shear strength of fiber reinforced concrete floor

[KnarfMan]

Hoping someone can help me here. I'm trying to find some design standards/equations for allowable "Punching Shear" (i.e., two-way shear) on a 6-inch thick concrete floor that only has synthetic fiber reinforcement (uniformly dispersed). Normally, I would follow the provisions of ACI 318.11.12.2 but this assumes that it is just "plain, unreinforced" concrete.

 

[GeoPaveTraffic]

The fibers do not add any significant structural strength to the slab. There only purpose is crack control, i.e. holding the cracks together.

 

[JAE]

Yes, what GeoPaveTraffic said.

 

[KnarfMan]

Hmmmm, well ... I have a maximum service load of 21.6K (steel leg) sitting on this 6" floor ..... following ACI 318, Chap.22 (struct. "plain" concrete) with phi=0.65, fc=4000 psi, load factor of 1.4 ..... I get a minimum base plate req'mt of 8.5" x 8.5"; and the fiber reinf. does nothing to help me reduce the base plate size?

The loading is from a series 38-ft high special rack/shelving system. The shelves come with a standard 4.5" x 4" base plate (6" x 6" plate is optional and extra). The manufacturer/supplier also informed us they have never had a problem with this rack system on 6" concrete floors in the past.

All comments welcome ..... TIA (thanks in advance)

 

[SlideRuleEra]

I consider synthetic fibers to be one of the best (concrete) inventions "since sliced bread" and have been using them for various projects since 1985...
but for your situation I completely agree with GeoPaveTraffic and JAE - they won't help at all.

http://www.SlideRuleEra.net

 

[JAE]

Think of it this way:

Fiber structural capacity = 0

 

[dik]

The loads are pretty high and depending on the soil and the leg spacing the slab should be checked for flexural tension.

The fibres, as I understand, helps with the retention of bleed water and the improvement is gained by a more uniform hydration of the concrete gel.

Dik

 

[miecz]

Agree with dik that the loads seem high. Flexural tension can be checked by the method shown in "Designing Floor Slabs On Grade" by Boyd C. Ringo and Robert B. Anderson, pages 43 to 45.

 

[Ron]

Yup....what they said about fiber. Remember fiber enhances the properties of concrete, but it doesn't reinforce it for anything other than creating a path break for cracking.

 

[Qshake]

If possible check the original content of the fiber. If the floor has anywhere near 2-3 bags per cubic yard you can bet that shrinkage cracking is all that the fiber is for.

If, however, you find that there is significant amounts of fiber (5-7 bags) or greater than 0.5%-2% then perhaps you may have some residual strength but I bet you'll be hard pressed to find a text on mechanics that will provide the necessary and reliable strength.

Regards,
Qshake

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