Steps in PT Slab

[greengold]

Hello,

I'm looking for information about stepping a PT slab. This would occur at a column gridline, and both the top and bottom of slab would step. Slab is generally 11" thick. I would appreciate any guidance on special design/detailing considerations at the step, whether special attention should be given to the change in P/A, any special reinforcing needed, etc. If you know of a good reference that addresses this, please point that out as well.

Thanks so much!

 

[JAE]

greengold - I don't have a direct reference but in the past we have used a staggered step in all concrete, whether its PT or mild-reinforced. What I mean by the staggered step is that the top surface slab-drop is usually pre-set by some required condition - a tile floor or other slab depression. The step in the soffit (bottom) of the slab is delayed for a distance so that you have a section of concrete that is thicker than the typical thickness = T + drop depth.

This gives you some room to allow your reinforcing, neutral axis, stresses, etc. to all transition to the lower elevation. We would use about 4 to 6 feet to do this but it sort of depends upon the size of reinforcing steel. I would suggest going at least 5 times the slab thickness as a minimum.

For PT design, you'd have a jump in your NA no matter what you do - just keep this included in your calculations.

 

[rapt]

greengold,

The main thing to allow for is the efefct of the change in centroid and effective depth of the tendon through and on either side of the step. The tendon cannot be at the top of the slab on both sides of the step and extra reinforcement will be required to provide sufficient capacity on the side where the tendon is low. Extra loadings(moments) will be applied at the changes in centroid depth and these must be allowed for in the analysis and design.

Good PT software (like RAPT) will allow for this automatically for you.