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RE: Steel Fibers Replacing Structural Reinforcing?

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> The steel fiber representative called me and told me that
> they would have to increase the slab thickness to 19" and
> use 60# of fiber per cubic yard. He invited me to use their
> software to "verify" the design. I guess I am a bit skeptical
> about using someone else's software that has a vested interest
> in the outcome of the design to produce a design.

The software only spits out what the programmers tell it to spit out.  And
if the programmer is working for a company that wants to sell steel

In their defense, my very limited experience with steel fiber design is that
the steel fibers (we were looking at little corrugated steel strips, NOT
poly fibers) do increase the tensile strength of the concrete.  But they do
NOT act as substitutes for rebar.  It's roughly the equivalent of designing
the mat as very high-strength un-reinforced concrete. 

The reinforcing you mentioned is about minimum bending steel
0.00333*(18"-4"cover)*(12").  Do you NEED this much steel to transfer the
forces, or did you just use minimum steel?  The strength provided by minimum
bending steel is only slightly higher than the cracking strength of plain
concrete.  If you use a slightly deeper section, increase the concrete
strength with the fibers, then use the higher load factors for un-reinforced
concrete (less certainty), then the fiber design could very well work.

> I told the steel fiber manufacturer that they would have to
> take full responsibility for the design and produce sealed
> stamped plans if they wanted to change my designs to steel
> fiber for the mat slab of the rack supported building.

This is a good start, and will shift some of the responsibility.  I'd
recommend providing an alternate foundation plan which shows all loads and
says that it is to be designed by others.  Charge more for this, of course.

Ultimately, however, YOU as SEOR have to review and "sign off on" the design
calculations done by the fiber representative.  Even if you write letters
documenting your unwillingness to use this product and that you take no
responsibility for consequences, you'll still be dragged into the lawsuit
fray IF something goes wrong.

> As far as the slabs on grade are concerned, I feel that if the
> client wants to use steel fibers in those areas that would be
> fine as long as they understand any limitations of the fibers.

This is the only part where I'm confused.  It's been a while since I've done
Industrial level design, but when I did the clients NEVER wanted to take any
chances with the structure.  Why endanger many millions of dollars in
equipment, not to mention operating losses caused by plant shut-downs and
repairs, to save a few thousand dollars in steel?  Even a few hundred
thousand dollars?

> There are also ... elevated composite slabs in this facility.

Elevated slabs MUST have minimum reinforcing steel (rebar), so fibers would
only be an added expense.

> That's the background, here is my question - Has anyone
> replaced structural reinforcing with steel fibers and are
> there any standards related to such a replacement?

Never in a built project.  Fibers were rejected in all cases where it was
proposed.  In one, there was an expansive clay subgrade, and the fiber
literature specifically excluded this condition (didn't stop them from
trying to sell them to us).  In the other, there was exposed concrete in a
exterior residential area, and the architect was concerned with rust

I don't know of any "standards" - I believe everything is proprietary, with
design values based on product-specific testing.

This got rather long, but I hope it helps.

Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

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