Cross Sections classification - Flat cross sections

[skilak]

Hi,

I am trying to design a flat 200x10 subject to compression using two methods of creating the section a) solid - user section b) Tools - section definition. When it comes to the cross section classification in the design module, the profile created using method a) has been classified as class 3 based on a rectangular section 200x10x5 (5mm thick wall rectangle), while for b) as class 4. Please could you explain and reference how you have obtained the class for rectangular profiles as it seems that no known standards (EC3, 5950, PN) to me deals with Flats for classification.

Best regards,

[skilak]

Apparently, the class of a section for method b) is 1 not 4, please see attached file.

[Artur.Kosakowski]

In fact in Robot it has been assumed that such sections are primarily intended to work under tension rather than compression especially that the code gives no rules for that. As far as I remember currently a) solid section will always get class 1 b) will be the structural tube of wall thickness equal to half of the width of the flat bar. 

The change we are thinking of for the future is to assume class 1 for flat bars under compression unless you force use of the elastic analysis (Configuration button) when the class will be assumed as 3.

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[skilak]

Unfortunately, the structure I have been checking was designed by somebody else. I would not use a flat for compression as well. Anyway, I have noticed that EC3 lacks a validation check which can be found in PN-90 i.e. slenderness <250 (to assure elastic boundary).

"solid section will always get class 1 " Well, elastic buckling is elastic therefore class 3 is OK as plastic properties are not much of help. Shall we assume that for any given solid section, within elastic theory, subject to compression, the cross section is always class 3?

"The change we are thinking of for the future is to assume class 1 for flat bars under compression unless you force use of the elastic analysis (Configuration button) when the class will be assumed as 3."  Well, the class ,up to 3, does not affect gross cross section and for solids the c curve shall be used (initial imperfection type) of 0.49 what gives Chi = 0.0049 in my case. As well, my Nb,Rd is almost the same as Ncr. (I have also done a FEM model of the flat and Ncr was the same;) )

Anyway, thanks for the thought.

Best regards,

[Artur.Kosakowski]

The idea of 'giving the choice' between class 1 and 3 is intended for situation when you have bending (e.g. self weoght) as well 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[skilak]

Concluding.

Regardless the section class the buckling curve for solid is "c" and corresponding imperfection factor alpha 0.49 not as in Robot "a0" and 0.13. I think it should be amended if relevant.

[Artur.Kosakowski · Accepted solution]

The use of the "c" for flat bars is tentatively scheduled for SP3 for Robot 2015 

Please note: Our quality assurance teams may need to reschedule some fixes. Please rely on the above timeline information as a guideline only and not a guarantee. We appreciate your understanding in this matter.

For now you can force/select the use of c in the bar type definition.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[skilak]

"tentatively" 😉

Please, if relevant, add as well a note that below slenderness of 250 or above non-dimensional slenderness of 3 as per EC3, the 2nd order effect due to buckling will be significant.

Best regards,

[Artur.Kosakowski]

Which point of the code do you mean exactly? IMHO using flat bars against compression you should be aware of the risk of buckling in the first place 

[skilak]

Hi,

EC3 does not say anything about slenderness limit. It used to be in PN90 B-03200 (4.4.1. d). But what I am suggesting is a warning calling for the use of buckling analysis +P-Delta if slenderness is above 250 (member needs to be divided into say 10 segments). In Robot, we can specify the slenderness limit and by default is 220.

"IMHO using flat bars against compression you should be aware of the risk of buckling in the first place "

Yes, we are aware but it would be great if the others could be warned;)

Best regards,

[Artur.Kosakowski]

The situation is that when you exceed the specified slenderness limit you get the corresponding warning during code checking of a steel element of a model. IMHO that is just enough. If I understand you correctly if you get above 250 you'd like to suggest running the 2nd order analysis but I think that such approach may actually be bit confusing. It can be e.g. then displayed for a bracing in a single story warehouse but not displayed for a mast with lattices and legs defined as chains of bars. In other words I think that the decision on including the 2nd order effect is actually not based on the slenderness of single elements of a model.

[skilak]

and where in the code, I mean EC3, I can find the slenderness limit?
I suggest the 2nd order because above 250 the curvature due to buckling is significant and you have bending involved. From the first principal buckling is a geometrical function (Euler curve, no force, no strength) and is endless and it does not take into account that the strut looks like a bow .................... ok, I think we shall end up over here.

[Artur.Kosakowski · Accepted solution]

Concluding.

Regardless the section class the buckling curve for solid is "c" and corresponding imperfection factor alpha 0.49 not as in Robot "a0" and 0.13. I think it should be amended if relevant.

Corrected in SP3 for Robot 2015.