Concrete slab in two levels - design/modeling? 2

[mats12]

How do you guys design concrete slab like this? How to model it? What about reinforcement?

 

[SteynvW]

There has been a couple of threads about this, but the consensus what you design the slab without
the step and detail the step "connection" so that moment can transfer though.

The step width (your is 20cm) should be at least twice the step height (30cm), in other words the step width should be 60cm.

Also take note the a step like this will create a slight increase in the deflection, just keep that in mind.

 

[mats12]

I dont know how to:

'detail the step "connection" so that moment can transfer through'

any suggestion?

Im confused - because of the geometry of the slab the load will transfer only in short direction (perpendicular to the step)... so the step (beam) actually dont help with bearing the plate. But it has to transfer moments (perpendicular to it) to the lower slab. Right?

It also bothers me becuase the step is near a support (concrete wall).

 

[SteynvW]

Here is a link from a previous discussion. This ought to help.

http://www.eng-tips.com/viewthread.cfm?qid=408814

 

[mats12]

tNX, but im dealing with a slightly different problem - my step is near support.

That changes things drastically.

If the step is in the middle of a span, then I agree - moments, shear forces and deflections are very similar. But not in my case when step is near support:

DEFLECTION:

MOMENTS, SHEAR and AXIAL FROCES:

Looks like step near support has good effect on deflection and lowering moments, but axial forces are huge.

 

[KootK]

The step width (your is 20cm) should be at least twice the step height (30cm), in other words the step width should be 60cm.

I've always seen this as twice the thicker slab depth (upper/lower). So 40 cm my way. Which is not to say that there's anything wrong with Steyn's method.

any suggestion?

The thread that Steyn linked for you has a strut and tie model of the situation that can guide your detailing. If that's insufficient for you needs, let us know and we can provide more detail.

Im confused - because of the geometry of the slab the load will transfer only in short direction (perpendicular to the step)... so the step (beam) actually dont help with bearing the plate. But it has to transfer moments (perpendicular to it) to the lower slab. Right?

Exactly right.

tNX, but im dealing with a slightly different problem - my step is near support.

It's the same problem in my book. Just less moment and less impact on deflection which is nice.

Looks like step near support has good effect on deflection and lowering moments, but axial forces are huge.

This is because you've pinned both ends of the slab against lateral translation. Are you sure that's appropriate in your case? I would think not based on your sketch. There's almost some unintended source of restraint but that is rarely considered by designers in these situations.

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.

 

[JAE]

We had a "situation" like this with a precast step system a while back. Here's a short summary of the findings:

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[JAE]

Also these documents were helpful:


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[JAE]

Also this one

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[mats12]

thank you very much! will look into that!

 

[mats12]

Kootk - strut and tie model you sketch in other thread is great. But are there any rules for forming that - angle of diagonals (45 degrees?), etc...

 

[KootK]

You local code proably has a limit on how shallow struts can be. Probably something around 25 degrees.

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.

 

[jdgengineer]

Not to hijack the thread, but how would your process change if the step height was exaggerated to 10' or so in a matslab with multilevel basent scenario? In this instance you obviously have more continuous support of the slabs. Would you model the step as pinned and try and avoid moment transfer?

 

[KootK]

Would you model the step as pinned and try and avoid moment transfer?

Generally, I'd model the step as pinned top and bottom and detail for some unavoidable moment transfer. I might also just treat the vertical bit as a support for the higher slab and a load on the lower slab. Sometimes I feel that it's justified to consider multiple load cases to try and bracket the problem.

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.