Diaphragm Chord Force Transfer

[bvbuf]

I’ve got a 520’ x 270’ warehouse with CMU shear walls only on the sides (spaced 520’ apart). All roof framing members span front to back (with intermediate beams). The back of the building is continuous CMU but the front of the building is mostly storefront and has several offsets (as large as three feet). Consider wind blowing at the back of the building, what is the customary way to transfer diaphragm chord forces across the front of the building at the offsets? Do I ignore this? Do I add diagonals at the offsets? Do a add members parallel to the back wall that "span across" the offsets?

 

[JRESE]

Assuming that I understand the roof diaphragm geometry, and that you are talking about a plywood roof diaphragm, maybe it would be easy to just run a row of solid 4x blocking and a Simpson CS continuous coil strap across over the roof sheathing. A CMST12 is good for 9.235 kips. Only comes in 40' lengths, so you would have to lap. I generally specify a tight nail spacing at ends and laps, with a larger nail spacing in the field. I don't know why you couldn't have two or more rows of blocks and straps if needed. What I like about this solution is that the chord is at the diaphragm level where it belongs, not down at the bottom of roof framing level.

Regards,

Jim Emanuel, S.E.

 

[jike]

Have you considered the possible need for an expansion joint in the 520 foot length?

Regarding the offsets, if the outer most member is discontinuous (at offsets) then use the 1st continuous member across all offsets as the chord. The members outboard of the chord will just go along for the ride.

 

[lkjh345]

I have had a similiar problem.

I set a row of columns just inside the offsets, and ran joists between the columns. The joists were set on a cap plate to the column so that their ends were only seperated by a small gap, then anlges ( with vertical leg turned down)were welded accross the gap to the top chords of adjacent joist shoes. This made a continous chord.

I used a joist loading diagraphm to give the joist supplier the necessary chord forces he needed to design the joists for.

 

[bvbuf]

Thanks everybody. Just to clarify - the roof is steel beam, bar joist and metal deck. And I do have an expansion joint.

 

[DaveAtkins]

If you don't have a continuous chord member behind the offsets, I would add angles between the joists to create a continuous chord.

DaveAtkins