Shear capacity of unreinforced concrete 1

[ZeroStress]

Hello

I am struggling to find the shear capacity of a concrete section with no shear or tensile reinforcement at all and was wondering if anyone can help me with it.

BS5400-4 and BD44/15 suggest a formula for concrete capacity which is based on the tensile reinforcement ratio (100.As/bd) but since there is no tensile reinforcement in the section, the formula doesn't seem valid for me.

Could anyone please help?

Thanks

 

[TheDW]

What kind of shear? Punching shear? Beam shear?

 

[ZeroStress]

It can be any type of shear, either punching or usual shear from beam loading?

 

[MIStructE_IRE]

One small shrinkage crack could change your answer completely. Personally I wouldn’t rely on this since shear failure is such a sudden and dramatic failure. Can you not get a couple of bars in somewhere??

If not, eurocode 2 gives good guidance on the use of plain concrete in section 12. They also go into shear resistance of plain concrete provided brittle failure can be excluded. I’m not convinced though..

 

[ZeroStress]

If I was designing a new concrete section I'd never forget to provide a min. reinforcement but unfortunately I am assessing an existing concrete section which was built some 70+ years ago.

Its a mass concrete leaf pier supporting a bridge and the record drawings indicate there is no reinforcement in the pier at all. The pier itself is 1.5m wide so it doesn't make any sense to me to say it won't resisnt even a tiny amount of shear force.

All I need to know is that in case of an impact on the pier, would it hold? If not, by what margine is it going to fail.

 

[JAE]

Not familiar with British standards but for ACI 318 (in the US) there is a chapter on Plain Concrete (i.e. unreinforced) where they offer several formulae for shear.

For beam action the suggest ΦVn = Φ x 4/3 x λ x sqrt(f'c) x b x h
where
λ is a factor = 1.0 for normal weight concrete
f'c is the 28 day compressive strength (psi)
b is the beam width
h is the beam depth
Φ is 0.6

For two way action ΦVn = Φ x (4/3 + 8/(3β)) x λ x sqrt(f'c) x bo x h
but not greater than Φ x 2.66 x λ x sqrt(f'c) x bo x h
where:
λ is a factor = 1.0 for normal weight concrete
f'c is the 28 day compressive strength (psi)
bo is the circumference of the shear area
h is the member depth
Φ is 0.6

This is an ultimate capacity which should be compared with factored loads (not service loads)

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[racookpe1978]

Doesn't the even the measured shear capacity of a "small" near-square piece of concrete in compression (shear) depend on the way that pier is supported below? If so, the theoretical (psi stress) levels would differ.)

To illustrate:
Case 1. 1.5 M x 1.5 m x 250 mm thick flat concrete block on a steel plate, uniformly and "perfectly" sitting on the flat steel test plate, being tested with a small sharp edged 12 mm x 12 mm bar pushed down from above with the piston.

Case 2. Same block, 250 mm thick, but sitting on sand or mud, under compression from a grouted flat plate 750 mm x 750 mm holding the bridge steel beams. With this, the whole block resists the weight, not just the little area resisting pressure as in Case 1 doesn't it?

Case 3. Same 1.5 m x 1.5 m concrete, but as a very deep 3 meter pier cast around a vertical steel shape 2 meters deep so the load is carried by contact on the side of the column and the sides of the concrete.

 

[NorthCivil]

Sounds like this might be a good application for applying an FRP wrap around the pier.

have a google for FRP wrap, and on the first page of results, you get this

https://www.asce.org/magazine/20170509-frp-wrap-gives-old-bridge-new-life/

 

[Agent666]

There are a lot of unknowns/uncertainties in assessing impact loads as well, sure code might give you a load but combined with uncertainties in the assessment of unreinforced sections and the lack of robustness in an unreinforced section to me adds up to a risk that should probably be addressed/mitigated. Another option might be to build another element in front to address the impact protection.

 

[BridgeSmith]

If you can't guardrail it, FRP, as NorthCivil suggested, would be a prudent move.

Axial compression in the concrete should help some, but estimating impact resistance of reinforced concrete is difficult enough (not to mention the difficulty in backing out the vehicle size and speed that might produce an impact of that magnitude); guessing what unreinforced concrete will do under impact loading seems like a fool's errand.

Even though the records may not show reinforcing, I find it unlikely that it doesn't have any. I suggest trying to get it scanned to see if there is actually rebar in it.

Edit: A crashwall or barrier to protect it is a really good solution if you can do it.