Double top plate on stud wall
Double top plate on stud wall
(OP)
I have a very long span floor (29')for a residential house. The floor joists are supported by existing 2x4 stud wall. The studs are spaced at 16" oc (HF #2). By using DL=15psf, LL=40psf, my analysis shows the single 2x4 top plate can not carry the load. I do not think it is right to consider double 2x4 working as a composite beam since it is impossible to provide that many nails if you calculate the shear flow between the two plates. But IBC does indicates 2x4 stud wall can carry one floor plus one roof ( I always feel strange it does not talk about the span limit). Would someone help me to explain why? Thank you!






RE: Double top plate on stud wall
Never, but never question engineer's judgement
RE: Double top plate on stud wall
If this is an interior bearing wall, if the joists lap over the top plate, then the load is spread out further to 6" instead of 3" if the joists are spiked together.
If this is an interior bearing wall with joists only on one side, hopefully it too hsas a rim joist like the exterior wall.
Mike McCann
MMC Engineering
RE: Double top plate on stud wall
That's correct, but the will act as two independent beams.
RE: Double top plate on stud wall
RE: Double top plate on stud wall
RE: Double top plate on stud wall
RE: Double top plate on stud wall
The truth is somewhere in between. Can you handle the truth?
Mike McCann
MMC Engineering
RE: Double top plate on stud wall
RE: Double top plate on stud wall
RE: Double top plate on stud wall
RE: Double top plate on stud wall
I don't think 29' spans are very common in residential construction, with wood, that is.
The sharing of load will depend on the stiffness of the components. If the floor below is stiff, then the centre partition will pick up a lot of load. If you don't want it to take load, you have to allow for the upper floor deflection in both the partition framing and the finishes.
RE: Double top plate on stud wall
Never, but never question engineer's judgement
RE: Double top plate on stud wall
RE: Double top plate on stud wall
RE: Double top plate on stud wall
"by using reduced live load ( normally I don't use this for residential"....
what reduction in live load? Your area per joist is less than 39 square feet...you don't get any reductions from Equation 16-24 or 16-25 (IBC 2006).
And your live load deflection is more like L/397. Based on the vibration studies we have done, and without going into yet another topic here, vibration will be a problem with a majority of occupants that use your floor. A good check is using the TJbeam freeware, and noticing that the proprietary "Pro rating" for a similar product, the TJI560 14" joist under similar conditions is 18. We design for 40 on low end residential and 50+ on upper end when using this software.
It is generally cheaper in residential construction to run more frequent supports (i.e. beam at center of building) than not. To get a perspective, your floor is approximately 21% wood (3 1/2" flange/16" o.c.). Adding a beam at centerline will reduce that by half and improve your overall floor performance.
Regarding previous (albiet mute) posts about using a rim to "spread load" remember that this only works when you connect to the rim via facemount hanger, especially for the end reaction you are dealing with. I would skip all of these unusal details and add a conventional beam with pads and posts at center line.
RE: Double top plate on stud wall
When I said reduction in live load, I meaned for the double top plate on stud wall supporting the floor joist, not for the joist design.
I start to realize it saves a lot of trouble if I keep the span shorter. But this one is already under construction and I can not do too much about it, except to make sure it works.
RE: Double top plate on stud wall
I am still trying to understand COEngineering's "If it is not lining up, as long as it is 3" away then it should not create any moment. ". Can anybody help out?
Thank you everyone for your advise.
RE: Double top plate on stud wall
Never, but never question engineer's judgement
RE: Double top plate on stud wall
My rule of thumb for a good, solid floor is a span no more than 18x the depth (or 1.5x in feet of the depth in inches). 12-inch, I try to keep at 18-ft or less. 14-inch about 21-ft, etc.
I agree regarding live load reduction - I wouldn't do it on residential - especially on the joists (where there isn't enough area) but even on larger beams because generally, the live load is a high percentage of the total load.
RE: Double top plate on stud wall
I agree no live load reduction for residential since the area is not big enough. IBC limits live load reduction when live load is 100psf. For residential, live load is 40psf.
