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# Metal Deck Diaphragm Shear Strength Question

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## Metal Deck Diaphragm Shear Strength Question

(OP)
If you are designing a metal deck diaphragm for lateral loading perpendicular to the deck flutes, what joist spacing do you use for SDI's tables? Would it be the deck span spacing (5'-0" in my example image) or 30' in my example image? Thanks.

### RE: Metal Deck Diaphragm Shear Strength Question

Its the spacing of the deck supports. 5'-0". This is independent of load application direction. Basically you have fasteners along each of those supports, thats whats giving your diaphragm strength.

### RE: Metal Deck Diaphragm Shear Strength Question

In addition, if your deck experiences net uplift due to wind, there will be combined loading on your deck fasteners that will impact your capacities. In this situation you can use one of many deck diaphragm calculators that are found online.

The manufacturer Vulcraft has a good one for free.

https://vulcraft.com/resources/designtools

### RE: Metal Deck Diaphragm Shear Strength Question

#### Quote (driftLimiter)

This is independent of load application direction.
This is absolutely correct.

Shear has no idea which direction it's going. Totally clueless.

### RE: Metal Deck Diaphragm Shear Strength Question

(OP)
So for a diaphragm 3'x80' in plan dimensions, where the deck spans the 3': Would you use 3' joist span in the tables for calculating the diaphragm shear strength?

### RE: Metal Deck Diaphragm Shear Strength Question

Sounds like a theoretical diaphragm... Yes, the 3 ft would be the span of the deck.

I don't have the code section memorized, but you should maintain some sort of acceptable aspect ratio on the diaphragm. 3'x80' is going to have some deflection issues.

Now you mention 3' joist spacing. Which tables are you using because the 'diaphragm shear strength' tables don't have joist spacing as the criteria. At least the tables that I have don't.

### RE: Metal Deck Diaphragm Shear Strength Question

The key to understanding this is to recognize that the members supporting the deck need to restrain a shear buckling mode of failure as shown below. Every support, with appropriate fastening, reduces the length of the potential buckling wave and, therefore, increases capacity with respect to shear buckling. Translating that to your specific questions:

#### Quote (OP)

Would it be the deck span spacing (5'-0" in my example image) or 30' in my example image?

The 5' deck span.

#### Quote (OP)

So for a diaphragm 3'x80' in plan dimensions, where the deck spans the 3': Would you use 3' joist span in the tables for calculating the diaphragm shear strength?

Yup.

Note that:

1) By the book, corrugated deck cannot resist axial loads perpendicular to the flutes that would fold it up accordion style. You have to distribute those loads out to discrete diaphragm framing members somehow.

2) You can deliver axial loads to the deck parallel to the flutes. And folks often do. This probably does create some degree of interaction between axial load buckling and shear buckling but, to my knowledge, that is not commonly considered and we don't have a robust means of checking it.

As with most things in structural engineering... one does what one can.

### RE: Metal Deck Diaphragm Shear Strength Question

Kootk, would you be willing to share the source for those photos? Appears like it could be an interesting read.

### RE: Metal Deck Diaphragm Shear Strength Question

KootK can you tell me where you got those photos?

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