Typical concrete floor slab thickness - over metal deck and bar joist?

[mdmiller]

I am curious what most people are using for a floor slab thickness when designing a non composite concrete floor poured over non composite metal deck and bar joists. I'm also curious to hear what most people are using for an on center bar joist spacing? There seems to be an unlimited number of combinations, but would like to use the most economical or most common. Floor joist spans are around 30 feet.

Parameters: Office (Live Load = 50 psf typical, and 80 psf at corridors, or 2000 lb concentrated load for office

Please specify Total thickness and how much of that is metal deck. (Example: 4" concrete slab thickness and 1-1/2" of that is metal deck)

Thank you!!

 

[slickdeals]

Your minimum slab thickness really depends on what kind of fire rating you require. For an office space, you require a 2 hour fire rating, which means you require a 4.5" NW concrete or 3.25" LW concrete over a 2" deck.

Just out of curiousity, why are you not using regular beams because vibrations can often be an issue on office spaces and I am not sure how good joists are in this regard.

 

[ARKeng]

My "typical starting point" is 5" total thickness with a W3 deck. Depending on architectural issues (often recesses) I might go to a W2. If I need some extra weight to help with vibrations I'll go with an extra 1" of concrete.

For joist spacing, I am usually in the 4.5' to 6' range.

The other reply mentioned vibration. We've used steel joists on floors successfully hundreds of times with not a single vibration complaint. We do run a vibration check on the SJI vibrate program though. Typically we wouldn't have any issues on 30' spans (problem range is typically 15' to 25'). As I mentioned above, increasing the weight or spreading out the joists can help.

 

[AHaddad1]

The first question you need to ask yourself is, what is the Fire Rating on the floor or Roof. Based on the fire rating requirement, then you can start picking and chosing. The second question is do you need to use Light Weight Concrete since the latter can improve the fire rating requirement.

It sounds like you are designing an office building; therefore, I will give some hints accordingly.

1) The typical fire rating is 2 Hours.

2) For 2 Hour FR, you can either use 3" or 2" Metal Deck with 3.25" Lightweight Concrete Fill (6.25" or 5.25" Total). , or 3" or 2" Metal Deck with 4.50" Normal Weight Concrete (7.5" or 6.50" Total).

3) Typical Construction is Steel Beams at the Floors (not Open Web Joists) and Open Web Joists or Steel Beams at the roof if there is no MEP penthouse.

4) for 30 foot x 30 foot bays at the floors, space your Beams 10 feet appart and stay with 3" 20 GA. metal deck, 3.25" Lightweight Concrete (Total 6.25"). I like to maintain 4.5Hz minimum natural frequency on the composite floor system. Design the Beams as Partially Composite and do not load the beam with too many studs to make it fully composite.

5) The Roof will not need concrete on metal deck if there are few Lightweight RTU's on the roof, in this case you may use Open Web Joists say 2 feet on center with a standard roof metal deck.

 

[mdmiller]

Thank you guys. I appreciate your comments!!