Step in flat slabs, not at supports

[sybie99]

Say you have a step in a flat slab, for example a 150mm step in the soffit and top of slab. Is there any reason why one could not step a continuous slab if the following criteria are met:
* You crank bars so that you have one continuous bar across the step
* The minimum radius of the bends in the crank bar are such that the bearing stress on the concrete is acceptable according to codes.
* the effective depth of the section is maintained, in other words the step in top and bottom are far enough apart that the perpendicular distance between a rebar and edge/face of concrete remains what it is elsewhere in slab, or the effective depth used in design.

For what reason can you not step a flat slab, and what could you do besides the above to make it possible?

 

[bowlingdanish]

Cranking bars is good for small steps, but larger steps require lapped Z bars to ensure moment continuity through the fold, unless you have excessively wide folds which the Architect won't be happy about.

 

[hokie66]

I try not to generalize in cases like this, but am usually comfortable as long as the horizontal distance between top and soffit steps are 4 times the step depth.

 

[Trenno]

In addition to what has already been said, you need to consider the stiffness of larger steps, as they will tend to act more like beams with slab either side. This all depends obviously on your support points in the immediate area.

 

[KootK]

I was taught that a cranked bar shouldn't generate an angular change of more than 2% unless the crank is tied back with ties etc. Obviously, the concern is with spalling out the concrete covering the inside of the bar bends as the bars attempt to straighten out. Applying this algorithm would make for an impractically wide slab fold in most cases so I vote for the z-bars or something similar to them.

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.