Two-way PT Slab with drops

[mxpengr]

We have a 2-way pt slab that has several drops from one end of the structure to the next. Some drops are a few inches - others are up to 4 feet. Most of the slabs we have done in the past have been relatively flat.

How do you handle drops in the PT Slab? What do you consider the maximum drop that will work well for a continuous tendon? What do you consider to be the maximum slope on the bottom of the slab for the transition from one elevation to the next?

Looking for some guidelines and general rules of thumb to get us heading in the right direction. Thanks for the input.

M

 

[dik]

By drops, are you referring to 'steps' in the slab or drop panels. If steps in the slab, then post tensioning likely will not work well with this system. Am I misunderstanding the question?

Dik

 

[jen4950]

Is your drop more than 6:1?

If you will recall, 45 degrees for stress distribution is a very fundamental behavior with concrete. If your material, construction considerations, or geometry appproaches limits that get even close to a transition that gets close to something that forces something more than this, I'd re-consider, or at least look really close at it. Think corbels and stirrups with regards to conventional concrete.

PT complicates matters in the sense that it amplifies errors, inconsistencies, and inexperience.

If your transition is at or more favorable than 6:1, there is less concern for sure, but always use your engineering judgement.

 

[mxpengr]

jen4950:

Thanks for the feedback. We are looking at making most of the transitions at closer to 12:1. The one big drop (step) is closer to 6:1. We are using ADAPT for FEM modelling of the slab and are assuming the transitions to be more like ramps than steps and then applying additional loads for the extra concrete on top of the "ramp" where the step occurs.

We are planning to provide some mild steel in the concrete step above the "ramp". Any suggestions, details or observations would be appreciated from anyone that has some practical knowledge to share.

 

[civilperson]

The locations of the tendons or cables through the drops will be a modelling headache. Friction losses through ducts or draping points for cables out of the linear alignment puts you at the leading edge of theory and development. Suggest some strain gages built in for monitoring actual vs. predicted.

 

[rapt]

mxpengr,

At those transition lengths you should not be ignoring the steps and treating them as ramps. Treat them as they are and add the minimum reinforcement required allowing for shear lag if you want.

civilperson,
Not if you have software that actually calculates tendon profiles properly allowing for sensible reverse curve radii and determining transition points from these. Obviously the definition of the "leading edge" varies depending on what you are used to doing and the calculation and design procedures you are used to using.