Where's the Diaphragm Chord?
Where's the Diaphragm Chord?
(OP)
I have a 160'x160', one-story warehouse bay with shear walls on the perimeter on 3 sides (the 4th side is a building expansion joint). The roof is an untopped steel deck. I have been able to check that the diaphragm meets acceptable deflection limits in the direction of one shear wall. My question is when the wind is blowing in the direction of the 2 walls, what is acting as the chord member of the diaphragm? And what force do I design the chord for? Typically, I would design the chord force for M/d, where d is the distance between extreme lateral elements, but there is only one lateral element in this case.






RE: Where's the Diaphragm Chord?
Usually chord members for a diaprhagm are steel members running along the edge of the diaphragm, either edge angles or beams.
RE: Where's the Diaphragm Chord?
As JAE alludes, it all depends on deflection compatibility of your diaphragm and the shear wall drift/defl ratios. There are code/spec limits on this relationship that determine whether you can assume a rigid or flexible diaphragm. If I guess from your description, you have CMU or tilt walls for shear walls, I doubt you will have such a compatibility. But I know engineers who neglect this and just model it as rigid.
RE: Where's the Diaphragm Chord?
RE: Where's the Diaphragm Chord?
BA
RE: Where's the Diaphragm Chord?
RE: Where's the Diaphragm Chord?
The chord on the open end will be the line of joists or the line of beams along that edge of the roof. If they are joists--make sure you put on the drawings that the joist supplier must design the joists for the diaphragm chord axial load, and develop a splice detail (a plate works nicely) to splice joist to joist. If they are beams, check them for interaction of gravity load bending and diaphragm chord axial load, and design the end connections for the prying that will be induced by the axial load. I would not be surprised if you need 5/8" thick connection angles on these beams, due to prying.
DaveAtkins
RE: Where's the Diaphragm Chord?
If you do get it to work, like Dave said, pay attention to your chord design. I like to use WF there so I can do the design and detailing and not put that on the joist mfr (axial plus uplift). Also, depending on if you have a canopy or storefront, a WF may come in handy there as well.
Let us know how it calcs out.
RE: Where's the Diaphragm Chord?
There are days when I wake up feeling like the dumbest man on the planet, then there are days when I confirm it.
RE: Where's the Diaphragm Chord?
There are many times when a building "wants" to be three sided (retail store with solid back and side walls and glass front) but these are usually quite long where the two side wall shearwalls are short and the eccentricity is small.
But in steellion's situation there is a 160 foot SQUARE building.
Even when considering a design using the three sides only, the second order effects on the building columns might get large and many engineeers don't take the time to check this - or don't know how to. The building will twist under lateral loading and all those leaning columns will add to the lateral force applied.
It just isn't something I would do at 160 x 160.
RE: Where's the Diaphragm Chord?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Where's the Diaphragm Chord?
RE: Where's the Diaphragm Chord?
There are days when I wake up feeling like the dumbest man on the planet, then there are days when I confirm it.
RE: Where's the Diaphragm Chord?
RE: Where's the Diaphragm Chord?
The torsion is 160*w*80. This is carried by the two walls perpendicular to the wind, so each carries 80*w (in opposite directions).
It should work, but I still wouldn't do it.
BA
RE: Where's the Diaphragm Chord?
RE: Where's the Diaphragm Chord?
Kieran
RE: Where's the Diaphragm Chord?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Where's the Diaphragm Chord?
RE: Where's the Diaphragm Chord?
DaveAtkins
RE: Where's the Diaphragm Chord?
That means the eccentricity of the c.g. of wind force to the shear center is greater than 80'. Hmmm...need to think on that for a while.
It doesn't seem like a good idea to leave this structure open on one side.
BA