Rebar compression formula for piles/mini piles.

[BigBar]

Hi, I am currently involved in specifying rebar's of grade 550Mpa Yield & 700Mpa UTS for a pile diameter 300mm. The bars to be used are 63.5mm in diameter and there will be three bars per pile. The usual practice here in Hong Kong is to have 4 No. 460Mpa yield 610Mpa UTS bars of 50mm diameter for this size pile but the design in this instance has changed.

I have been asked the compressive strength of the steel by the designer of the pile but as I have only worked in the past with the tensile strength I would be most grateful if anyone could tell me a formula to calculate the compressive strength of the steel based on information that can be obtained from a destructive tensile test of the bar or grade and diameter of the bar. Also would buckling be a factor in this instance or would this be checked by the surrounding concrete and ground. Are there any other parameters that should be considered when using this alternative reinforcing specifactions?

I would be most grateful for any comments anyone may have.

Cheers Big Bars.

 

[ishvaaag]

1) Lots of formulae for piles are very restrictive about how much to count compression on the steel rebar as a contributor to the structural compressive capacity of the steel. This uses to be (at service level) from 20 to 30% of yield strength.

2) For more complex analyses where tensile strength is an issue, such in laterally loaded piles one may resort for the tensile branch to the real stress-strain diagram.

It is contrarily a practice for pure compression columns, -and it would be sound to extend it to piles- never account more than 400 MPa compressive (limit) strength or so, since

sigma= E·epsilon=200000 MPa·0.002=400 MPa

being the limit pure compressive strength of the concrete (when the whole section is equally compressed) attained at 0.002 deformation it is to be surmised that the concrete will fail and spall at this stress, and going further up on limit srength on the steel, even if the yield strenght is bigger, simply won't work because at such levels the column is understood to be distroyed when a controlling hypothesis is pure compression.

As you see this european practice has its common sense and it would be better to give it a thought.

 

[RiBeneke]

I support what ishvaaag has written. In addition ...

The buckling of bars in compression should be restrained in an outward direction (away from the centre of the pile or column) by stirrups (ties).
Theoretical research and tests have resulted in codified rules for these stirrup sizes and spacings, but in most cases, these rules assume conventional rebar up to 450 MPa yield strength. Therefore you may have a problem here too.

My intuition also tells me that 4 bars would be better than 3 bars for overall pile buckling restraint.