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


Modulus of Rupture

Modulus of Rupture

Modulus of Rupture

For the slender wall method (Chapter 14.8 in ACI 318-08) the modulus of rupture is listed as 7.5*lambda*sqrt(f'c).

For structurally plain concrete (Chapter 22 of the same code), it is given as 5 * lambda * sqrt(f'c).

Why are these values different? Why is the modulus stronger for the slender wall method?

RE: Modulus of Rupture

I would think that for plain concrete you might want to be a little more conservative, where with slender walls, you have reinforcing that can mitigate an overload and also maybe some compressive load... also with pavements, depending on the application, you could be looking at 8 or 9 as the factor.


RE: Modulus of Rupture

Agree with dik. Axial compression of the slender wall likely plays into that as well.

RE: Modulus of Rupture

I have also had this question in the past.

I had thought about conservatism, but there are phi factors present for this. The code also requires plain concrete to be in overall compression.

To add more to this, if I pour my little beam on the job site and test it in three or four point bending I will be testing a plain concrete beam, regardless if my pour is actually reinforced. Is the resulting modulus of rupture from the lab analogous to ACI 14.8 or ACI chapter 22?

RE: Modulus of Rupture

I don't think that Chapter 22 states that the modulus of rupture is 5*sq rt f'c. What it says is that the nominal moment capacity is S*(5*sq rt f'c). You can make the case that the implication is that the modulus of rupture used is 5*sq rt f'c, but I'm not sure I see it that way. It's likely just additional conservatism built into the equations.

RE: Modulus of Rupture

The 7.5*sq rt f'c is for deflections only. It isn't a strength calculation.

MacGregor & Wight note that the mean modulus of rupture is 8.3*sq rt f'c, so the 7.5 makes sense for deflections. 5*sq rt f'c is closer to the lower bound value, which would make sense for strength calculations. MacGregor & Wight have a 3 page explanation of it if you're interested.

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!


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