Overturning moment on shear wall
Overturning moment on shear wall
(OP)
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.
Thanks in advance.






RE: Overturning moment on shear wall
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
RE: Overturning moment on shear wall
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.
RE: Overturning moment on shear wall
DaveAtkins
RE: Overturning moment on shear wall
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
RE: Overturning moment on shear wall
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.
RE: Overturning moment on shear wall
RE: Overturning moment on shear wall
DaveAtkins
RE: Overturning moment on shear wall
I am not sure I follow. Can you elaborate a little?
RE: Overturning moment on shear wall
DaveAtkins
RE: Overturning moment on shear wall
csd
RE: Overturning moment on shear wall
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.