Overturning moment on shear wall 2

[AlpineEngineer]

I have a theory question for you guys. As I look through some examples of shear wall calculations (stick built walls)I see that the in calculating the overturning moment the lateral force is applied at the top of the wall, thus assuming the top is unrestrained. Also, the IRC deflection eqns assume the top is unrestrained if I'm not mistaken. Unless you have a gable end situation it seems to me the truss connection to the top plate restrains the top of the wall and the lateral force should be applied at mid height which would result in a lesser overturning moment. Am I missing something?
Thanks in advance.

 

[csd72]

Alpineengineer,

This ass umption is basically correct, except the situation is more complicated than that.

The whole thing will behave as a T shaped frame with the truss forming the horizontal and the shear wall forming the vertical. The truss will give some bending resistance to the top of the shear wall but it will not be as muchas at the base. The effective load point will therefore be higher than mid height- an analysis would be required to determine exactly where.

You will also have an additional push/pull at the end of the truss from this moment.

Much easier to design it as cantilevered from the ground.

csd

 

[JKStruct]

I think it should be designed as a cantilever. The shear wall is loaded at the top by the trusses, so I don't know that the same truss loading the wall can be used to brace it in such a way that the load can be assumed to be applied at midheight.

I think this is somewhat analogous to the previous thread on this board regarding whether the compression leg of a shelf angle supporting brick can be assumed to be braced by the brick that's loading it. I believe it was JAE who said this is something like two drunks trying to hold each other up... haha.

Maybe I'm not visualizing the question entirely. I went to happy hour.

 

[DaveAtkins]

In theory, you are correct. But the moment at the top of the shear wall, which you are throwing back into the roof trusses, must go somewhere. Where?

DaveAtkins

 

[csd72]

Alpineengineer,

Sorry, I misread your post. The above comments apply to the end walls where there is a parallel gable truss over.

If your trusses are perpendicular to your shear wall there is no top fixity and the wall is a full height cantilever.

csd

 

[jike]

The diaphragm which is transfering lateral load thru the trusses or thru shear blocking to the top of the shear wall cannot then in turn restrain the wall (against shear in that plane). Like Dave said, the force has to go somewhare. You have to determine what is holding up what? You cannot have it both ways!

You might be confused by how much load actually gets tranferred to the shear wall. With wind load on the end wall, half goes up to the roof and half goes down to the foundation. The half that goes up to the roof then gets applied at the top of the shear wall. The OTM in the shear wall should be about the same as taking all the wind load at the half height of the shear wall but the shear would be double the actual amount. This all assumes a flat roof. With gable trusses, a larger percentage goes to the roof then to the foundation, but I have just used the flat roof for illustration purposes.

 

[Lion06]

The wall is only braced at the top for OUT-OF-PLANE loads, not in-plane loads. The reason for this is as others have stated but let me give my own take on it. The out-of-plane wind loads gets transferred to the ground and the horizontal diaphragm (floor or roof). The only reason it can transferred to the floor or roof is because the floor or roof is spanning between the shearwalls. The shearwalls are bracing the floor for the walls receiving out-of-plane loads, not the other way around.

 

[DaveAtkins]

I think what AlpineEngineer was asking is, can it be assumed that there is some fixity at the top of a wood shear wall, like there is at the top of a CMU shear wall (if you design for it). It's not typically done for wood shear walls--and so I think it would be quite a stretch to assume the top has fixity.

DaveAtkins

 

[Lion06]

DaveAtkins-
I am not sure I follow. Can you elaborate a little?

 

[DaveAtkins]

Envision the top of the shear wall moving laterally, but still fixed for "Z-Z moment." The point of inflection is at midheight of the shear wall, not at the top. The wall is in double curvature.

DaveAtkins

 

[csd72]

The answer to these types of questions is simple. If you can prove it is fixed, design it as fixed if you cant, then design it as pinned.

csd

 

[haynewp]

I just came across the same situation today with a shearwall that is only 1/4 the length of the truss it attaches to.

The sheathing will cover the outside of the truss at the exterior and I see that there is some stiffening of the shearwall by the truss. The truss is normally not designed to take this fixity but it will take some anyway, and it will eventually get transferred to the truss supports at each end. But I am not sure how to quantify the amount of stiffening since the truss may be more flexible for in-plane lateral than the shearwall depending on the specific situation.

So I guess the safest and easiest thing to do is put the force at the very top of the shearwall and not worry about having them design the truss for the moment from the top of the wall or having to figure out relative stiffness between the wall and truss. Though I guess if the truss is stiff enough it could overstress the nailing around the top perimeter of the shearwall due to the fixity that is not accounted for. But likely the same nailing is specified for the entire shearwall based on the higher shear and moment you calculated at the bottom of the wall.

But this is in reference to a shearwall that is not supporting the truss (ex. truss is supported by a beam at each end). If the truss were supported on the shearwall (bearing wall case) then counting on the truss to fix the top of the shearwall would be like picking yourself up off the ground by your shoe laces or something like that.