Ties distribution in columns 2

[Matt_Struc]

I have recently noticed that a lot of structural engineering companies based in Australia give this attached typical details for column ties between floors.
You notice two main thing in that detail:
1) 3 sets of ties at 80 below every slab
2) half the ties spacing at lap length.

I could not find any reference to the above and to be honest it seems arbitrary to me.

my note on tie spacing is the following:
1) For columns with concrete strength equal and under 50 MPa distribute ties equally as per AS3600 c10.7.4
2) design ties to resist shear in columns which most cases would be uniform for lateral loads and thus again tie spacing could be less but still nonetheless uniform throughout the height of the column.
3) Australian concrete detailing handbook recommends adding at least two ties to support bars below ( my take on this, that bars could bend if unsupported during construction)
4) ACI 318-11 7.10.5.2 requires to add bars to resist the horizontal competent force developed in offset bars and place the added ties no more than 6 in from the offset.

Note the detailed of column i attached is for a column not part of a moment resisting frame or high strength concrete column.
This detail is used by too many companies i have seen, and i can't seem to be able to explain their logic!
Thanks for your response in advance and looking forward to hear from you.

 

[MotorCity]

I am not familiar with the Australian code, but I believe the closer tie spacing above and below the floor serves a structural function. The column shear is typically higher at or near the floor level, as opposed to mid-story height. They are intended to resist story shear from lateral loads

 

[Matt_Struc]

Dear Motorcity, i believe shear from lateral loads would be uniform in anyone column, as the moment diagram is linear thus shear is constant. Therefore, adding ties at the ends of the columns only doesn’t make sense from shear point of view.

 

[hokie66]

The closer spacings serve a couple of funtions, in my opinion.

1) The ties restrain the lateral forces resulting from the bar splices. That includes both the restraint of the offset bars, and also the stresses created by the laps themselves. Consider a strut and tie analogy to see what I mean.

2) The ties add robustness for seismic loading. Most codes now, even in low seismic areas, require consideration of robustness at joints.

 

[rapt]

Mat Struc,

The extra ties below are explained by 10.7.5.5, similar to the ACI requirement to provide lateral restraint to the sloping bars at the bottom of the lap.

The ones above are possibly because of a rule in Section 13 on compression laps, or maybe the halving of the tie spacing for bundled bars (though I would not call a splice a bundled bar situation).

 

[Matt_Struc]

Dear hokie66 and rapt, thank you for your great responses. I thing that's exactly what it is.
The bottom bars are to take the horizontal force developed in the offset bars ( I was thinking the offset always happen to be in the slab which is why I didn't see this explanation)
And rapt, although I agree with you about the definition of bundled bars, at least now I can go to sleep happy knowing at least the logic other engineers are following.
Hokie66, I definitely agree with you there should be a min amount of ties added for robustness, the question is how much. Have you come across any clause in concrete standards stating the amount?

 

[hokie66]

No. I think engineering judgment rules. Using half spacing over the length of the lap is simple, and not subject to interpretation by the rod busters.