Replacing Shear Stirrups in Beams with Steel Fiber (Helix) 3

[Leo Baldwin]

Hello,

I am not with Helix; just wanted to make sure no one was looking at plastic fibers.
There are a few studies out there but I am not sure if this is the way to go? Can anyone give me their opinion on this please?

Thanks!

 

[jayrod12]

Definite no go for me, I don't have a code reference forbidding it but I can't see this actually working.

Are you relying on the compression steel for capacity because then it would be a code mandated no.

 

[Leo Baldwin]

Jayrod - did not know that compression steel negates fibers. Thanks!
This is for a one way slab. Top slab is usually 3"-5" thick and is monolithicly poured with a series of concrete joists, 4" wide x 10" deep each at 24" o/c.
one of the studies:
Link

 

[cliff234]

Nope. To my knowledge the use of steel fibers to enhance the shear strength of concrete beams is not permitted by ACI 318.

 

[Leo Baldwin]

Luke frolm Helix just got back to us.
He says: Helix could be used for this application. Its covered in ACI 318 Section 9.6.3.1. This allows the use of deformed steel fibers at a minimum dosage of 100 lbs/yd to replace minimum shear reinforcement. While Helix meets the requirements, the minimum dosages may present a challenge.

That is s a high dosage. Normally they are 10 maybe 20 pounds per cuyd.

Just reporting for anyone else that may be interested.

 

[TehMightyEngineer]

I looked at this a bunch to look at eliminating reinforcement in our manhole structures. We determined that the dosage requirements were uneconomical, that corrosion of the fibers with minimal clear cover may cause spalling or other issues, that there was a safety concern with having that many sharp objects potentially protruding from the surfaces of the structures during finishing, and that it added difficulty in mixing, pouring, and finishing.

We use polyprop. fibers for impact resistance in a lot of our un-reinforced or impact vulnerable structures (pads, tanks, light pole bases, etc.). The dosage requirements are low and the benefits make sense.

The only place I've seen steel fibers to reduce or replace reinforcement make sense was a slab on grade setup.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

Thanks for your contributions on this TME/LEO. Made for a canned answer to questions I had to answer this morning.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Agent666]

European codes allow it. I believe that you need to demonstrate that you have a minimum of something like 1MPa residual Shear strength due to the fibres and you can use this to replace the minimum level of conventional Shear reinforcement.

We use it in NZ to replace Shear reinforcing in waffle type slabs on grade (thin 100mm wide ribs at ~1100 ctrs each way with polystyrene pods between and thin slab poured over). The idea is that it's more cost effective to add the fibres than tying off thousands of Shear links throughout the slab as it minimises the on site work.

Check out dramix fibres as they have a number of standard use cases where they can replace or supplement conventional reinforcement with fibres (both Shear and longitudinal reinforcement). Typically for foundations is where I believe they currently see it being used.

 

[cliff234]

I was unaware that ACI 318-14 addressed the use of steel fibers to increase shear strength in concrete beams, however upon reading section 9.6.3.1, it appears that the only benefit in using steel fibers is that designers get out of the requirement of having to provide Av,min when Vu > 0.5 x phi x Vc. When Vu > phi x Vc, designers still need to provide conventional shear reinforcing. In other words, steel fibers may not be used to provide “Vs” shear strength.

Regarding the use of steel fibers to get out of having to provide Av,min when Vu > 0.5 x phi x Vc in building structures (those are the structures that I design), I doubt that this would be an economical option (at least in the U.S.) because you would have to provide the fibers in both the slabs (where they are not needed) as well as in the beams (where they may be needed), because slabs and beams are typically poured monolithically.

Perhaps steel fibers might be an option in non-building structures, however I don’t see them being economically viable in building structures (at least not at a dosage rate of 100 pounds / yard). My guess is that the cost of Av,min stirrups in the limited areas where they are needed would be much less that the cost of 100 pounds / yard of steel fibers throughout the floor system.