Double top plate on stud wall 7

[ykmz]

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!

 

[COEngineeer]

Well, if the studs can take the axial load then I would just make them try to line up the joists with the studs. Then the double plates will only act to resist lateral load (chord force). If it is not lining up, as long as it is 3" away then it should not create any moment.

Never, but never question engineer's judgement

 

[msquared48]

If this is an exterior wall with a rim joist, the rim joist would help spsan out the load between the studs.

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

 

[sdz]

" I do not think it is right to consider double 2x4 working as a composite beam"
That's correct, but the will act as two independent beams.

 

[ykmz]

It is an interior bearing wall with joists only on one side. But it does not have rim joist. There is blocking between the floor joist, which I do not think it would help to distribute any load. I could prove it works for bending by using reduced live load ( normally I don't use this for residential )and just single 2x4 top plate. But it would not work for shear even with reduced live load. I am wondering why I can not use double 2x4 top plate for shear analysis since it is there. I do not quite understand the 3" and 6" Mike and COEngineer mentioned above.

 

[ykmz]

It took me a while to agree with sdz. I used to think the plate on top will carry all the load until it fails, then the load will be carried by the one at bottom. But the more I think of it, the more I agree with sdz. Once the top plate carry the load and deform, the bottom plate will deform the same amount, which makes the two plate each carries about half of the load. Any disagreement or comments?

 

[hokie66]

If you need a further endorsement of sdz's post, you have it. The two top plates will work together, but not compositely. But you need to watch where splices occur.

 

[msquared48]

In actuality, depending on the nailing and any gluing, partial composite is more likely. It's just that using two independent beams is conservative, and full composite is unconservative.

The truth is somewhere in between. Can you handle the truth?

Mike McCann
MMC Engineering

 

[ykmz]

Thanks hokie66. It does give me more confidence on my understanding. BTW, is 29' floor span commonly used in residential? There are nonbearing walls below and cross the long floor joist at middle of the span. The inteior nonbearing wall is built on existing 2x6 T&G plank floor. Will it be a concern that the nonbearing wall will support the floor joist even it is not intented to?

 

[Ron]

Once initial deflection occurs, composite action is mobilized, though weakly. As Mike noted, the two should be "spiked" together. If the spiking occurs at a reasonable interval, i.e, nailing the top plate together at 6 inches or so, staggered, then some composite action occurs.

 

[beton1]

Long term deflection will be a concern and accidental load sharing can happen, vibration may also be a factor with this span. Back to the original question, can you add studs or double up to reduce the problem/issue of the double plate? Agree with trying to line up joists with studs, but if that cannot be done, or is already built, then you will need to do something to beef it up so you can sleep at night.

 

[hokie66]

ykmz,

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.

 

[COEngineeer]

29' foot you will probably need some kind of truss system that is 24" tall. Does the architect give you 24" of joist space between floors? If it is only 12" then you are out of luck unless you want to use LVL joists and the will be very very expensive especially if it is 12" O.C.

Never, but never question engineer's judgement

 

[ykmz]

I was using 14" I joist (LPI56). It is such long span since I was trying to reduce concrete work in the crawspace. The joist itself works. I think the vibration is not a problem since the deflection was controlled at l/480 and there is blocking between the joist, which I think would help the vibration. I just never calculated the double top plate before until now. The wall and joist are all built now. If I use uniform load to calculate double top plate by considering each top plate carry half of the load, the top plate works. Regarding the nonbearing interior wall, the maximum joist deflection at wall is about 0.5". The wall is on 1.5" thick T&G plank floor, which is not that stiff. Is it fair to say once the nonbearing wall start to pickup the load, it will deform, then the two bearing wall (which has foundatin underneath)at end of joist will start to pickup all the load?

 

[ykmz]

It's all built so line up stud under joist is not feasible. There is no splice in the double top plate.

 

[bigmig]

Ykmz,

"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.

 

[ykmz]

Thanks bigmig. Would you take a look at the attached copy from LPI56 catalog? It indicates for DL=15psf, LL=40psf and live load deflection controlled under L/480 the span can be 29'-11" (@ 12" o.c.). I always used the catalog to choose the joist and never used software. Is that something I should do in the future?

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.

 

[ykmz]

Forgot to attached the copy from LPI56 catalog. Here it is.

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.

 

[COEngineeer]

double plates are 3" thick, if you put a point load 3 " away from your stud, it should act almost like if you were putting the point load right in the middle of the stud.

Never, but never question engineer's judgement

 

[BTL]

L/480 is an ok starting point but for 29-ft, that is 3/4-in live load of deflection and over 1-in total load deflection. If there is tile, you probably want to go at least L/600 lest the tile crack. Furthermore, if this is over a crawlspace (or another area where there is no directly attached ceiling) you will be losing another floor performance enhancer.

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.