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# Calculating Outward Busting forces for cranked rebar in circular columns

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## Calculating Outward Busting forces for cranked rebar in circular columns

(OP)
How do I calculate outward busting forces in a circular concrete pile at positions of cracked longitudinal rebars. This column is 800mm dia with 15No dia 32mm bars to enable splicing. The pile is 23m long. The rebar has 2 spices. Links are a spiral dia 16mm bars at 250mm pitch. Because there are so many bars (15No) this makes the tie bar angle so wide that trying to resolve the forces seems to require quite some substantial additional links to counter the outward busting force. Is it allowed to just resolve the forces in this way and provide as much tie reinforcement as is necessary? Is it possible for someone to help me with an example of how to calculate the busting force or guide me in the right direction? Thank you

### RE: Calculating Outward Busting forces for cranked rebar in circular columns

Why crank the bars? I agree that restraining the horizontal force at a crank of a large bar is problematic unless there is a horizontal concrete element at that point. But it you just lap the bars around the perimeter without cranking, I think the problem is much less.

### RE: Calculating Outward Busting forces for cranked rebar in circular columns

Force in bar X Crank H/V = Outward Bar Force.

Add up the contribution of all the bars to hoop stress in the ties and you'll have the demand on the ties.

I think it's allowed to reinforce for crank busting forces in this way but you'll have to give some thought to how you'll integrate your concentrated local ties with the spirals above and below the cranks. Hokie's method sounds much simpler if it will work for you.

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.

### RE: Calculating Outward Busting forces for cranked rebar in circular columns

(OP)
Thank you so much for the tips.
Hokie66, the reason for cranking the bars has been requested by the contractor. The original cage had full length bars but the contractor prefers to do the cage in segments using casings a relatively smaller crane for lifting the prefabricated cage. Cranking the large diameter bars rather than simple laps side by side appears to give more spacing between bars for concrete to flow. My understanding is that for large dia bars the gap between the bars should be at least 100mm for concrete to flow?.
KootK ,I have applied the formula, does give some reasonable ammount of rebar area for ties. Should be able to continue the spiral links over if I use smaller diameters for the ties. Much appreciated.

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