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Double top plate on stud wall
7

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

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

RE: Double top plate on stud wall

2
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

RE: Double top plate on stud wall

" 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.

RE: Double top plate on stud wall

(OP)
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.

RE: Double top plate on stud wall

(OP)
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?     

RE: Double top plate on stud wall

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.

RE: Double top plate on stud wall

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

RE: Double top plate on stud wall

(OP)
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?        

RE: Double top plate on stud wall

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.

RE: Double top plate on stud wall

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.

RE: Double top plate on stud wall

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.

RE: Double top plate on stud wall

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

RE: Double top plate on stud wall

(OP)
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?        

RE: Double top plate on stud wall

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

RE: Double top plate on stud wall

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.
 

RE: Double top plate on stud wall

(OP)
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.

RE: Double top plate on stud wall

(OP)
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.

RE: Double top plate on stud wall

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

RE: Double top plate on stud wall

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.

RE: Double top plate on stud wall

(OP)
It is two story residential and the floor joists are at the second floor.  I do have tile at bathroom and the bathroom is at the end of the joist so deflection is not that big there.

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

This is one of those situations where you must "pull out all the stops."  Is the double top plate failing in bending or in shear?  Have you used all of the multipliers allowed by the NDS?  For example, there is a rather generous multiplier for shear (2.0 as I recall) which I believe you can use.  Have you considered how much friction there will be between the two plates due to the imposed load?  This will help with composite action.  Have you calculated how much load the gypsum wall board can support?  It will be attached to the double top plate on each side of the wall and can take a small amount of gravity load.  How much overstressed is the double top plate?  10% or 100%?  Do everything you can to justify what has been built before you make the contractor tear down wall board.

DaveAtkins

RE: Double top plate on stud wall

(OP)
Thanks DaveAtkins.  I do not see split on the top plate but the contract did use long nail when he nail the OSB to the top plates.  This wall is also a shearwall so it has 1/2" OSB on one side.  How do you calculate how much load can the OSB take?  When I use uniform load (plf) to check the double top plate, it is okay in both bending and shear.  But when I consider each joist as a concentrate load on continuous beam, my shear stress can be 55% over ( bending is 5% so I call it is okay).  
 

RE: Double top plate on stud wall

I still dont understand why you cant have the joists line up.  It is standard practice!   

Never, but never question engineer's judgement

RE: Double top plate on stud wall

I think you have to bite the bullet, somewhere, somehow.
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

(OP)
You mean lining up the joist is standard practice for framing contractor?  If that is the case, we do not need to worry about the double top plate for gravity load at all!  From your experience, does the framing contractor always follow the standard practice?   

RE: Double top plate on stud wall

Let us review the situation.  Your studs are spaced at 16" centers.  Your joists are spaced at 12" centers (correct me if I am wrong).

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

check the cross grain compression on the wall plate, this wil probably now be critical.

RE: Double top plate on stud wall

The double plate should be the least of your concerns with your scenario.  The problems will come from the floor joists.  You will have some serious spring board action in your floor and the satisfaction of the owner will be in doubt.  You will want to get your hands on the joist software packages (free from the manufacturers).  Hard to get a feel for spans and performances when using the catalogs for design. I would never consider 14" depth for that kind of span.  Even 16" deep is pushing it.

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

ykmz,

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

That interior non bearing wall is a problem unless it is detailed correctly.  If that wall takes load, the I-Joists go into reverse bending putting the bottom flange in compression.  Those elements are not typically designed to be unbraced, so you could have a serios lateral torsional problem.  Detailing the joist over wall will require a slip joint vertically but provide lateral support to the wall.  Further, the drywall has to be detailed to slip, so the ceiling-wall joint wont be solid.  It will move.  Also, any trim attached to the wall can not be attached to the ceiling.

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

(OP)
Thank you everyone for all the inputs.  BAretired, my joist actually is 29' span plus 1' cantilever.  At end of cantilever there is one 10' nonbearing wall, which added some additional load to the joist.  Since it is cantilever, it helps the joist, but it does add load to the double plate.  The load I calculated is a little bigger than yours (936lb per joist end).  With 4" from end, the shear is 936*12/16=702lb.  Shear stress=1.5*702/3.5/3=100psi.  I never used load factor design for wood.  Where does it show in code?

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

Actually, vibration should get better when everything is installed and furniture is in place.  It's called "damping."

DaveAtkins

RE: Double top plate on stud wall

Keep in mind that I am not using the same code as you.  My code is CSA Standard 086-01 "Engineering Design in Wood".  We have been using Limit States Design for wood in Canada for quite a number of years now.  

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

A good carpenter will always align the joists with the studs if they are at the same spacing.  Unless it is a roof - then they will align the rafters with the studs and ceiling joists will be 1-1/2" off the studs.

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

Don't forget all the other things going on that will help your top plates that we don't account for.  Including additional strength from interior/exterior sheathing which will produce more composite beam action, etc.  There are lots of contributing components we ignore in residential construction.  

I agree, the truth is somewhere in the middle.

RE: Double top plate on stud wall

My question is. If this is outside wall, why isn't it a 2x6 stud and top plates?  

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Double top plate on stud wall

It's an interior wall with joists on one side.

Best regards,

BA

RE: Double top plate on stud wall

Do you really think a double top plate will fail under 14.5' of tributary floor load?

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.   

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