×
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

Incorrect formula for composite design in AISC?

Incorrect formula for composite design in AISC?

Incorrect formula for composite design in AISC?

(OP)
Hello everyone,

I have a question about the formula to compute the moment capacity of a composite beam when the PNA is in the flange of the steel beam. I am perplexed by the formula given by AISC to compute moment capacity. First the stress distribution that AISC bases their formula off of is shown below.



Based on the distribution above, AISC presents the formula below for the moment capacity of a composite section.



AISC is saying since both the portion of the flange in compression and the portion of the flange in tension have the term C/2 with opposite signs (see distribution above), the two terms cancel out. While this is true for the resultant force, the same logic can not be applied for summing moments. Since the two C/2 terms have different moment arms about the plastic neutral axis, the moment caused by the two terms will be different. From what I can tell steel textbooks seem to agree with this logic (at least McCormac does). I think this equation needs to be amended, or I need to refresh my statics... Am I missing something here? Is it possible that AISC decided the difference between moment arms is negligible?

I came across this discrepancy looking at the AISC Design Examples for composite section. Specifically, I was looking at example I.1 which looks at the design of a composite girder. In the example the plastic neutral axis is in the web, but the formula shown above is utilized to find the moment capacity.

A follow up question relating to the composite section moment capacities in the steel textbooks. Most of the textbooks double the compression force in the flange of the beam so they don't have to subtract anything from the tension force (i.e. T = FyAs instead of (FyAs - flange compression force) since I added more compression in the flange). I have considered both adding and not adding the additional compression in the flange, and both approaches give a different moment capacity due to the reasoning above (different moment arms). How do you choose to compute the moment capacity of a composite section when the plastic neutral axis is in the flange of the beam?

Thanks


RE: Incorrect formula for composite design in AISC?

At a quick glance, I agree with you. Equation 13-10 appears to be overestimating the contribution of the steel beam.

BA

RE: Incorrect formula for composite design in AISC?

I've calculated the moment long-hand before, and I get the same answer as the equation. I haven't gone to the trouble of deriving the equation. Give it a try yourself and see.

RE: Incorrect formula for composite design in AISC?

Quote (OP)

While this is true for the resultant force, the same logic can not be applied for summing moments.

I think that the formula checks out. See the derivation below for the moment effect of the tensile forces in the steel about the d2 axis. The trick is to recognize that both C/2 forces are really halves of the same force C and, therefore, are located at the same vertical position. If that doesn't clear things up for you, report back and we'll kick it around some more.

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: Incorrect formula for composite design in AISC?

As long as the PNA is correctly located so that the sum of horizontal forces above is equal to the sum below (the definition of the PNA), you will get the same correct moment capacity no matter what location you choose as the datum to sum these forces about.

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