Fiber vs WWF

[manofstl]

So, without telling anyone, Contractor decided to substitute fiber reinforced concrete instead of the Vulcraft metal deck with concrete/6x6WWF I spec'd for a second floor. Now he calls, wanting approval from yours truly so the job can proceed. I checked with Vulcraft to see if the FRC is an acceptable substitute, and they (understandably) have no position either way.

Anyone run into this scenario before?

 

[haynewp]

Is the steel deck composite or non-composite deck?

 

[manofstl]

Non composite

 

[WillisV]

Manofstl,

Looking beyond the obvious contractor issues you are facing, the use of steel fibers is adequate as a substitute of WWF for temperature and shrinkage reinforcing according to the May 2003 update to the Steel Deck Institute' design guide.

"Cold-drawn steel fibers meeting the criteria of ASTM A820, at a minimum addition rate of 25 lb/cu yd (14.8 kg/cu meter) and possessing an average residual strength of at least 80 psi (550 kPa) when tested in accordance with ASTM C1399, may be used as a suitable alternative to the welded wire fabric specified for temperature and shrinkage reinforcement"

See

http://www.sdi.org/30amend1.htm

for the full statement.

 

[haynewp]

If it was composite deck I would be less concerned, but as you see in the Vulcraft manual, there are minimum areas of steel that are required and notes regarding when the mesh should be draped for non-composite deck.

I've never heard anything good about substituting fibers for mesh. My understanding has been to count on fibers (particularly synthetic fibers) only for increasing the aesthetic value of a slab.

 

[WillisV]

Steel fibers work great for temperature and shrinkage reinforcing especially for slab on grade. This is not a suprise considering that most anyone who has seen where the WWF actually ends up in almost all placements (on the bottom of the slab). I would not spec it, nor is it code approved for, a replacement for required flexural reinforcing.

My previous post is valid (as the SDI document states) for composite slabs only.

 

[SlideRuleEra]

A lot depends on the reason for specifying WWF. WillisV has a good reference for temperature and shrinkage. Another resource, if the WWF was for structural purposes, is the Wire Reinforcement Institute at

http://www.wirereinforcementinstitute.org

This publication (free .pdf download) may be helpful

http://www.wirereinforcementinstitute.org/HTDocs/PDFs/TF 205-R-03.pdf

http://www.SlideRuleEra.net

 

[JAE]

Just check your deck design for non-composite capacity carrying the concrete. Many Vulcraft non-composite tables are based on using the WWF for actual concrete reinforcing. If the new use of the form deck without reinforcing is OK per the tables then the fibers may be OK structurally as the deck simply takes all the structural loading and the concrete is just a mass going along for the ride.

If the deck by itself can't take the loads without the WWF reinforcing, then you've got a problem...or rather, the contractor has a problem.

 

[ChipB]

Well, Vulcraft's manual doesn't call it out, however, another metal deck company (CSi) states in their fine print, something about reducing the tabulated loads by 10% if WWF per ACI isn't used for their COMPOSITE deck. To me, that implies it is essential in form deck.

 

[JAE]

ChipB - how does a note about composite deck imply anything about non-composite form deck?

 

[TPNY]

If there were any cylinders poured with the steel fiber mix, maybe you could use those tensile test results (steel fiber doesn't help that much in compression) to check the design of the concrete (assuming that the deck does not support anything, structurally). I have read that adding 1.5% (by volume) of steel fibers to concrete mix can increase tensile strength up to 40% and flexural strength up to %150.
If there were no cylinders poured, but you knew the type of fibers used and the amount, you could possibly determine a ball-park strength of the concrete and use those numbers to check the design.

 

[crossframe]

manofstl,

You've been given some pretty interesting suggestions here. Is there any reason the contractor cannot be compelled to prove to you that his field change results in a satisifactory product? It was his change, so I'd say the burden of demonstrating acceptability is also his.

