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!

 

[ykmz]

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.

 

[DaveAtkins]

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

 

[ykmz]

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).

 

[COEngineeer]

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

Never, but never question engineer's judgement

 

[beton1]

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.

 

[ykmz]

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?

 

[BAretired]

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

 

[csd72]

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

 

[FSS]

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.

 

[BAretired]

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

 

[VTPE]

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.

 

[ykmz]

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?

 

[DaveAtkins]

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

DaveAtkins

 

[BAretired]

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

 

[enginerding]

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.

 

[JrStructuralEng]

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.

 

[dicksewerrat]

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

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[BAretired]

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

Best regards,

BA

 

[JrStructuralEng]

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.