Lateral Load Distribution To Non-orthogonal Shear Walls For Flexible Diaphragm

[lacyma1]

I am running into an issue analyzing a structure that will have non-orthogonal shear walls. I figure that for a rigid diaphragm I could use the projected length of my walls in my primary directions to approximate their stiffness and distribute my loads that way, but for a flexible diaphragm where load distribution is based on the tributary area of my lateral force resisting elements I don't know how to approach the issue. What is the best method for finding the tributary areas of my shear walls?

Thank you

 

[hardbutmild]

You could find the mid point between the ends of the two neighbouring walls and their beginnings. The line connecting those two midpoints is the edge of a tributary area. Repeat this for all of the walls.
I think this is good enough in most cases (if I'm wrong I'm sure someone will correct me).
Don't forget to take all of the loads into account.

 

[BAretired]

It really is a judgment call. When in doubt, be conservative.

BA

 

[DaveAtkins]

Keep in mind that a shear wall can only resist diaphragm shear parallel to itself. So, when the non-orthogonal shear walls resist diaphragm shear, they will introduce two force components into the diaphragm. You need to account for what this does to the diaphragm.

DaveAtkins

 

[lacyma1]

So if I'm understanding this right:

If a load comes in at any angle other than parallel to a shear wall, that shear wall will resist a portion of the load based on the angle the force comes in at. We resolve the incoming force into a force parallel to the shear wall in question and a force perpendicular to that shear wall. The force perpendicular to that shear wall will need to be resolved by shear walls with different orientations.

Is this baked into the ASCE 7 requirement for simultaneously applying 100% of the seismic load in one direction and 30% perpendicular? Or should the component forces from a skew wall be applied in addition to that?

 

[JoshPlumSE]

Is this baked into the ASCE 7 requirement for simultaneously applying 100% of the seismic load in one direction and 30% perpendicular? Or should the component forces from a skew wall be applied in addition to that?

No, this isn't related to the direction of the earthquake. It's just a statics thing. The shear wall can resist shear, but can't usually resist force perpendicular to it. Therefore, the shear in the wall is larger than you think because of the skew....

 

[Deker]

For a quick and dirty design use the shear wall midpoint when determining diaphragm tributary areas. For diaphragm design, rather than assume the reaction occurs as a point load at the midpoint of the skewed wall, treat the reaction as a distributed load over the projected orthogonal length of the wall. This method (or any other traditional flexible diaphragm design method) may NOT be conservative depending on the skew of the wall.

In addition to the advice given by others above, see thread507-426571.

 

[HTURKAK]

I will suggest the structural analysis which consider the relative stiffnesses of diaphragms and the vertical
elements of the SFRS. You may model the diaphragm with FEM and set one of the orthogonal axis parallel to the one of the walls.

ATtached the document ;An alternative procedure FEMA P-1026 (Seismic Design of Rigid Wall-Flexible Diaphragm Buildings)

 

[DaveAtkins]

For example, if the shear wall is at a 45 degree angle to the direction of the applied force from the diaphragm, the force in the shear wall will be 1.414 times the applied force. As JoshPlumSE stated, this is to satisfy statics, and has nothing to do with seismic code requirements.

DaveAtkins