×
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

Relative Stiffness of Steel Frames and CMU Shear Walls

Relative Stiffness of Steel Frames and CMU Shear Walls

Relative Stiffness of Steel Frames and CMU Shear Walls

(OP)
I've got a steel framed multi-story building.  I'm using ordinary concentrically braced frames (or at least trying to).  I've put frames everywhere I possibly can in the structure, but the foundation forces are still unacceptable.  A face of the building is CMU, as are two stair towers and the elevator shaft.  In addition to my OCBF's, I'd like to engage these CMU walls.

What would be the relative stiffness between the OCBF's and CMU shear walls?  What procedure would be used to determine both?  E of steel and E of CMU are quite different (roughly 29:1), and I don't think my OCBF's will take 29:1 times more load than the CMU shear walls.

RE: Relative Stiffness of Steel Frames and CMU Shear Walls

Well,

Assuming you have rigid floor diaphragms, you should distribute the load based on the stiffness of the elements, such that they deflect the same amount.  The stiffnesses depend on E as well as I.

To determine the relative stiffnesses, model the braced frame with an arbitrary load (say 10K) and model the shear wall with the same load.  Calculate the deflections, and the ratio of the deflections will give you the relative stiffnesses of the walls/frames.

If the center of mass and center of stiffness aren't the same, don't forget to take torsion into account.

RE: Relative Stiffness of Steel Frames and CMU Shear Walls

It depends on the inertia also. You could put a unit load on the cmu stair and elevator shafts and get the deflection as compared to the deflection of the braced frames under a unit load at the same height, figure the  stiffness=1/deflection. Then compare the relative stiffness of the cmu versus the braced frames. The cmu will have shear and bending deflections and the braced frames have tension and compression contributions.

You have to get the inertia and modulus of elasticity of the cmu shafts and if you can input this into a computer model or figure the deflections by hand as compared to that of the braced frames.

RE: Relative Stiffness of Steel Frames and CMU Shear Walls

Distribution between the brace and CMU walls are based on the relative stiffnesses of the two systems per the two responses above.

If the CMU ends up veing very rigid compared to the OCBF, consider using a two stage static analysis and deliver all of the load into the CMU walls.  I would still design the OCBF for the force distributed per relative stiffness.

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