 

[manofstl]

I am very thankful for the perspective you guys have shared. I lurk this site a lot and have learned a lot, too, just by reading the broad view of responses to different issues.

I am going to recemmend a complete design review using JAE's suggested approach.

 

[JedClampett]

I've been burned before by using the Vulcraft WWF recommendations. From what I now gather, unless the size WWF is in stock, it's not worth running a batch until its about 40,000 square feet. So I use the Vulcraft WWF recommendation and invariably get a call asking to substitute another size WWF or regular reinforcing. Vulcraft makes no consideration of the "standard" WWF sizes and gages.
I have no idea if that's what happened here, the size WWF not being in stock, but it's a good thing to be aware of.

 

[LPPE]

Jed- I thought the Vulcraft 6x6-W1.4xW1.4 and the 6x6-W2.1xW2.1 were quite common sizes.

 

[ChipB]

JAE,
Well, we all know (or should know) the metal deck is used to be part of the tension steel in composite deck. Therefore, if the manual is calling out WWF in addition to the composite metal deck, or to reduce the loads by 10%, by deduction the form deck w/o reinforcing is a screwed up situation. I'd probably make them remove the concrete and put it down the way I spec'd.

You could potentially calculate the cracked moment of inertia for a 12" wide concrete section for your design loading condition. Given your deflection at midspan has to be exactly the same between the metal deck and the concrete, you could determine how much of your design load is carried on your concrete and how much is carried on the metal deck. It would be similar as designing a flitch plate beam. (I think) There just isn't anything between the concrete and mtl deck that would cause the mtl deck to go into tension, so you would have to design it in bending. This is making my head hurt.

 

[structuresguy]

Wow, I can't believe how many of you are talking about COMPOSITE deck and not helping out the original post writer at all. While WWF vs fiber in composite deck is an interesting discussion, it has no value here when trying to answer the original question.

JAE hit it right on the head in his above post. Without WWF in a non-composite deck, the concrete is basically unreinforced. Hence, the deck will have to take all the structural loads by itself.

IF you really want to help out the contractor, contact the fiber manufacturer and have their engineer sign and seal a statement claiming that the deck as constructed meets the design required loads. However, I doubt you will find a fiber engineer who will do this...

 

[TPNY]

The problem with allowing the metal decking support the slab, is probably going to be the deflections you'll get. What is the slab for, offices?, condos? My experience with unreinforced concrete "supported" by the structural integrity of the metal decking, is that is cracks all over the place and you can feel it under carpet. It becomes a huge maintenance issue. Consider calculating deflections and flexural capacity of the fiber-ized slab to determine if there is going to be a lot of cracking, which could cause the owners to track you down in a few years and make you (or whoever okays the fiber) pay for the replacement of it anyways.

 

[JAE]

ChipB - Form deck can indeed be used to take all the weight of concrete without any structural help from the concrete.

Vulcraft even has tables for formdeck that provide a maximum allowable uniform load.

Concrete without reinforcing, sitting on formdeck, can be acceptable if the deck is properly designed to take the weight of the concrete and any subsequent live loads.

No sharing of the load, or concrete tension reinforcing is necessary. The deck is simply a structural element with enough stiffness and strength to support the dead concrete weight. The lack of fibers or WWF or rebar simply affects the cracking of the concrete on the floor. But the cracking doesn't adversely affect the structural performance of the metal deck as it is taking all the load and doesn't care a hoot what the concrete is doing.

Composite deck is a TOTALLY different structural system and behaves as such, linking the deck and concrete into a system where the concrete and deck are inter-dependent, transferring horizontal shear to resist flexural bending.

 

[haynewp]

JAE, maybe I am misunderstanding your post.

I think those "allowable load tables" are based reinforced slabs. The form deck acting alone has a "construction span" table that is applicable only.

The forms themselves may be able to take the total dead plus live loads without help from anything else, but I think that when calculating this load, you have to base it off of the construction table and not the uniform load tables.

http://www.vulcraft.com/catreq/deckcat.pdf