AS/NZS2327 composite beam penetrations tee compactness requirements!

[Agent666]

Hi all

Does anyone have an example applying the provisions in AS/NZS2327 appendix C clause C3.1.3 relating to openings in steel composite beams? This is with respect to determining the compactness of the tees at an unstiffened opening, in particular for non-compact and slender tees.

Quite frankly I'll admit I'm super confused on this one!

I've read what they wrote above like 100 times and flipped and flopped on how to interpret or define the compactness criteria.

I've read through SCI P355 on which the provisions are (loosely) based, but all the examples given are simply stating the tees are Class 2 or Compact sections as its based on Eurocodes. Also there are slight differences in some of the equations regarding the limiting web depth (presumably intentional, but equally could easily be an error as left off some terms being squared in the square root) So the non-compact or slender provisions don't seem to be covered to aid in interpreting them in P355. P355 also implies 'no limit' on web depth on some of the checks. but it also implied that these limitations only apply to the tension tee, which AS/NZS2327 doesn't really say apart from modifying the compactness under some conditions (the relevant clause in P355 is as follows)

I think the thing confusing me is how do you initially confirm whether you are compact/non-compact or slender, or is these limits attempting to do this by providing a max outstand for a given classification. However if you end up in the 'no limit' bit (effective length of opening <32*ε*t_w for example), are they implying no limit on depth but it's then just considered as compact..???????? In terms of AS/NZS2327 are you supposed to evaluate the compactness of the web in the tee using the table 3.4.3.3 of which relevant part is shown below then see if any of the C3.1.3 equations limit the depth for that compactness?

The 'depending on ratio of the effective length of the tee at the opening, L_t, to the outstand depth h_w,t' implies some comparisons are required to be made with a L_t/h_w,t ratio which I'm not seeing/understanding in the context of what is written?!

P355 also implies if you're non-compact or slender then you need to be checking the elastic bending capacities of the top and bottom tees above and below the web opening, not the plastic bending capacity. AS/NZS2327 seems to be silent on this. No idea if this is intentional or not? Anyone know?

The new standard has not gotten to much airtime in NZ, people still seem to be using NZS3404 for composite design, despite AS/NZS2327 being required by the building code for quite some time now.

Overall the new composite standard contains some good stuff. It's just really let down by the fact it seems to have so many inconsistencies and errors in the notation and methods. It's like a dyslexic blind person edited it.... they even amended it recently and left a lot of the errors in place which is super frustrating. Anyone else finding this standard really frustrating?

While we're ranting, the mixing and matching of Eurocode notation and Australasian notation is likely to do your head in if you've never dealt with Eurocodes before.....

 

[captain_slow]

@Agent 666

I had to muddle around with this a few months ago and my takeaway was that the entire Appendix C is just (incompetently) copied from SCI P355; SCI P355 should be read alongside Appendix C as Appendix C contains a lot of blanks and makes zero sense on its own. They attempted to flip some of the notation to be in-line with AS notation but then even that is not consistently done throughout.

In regards to compactness of T-section I ended up simply adopting the table from SCI P355 for compactness of the web:

Class 2 = compact web
Class 3 = non-compact web
Class 4 = slender web.

With the flanges as they say the top flange can be considered compact and for the bottom flange you should turn to your governing steel design standard (AS4100 for me and I suppose NZS3404 for you).

As you say there is some additional text which seems to imply that classification of compactness can be upgraded (or downgraded depending how you look at it) if distribution of plastic stresses meets some certain conditions. I decided to not mess about with all that given how rough the design guidance is. I just accepted my section classification as it emerged from the table.

In regards to the elastic/plastic capacity question - that's just based on definition of what compactness is, right? A compact section can develop its full plastic capacity and so the design check is done that way; a non-compact section can develop yield stress at the outermost fibre but otherwise can only develop elastic capacity as it can locally buckle beyond that. A slender section is the one where you expect local buckling effects to be dominant and so presumably even the elastic capacity check has to be appropriately amended to take account of that.

Apologies if these ramblings are not particularly helpful but now you have a second opinion to verify that the Appendix is poorly written (ahem....copied) and you are not going crazy.

 

[Agent666]

I had to muddle around with this a few months ago and my takeaway was that the entire Appendix C is just (incompetently) copied from SCI P355; SCI P355 should be read alongside Appendix C as Appendix C contains a lot of blanks and makes zero sense on its own. They attempted to flip some of the notation to be in-line with AS notation but then even that is not consistently done throughout.

Couldn't agree more. Definitely to be read in conjunction with P355. There are a few extra squared terms missing in some of that upgraded compactness equations and the odd extra quare root or 32/36 constants being swapped in some of the 2327 provisions which I've just taken to be outright mistakes and not intentional. Basically doing as you did and following P355 in some areas.

Yeah good point on just ignoring the upgraded compactness if it makes no sense. I guess getting to compact means you can use the plastic vs elastic requirements which squeezes out a bit more capacity for the vierendeel checks.

Apologies if these ramblings are not particularly helpful but now you have a second opinion to verify that the Appendix is poorly written (ahem....copied) and you are not going crazy.

Nah helpful to know I'm not the only one who has found the standard hard going.