Expert: Dan Griffin - 1/23/2012

Question
QUESTION: I have a project of building a 33" X 33" elevated concrete deck, 4" thick.  10" I Beam running north & south is on 11' centers, supported by 8" pipe. 8" purlin running east & west on 4" centers are welded to the I beam. 1 1/2" welded steel decking is the bottom form.  This will be an outdoor patio with minimal live load. This is your quote from your answer to a question on the same subject in 2008. "Another common method  places bar joists about 2' apart with welded steel decking creating a composite deck often with no additional steel".  Another knowledgeable builder has told me the same thing about rebar being unnecessary. Just trying to ease my mind because I always use rebar

ANSWER: Steve,
You indicate purlins on 4 inch centers.  I suspect you mean 4 feet.  You mention welded steel decking.  I assume you mean "F" deck or similar, not metal building sheeting - you would be well advised to use galvanized for an outdoor installation.  Remember that concrete weighs 4,000# per cubic yard, so each 3'x3'x.333' section of your 4" slab weighs just under 500# plus water plus weight of m(a/e)n working.  Respect the weight!

Rebar can do more harm than good.  Let's look at #4 bar - bottom bar 1/2", top bars 1/2".  If you could maintain the bar on chairs at center (not the proper location for the steel) you would have an 1 1/2 of concrete cover above and below which is the absolute minimum for exterior formed concrete.  If you want to reinfore, I would be more inclined to use "hog wire", a heavy welded version of remesh.  It is made in galvanized in 4 gauge and 6 gauge which are notably heavier than the rolled up 6x6 10/10 remesh which is technically NOT a reinforcement.  All reinforcement should be on chairs to control the install depth, preferably in the bottom 1/3 to aid concrete in tension, its weakest property.  4 gauge is smaller than 1/4".

I hope this helps you make your decision.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your response.  Installing the welded steel decking (inverted) with the ribs on the framing allows 2 1/2" of concrete above the wider dimension of the decking, in order to reduce weight. This sound OK? Also what about using "light" concrete and or fiberglass re-in forced concrete instead of the hog wire?

Answer
Steve, the 2 1/2" is typical.  Yes, I would put fiber mesh in the mix.  

If you are planning to pump, make sure you work out concrete details with the pumper and the concrete company.  Many pumpers use chip mix concrete with smaller aggregate.  Ask them for recommendations as they will have local experience and information.  My first reaction would be to stay with conventional concrete with air entrainment.  You might consider a plasticizers to help flow out and reduce the need for a vibrator.  Discuss having a water proofing agent in the concrete design mix.  Have a plan for curing - I would use Conspec 21.

Now all you need is strong backs.