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Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

(OP)
HI All

I have a circular suspended slab (23meters in diameter). The structure underneath is to be steel beam framing with metal deck over. The secondary beams are 90 degrees to the metal deck form. The primary beams are on the edges and the secondary beams frame into these. All top flanges are at the same level.

My analysis shows that top reo only is needed because the metal deck acts as bottom reo.

My question: How do I orientate the top reo?? I have shown a way on the attached(not sure if correct or practical).

I also have seen reo placed radially (for example water tank slab)...but I don't think it will work in this case since the metal deck profile is not radial - I could be wrong though!

My attached solution would mean that I would calculate the top reinforcement as a vectorial component.

Any ideas are welcome.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

I think the way you show is practical, and the only solution.

The vector components make sense.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

The circle perimeter is a beam, correct? Shouldn't it be straight segments, to minimize deck overhang (cantilever)?

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

There's been discussions on the composite action of metal deck on these forums and most of us don't trust it. I'd put in enough bottom reinforcing to carry your load. The metal deck people will tell you it acts compositely, but if you examine what they're counting on, it's pretty weak.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

I would use a heavy enough gauge of deck to reduce the reliance on bottom rebar, and more beams to reduce the span of deck. I didn't see the 23 meter diameter initially.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

(OP)
HI AELLC and JedClampett

Just to confirm there are edge beams on the perimeter ( which are not shown on my sketch). The overall diameter is 23m.

Not sure if I understand the "bottom bar" comments being made? The metal deck that I am using has ample capacity to do the job. Not that in my sketch the ribs are shown in black and the top mat of reo in red. My only concern is the top mat of reo.

Even if I do the vectorial sum method for the "As" in the top...how does one know that the angled bars are able to go back 0.3L ( which is normally the case for top reo. That is, As you have probably done in the past, negative reo( on the top) goes back 0.3L. If the bars are angled relative to the span...how do you get the 0.3L???

regards

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

(OP)
Or what about actiually having isolation between each of the 6 internal edges such that the top mat does not go thru to the "next" section. And kepping the top reo parallel with the ribs but stopping at the main primary beams.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

Jed - not sure where you are coming from on doubting whether composite metal deck provides adequate strength.

I've seen many of these deck/slab assemblies tested in a lab and they all perform very well. In fact the variability/scatter of results is
about the same as shear failure in concrete beams.

Now using smooth formdeck -I would agree with you - don't count on natural bond between metal and concrete.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

Where I started, 35 years ago, we used composite deck (called in those days, Robertson Deck), but never counted on its reinforcing value. This was design for the power generating industy, which might be more conservative. I thought there were some like minds on this forum, but I couldn't find any threads about it.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

geopat69,

I would keep the top bars in orthogonal directions as you have shown. Much simpler to place that way. Where the bars are parallel to the deck, only one layer will work flexurally, so figure your "d" based on the lowest top bar. Where they are at 45 degrees to the deck, both work, with different effective depths. Don't separate it as suggested in your latest post. Too complicated, and losing continuity is not a good thing.

By the way, I would agree with Jed about the use of composite deck if this is in an environment where corrosion will be an issue, now or in the future. When composite deck corrodes substantially, there is usually no option but to demolish and reconstruct.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

hokie66 - yes I forgot to mention another condition where I would agree with Jed and not depend on composite metal decking with the concrete slabs - those conditions you cite where deterioration is a major probability.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

(OP)
hokie66

thanks mate. So essential work out the components of the top steel when not parallel to the deck.( which is what I first thought).

My only worry with that is the 0.3L fo rthe negative moment. This usually for steel in line with the moment diagram! now the steel is at an angle ( so does that mean more than 0.3L)?. Maybe it doesnt matter because I would be developing the bar at an angle instead of in line.


RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

Surely you wouldn't cut bars for this. Provide a continuous top mat in both directions. You need it in the opposite direction for crack control anyway.

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

(OP)
HI hokie66

I did not explain it very well. The mat would be continuous in both directions. But if the mat is inclined to the metal deck, to comply with the 0.3 L requirement, this would not be achievable because the continuous bars are at an angle to the "0.3L" direction. i.e the 0.3L measured is in the direction of ribs...but the bars are at an angle to this.

I do personally think that it is largely irrelevant because the "0.3L" just needs to be satisfied in any direction and not necessarily in the direction of span of the deck.


Did that make sense?

RE: Urgent Help! Reinforcing a Circular Slab with Metal Deck Form.

Your top bars will always extend more than 0.3L from the support centreline, because they are continuous. Remember, when the bars are inclined to the support, you have two of them, at two different angles. Added together, they work.

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