Evaluation of lateral restraint at compressive flange

[Mohanlal0488]

Greeting Fellow Structural Engineers

I generally use SANS10162 for most of my steel work designs, however I do use AISC and BS/EN standards for design aspects not covered in SANS10162.

For beams SANS10162 requires that the lateral restraint be provided on the compression flange, however this is as far as this statement goes. Does anyone know of any literature or codes of practice which actually deal with the mathematics behind this?

For example, I would assume that if one has a relatively deep beam (supporting beam), with tie beams (much smaller then the supporting beam) and bracing such that it is positioned on the tension side of the supporting beam, this would not constitute as a lateral restraint by definition, however I think that it would offer restraint to some degree, and I would be interested to know how this could be taken into account.

 

[canwesteng]

The mathematics of it are in the commentary for the AISC steel specifications, available for free online.

 

[phamENG]

To piggyback on canwesteng - look at Appendix 6 in AISC, both the body with equations and the commentary to help you understand the principles behind it. Should answer most of your questions. Once you've had a look, come back with more specific questions if you still have them.

 

[r13]

In the US, please find the article titled "Bracing for Stability" by Joseph A. Yura.

 

[Tomfh]

There was a massive thread on beam bracing a year ago:

https://www.eng-tips.com/viewthread.cfm?qid=459248

Many of us are still suffering PTSD from it.

Basically you need to follow your code rules for where to brace (Eg compression flange if that’s how your code define it), or if your code allows it you can run a finite element buckling analysis based on actual brace locations.


Bracing away from compression flange often enhances buckling resistance (Eg bracing the bottom flange of a simply supported beam under gravity load), but is less reliable and codes won’t support it unless proven with analysis (which other engineers may not believe).

The mathematics of it are complex. The rule to brace the compression flange is a simplification designed to produce acceptable results in most cases, as opposed to being a hard mathematical limit which constrains all beam buckling.


Note that to truly brace the beam you need to brace both flanges, and some authors take that approach. This is especially the case with continuous beams with moment reversal.

 

[Agent666]

Note that to truly brace the beam you need to brace both flanges

Which can also be achieved to some degree by laterally bracing one flange with a detail that also restricts the rotation of the entire cross section.

AISC have quite a few videos on YouTube relating to stability and flexural torsional buckling, some of which even go so far as deriving the fundamental buckling equations in AISC if you really must know where the equations came from. They are well worth a look as collectively they give a good background knowledge to buckling in general.

I'd also suggest working through the stability fun modules from mastan2, they are a series of self study modules which aim to teach you the basics and build on them via practical examples. Mastan2 is a free program, somewhat academic, and sometimes user interface isn't that intuitive (you have been warned) but it is well worth working through the modules to gain a deeper understanding of the use of buckling analyses. You can find it here if you're interested.