Concrete Bearing 2

[SlenderBeam]

I'm trying to understand the reasoning in the bearing capacity calculations used in a number of concrete codes, where confinement can be taken to aid the bearing capacity (i.e. by a factor of sqrt(A2/A1), within limits).

Does anyone have any papers/tech info in regards to the testing (or similar research) which has been carried out to formulate the American ACI / Australian AS3600 approach to concrete bearing surfaces?

 

[Lion06]

I don't have any papers, but it's a result of the confinement that the additional concrete provides.

 

[ishvaaag]

You can give a look to bidimensional confinement enhancement of concrete compressive strength in many classical texts on reinforced concrete, Park & Paulay, Montoya-Meseguer-Morán between others. AISC LRFD code uses that and may give references.

The concept is easy to understand, the lateral confinement of the surrounding concrete creates a triaxial state of compressive stresses under a column applying compression. f'c and fck are derived from unconfined compression tests at which the average value of compression attained before rupture is lesser than in the confined state.

By the way it is the same reason because coefficients of conversion between 8" cube strength and 6"x12" cylinders' stregth is required, the cube being stubbier shows bigger restraint at its core and its enhanced compressive strength there pushes up the average strength.

 

[AHaddad1]

I cannot recall the specific research papers that lead to this conclusion; however, I do offer the following insight.

The allowable bearing pressure is 0.35fc due to shear cone failure of the supporting concrete, say below a column base plate for example.

This was proven via sample testing and thus the Sqrt(A2/A1) which is limited to 2.0 maximum.

The 0.35fc can be increased to 0.70fc upper limit based on the additional concrete confinement created by the additional concrete around the bearing contact surface. why? because the diagonal shear failure plane has increased with the increase of the additional concrete confinement.

A similar behavior is footings on soil, look at Boussinesq influence charts for spread footings on soil for example to understand how the footing bearing stress can be reduced by taking some of the isobars around the footing out. The isobars are contributing to the bearing capacity. A similar approach for the concrete bearing stress applies.

 

[hokie66]

The research which leads to bearing pressure increase due to confinement would be quite old, as it has been in the codes as far back as I can remember, and I am quite old. I think of it in terms of Poisson's ratio. As the concrete is compressed in one direction, it wants to spread in the orthogonal direction, but can't because of the restraining surrounds.

I disagree with AHaddad1 that it is related to "shear cone" failure. Bearing failure in concrete should be thought of as a crushing failure, while shear failure is a diagonal tension failure.

 

[AHaddad1]

hokie66,

There are three types of concrete failures

1) Diagonal Tension Failure in Long Span Beams.
2) Shear Compression Failure in Short Span Beams.
3) Splitting or True Shear Failure which almost eleminates the diagonal tension concept.

In the above case, I believe we are closer to 3) than 1)

See Pages 6) & 7) in the following URL

http://www.edutalks.org/technical articles/dILEEP KUMAR.pdf

Take Care

 

[AHaddad1]

Allow me to reiterate and expound on my last post:

1) Diagonal Tension Failure in Long Span Beams.
2) Shear Compression Failure in Short Span Beams.
3) Splitting or True Shear Failure which almost eleminates the diagonal tension concept. Based on which the 0.35fc was established.
4) The Upper crushing limit is what hokie66 is referring to which is the 0.70fc

Between 3) and 4) is where the SQRT(A2/A1) factor applies.

 

[hokie66]

AHaddad1,

Not sure which code you are quoting with the .35 and .70 factors. ACI?

The Australian code AS3600, depending on the area ratio, allows ultimate bearing stress to be increased up to a limit of 1.2 fc'.

Again, I think the OP was asking about bearing, not shear. It is the same reason that you can pass column loads through floors of lower strength concrete than the columns, within limits.

Concrete strength fc' is based on testing specimens, which fail in shear, but these are unconfined. Bearing is different from shear.

 

[AHaddad1]

hokie66,

SlendeBeam was asking about the factor SQRT(A2/A1)

Where,
A2 = is the Area of the supporting member
and A1 = The Area of the contact bearing.

This factor is expressed in the AISC ASD 9th Edition for steel columns base plate design.

Say you have a steel column base plate over a concrete pedestal where A2 ~ A1, in this case SQRT(A2/A1) = 1
Therefore, the maximum allowable concrete bearing stress is 0.35fc

Another situation if the Steel Column base plate is bearing over a footing where A2 is much larger than A1, in this case SQRT(A2/A1) could be much larger than 2; therefore maximum 2 can be used and in this case the allowable bearing pressure is 0.7fc

1.2fc in factored load (strength) design equates to 0.70fc for allowable stress design.

Hope this clears this up...

 

[hokie66]

Yes, that agrees with our concrete code. Allowable stress design is no longer used in the Australian concrete or steel codes.

I think SlenderBeam understands how to use the code, but he was asking for references to research on which the bearing provisions are based. I don't think we have answered that.

 

[AHaddad1]

hokie66,

The Factor formula SQRT(A2/A1) derivation is very simple but may have research papers that I am not aware of.

I will draw you a sketch and attach it in my next post when I have some time to show how the formula can easily be derived.

 

[AHaddad1]

Here is the quick sketch (Not to Scale)

For simplification, I tried to illustrate the One Way behavior as I understand it under bearing.

 

[SlenderBeam]

Would it be fair to say that the inner envelope is analogous to the failure which is observed in the testing of cylinder strength?

i.e. splitting / shearing failure of the edges.

Typically, this leaves two 'cones' of concrete.