How does the 2nd floor bearing wall load be transfered to wall below? 4

[ykmz]

It is hard to understand some common details on residential construction. Here is one. Most roofs are built with roof trusses, which can have long span like over 20 or even 30 feet. The bearing wall take quite a big load. But when the floor joist underneath is parallel to the bearing wall, the wall only rest on rim joist which is just 1.25" wide. Although there usually has blocking at 32" or 48", that means the bottom plate will span that long. Unless you consider the wall work as a composite beam, I do not see how it works. Does anybody ever worry about this before?

 

[enginerding]

Did you actually check the stresses on the rim? If the truss can transfer the load at 24" on center through a 1-1/2" wide member, the 1-1/2" wide continuous rim should be able to take it just fine...

 

[ykmz]

The rim joist is fine if it takes all the load. But the 6" wall's center of gravity is off from the rim joist. It is not that straight forward for me to see the load go to rim joist instead.

 

[enginerding]

Take a ruler in your hand - hold it vertically. On the bottom, put one finger toward one end. At the top, put one finger in the middle - or even at the other end. The ruler is stable. The only condition that is required is that both the top and bottom be prevented from moving laterally.

Basically you will get a small couple from the vertical load eccentricity that will be counteracted by a couple in lateral loads at the top and bottom of the wall.

 

[mark1234]

There is a valid point to this question.

I have been contacted by another engineer trying to figure out why in a four story wood framed building the first floor walls are bowing out up to 3/4" in only the parallel conditions.

I haven't been out to the job site but suspect a combination of inadequate nailing, blocking, eccentricity, etc. has created this anomaly.

 

[enginerding]

It would bow inward if this eccentricity were the problem.

 

[DaveAtkins]

enginerding is correct. A rim joist can transfer the load from the wall above to the wall below--no problem. The tiny amount of bending moment induced by the eccentricity between center of stud and center of rim is nothing compared to the wind load moment on the stud.

DaveAtkins

 

[ykmz]

I have a feeling it is rather complicated. It has eccentricity and wall gets some restrain from top and bottom. The load is taken partially by rim joist and partially by bottom plate, or composite action of the wall. It is combination of everything. Usually the load in residential is small so there is hardly any problem be found. But using a simplified engineering analysis seems not enough to prove it works.

 

[csd72]

Enginerding,

I cannot see how the walls would bow inward if the rim joist is on the outside, it must bow outward surely?

 

[BAretired]

The top of the rim joist is laterally restrained by the floor deck which is (hopefully) nailed to it. The load from the roof comes down the stud. The stud is supported by the rim joist which is outside the center of stud. The eccentricity is resisted by an equal and opposite and very small horizontal force at roof and floor level to restore equilibrium to the system.

If the load is too large for the bearing area of the stud overlapping the rim joist, the designer is at liberty to add a second rim joist or some squash blocks under the studs.

Assuming that no such additional compression filler is added, the studs between the floor and roof will bow inward as a result of the eccentricity.

Best regards,

BA

 

[csd72]

I think we are getting our actions and reactions mixed up here.

With a two story wall, and the rim joist on the outside, the upper joist has a reaction which is inside the rim joist and hence wants to rotate the top inwards and the bottom outwards with the reactions opposing these forces. The bottom wall is the opposite of this with the top tending to rotate outward.

Hence with no reaction at the first floor the wall would tend to bow outward on this. Do your own freebody diagrams and you will se I am right.

 

[enginerding]

csd72-

I don't think you're talking about the same thing here. This post is referring to the walls that are parallel to the joist, not the walls supporting the joists.

 

[BAretired]

The attached sketch illustrates the situation. The upper stud is loaded centrally at top and on the outside at the bottom. This requires horizontal reactions, equal and opposite at roof and floor.

The lower stud is loaded eccentrically top and bottom. No horizontal reactions are required for equilibrium but a constant moment occurs through the lower stud.

Both upper and lower studs bow inward as a result of the eccentricity.

Best regards,

BA

 

[COEngineeer]

I always put blocking at 24" O.C. at wall parallel to the joists. So actually the rim and the blockings will transfer the load. I also specify LSL rim board. It is a good idea anyway to put a lot of blocking to transfer your lateral load to your floor diaphragm. Make your blocking 24" O.C.!

Never, but never question engineer's judgement

 

[JrStructuralEng]

I would suggest inward bowing as well.

 

[csd72]

I still fail to see how you could possibly get inward bowing in this situation. The second floor would want to bow outward between the blocking - simply by first year statics.

The eccentricity of the load would induce a clockwise rotation in the topwall and a counterclockwise rotation in the bottom wall.

If correctly fixed then the floor diaphragm would resist this.

 

[BAretired]

csd72,

I think we are talking about different things. I am talking about the studs bowing inward between second floor and roof and also between main and second floor. Please see the cross section in my previous post.

You are talking about the second floor bowing outward. I would agree with you if this is the only lateral force acting on the second floor. If the joists are aligned throughout the floor, the detail on the opposite wall is presumably the same. In that case, the floor experiences a slight tension but no bowing.

Best regards,

BA

 

[csd72]

okay, agreed.

This is why engineers talk better in sketches.

 

[mark1234]

When I posted my scenerio I think "bowing" was the wrong description.

The first floor walls at top plate have moved outwards up to 3/4". T.O. second floor is at zero, T.O. of first floor double top plate at 3/4".

This is a 4 story wood framed building, S.O.G., and only happening on the 1st floor.

 

[BAretired]

mark1234,

I tend to agree with your earlier assessment that the problem likely stems from inadequate nailing and blocking. It is a serious deficiency which must be corrected.

Best regards,

BA