Tuesday, 11 August 2015

FAQ

"What is the difference between Global stability check, and Member check function? How can I get the buckling length of a certain member?"


In Eurocode 1993-1-1, and so in ConSteel, there are 3 methods to verify the stability of a model:

  • Imperfection approach (described in Section 5.2 and 5.3)
    • The structural model is subjected to appropriate geometrical imperfections and after completing a second order analysis, only the cross section resistances need to be checked
  • Isolated member approach ( described in section 6.3.1, 6.3.2 and 6.3.3)
          The method is based on two essential simplifications:
    • Structural member isolation: The relevant member is isolated from the global structural model by applying special boundary conditions (supports, restraints or loads) at the connection points which are taken into account in the calculation of the buckling resistance
    • Buckling mode separation: The buckling of the member is calculated separately for the pure modes: flexural buckling for pure compression and lateral-torsional buckling for pure bending. The two effects are connected by applying special interaction factors.
  • General method (described in section 6.3.4)
          The basic idea behind the general method is that it no longer isolates members and separates             the pure buckling modes, but considers the complex system of forces in the member and                     evaluates the appropriate compound buckling modes. The method offers the possibility to                   provide solutions where the isolated member approach is not entirely appropriate:
    • The general method is applicable not only for single, isolated members, but also for sub frames or complete structural models where the governing buckling mode involves the complete frame.
    • the general method can examine irregular structural members such as tapered members, haunched members, and built up members.
    • The general method is applicable for any irregular load and support system where separation into the pure buckling modes is not possible.
However, in pure cases (pure compression, or pure bending), buckling length calculated from general method can be equated with isolated member approach. In the following, a "how to" example will be shown on a pure compression column:

 Example:

Determination of alpha critical factor with buckling analysis:
Alpha critical factor: Minimum amplifier for the in plane design loads to reach the elastic critical resistance of the structural component with regards to lateral or lateral torsional buckling without accounting for in plane flexural buckling.


Elastic critical normal force can be expressed, with the multiplication of alpha critical, and Ned. With a substitution to the Isolated member approach formula, buckling length can be calculated. 
In this case, the buckling length is 2088mm, which (with the 3000 mm whole column length) gives back the well known 0,7 effective length factor by the buckling shape of a top-pinned, bottom-fixed column.

Summation:

If you want to read more in this topic, please click the following link:

No comments:

Post a comment