Steel deck design

[LQQD]

Hi all,

Im designing a composite steel floor deck slab to the ANSI/SDI C-2011 for the first time. After reading the standard i searched for some design examples and found some from "CMC Joist & Deck" and other from "United Steel Deck". There are some major diferences between them. It seems the specification design methods are not those followed in the CMC ad USD examples.

According to the specification C-2011 there are 4 ways of designing the composite slab:

a) "Prequalified section method" Appendix 2
b) "Shear bond method" Appendix 4
c) Full sacale performance test
d) Other methods approved by the building official

I hace two major problems followind a) and b), i don't consider c) or d) an option right now. On applying method a) i would need embossments data wich i don´t have. On following method b), i should have Vt which is the tested shear bond resistance which, again, i don´t know. Neither of the "exaples" use this information, embossment geometry or the Tested shear bond resistance.

I think maybe the "examples" may be outdated and used a previous version of the standard, i tried to download the 2006 version from the SDI (Steel Deck Institute) site, but the email with the download link doesen´t arrive....

On the examples i noticed the design is simpler and has some similarities to what the specification requires. On the examples the allowable flexural strenght is determined by yielding of the steel deck before the composite behaviour, untill concrete gains resistance. For the composite behaviour there are two types of verification, the first should be used if no stud will be used and the second allows a grater deck strenght. The "first" verification limits the stress of the composite deck to the yielding stress taking into account the cracked section properties. The second verification allows full plastification of the composite deck and is intended to use with a number of studs to be determined. Between this two allowable resistance, a linear interpolations of number of studs can be made. There reason wich i am not comfrtable with is that it seems there is no calculation of shear strength between concrete and deck surface to assure a composite behaviour.

One other difference i noticed, was that the standard includes a load pattern which is not taken into account neither on the examples, or on a figure from the own specification showing criticall load distributtion pattern for different span configurations. The load which is not included is the Wcdl, it is a "uniform construction live load (combined with bare deck), not less than 50 psf", again i think it may be some new requirement to the new, 2011, specification.

For the moment i just need a rough estimate of the deck size, do you thik using the examples procedure and adding the Wcdl load would be ok?

I would be gratfull if anyone with some more experience could help me with this!

Thanks!

 

[KootK]

The usual process is to use capacity tables to determine deck capacity for uniform loads and, where needed, back calculate moment and shear capacities from that.

Are you trying to determine capacity for a deck product that is not supported by product design literature? Don't get me wrong, your efforts to calculate and understand deck capacity are admirable. And I'll be interested to see what you come up with. However, if you're just looking to specify deck for a project, the tabulated capacities are definitely the way to go from a design efficiency perspective.

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.

 

[JedClampett]

Reinforcing is cheap. Skip all the composite action effort and add reinforcing to carry the loads.
As an aside, I've never seen a project where we trusted the composite deck action. I'm sure there's great engineers who do, but I've never seen it.

 

[XR250]

@JedClampett:

You don't trust composite slabs when composite deck is used or when just using standard deck and counting on the bond?

 

[JedClampett]

Yes

 

[XR250]

Which one? - both I assume.

 

[JedClampett]

Both. I don't know if you've looked closely at what develops the composite action, but it's pretty shoddy. Basically they're shallow dimples in the deck at a regular spacing. I know it's been tested and put through the wringer, but I just don't trust it.

 

[XR250]

Fair enough. Have you ever heard of any failures occurring?

 

[LQQD]

Thank all very much. KootK, yes, i was trying to desgin the steel deck for spans grater than those which appear on the catalogue so i intended to investigate how it works. I think i have a trust issue on catalogues in general, that's also why i was doing my own numbers. Usually i design to steel profiles that don't appear on strength catalogues so im used to do my own calculations. Also, i have sometimes found error on catalogues and manuals, it is well known most of this type of documentes ussualy have "erratas". And the disclaimer stating that the values of the document dosen't exempt the engineer of doing his verification is enough for me to creep out and do my own verification.

More details on how the bond works can be found on the ANSI/SDI C-2011 specification, it is free and can be found on the SDI website. As Jed says it relies on embossments geometry. Haven´t found a single design manual or example on the web that takes explicitly this into account, maybe they are outdated, this freaks me out!

For the moment i think i will fall on "blindly" following a design manual, or catalogue, while i keep researching on the subject.

Thanks again!

 

[XR250]

I guess I need to lose sleep over the hundreds of composite decks I have specified

 

[KootK]

A while back I was looking to extend the application of a major composite deep deck product beyond the tabulated values. I called them and they said that the engineer who calculated the capacities had been on vacation in Australia for the last three years and that no one else on staff knew how to work out the capacities. I couldn't believe that they even told me that. I would have at least lied and said he was in the bathroom or something.

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.

 

[XR250]

Nice.
I bet all of these tables are based on some limited testing that was done by one agency. Maybe I will start adding belt and suspenders rebar.
FWIW, I never specify deck thicker than 18ga. I did 16ga once because the contractor did want to shore it. The deck barely had any dimple impression at all. I guess it was thick enough that the machine could not deform it adequately.