RE: Double top plate on stud wall
DaveAtkins
RE: Double top plate on stud wall
RE: Double top plate on stud wall
Never, but never question engineer's judgement
RE: Double top plate on stud wall
If you add a beam, as has been stated, you reduce the load on the double plate, and reduce deflection/vibration in the joist floor system. If, as you state, deflection is not a issue (tiles at end of span), you still have to deal with the double plate.
RE: Double top plate on stud wall
RE: Double top plate on stud wall
You are designing for a 29' span with a LL of 40 psf and a DL of 15 psf. The unfactored reaction on each joist is 29*(15 + 40)/2 = 798#.
Assuming that every fourth joist aligns with a stud, the worst case for bending in the top plate is when the joist reaction is midway between studs. The simple span moment in that case is 798*16/4 = 3192"#, i.e. 1596"# per 2x4. The section modulus for one 2x4 is 3.5*1.5^2/6 = 1.312in^3. So the bending stress in each 2x4 is 1216 psi.
In that case, the unfactored shear is 399# and the maximum shear stress is 399/(2*1.5*3.5) = 38 psi.
When the joist reaction falls 4" from the center of a stud, the maximum shear is 798*12/16 = 598.5# and the unfactored shear stress is 57 psi.
In addition, you have OSB nailed to one side of the top plate. I don't know what kind of material you are using, but for #2 SPF, where's the problem?
Your spans seem a bit long for comfort. If the first and second floors span 29' and you have partitions between these floors, my suggestion is to take advantage of load sharing and tie the two floor systems tied together using partitions and perhaps a few strategically placed strap ties. An added bonus is that you will damp out vibrations in both floors.
Best regards,
BA
RE: Double top plate on stud wall
RE: Double top plate on stud wall
Be careful with that composite action thinking with the two plates. We did a garage test with a nailed together double plate and you would surprised how quickly the nails start to slide. You get some serious shear flow when you crunch the numbers, and our crude testing certainly backed it up.
RE: Double top plate on stud wall
In my earlier shear stress calculation, I forgot to multiply by 3/2 for maximum shear stress, so the maximum unfactored shear stress if 57*3/2 = 85.5 psi. Assuming load factors of 1.25 and 1.5 for dead and live load respectively, the factored load is 79 psf and the maximum factored shear stress is 123 psi which is okay for #2 SPF.
I agree with FSS that a joist depth of 14" seems very shallow for a 29' span but properties of the LP156 joist are not given, so it is difficult to check. It might be a good idea to follow up with the supplier and ensure the joists are adequate for both deflection and vibration.
Best regards,
BA
RE: Double top plate on stud wall
Personally, I would put a foundation under the interior wall and span the floor half the distance. In the long run, the owner will be a lot happier with the performance of the floor. Or put a steel beam and some posts.
RE: Double top plate on stud wall
I went to the site this morning and walked(even jump)on the floor. I do not feel any vibration. There are blocking for the floor diaphragm between the joist, maybe that helps a little? Would it get worse when everything is installed and furnitures are in place?
RE: Double top plate on stud wall
DaveAtkins
RE: Double top plate on stud wall
A 14" deep Trus Joist TJI 560 is rated to span 29'-6" with L/480 deflection and 20 and 40 psf for dead and live load respectively which tends to confirm your design . The top and bottom chords on that joist are 3.5" wide by 1.375" deep.
Vibrations will not get worse when furniture is in place. In fact, furniture will tend to dampen vibration.
Best regards,
BA
RE: Double top plate on stud wall
I always hear complaints from the carpenters when studs are called out at 16" oc and trusses are 24" oc. It just bothers them not see everything lining up perfectly.
RE: Double top plate on stud wall
I agree, the truth is somewhere in the middle.
RE: Double top plate on stud wall
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Double top plate on stud wall
Best regards,
BA
RE: Double top plate on stud wall
There are hundreds of houses I've seen with 2x4 walls carrying 3 floors (~36' of total tributary floor load) plus roof/snow load in our city. And these houses are over 100years old and we get a good amount of snow up here in Canada land.
I would say if you are really worried about it, put vertical 2x4 blocking directly below the top plate in each joist space, or, even worse, add a stud every other space creating a ~12"o.c. spacing. I really can't see the top plate failing before the stud in a case like this...but that might help you justify it.