Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • 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!

Join Eng-Tips
*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.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

Deflection Equation for Composite Beam with Moment Connections

puffy333 (Structural) (OP)
24 Sep 08 9:59
I need some help with a composite beam that has moment connections.  The examples in the ASD manual calculate deflection of a composite section as ML^2/161I, but their example follows a beam with a pinned connection.

Do the steel beam connections themselves play a part in the derivation of this deflection equation?  And if they do, how can I find/derive the equation that takes into account a fixed connection as opposed to pinned?

Any help would be great.

frv (Structural)
24 Sep 08 17:53
You are describing an indeterminate structure. Time to dust off the old structural analysis books.

For a single member, moment-area is probably straightforward.
miecz (Structural)
24 Sep 08 20:12
For an approximate solution, figure that a prismatic beam with fixed ends deflects around 20-25% of a beam with pinned ends.  But a composite beam with moment connections can really get complicated, as the composite moment of inertia will vary depending on whether the beam has positive or negative moment.  The negative bending moment of inertia will vary depending on whether the concrete is cracked or not, and on how much longitudinal slab reinforcing contributes to the section.
PanamaStrEng (Structural)
25 Sep 08 7:29
Is the slab part of the continuity, or is only the steel beam connected with moment connections, while the slab is disconnected to the support and only composite to the beam at this point?
puffy333 (Structural) (OP)
25 Sep 08 9:00
Thanks, meicz, I knew this one was going to be over my head.

PanamaStrEng: Only the steel beam is connected to columns with moment connections, and the slab is only composite with the beam.

csd72 (Structural)
26 Sep 08 14:14
I have seen a method that engages the tensile capacity of reinforcement placed in the concrete slab but this is generally only applicable to spread moments between the beams, transfer to the columns is more complicated.
msquared48 (Structural)
18 Oct 08 23:08
Even considering equations 32 or 33 in the AISC manual, the solution is difficult due to the varying E and I along the length of the composite member.  

I agree that for a reliable solution, a computer analysis is best here, breaking the span up into segments of appropriate E and I values

Mike McCann
MMC Engineering

haynewp (Structural)
23 Oct 08 19:37
It depends on how accurate you want to get with this of course. You can use moment area method with taking into account the inertia change if it happens that the concrete is cracked.

I have done this a couple of times before with concrete beams, you can use moment area to find deflections prior to cracking, at cracking, at yield, and at ultimate by plugging in the inertia changes along the beam at the affected locations then using moment area. Or you can do it with a computer program.

If it is a new beam, what I would do is design it to satisfy gravity deflection requirements using a pin-pin assumption and go home.


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!

Back To Forum

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