15 Dec

Rational Buckling Analyses to AS4100 or NZS3404 (Part 4)

In the last post we looked at the basic method of undertaking a flexural buckling analysis using Mastan2 and interpretation of the results. We compared a couple of the built-in equations in AS4100 & NZS3404 relating to the \alpha_m factor and got reasonable agreement.

I stressed that getting the elastic buckling moment out of a buckling analysis does not equate to determining the design capacity. We briefly mentioned the \alpha_s factor in passing. This is a scaling factor which is determined from the reference buckling moment M_{oa} and the nominal section capacity of the member in bending M_{sx}, it is intended to essentially account for a number of 2nd order effects. Basically converting our theoretical buckling value to a “design” value that accounts for real-world things like the fact that all members have initial imperfections (they are not perfectly straight), all beams have some degree of residual stresses (which results in some regions of the section yielding prior to others and resulting in reductions in stiffness with respect to the resistance to buckling).

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25 Nov

Rational Buckling Analyses to AS4100 or NZS3404 (Part 3)

In the last post in this series we looked at a semi-real scenario where a rational elastic buckling analysis was undertaken in which we were able to determine the axial capacity of a system of members. The exact type of system of columns is sometimes referred to as a ‘lean-on’ system, whereby the column carrying no load helps to increase the capacity of the supported column. Basically if we make the supporting column (or in other words the bracing system for the RHS) stiff enough it has the effect of producing a higher mode of buckling in the supported RHS column.

Basically something where it would be hard to guess the exact behaviour and hence capacity, becomes easy peasy. It would be hard to guess the exact effective length factor k_e (in AS4100/NZS3404 terminology) or K (in AISC360 terminology) for anything but the simplest of scenarios.

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20 Nov

Rational Buckling Analyses to AS4100 or NZS3404 (Part 1)

This post was inspired by a rather epic post at Eng-Tips forum, which came out of a seemingly simple request for some help on the segment length to consider for a continuous beam design to AS4100 (the Australian Steel code). Fast forward several hundred posts of debating issues of code interpretation, debates over critical flange definitions, everyone telling each other everyone else is wrong, backtracking, changing of minds, notionally proposing re-writing code clauses to suit particular sides of the argument, etc, etc. At the the end of it all there may or may not have been any real agreement reached, not unusual once engineers get to arguing.

You know when the original poster bows out at post #11, and the thread carries on for 200+ more posts that it’s a hot potato and there’s going to be a few virtual knife fights going down before it’s all said and done.

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21 Aug

Inflection point != point of restraint

It’s a question that seems to come up from time to time in my experience, can you take the point of inflection as a point of lateral restraint for the restraint of structural steel members under flexural forces?

The answer is of course… NO!

Let me say that again… NO, NO, NO!

Some people don’t ask obviously before doing, they just do it and are none the wiser.

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