AS3600 Clause 10.8 - Transmission of Axial Force through Floor Systems

[Just Some Nerd]

Hi all, had a couple of questions regarding this clause. I'm early in my career and don't yet always grasp the intent behind the wording in the code (though having read past discussions on this forum, maybe not that uncommon?)

First off, when assessing the joint using the effective concrete strength are we looking solely at the vertical force transmitted through the slab i.e. without consideration of any sort of moment (mainly the minimum) that we would be considering for the column itself? The clause itself only ever refers to the axial transmission but I have a hard time justifying to myself we should be ignoring a moment that the columns themselves must consider.

Secondly, Note 1 at the end of this clause states that confining reinforcement may be used to increase the effective strength of the concrete, and the commentary suggests doing the same. What sort of methods/resources would you use to calculate the increase in effective strength for a certain amount of confinement provided? Calculating a confinement pressure is simple enough given the equations given in Clause 10.7.3, but I am unfamiliar with how one would determine what increase in effective strength arises from this confinement.

 

[rscassar]

item 1, yes, the minimum eccentricity should be consider. The column above and below both need to consider the minimum eccentricity so that should go thru the slab as well.

Second point is a good question. People have asked me that question a few times about how to increase the effective compressive strength of the concrete using confining reinforcement and I don't know. There is nowhere in the code where the compressive strength goes above f'c except for section 10.8 and that clause about bearing surfaces which I don't really like. As far as I know there is not really a precedent for it.

 

[Trenno]

There eurocode will allow an increase (~10%) in compressive strength when confinement is provided.

 

[Just Some Nerd]

Thanks for the answers rscassar. I imagine others have been asking you that question for the same reason I was in seeing if there's any good ways to avoid doing puddle pours - I personally don't trust builders/contractors not to mess up puddle pouring (or in general) but I've not yet come across any good ways (i.e. not just adding more columns and/or making them significantly wider) to deal with the significantly reduced strength once a building is tall enough in height.

Trenno - are you referring to Clause 3.1.9 in EN1992-1-1? It seems that clause gives about 5% increase in concrete strength for every 1% (of f'c) in confinement pressure, which to me and the projects I work on seems pretty low compared to the equations in AS3600 10.8, unless we are meant to be considering the effect of the confinement on top of the calculated f'ce values from 10.8.

 

[SRL V1.0]

Priyan Mendis' book, "Design of High-strength Concrete Members", Section 4.4 "Laterally confined concrete columns-calculation of confinement pressures" includes the equation & example below.