irregular home plan geometry

[tricalcim]

conventional light-weight construction of a wood 2-story house in california. in plan view the house has a stair stepped looking geometry (5 steps), and is about 80' wide and 40' deep, wind controls.

Q: what is a strategy for shear wall, chord force, and drag strut analysis. That is, would you break the house up into individual rectangles. Or??

 

[ERV]

Being that the governing code in California is still UBC 97 (CBC 2001, CCR Title 24 part 1), yes I would analyze it cut into individual boxes as much as possible. Basically all that is needed is drag struts from your "steps" to tie through the diaphragms (which would probably be considered rigid). I believe you may do the wind analysis based on ASCE 7-95 (simple).

 

[wcand]

I agree with ERV. I would lay this out on a grid in plan view. If the horizontal diaphragms are not considered to be rigid then loads must be distributed to adjacent shear walls from the flexible horizontal diaphragms. If there are no resisting elements to distribute to then transfer to the next load line with vertical shear elements. Also keep in mind the length to depth ratio for floor diaphragms is 3:1 {span to depth) to prevent excessive diaphragm forces and deflection.

 

[tricalcim]

ok thanks, so now can you help me take this a bit further - I'm gonna try to paint a picture here.
start at point a(0,0) and draw a line due north 45', this is point b. from here go 25' due east, point c. move 6' due north, point d, now 16' east, point e, from here continue north 46', point f, from here 24'west, point g, now 5' south to point h, and close by goinging 17' west - should end up where you started (0,0).

anyway, you'll notice that the strut from shearline c-d does not line-up with the wall line g-h. there is an 8' discontinunity in the chord at g-h and c-

 

[nevsam12]

cd and gh will not act as shear walls because their lengths are too small compared to the total shear wall length of ab, and ef in the North-South direction. This is basic stiffness priniples. Diaphragm will span full width from ab to ef.

If you prefer you can always design cd and gh as shear walls, but to guard against what I said above, I will design ab and ef for full load.

 

[AlohaBob]

The shearwall lines do not have to correspond with the exterior geometry. Perhaps you have interior lines that may serve as shear walls.

I think the typical residential analysis considers plywood diaphrams flexible and tributary area loads the shearwalls.