6x6 welded wire mesh as "chairs"

[jmims912]

I'm interested in input concerning a reinforcing design as follows:

The reinforcing steel in the topping slab of a double T precast cambered deck is has transitioned through several changes and is final at:

#5 steel bar spaced at 30" over 6x6 wwr mesh resting directly on the deck. Topping slab will be approx 4" at the walls and (due to cambered deck) 2 1/2" at centre of 80' span. I'm just not sure of 1) mesh at what is basicly a cold joint and 2) coverage of #5 bar.

Any thoughts, Yall?

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[civilperson]

Steel resting on the surface before topping is the surest method of knowing the vertical location for that steel. A low/medium frequency compressed air driven vibrator made out of a series of closely spaced pipes will assure good consolidation when used on leveled/placed mix. A chair would be required only if you desire a higher location of the steel in the topping.

 

[jike]

If the mesh is on the deck, it sounds like you intend the contractor to pull it up after pouring (although the topping slab is quite thin). I would not count on knowing exactly where it winds up.

What is the purpose of the #5 bar? If it is supposed to be a support bar, shouldn't it be under the mesh (or did I mis-understand)?

The mesh should run thru the construction joint and lap with the mesh in the adjacent pour.

How about using fibers in the mix?

 

[jmims912]

Civilperson,

To be sure that I understand...the re-steel mat is OK to be sandwiched in the cold joint between the slab placements?

Or is it expected that the reinforcing will float enough to get bonding and coverage by vibtrating the mix? I can conceive of this, but wouldn't expect it to actually happen...

I'm intrigued by the vibrator device you refer to...I've not seen this...would love to know more...

 

[jmims912]

Jike...Thanks for the reply

The steel is not to be pulled up into the fresh concrete...there is not sufficient depth to the topping slab placement. Coverage at the surface is already 3/4"-1", at best.

And, well...there is this...On this project I am in an observation, report and (limited)suggest/advise roll only...unfortunately the GC's Pre-cast Eng/Mfg Sub controls the design and the GC is going with what they've paid for as primary incentive for quality for design.

Did I say that I despise "Design/Build" Projects, especially where Independent QC is contracted by the GC.

Never-the-less...

I've been told that the #5 bar is the primary reinforcement in the topping slab, therefore is, now, on top of the mesh and the mesh is to support the #5…and "minimize the damage below the floor if the floor broke up" (?!?!) so the mesh being in the cold joint doesn't matter. It's all said to be "a composite design"...yeah, so? (There seemed to be the implication that "composite design" explains why the composition doesn't matter as much.…..Words!! "Let's build it anyway we want, just use some creative words.")

A little history, though...The Pre-cast specs originally called for the mesh to be over the #5, with another layer of bar(#4) running perp to, and under the #5. I questioned the 1/4" coverage that that would leave at the surface and the #4 bars were removed from the picture.... a continued show of concern seemed to lead to the flip-flop of mesh/bar...I'm not really sure whether any of this made a bad design worse or made a worse design "not-as-bad -as-it-was"...either way, it doesn't seem to me to approach "good".

Oh, listen, I'm very open to being wrong here...matter of fact, Please, somebody, convince me of it...is it supposed to be this complicated?

 

[TXStructural]

Reinforcement along the cold joint will act as a wedge on a splitting plane. The bar needs enough cover to develop, and needs to be supported, not "pulled up."

You could place smaller bars more closely, or use an appropriate WWR in lieu of any bar. Simply design the mesh to provide the require area of steel. The mesh must be supported on chairs or dobies/blocks.

For example, #5 @ 30" grade 60 is roughly equivalent to 0.1 lb/sq ft grade 80.
WWR 6x6 D5xD5 provides this reinforcement (although the cross wires do not necessarily need to be the same.

Also, be aware that the connections between the double tees take a great deal of force and fail with great regularity. If they fail, the differential motion between the tees tends to break the WWR out of the topping slab.

 

[jmims912]

TXStructural... TNX for the input... I understand...and agree. That is how I saw it as soon as it was beginning to be constructed...it just did NOT look right.

Note that I'm just a lowly Eng Tech with the ITL hired by the Design/Build General. Meaning that our input is limited (although I always push the limit, if I'm pretty sure or suspicious of something). The dispecable current trend of the ITL not being hired by and representing the owner directly puts us in a sometimes unbearable conflict. I could go on...but I'm sure you get it...$$$

Again, I appreciate your input...This has been one project where I believe the "the factor of safety" will allow all to fair well...it is truly a shame to see what appears like the design team(and remember "design/build" (with all the inherant conflicts in THAT) shift to relying on such...maybe I'm wrong...maybe I'll never have to point out the entries in our records...maybe...

Oppps!! Was I ranting off subject...oh well, I feel a little better...Tnx