Steel deck span condition

[thoughtofthis]

Steel roof and floor deck capacities are typically presented as a function of span condition. Most calculations I've seen assume a 3 span condition to get the most out of the deck; however, if you don't have supports in multiples of 4, wouldn't you be required to design some of the deck to a double or single span condition? When that is the case, how do you convey this in the contract documents?

 

[BSVBD]

I often design metal deck by applying the design loads, to a simple or single-span condition, to determine the required Sx and then choose the deck based on the given Sx/Sp/Sn of the proprietary, SDI endorsed load chart and/or section properties. Simple span is certainly conservative, but, for simplicity, it satisfies most of my concerns.

You should design the metal deck for all applicable conditions. If you have simple-span conditions, then "Yes", design it for simple-span.

The contract documents should provide an explicit framing plan that would show all span conditions for the metal deck to be furnished.

If you feel that either your spec book or your General Structural Notes sheet does not provide sufficient information to draw necessary attention to what you want understood, i will often provide an additional note or specification within a box, near enough to the framing plan to clarify a special condition.

Is there something else you wish to convey that i may not have covered?

 

[KootK]

My experience is less encouraging. I've seen three span condition specified on pretty much every drawing, ever. And that would seem to be regardless of whether or not it's mathematically possible to have three span everywhere. I've only seem one, very special, project where the EOR drawings actually showed the start/stop locations for the deck.

On occasion, I've specified three span for the first sheet installed from the edge of the roof and two span elsewhere. Usually, I'm wanting the three span for a governing condition that includes snow drift.

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.

 

[thoughtofthis]

Thanks for your responses. My experience has been very similar to KootK. I've never seen anyone specify a condition other than 3 span even when the number of supports don't add up. I guess the underlying question is why does that need to be on the drawings? My only reference is that span condition is a parameter in MasterSpec and I've seen engineers put it on the drawings.

 

[jayrod12]

I will typically design around the 3 span condition. I will make sure that one of the gauges of deck will work in the one span condition but I won't specify the different gauge on the drawings. The steel deck is sized in the shop drawings and it is that engineer's responsibility to design it in the end.

 

[SlideRuleEra]

The use of only 1, 2 or 3 span conditions for steel decking is a legacy from pre-software times.

Of course single span is conservative.

Two spans, less conservative.

Three spans represents the best (most optimistic) case. Three spans was considered representative of any additional number of spans, say 4, 5, 6, or more. Precise (hand) calculations for 4 or more spans was not considered necessary for engineering purposes - 3 span numbers were good enough. This is not an unreasonable assumption - compare shear & moment results of 3 span and 4 span conditions - not much difference.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[XR250]

Sounds like we should design for 2 and specify 3

 

[BSVBD]

Despite my previous reply, (which was intended to provide an answer to the OP two questions), I rarely specify anything other than 3-span requirements unless the framing plan explicitly shows a non-negotiable, simple-span condition. I then design accordingly.

Near the framing plan, I typically show only the deck profile, with a simple spec, (i.e. 1.5B22), with number of fasteners and layout.

With a satisfactory framing plan, it should be understood that i/we/the engineer has (at least) considered all scenarios when providing the depth, profile and gage.

 

[spats]

I usually design for 2-span, and specify that single span conditions are not acceptable in the specs/drawing notes. I also review the deck layout shop drawings (specified submittal) and make sure the layout does not contain any single span conditions, which it almost never does.

 

[DETstru]

Usually I choose my deck via the tables using 2-span. Then I go through my plans to see if there are any locations that would end up being simple span. I check those separately to determine if my gauge is still ok based on the individual load cases. If I find many instances cause me to up the gauge, I up the whole thing (only happened once).