01 Feb

Rational Buckling Analyses to AS4100 or NZS3404 (Part 5)

So if you’ve been following along with this series you now hopefully know a little bit more about some of the methods involved in undertaking buckling analyses. Pat yourself on the back for making it this far through my ramblings I guess (if nothing else).

It’s pretty simple, and in most cases it will yield a capacity very similar to the code hand methods, albeit via a completely different path. It’s always good when different methods yield similar results.

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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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24 Nov

Rational Buckling Analyses to AS4100 or NZS3404 (Part 2)

In part 1 of this series we briefly explored the requirements related to calculating the capacity of a column via the use of a buckling analysis. Introducing the general methodology to follow and showing agreement with the normal hand methods for a known k_e.

In this followup post we look at the power and beauty of this method in being able to assess any complicated design scenario, typically ones that don’t fit in the mold of the typical idealised restraints.

NOTE – something is wrong with the formatting in this post, on mobile devices something pushes the text off the edge of the screen in portrait mode, try reading the post in landscape mode. If anyone can help sort it or knows what is wrong please post a comment…. my CSS skills are rubbish which I suspect is the problem 🙂

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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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15 Aug

Improving Excels spellchecker with highlighting

Excel does many things well, but it goes without saying many things could be vastly improved, or at the very least shown a bit of love by Microsoft.

One particular thing in Excel that that causes me some frustration is Excels inbuilt spellchecker.

Why? Because it’s infinitely more shit than the spellchecker in any of the other office products for a start.

This personal view is primarily born out of using Excel for the purposes of recording text based queries and responses as part of structural peer reviews. I use it as a log of sorts to log questions, responses and the like. The tabular nature of Excel lends itself to this application.

I’ve had logs that stretch to 40 odd A3 sized sheets full of queries when stuff goes a bit pear shaped and the designer has a ‘few’ (read ‘a lot of’) items to address, confirm, clarify, etc.

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03 Aug

Parabolas, Rectangles, Triangles & Snakes (Part 9)

Part 9 – Verification

Part of the verification process in development of a spreadsheet or software tool involves what I like to term the process of idiot proofing. Some end users are considered as potential idiots, asking yourself to think like this idiot can be a challenge, as it doesn’t come naturally to some. Luckily as engineers we are good at solving problems, identifying risk and considering all eventualities, and are able to either test for these eventualities or provide suitable error handling to avoid giving back an incorrect, but potentially believable result to the end user.

You’ll essentially need to consider things no sane person would do, and then do these things to ensure your code, calculations, etc, all work as intended. You know that guy who uses a spreadsheet to design outside of any fundamental assumptions it might be based on, or intentionally decides some course of action is conservative when it really isn’t. Think like this guy, what will go wrong will go wrong, and you’re trying to head this eventuality off at the pass so to speak.

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25 Jul

Finding an equation for the Moment of Inertia of a complex tapering member using Python

Often in engineering you need to make simplification in analyses that make your life easier, often applying some judgement based on experience that involves some simplification in the analysis of a structure for example. Sometimes you do this in the name of achieving conservatism, other times to get a reasonable/faster answer that is deemed close enough to the true solution to be accepted as good enough.

Sometimes peoples understanding of engineering principles lets them down here, and the end result is they made it simple, but they also made it wrong or unconservative. Don’t be this person.

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