Decking Diaphragm Question

[SteelPE]

I am currently trying to rough out the design of a small single-story retail building. I am check the diaphragm and I am calculating a demand of roughly 850 plf (allowable) which is quite high. Currently, due to loading issues, I have open-web steel joists spaced at 3'-0"+/- o.c. I opened up my old Vulcraft deck Catalog and can see that an 1 1/2" 18 ga deck has the capacity of 983 plf (using 5/8" dia puddle welds and 2-#10 TEK fasteners per span).

All is fine.... and I am not quite comfortable stressing the diaphragm this much (even though it will work).

So, on a whim, I think my information is old and I go to the manufacturer website to figure out if this information has been updated.... I find that there is now a calculator for deck diaphragm capacity and I run my deck through the calculator and it returns a capacity of 1495 plf

I am not sure why there is such a drastic change in capacity. The only thing that pops up in my mind is that is seems like they are using decking material that has Fy= 50ksi vs the old decking that used to be Fy= 33ksi. Is there such a reason for this drastic change? I just want to make sure I am comparing apples to apples here.

 

[XR250]

That would be my guess. I ran into the same issue with un-shored spans etc on composite deck.

 

[lexpatrie]

Somewhere in the thickness the Fy changes, for one. I think the thicker stuff tended to have the lower yield stress (33 ksi?) back then.

If the older document is old enough it may be for a different or higher factor of safety, (2.35 in the upper one), there's the 1/3 stress increase potentially in there as well, and the new table might be nominal (without a factor of safety) so it's for both ASD/LRFD.

 

[SteelPE]

From what I can tell, for 18ga, the steel thickness did not change (t = 0.0474 inches). So I don't think that is the issue.

The second table lists ASD, so we appear to be using the same methodology (not an ASD vs LRFD thing).

I am not sure what the FOS is in the second chart as I find the detailed calculation a bit confusing.

I have an email into a contact at the manufacturer.... but I don't know if they will give me a response or not.

 

[bones206]

The newer table states in the header that the values include the safety factor, i.e., divided by omega.

The ratio of allowable shears is almost identical to the ratio of 50/33.

 

[SteelPE]

So the mfr is saying the increase in capacity is due to the increase in the yield strength of the material........ makes me now wonder what other manufacturers are using for their yield strength. I don't want to rely on these numbers only to find some other manufacturer is using a lower yield...

 

[bones206]

I call out the yield strength along with the gauge and profile on the plans.

 

[sandman21]

So the mfr is saying the increase in capacity is due to the increase in the yield strength of the material........ makes me now wonder what other manufacturers are using for their yield strength. I don't want to rely on these numbers only to find some other manufacturer is using a lower yield...

50ksi has been for a while. You would be specing the type of product used you should give them a fy if you are leaving it generic not a specifying a product.