Fiber vs WWF
Fiber vs WWF
(OP)
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?
Anyone run into this scenario before?






RE: Fiber vs WWF
RE: Fiber vs WWF
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
My previous post is valid (as the SDI document states) for composite slabs only.
RE: Fiber vs WWF
www.wirereinforcementinstitute.org
This publication (free .pdf download) may be helpful
h
www.SlideRuleEra.net
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
RE: Fiber vs WWF
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
I am going to recemmend a complete design review using JAE's suggested approach.
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
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...
RE: Fiber vs WWF
RE: Fiber vs WWF
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.
RE: Fiber vs WWF
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
RE: Fiber vs WWF
The Vulcraft catalog, for example, has three tables, not two.
One table is titled, "Max. Construction Clear Spans"
The second is, "Reinforced Concrete Slab Allowable Loads"
The third is, "Allowable Uniform Load"
I'm talking about the third table.
The first is a construction span table, providing a maximum allowable span for specific thicknesses of normal and lightweight concrete.
The second works with the first table, and provides the max. loads for various spans of reinforced concrete slabs. This table requires reinforcing as the deck here is simply a temporary form.
The third table is a generic tabulation of various deck gages, spans and limit states (strength and deflection). The formdeck can be designed from this table directly (without the first two) simply by ensuring the strength (Fb = 36,000) is OK, and then selecting one of two deflection criteria (L/2400 or L/180). The W1 value in this table corresponds to the first table.
You can see that the W1 value is the same as the constr. clear span table by comparing 1.5C24 / 1 span / 4'-6" span - gives 60 psf as the max. The first table, for the same parameters shows that a 61 psf weight (5 1/2" total thickness) is the limit.
RE: Fiber vs WWF
RE: Fiber vs WWF