Diaphragm Question
Diaphragm Question
(OP)
I have a project in which I am designing precast concrete shear walls for loads given by an EOR. The only attachment of roof deck to walls is with a ledger angle at the wall only. No other collectors or drag struts are provided. I've created a simplified illustration for discussion purposes. The building is low seismic SDC "B"
The load the EOR has provided, divided by the length of wall (ledger angle) put the deck shear at double the allowable shear capacity (25000#/25ft = 1000plf >> 500plf NG!). I've been presented with the argument that the deck shear is actually the shear wall load divided by the total depth of the roof deck and the individual fasteners of deck to ledger can take the load, thus it is fine (25000#/75ft = 333plf < 500plf OK).
I have my opinion... what are your thoughts?

The load the EOR has provided, divided by the length of wall (ledger angle) put the deck shear at double the allowable shear capacity (25000#/25ft = 1000plf >> 500plf NG!). I've been presented with the argument that the deck shear is actually the shear wall load divided by the total depth of the roof deck and the individual fasteners of deck to ledger can take the load, thus it is fine (25000#/75ft = 333plf < 500plf OK).
I have my opinion... what are your thoughts?







RE: Diaphragm Question
RE: Diaphragm Question
However, without collectors in the (apparent) open areas, your collectors (and you DO need collectors) at the 25' wall need to be sufficient to transfer the total reaction.
RE: Diaphragm Question
RE: Diaphragm Question
With precast wall panels and a 75' span, i assume you have OWSJ with metal deck.
Can you clarify?
RE: Diaphragm Question
@BSVBD, I've simplified the geometry and problem down so it is not recognizable to an actual project. The main question I wanted to discuss was regarding the lack of collectors. I agree with the comments so far that in order to use the full deck depth, collectors would be needed. We are at an impasse with the EOR. This is just one of many problems we see...
RE: Diaphragm Question
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RE: Diaphragm Question
Mike McCann, PE, SE (WA)
RE: Diaphragm Question
My argument is there is a concentration of shear in the deck as the load "approaches" the shear wall. Some other failure mode will happen; the deck will buckle, the fasteners at the ends of the ledger angle will fail first, something...
RE: Diaphragm Question
You still need the ledger angle for the full length of 75'. Fastening the angle to the wall will, by default, serve as collectors, and then the angle will be the drag strut to transfer the load to the 25' shear wall.
I agree with the description by msquared48.
RE: Diaphragm Question
BA
RE: Diaphragm Question
RE: Diaphragm Question
RE: Diaphragm Question
1) It's either full depth diaphragm at the shear wall with a drag strut or it's a partial depth diaphragm just like you've proposed the numbers.
2) You can do a partial depth connection to the walls but it requires a) a high demand transfer diaphragm and b) some EW chord members as shown in the sketch below. If your EOR doesn't have these things, then he or she doesn't have a partial depth diaphragm no matter what their preference. In theory, with the deck oriented in the right direction, you could eliminate the EW struts and use the deck in in-plane bending instead. That involves a lot of deck detailing that likely isn't present however. And, in general, I simply dislike the concept.
Time to embark upon some EOR educating it seems. And yeah, if it's a 25' angle, it's a 25' angle sitting on top of a real framing member.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Diaphragm Question
BA
RE: Diaphragm Question
We are passing our concerns on to our client and recommending to the Owner they get a 3rd party peer review.
RE: Diaphragm Question
Mike McCann, PE, SE (WA)
RE: Diaphragm Question