×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

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!

*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

Stiffness of concrete plate

Stiffness of concrete plate

Stiffness of concrete plate

(OP)
Risa3D V17 doesn't seem to have a way to add reinforcement to concrete slabs (plates). Reinforcement has a large influence on the bending stiffness of a concrete section, so it seems like an analysis of a concrete shell structure in Risa3D would be inaccurate. How can the stiffness of the reinforcing bar be included in a concrete shell model in Risa3D?

RE: Stiffness of concrete plate

The best you can do is adjust the E value or plate thickness to give yourself an equivalent stiffness.

Note: RISA also does not account for cracking of the concrete either. I would argue that cracking has a much larger effect on stiffness than the reinforcement. Now, for concrete members (or even walls), you can specify an I_cracked factor to modify the stiffness due to cracking. But, that's not available for plate / shell elements that are used for modeling slabs.

RE: Stiffness of concrete plate

(OP)
I did a quick hand calculation of an RC beam and showed that stiffness effects due to rebar can be greater than the stiffness of the gross concrete section in bending, so it certainly seems that the modulus of elasticity of the concrete shell material should be increased to account for this as you suggested. Of course, an RC section is not isotropic and will have varying stiffnesses in different directions. Does RISA typically recommend modifying the modulus of elasticity to account for bending effects only, since that will often be the dominant action of concrete shells (usually slabs or irregularly shaped walls).

Edit: The cracked MOI should be easier to take care of since shell thickness can be taken from the top of the section to bottom rebar for a shell element supporting monotonic loads.

RE: Stiffness of concrete plate

Mega -

I don't know that RISA has a recommendation one way or the other on how to handle this. I will say that, during my time working for RISA, we did not generally get many requests for considering reinforcement in the stiffness of beams / columns / walls or slabs. My belief is that in general structural analysis, most engineers are comfortable using the gross section (or some multiple of the gross section) for the equivalent moment of inertia of the concrete element.

Your point about the behavior of concrete slabs being non-isotropic is a good one. The plate element that RISA uses internally is sometimes modified to be orthotropic when it is used in wall elements. But, that is all automated. And, the program never gives the user the ability to unlock orthtropic behavior for plates.

Obviously, if these items are important to you, then you could send a message to the RISA folks and request the feature be added. Though, I have a feeling they would not give that request much priority.... at least not unless a lot of people are requesting it.

RE: Stiffness of concrete plate

The stiffness reduction due to concrete cracking is far greater than the increase in stiffness due to the reinforcement, especially for typical lightly reinforced slabs.

If you really want to model the increased stiffness, increasing the E value would be the easiest way to do it, but I'd suggest double checking the cracked stiffness, which can be lower than the unreinforced uncracked short-term stiffness by a factor of 5 or more.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

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! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close