What if wrong Crank bar length for slabs is given?

[Mechanicslearner]

Hello,

Upon execution we provided crank bar length as 0.25 L for slab instead of 0.15 L as per IS456 . May I know the consequences for this mistake and how to resolve it? The slab is for residential building.

 

[JAE]

If the 0.25L refers to the extension of the top portion of the bar into the next adjacent span, then the extra length should not be a problem.

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[hokie66]

A sketch would help. I suspect JAE's interpretation is incorrect. The word "crank" suggests to me that the top bar is turned down at that point.

 

[JAE]

We’ve always used the terms crank bar, truss bar, and bent bar to mean a longitudinal bar that extends from one end of a beam, at the top, then bends downward into a bottom bar for the center span area, then turns back up into a top bar for the remaining end of the beam.

The crank bar either starts as a top hook at a discontinuous end or extends over the support and some distance into the next span for interior support locations.



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[hokie66]

Yes, same as my definition. But if the crank of the top bar is too far into the span, the bottom moment capacity could be affected adversely.

Cranked bars, particularly in slabs, are uncommon and unnecessarily complex these days.

 

[JAE]

hokie66, I was assuming that the bend (turn-up) location was in the same location - I just assumed that the extension was too long.
However, I agree that if the turn up was also mis-positioned then there could be a problem.



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[Mechanicslearner]

Yes hokie66, crank length here refers to bar being bent at 0.25 L of slab. So if the bar to be bent at 0.15 L is now at 0.25 L . That does not impose any problem?

 

[hokie66]

I think I answered that question. Cranking the bar at the wrong place can potentially affect the positive moment capacity (requirement for bottom steel in that area). But that would depend on the actual situation, not a generalization.

 

[Mechanicslearner]

So you think increasing bottom steel area would be one of the solution? Its residential building

 

[hokie66]

Maybe, but best to ask the responsible design engineer rather than strangers on the internet.