Concrete Bearing Stress

[SteynvW]

Hi All

Does ACI, BS and Eurocode allow an engineer to
increase the bearing capacity of concrete with the use
of reinforcing?

Let's say for arguments sake a concrete column
is placed on a perfect inelastic material and the bearing
stress at the bottom of the column is to high for the conrete
to handle, but reinforcing is also going through the same plane.

Are you allowed to increase the bearing pressure of the column
because of the steel using proportional elastic theory?

 

[Teguci]

If the reinforcement is through the plane (goes between the column and the unobtainium bearing plate and is anchored on both sides in compression) then the reinforcement will take some of the axial load and provide for a higher load than allowed with just concrete. Further, due to concrete creep under load, the reinforcement will, over time, take more than just the proportionate amount of axial load (and must be considered when proportioning the longitudinal reinforcement) ACI 318 10.9.

If the reinforcement is parallel to the plane (confines the concrete above the unobtanium bearing plate) it stands to reason that you should be able to get a higher compressive bearing capacity of the concrete similar to ACI 318 10.14 - EXCEPT: Like a corbel, the bearing edges of the concrete aren't confined, will chip and at the least reduce the bearing area. More than likely it will also cause a maintenance problem.

 

[bkal]

BS811 states:
3.9.4.13 Concentrated loads
When these are purely local (as at beam bearings or column bases) these may be assumed to be immediately
dispersed provided the local design stress under the load does not exceed 0.6fcu for concrete grade 25 or
above, or 0.5fcu for lower-strength concrete.

In case of EC2, there are several provisions, but they do not look directly applicable. Clause 3.1.9 gives strain-stress relationship for confined concrete. Clause 6.7 talks about partially loaded areas and the section on strut-and-tie models allows increase in strength by 10% in the node in case that concrete is confined. However, I do not think that any of the above is applicable to your case, for similar reasons as Teguci has mentioned.

 

[KootK]

Does ACI, BS and Eurocode allow an engineer to increase the bearing capacity of concrete with the use of reinforcing?

Yes. As Teguci mentioned, there are two primary options:

1) Rebar parallel with the applied axial load and crossing the connection plane. This is absolutely allowed and is, in fact, how most concrete column to concrete footings are detailed. At minimum, the crossing bars must be developed for the expected compression force on either side of the joint. Sometimes, consideration must also be given to where the load goes after it is delivered to the compression dowels. In the case of a footing connection, it becomes punching shear in the footing.

2) Rebar perpendicular to the applied load providing confinement to the compressed concrete. While there's no doubt that such a technique would be effective, quantifying it with code provisions isn't straight forward (at least not using any of the codes familiar to me). Some ideas:

a) Seismic literature contains some guidance with respect to the increase in f'c that can be expected when confining rebar is provided.

b) Based on similar recommendations for anchor bolts, post-tensioned cable anchorages, and footings on bedrock, it would seem that providing a confining force equal to about 1/4 of the applied axial load is appropriate.

As with the parallel rebar option, it is prudent to give some consideration to where the load has gone once it gets beyond the confined concrete.

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.

 

[SteynvW]

I understand.

Thank you all for the helping comments.