by calculation. But floor joists are 16" OC and roof trusses are 24"OC. So do the studs have to line up all floors? if so, all floors need to have studs @ 12" oc? Do the joists and trusses need to line up with studs? Please help. thanks.

 

[structSU10]

If you can get your top plate / rim joist to span they don't have to align.

 

[Ron247]

If they do not line up, you need the top plates (usually 2 pieces) to be able to span the distance with the point load on them.

By the way, you have the same issue if the walls, floors or roof are spaced the same. You have no way of knowing the contractor will start them all at the same location so you can still have a floor joist in between two wall studs.

 

[Engin1]

The 2-2x6 top plates may take one floor load, but for 4 floors. I doubt. I am thinking to put continuous rim board to distribute the load from above. See the detail from LP. Any comments on this detail?

https://files.engineering.com/getfile.aspx?folder=f80e14ac-c9cc-457e-afdc-44e7edccce63&file=pic.png

 

[structSU10]

I generally use the rim board detail, but in a pinch have used PSL top plates.

 

[dhengr]

Engin1:
You have to do a set of std. calcs., following the NDS code, using the grades of lumber you typically use, to set some limits for all of the various limiting design conditions. What concentrated roof truss, joist or stud load is the max. for a single and double top pl., with the pl. bending as the design issue, when the studs below are at 12, 16 and 24” o.c.? What is the max. compression load on a 2x4 or 2x6, stud or joist before compression perpendicular to the grain limits you? Do these in an iterative process so you can see when you are reaching each design limit. You kinda have to keep these values on the top of your head, and in your current wood design file, so you know when you are reaching the limit for a given condition. See if you can find a source for 3x and 4x lumber, we used to use 3x4’s and 4x4’s for studs on the lowest fls. for some cases. Thus, we could maintain the 16” o.c. stud spacing. You have to pay particular attention to large jambs at large openings, and we usually tried to stack those and block them in the fl. jst. depth space, and we used a jamb table, J1, J2, J3 to call them out on plan. The rim joist idea works to help with the top pl. bending problem and with the distribution of loads over small openings. But, ultimately compression perpendicular to the grain always becomes the killer.

 

[Ron247]

Engin1: In my locale, I have to be cautious not to use Southern Pine studs unless S-P-F won't work. Reason is, all the precut studs in my area are S-P-F. I see many projects specifying Southern Pine and then S-P-F actually gets installed. The local code official have never flagged that practice. Attached is a spreadsheet output I have been working on and have not fully debugged yet. I used a simpler version of this sheet for projects similar to yours where stud loads vary from floor to floor. The more elaborate sheet is for 9' studs but I can easily change the stud size, length, spacings and species. This may help you some but you need to hand calc any, but please compare to my sheet is see if the answers are close. As I said, I never fully debugged it but had to shelve the development to get some paying work done. It does give you a single sheet that shows how the allowed loads change with spacing, wind and brace points

I think the rim board detail would only work if you "hard connect" the joist to the rim. If not, the joists still bear on the top plate and cap plate. In your detail, the rim joist is not directly connected to the joist other than one 8d common or 10d box. During installation, the joist will be set on the wall first and then fastened to the rim board.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1564009025/tips/MSS-Wall_Stud_Charts_2015_NDS_NO_LOGO.xlsm_q5atuk.pdf[/url]

 

[Ron247]

Also, the rim board does pass the loads above down to the studs because the wall above is directly over the rim, but the load from the joist itself does not pass through the rim board. It is the one that loads the top and cap plates.