non perpendicular shear walls

[kano23]

Hi everyone,

I've been asked to design a two story dwelling, but this one has not perpendicular (Orthogonal) walls, most of these walls has an inclination of 45 degrees in plant(floor). It is feasible to design this structure with wood shear walls or is advisable to use another seismic resistance system. In case of being walls of wood, it is necessary to analyze applying simultaneously 100 % of the seismic force in a sense and 30 % in other one??
What about seismic behavior??. Diaphragm is flexible.

Thank you for your help...

 

[wikidcool]

Sounds like you may already know this, but UBC (IBC does too I'm betting) has provisions for how to handle out of plane loading on shear walls......

"The requirement that orthogonal effects be considered may be satisfied by designing such elements for 100 percent of the prescribed design seismic forces in one direction plus 30 percent of the prescribed design seismic forces in the perpendicular direction. The combination requiring the greater component strength shall be used for design. Alternatively, the effects of the two orthogonal directions may be combined on a square root of the sum of the squares (SRSS) basis. When the SRSS method of combining directional effects is used, each term computed shall be assigned the sign that will result in the most conservative result."

It applies to all construction types. If 'most' of the walls are at 45 degrees, could you shift the axis to the walls instead of the outside of the building, and adjust the any wind loads, etc. for the non-perpendicular surface area? The results would prob. be a little conservative, but not too bad.

Earthquakes don't happen "in plane" anyway. We just design that way.

 

[csd72]

This would be workable if you have walls perpendicular to the 45 degree walls, if all walls are paralel then you will have a problem when the wind is perpendicualr to these walls.

Otherwise wikidcools comments are all good.

csd

 

[sdz]

Long answer required here.

Suppose the building sways 10mm at top of wall level. A wall at 45 deg will lean out of plane by 7mm and in plane by 7mm. Compared to an orthogonal wall with 10mm racking the racking force developed in plane will be less but by how much. Assume racking force proportional to displacement and you get 0.7 times.
This force is acting at 45 degrees to applied load and so 0.7 of it effective in resisting that load.
Total effectiveness compared to an orthogonal wall is 0.7x0.7=0.5.

Generalizing, the effectiveness of a non-orthogonal wall is cos(angle)^2.
Wall parallel to applied load; angle=0; effectiveness=1.0
Angle=30 deg; effectiveness=0.75
Angle=45 deg; effectiveness=0.5
Angle=60 deg; effectiveness=0.25
Angle=90 deg; effectiveness=0.0

 

[csd72]

You may also induce twisting or lateral sway forces due to the lateral component of the reaction.

csd

 

[Lion06]

Why not use only the walls that are orthogonal to each other as shearwalls? You can leave the other walls in place, just don't use them as shearwalls. Do you have enough shearwalls to be able to do that?