Use of composite deck under a roadway 5

[VA-Struct-Engr]

Hello all!
I am working on a project where a raodway will be built on top of our composite steel framing. Adequate waterproofing will be provided between the concrete over metal deck and gravel/asphalt etc above. We will design the concrete metal deck slab as multi-span continuous slab, i.e. top bars at each steel beam (roughly 10ft apart). We are debating if the composite deck should be used as the positive reinforcement for the slab or should we provide bottom bars and use the deck as a form deck. We will specify G90 galvanized deck for durability per AISC design guide 18 recommendations. I am wondering what other engineers have done in similar situations. Floor below (basement level) is used for parking.
Additionally, should I be concerned that vibrations from vehicles may loosen the bond between the composite deck and concrete over time if deck is used as positive reinforcement? Has anyone come across this before?

Thank you for your thoughts in advance.

 

[jayrod12]

Use the deck as form deck. Do not use it as the positive reinforcement. Even galvanized steel corrodes in time. Can't remember how often we've seen fully rotten galvanized deck. This is usually in parkade stairwells that are protected from the elements but have road salts and moisture getting tracked in on shoes.

 

[dik]

Most deck manufacturers don't recommend composite deck for moving loads.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[r13]

Agree with both comments. Vibration itself may not harm the composite deck, but the cause of it may - high frequency of repeat stress reversal causing fatigue of the studs.

 

[VA-Struct-Engr]

@r13, are you saying that we should not design the steel beams as composite beams because of vibrations? Could you please elaborate?

 

[r13]

You can, but have to keep the stress low, means a much stiffer structure is required than would be if it subjects to static load only.

 

[jittles]

Agree with others to use deck as form-only.

I am not sure what r13's reference is, but I'd be curious to know. I imagine AASHTO has guidelines for design of composite steel beams and not just a prohibition of using them.

 

[r13]

jittles,

You can, ....

I don't know where the notion of "prohibition" came from, please address the source.

 

[jittles]

I perhaps incorrectly extrapolated from your first comment that it could harm the deck somehow.

Not a prohibition but just limitations on the use? Apologies.

 

[r13]

Not a problem. You should consider fatigue on the anchorage, and the deck system for cyclic loading effect.

 

[STrctPono]

So this is essentially a bridge?

I don't know if we have any VA bridge engineers on this site but every state is different. We don't use stay-in place forms for our bridge decks but I know lots of States that do. I am not aware of any State that allows the form to be used for positive reinforcement capacity for the reasons outlined by dik. However, reading through AASHTO, I don't see anywhere where they explicitly forbid it either. My opinion would be to add additional reinforcing in the deck to resist the live load positive moment. The argument for not allowing any stay-in place deck formwork is because it hampers inspection work and doesn't allow the agencies to properly assess any damage and spalls in the concrete soffit.

AASHTO certainly does not discourage the use of composite girder design... quite the opposite they strongly discourage the design of non-composite girders.

You should not rely on stay-in place deck forms to provide adequate bracing to girder top flange prior to deck hardening.