Diaphragm Question 1

[PMR06]

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?

 

[mike20793]

If there are no clearly detailed collectors, then I agree with your shear calculation; but if that ledger angle is intended to act as a collector (which I think it is and they really do need a collector) or the EOR intended the adjacent wall panels to behave like a collector (he should have indicated), then the shear will be the 333 plf. This is one area that I find many engineers lack clarity on their drawings, especially since collectors/struts require special inspection in many cases.

 

[BSVBD]

I consider that the deck shear should be the shear reaction divided by the total depth of uninterrupted deck. You also need to consider Diaphragm Aspect Ratios, which, it appears you are approaching a questionable threshold, in my opinion.

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.

 

[sandman21]

You would need to have collectors on each side of the shear wall to use the full diaphragm depth. The way your case works is outside the connection from the deck to wall ledger the deck shear see the full diaphragm depth since you are only using the the wall length shear load will be transferred to the wall within that section.

 

[BSVBD]

What type of roof deck do you have and how is it supported?

With precast wall panels and a 75' span, i assume you have OWSJ with metal deck.

Can you clarify?

 

[PMR06]

Thanks for the comments so far.

@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...

 

[JAE]

I agree you need collectors to "collect" the shear over 75 ft. vs. 25 ft.

Check out Eng-Tips Forum's Policies here:
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[msquared48]

But it sounds to me that the ledger angle is functioning as the collector since the metal deck is welded to it. You just have to provide an adequate attachment of the angle to the 25 foot shear wall to develop the shear force, as well as maintain the continuity of the angle.

Mike McCann, PE, SE (WA)

 

[PMR06]

Lets say the ledger angle is 25ft long and continuous, and the strength of all the individual fasteners from roof deck to that angle can transfer the load shown (25kips). Outside that, the framing is for gravity only with no connections intended to use the gravity framing as collectors. The gravity framing is spaced, and side lap fasteners provided such that the assembly provides the allowable capacity shown.

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...

 

[BSVBD]

I deleted my own post since i did not accurately read that there was a ledger angle supporting the metal deck.

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.

 

[BAretired]

Continuity of the angle is important and should be stated as a requirement on the drawing.

BA

 

[BSVBD]

If you need to, read my deleted post. Then ADD a steel joist in lieu of the ledger angle, and design the collectors as i stated in the deleted post.

 

[jayrod12]

Does anyone else think an angle spanning 25 ft is suspicious as well?

 

[KootK]

My thoughts:

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.

 

[BAretired]

Agree with jayrod. An angle spanning 25' is suspicious.

BA

 

[PMR06]

@Jayrod12 & BAretired, the 25ft ledger angle in my example would be welded to embed plates in our precast wall 4ft oc max. Outside that would be gravity framing only, with no other collector elements dragging load into the walls.

We are passing our concerns on to our client and recommending to the Owner they get a 3rd party peer review.

 

[msquared48]

Glad you have an angle on how to solve your problem.

Mike McCann, PE, SE (WA)

 

[PMR06]

@msquared, a well crafted pun definitely deserves a star!