huangyhg

lateral force restraint through out-of-plane bending
i am designing a building that has a stiff flat roofed cmu box with multiple interior cmu shear walls. the adjoining bay is a higher sloped roof with almost zero lateral strength due to steel stud walls at each end with large openings. the sloped roof is long and slender with an aspect ratio of 4.5:1. the low roof is at 15'-6" and the high roof is at 20'-0".
i am looking at dumping the lateral force into the common 8" cmu wall through out-of-plane bending. is there any reason this would not be acceptable?
this project is an sds cat. d.

if grid 4' can take the bending and you put
i am surprised that i have only gotten one response to this post. was it due to being asked over the weekend? anyone care to comment on whether this is a good idea or not?
is there an r value for that?
it would be most similar to an inverted pendulum system. that would indicate an r of between 1 and 2.5. these system's typically have very low redundancy whereas this wall will provide almost continuous support. i am planning to use 2.5
i would use r=1.
can you give us some more information?
1. to clarify, are you proposing that the out of plane lateral force on the 30' high wall will travel through the sloped roof into the top of the 8" cmu common wall?
2. will the walls of the adjoining bay (framed w/ metal studs) perpendicular to the 8" cmu common wall be able to take/transfer the in-plane shear?

3. what about the lateral force in line w/ the 8" common wall? how will that be resisted? with a flexible diaphragm you'll have a substantial amount of in-plane shear in the 30' high end wall.
question #1 yes. i will be using the lh joists as collectors to transfer the seismic and wind loads to the top of the cmu. did i say omega o? ;>)
question #2 no. the walls perpendicular to the cmu bearing wall at each end of the sloped roof are all overhead door. there are 12" piers with a 16' x 16' door. they will just go for the ride. as an added problem the sloped roof has an aspect ratio of 4.5:1 so it is too long and skinny to work as a diaphragm even if the end walls had capacity. the simple truth is that the load is going to the cum wall on grid 4` regardless of how i try designing it because it is much stiffer. the wall to the adjoining bay is a clerestory wall with a high roof at 30' and a low roof at 20'. you can't get much better hinge.
question #3 grid 4` wall is designed as a special reinforced masonry shear wall. it has loads of in-plane capacity. the end wall of the 3'-4' bay will be balloon framed for out-of-plane loads but nothing for in-plane.

haynew, how do you justify an r of 1. ordinary plain concrete and urm are designed with 1.5.
plain concretee and urm are not even allowed in sdc c or worse. on second thought, i probably wouldn't even use out of plane bending on that cmu for lateral resistance in sdc d at all.
i find it strange that you would not consider using this continuous wall taking a relatively light seismic load when no one would think twice about designing a cantilevered rc slab with a wall load on the end of the cantilever. it is strange how we can be so comfortable with continuous large load and be scared spitless about a load with a 2%-50yr.load.
my cs is .268 w the roof dead load is 15 psf so 25' x 14' x 15psf x .268 = 1.41 kip per joist. with over-strength factor of 2.5 the collector load is 3.517 out-of-plane at 14 foot spaces along the wall. i ran a trial design and it looks like #5 @ 24" each face will carry it. am i missing something?

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