×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Concrete slab in two levels - design/modeling?
2

Concrete slab in two levels - design/modeling?

Concrete slab in two levels - design/modeling?

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

RE: Concrete slab in two levels - design/modeling?

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.

RE: Concrete slab in two levels - design/modeling?

(OP)
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).


RE: Concrete slab in two levels - design/modeling?

(OP)
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.

RE: Concrete slab in two levels - design/modeling?

Quote (Steyn)

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.

Quote (OP)

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.

Quote (OP)

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.

Quote (OP)

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.

Quote (OP)

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.

RE: Concrete slab in two levels - design/modeling?

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: Eng-Tips.com Forum Policies

RE: Concrete slab in two levels - design/modeling?

(OP)
thank you very much! will look into that!

RE: Concrete slab in two levels - design/modeling?

(OP)
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...

RE: Concrete slab in two levels - design/modeling?

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.

RE: Concrete slab in two levels - design/modeling?

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?

RE: Concrete slab in two levels - design/modeling?

Quote (jd)

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources