Form Deck

[SteelPE]

Recently I was asked to justify some load tables that can be found in manufacturer’s deck catalogs. In particular the client was requesting information in regards to the capacities of non composite deck (from deck). In digging through the various catalogs I have pretty much been able to recreate the tables (within a few psf). However, in order to generate the slab capacities shown in the catalogs I have to ignore the first bay which tends to have higher moments. They are using moment coefficients of 1/12 WL^2 for negative moments and 1/16Wl^2 for positive moments. From what I can tell the first bay should be subjected to a positive moment of 1/11Wl^2.

Has anyone noticed this and can anyone justify ignoring the first bay in their analysis? This just doesn’t happened across one mfr, but at least two so far and it may have something to do with the SDI which I don’t have.

 

[XR250]

What if they end up installing it as simple spans in some areas?

 

[SteelPE]

XR250

Well, that onus is on the EOR as the tables specifically state that they were developed using tripple span conditions and the EOR should reduce the loads otherwise (hence the questions). So, that's really your problem. I'm just wondering why they neglect the first bay positive moment in the charts?

 

[XR250]

hey, I'm not the EOR :>

I guess my question was more philosophical. We always specify 3 -span conditions, but it probably does not always occur especially if they are running out of material. Same goes for plywood roof or floor deck. The span ratings for that are based on a 2-span condition but I don't know anyone who puts that in their specs or checks that it occurs in the field.
Did not mean to distract you from your question which I do not know the answer to.

 

[SteelPE]

Sorry if I sounded harsh.

The point is, in this case, you would need to verify during the shop drawing review phase that the deck is indeed in tripple span, if not, change it, if you can't then calculate the capacity of the floor in single or double span. If the capacity is less than expected then mark up the drawings such that the system will work (increase the slab depth, add additional members, increase the reinforcing etc). All easy enough to do.

 

[DETstru]

My understanding is that the load capacity for 3-span deck is based on the two end spans, not the middle span.

If I'm taking a deck to near max capacity (for 3-span) I usually comb through the plans and try to figure out if there is a condition that would force a 2-span or single-span condition and be sure the deck still works.

 

[SteelPE]

DETstru,

That's now how the tables I have reviewed from two different manufacturers are calculated. They both ignore the end spans.

I usually don't push the deck to capacity as I know the placement of the reinforcing is never the best. In this instance I needed to verify how the charts were created so I could adjust some capacities. I was surprised how they ignored the end span condition.

 

[DETstru]

Just popped open my Vulcraft catalog and it seems like you're right. They just blanket the 1/16 factor for all positive moment capacities.

Of course they give themselves a nice CYA note:

Other conditions may require further analysis.​