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

Related Articles


Multiple Section Moduli with Non Composite Bending

Multiple Section Moduli with Non Composite Bending

Multiple Section Moduli with Non Composite Bending

Here is the background. With design of glass facades for buildings you often need to strengthen the vertical members (mullions) that transfer the wind loads to the floor slabs by inserting steel cross sections that fit snugly so the steel member and the aluminum member act together, but they are non-composite. This is done for controlling deflection or reducing the stress on the aluminum section.

For deflection behavior it is a simple matter to multiply the steel moment of inertia by the modular ratio add it to the aluminum moment of inertia.

The question I have is, for this situation, how does one combine the section modulus of the steel and aluminum when computing stresses? It seems overly conservative to simply add the two, but it doesn't make obvious sense to multiply the steel by the modular ratio either.

Thanks, in advance.

RE: Multiple Section Moduli with Non Composite Bending

If they are non-composite then the procedure you describe using the modular ratio is not appropriate and the steel and aluminum will only share load based on relative stiffness assuming an adequate load path from the aluminum to the underlying steel.

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering

RE: Multiple Section Moduli with Non Composite Bending

For each of the two components M = CEI where C is the curvature.  Hence
    Mtotal = Ms + Ma = C[EsIs + EaIa]
because (we assume) the curvature is the same for each component.

RE: Multiple Section Moduli with Non Composite Bending

Agree with Celt83. Stiffness is measured by EI, so the difference in material properties is already taken into account.

Using two dissimilar metals in contact with each other presents a potential galvanic corrosion problem. Are you planning to wrap the interior shape with plastic strips or similar material to prevent contact?


RE: Multiple Section Moduli with Non Composite Bending

You compare the relative stiffness (EI) values of the two members and apportion the load to the two according to that comparison.
Each will deflect exactly the same since deflection is directly related to the two EI's.

You can then just take the portion of load attributed to each and calculate the stresses for each.

Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies

RE: Multiple Section Moduli with Non Composite Bending

What you need is the separate forces in each member. The load is shared based on stiffness. I like to do it as a ratio, and then apply that percentage of the total load to each component. for example if the steel is represented by EsIs, and aluminum by EaIa, the moment in the steel for a simply supported beam with uniform load would be M = (w*L/8)*(EsIs/(EsIs+EaIa). That ratio of stiffness can be used for each force, and then stresses checked for each individual component.

RE: Multiple Section Moduli with Non Composite Bending

Thank you, that is very helpful.

Regarding BAretired's question about galvanic corrosion. So far as I know, it is standard practice in the industry to not use any wrap. The steel is protected from heavy corrosion by being contained inside the aluminum. Also the relative amount of aluminum (which would act as the anode) is such that galvanic corrosion is negligible over the whole member over the expected life span.

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