Max SDI Construction Span

[jcox]

My question is simply this. If SDI limits construction spans to a certain maximum span, is ther any practical application of a cold-formed steel deck beyond this span. For example: 20 ga B-deck 3 span condition can tolerate 53 psf for equal spans of 8'-6", but max SDI construction span limits you to 7'-9". The way I interpret this is that no matter what the capacity of the deck may be, you are never allowed to go beyond the max construction span. I am primarily talking about situations where the steel deck is the roof.

 

[TFL]

factory mutual has fire rating of roof deck. they typically limit their certification to 6'-6" for 1.5" 20 ga b-deck. this is what we use as a maximum i think because it impacts the insurance rates for building

 

[JAE]

The SDI construction span limits are based on 26 ksi max. stress and a max. deflection of L/240. If you want to use the 8'-6", then shoring during construction would have to be provided to utilize the capacity of the 8'-6" condition.

Most always, shoring is a higher expense than increasing the gage.

 

[UcfSE]

On page 10 of the Vulcraft steel deck catalog construction loads are shown that are used to compute spans for the construction loads given so that maximum deflection does not exceed L/240 and maximum stress does not exceed 26ksi. The constr. loads include point loads as well as uniform pressures. Maybe that's why the difference in the max span for construction and max span for uniform loading. It probably means that if you exceed the construction span then the contractor needs to shore the deck during construction. If you think you might have higher construction loads than those used to derive the spans in that column in the SDI table then you have to reduce that span accordingly.

 

[JedClampett]

I think you could exceed the construction span if you were to shore the roof. I'd advise against it. There might be some future work done on the roof where this span is exceeded.

 

[jcox]

Thanks you for your replys. Doesn't shoring only apply to situations where you are pouring concrete on top of your deck? If the steel deck is going to be your roof deck, then it seems to me that any loads listed beyond SDI cons't max is just there to nicely fill out a table, and is really meaningless for design purposes. Unless, of course, your willing to shore up the roof everytime there is a construction project.

 

[JAE]

No, shoring is a means to reduce stresses and deflections on loose deck during construction when 300 lb. Bubbas stand at midspan. Once the deck is fastened to the supports, and the design for the deck is then changed to a "uniform" load, the SDI criteria don't apply.

But to an extent, the tables that go way out there to large spans are probably not used that much for practical and cost reasons...but they may provide design information for other uses besides "roof" deck.

 

[UcfSE]

LOL @ 300 lb Bubbas

 

[Lutfi]

Hey JAE, do not forget the lunch boxes full of moon pies for Bubbas.

I thought I lighten things up.

Regards,

Lutfi

http://www.cdeco.com

 

[JAE]

I believe moon pie loading was dropped from the last ASCE7 after hours of tough, frazzled committee discussions.