How do you pick your metal floor deck? 1

[StructEngineer90]

At my previous company, we had always specified 3" composite floor decks in the typical office type buildings. As a young engineer, I never thought to question why 1 1/2" decks were never used.

At my current company, one the engineers has always specified 1 1/2" composite metal decks and only went up to 3" for unique cases. I've asked them why they specify the way they do and their response is basically "that's always the way I've done it".

Another engineer there never specifies composite metal decks and typically defaults to 1 1/2" metal form decks. He basically told me the same thing for his reasons and added that he doesn't trust the composite action of the deck and slab.

The discrepancy is kind of driving me a little crazy. Is this just kind of a case where there is no real "right" answer? Or are there rules of thumb for each type of deck?

 

[jdcollins]

There is definitely some engineering judgement/preference going on, and it can be for any number of reasons. However, unshored/shored construction, general loading requirements, vibration attenuation, and getting your required fire rating are all the typical items that need to be taken into account.

Sometimes 3" deck with 3" topping is sufficient. Other times it makes sense to use 1-1/2" deck with 4-1/2" topping.

 

[r13]

Each of the above have a cost differential for all design parameters hold the same. Usually you select the thickness based on the strength/deflection table from the manufacture, and select the most economical one. But after certain amount of practices, you might notice that either the 3", or the 1 1/2" deck fits most of your projects, then you'll grow use to it without spend time to evaluate. Nothing really wrong though. Unless very light construction, try to avoid form deck, which tends to have more problem later on.

 

[calvinandhobbes10]

I often use 2” for composite.
If no fire rating, start with 4.5” total. If fire rating, use whatever I need to achieve.
1.5” usually too many beams for my liking, 3” only if the deck span is right to counter the extra concrete.
Agree its a gut-feel decision.

 

[JedClampett]

Randomly pick one that works.
What's life without a little whimsy?

 

[milkshakelake]

I tend to always use 2" to 3" composite decks because it will increase spacing between beams. Fewer beams = fewer connections = cheaper structure.

For cold-formed steel joists, I use 9/16" decks because joist spacing is usually 16".

 

[KootK]

1) This seems to have more to do with the perception of what generates the best economy more so than what actually does.

2) When I stopped doing work in WI in 2007, it seemed to be mostly 1.5" composite.

3) When I resumed doing work in WI in 2016, it seems to be mostly 3" composite.

4) In Alberta, it seems to be mostly 1.5" deck.

5) In addition to the economy of having fewer beams and fewer beam connections, my sense is that the shift towards 3" decks may be, in part, a result of engineers paying a bit more numerical attention to vibration issues than was the case in the past. The extra deck weight can help with that a fair bit.

6) I did an exercise in replicating an existing project a few years back that led to a 3", non-composite deck on shallow OWSJ at 32" oc. It seemed as though every damn plumbing riser landed on top of an OWSJ. Never again.

7) From an intelligent project management perspective, I think that a robust strategy is:

a) When in Rome...

b) If you've got project team members who can validate costing, show them stuff early and often.

c) Show your client, and the team, more confidence in your choices than you may actually feel. Clients tend to react well to this and it will slow down the rate at which you'll receive frivolous requests for rework.