X-member (tie rods) receives compressive forces from earthquake design combination – how to handle correctly?

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Hello everyone,

I have modeled horizontal X-braces made of flat steel in my RFEM model, which are structurally designed as pure tension members (no compression capacity intended).

Under the normal load and design combinations, the members behave as expected. However, in the design combination for earthquake, I am getting compression forces in the X-braces.

This is not entirely understandable to me, as these elements are structurally only able to carry tension forces.

My questions:

  1. Where do these compression forces in the earthquake combination come from?

  2. Is it statically correct to neglect compression forces in pure tension members during earthquakes?

  3. Is there a clean modeling option in RFEM to ensure that the members only carry tension forces even under dynamic loading?

  4. Should nonlinear members generally be considered for X-braces in the earthquake case?

I want to represent the model statically correctly and according to standards and would appreciate any advice on how you handle such cases in practice.

Thank you very much!

3

Hello Mzabi2025,

A very interesting question, which is indeed asked repeatedly. :face_with_monocle:

This can be explained with the basics of modal analysis. This is a linear calculation method. That means: Nonlinearities are not taken into account. Among these is the tension rod, as it fails under compression load.

In the online manual of the add-on "Dynamic Analysis," a solution for this is given through the use of initial conditions. Also, on our website, a technical article is available, which deals directly with your question, related to RFEM 5.

Best regards,
Marc Gebhardt

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4

Hello Marc,

Thank you very much for the help.
Could you please show me how to do it using an example from RFEM 6 with earthquakes?
I tried, but unfortunately without success.

Best regards
mzabi

6

Hi Mzabi2025,

if you want to determine the total shear forces of the earthquake situation, for example to compare them with the wind load, it may be sufficient to neglect the nonlinearities:
https://www.dlubal.com/de/support-und-schulungen/support/faq/005667?srsltid=AfmBOoor7dSpbFVQn6cIbniTRbhk_DbbfhpBTbIwedlHp-UKm4u6hkcB

Since in the modal analysis and the response spectrum method the tension members are linearized as truss members, for an accurate consideration, manually failing members (tension members where compression is expected) must be removed (deactivate the object by structural modification).
https://www.dlubal.com/de/support-und-schulungen/support/knowledge-base/001833?srsltid=AfmBOoqm5LILVo1rTuUZQqXvjlxOdJwocuLDsoRTgKGC49Yp03hw4uvi

For X-bracing, the following approach can also be effective:
No matter in which direction the earthquake acts, one of the X-bracing members will always be active in tension. However, due to the linearization, both members are active and in most cases take on equally high forces. Now you can transfer the forces of the actually failing compression member to the tension member and design it accordingly. However, this must be done manually. If you want to model it even more precisely, you can adjust the stiffness of the members so that together they represent the stiffness of a single tension member.

Best regards
Thomas Eichner

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