Lateral Torsional Buckling of monosymmetric I-beams per SP 16.13330.2011

[Anonymous]

Hi Guys,

I found an issue in Robot with lateral torsional buckling check of monosymmetric I-sections per SP 16.13330.2011. It seems that it incorrectly calculates B and C parameters used in formula Ж.9 - see attached spreadsheet for manual calculation.

Concearning B - When load is applied to top flange in the middle of the span B=Delta-1, however in Robot B=1=Delta which corresponds to load applied to bottom flange.

When the load is applied to bottom flange the situation is opposite

Concearingn C - I thought the issue might be in calculation of Jt (torsional moment of inertia) which is an old story and was discussed here, but actually even if I take the value which Robot gives and use in manual calculation C is still different from Robot.

[Anonymous]

Sample model attached

[Artur.Kosakowski]

Hi Andrey,

Thank you for posting this information

The first issue is caused by using the first "line" in the situation when the "second" should be used and vice versa which need to be corrected. 

For the It value - it is incorrectly displayed as 211 cm4 in the note but internally it is calculated as 219 cm4 instead and this later value is used in the bar verification process. 

[Anonymous]

Hi Artur,

Thanks.

Concearning B - understood.

Concearning C - As you can see in pdf file with manual calculation initially I took Jt as 219 cm4 and C=0.91, while robot gives 0.886. Robot does not give values of intermediate coefficients used to calculate C so I can't tell from where this difference arises.

[Artur.Kosakowski]

Robot follows these rules:

n = J1/(J1 + J2) = 5461.3/(5461.3+1333.3) = 0.8038

alfa = 0.385*It/J2*(Lb/h)*(Lb/h) = 0.385*219.2/1333.3*(6/0.63)*(6/0.63) = 5.7415

N = (1.0-n)*(9.87*n + alfa) = 0.33* (1.0-0.8038)*(9.87*0.8038 + 5.7415) = 2.6834

C = 0.33*N = 0.33*2.6834 = 0.886

As far as I can tell the difference originates from this formula:

You use hf = 0.67 (distance between axes of flanges) whereas Robot assumes h = 0.63 mm being the total height.

It seems that the code is not precise in this part:

as in some part of this chapter h is assumed as the total part and in some as the distance between flanges. Could you clarify why you think the distance between flanges should be used for the formula indicated above? Thank you for your help. 

[Anonymous]

Yes, Artur, you are right - the code is ambigous on this issue. However, there is a clue which I think is a key for the answer. From the beginning of appenix Ж, in item Ж.2 it is said that h - full height of section for rolled profiles, and distance between axes of flanges for built-up section:

Next, in Ж.3 it is said again in a) Rolled profiles, h - full height, б) Built-up sections, h - distance between axes of flanges:

And finally in Ж.4, which applies to build-up sections only it is again said that h - distance between axes of flanges.

After that there is no indication what h is, except Ж.5, where is is said that h should be defined according to given appendix.

Summing up all above mentioned I would say, that there is a consistent pattern for h - definition: Rolled profiles h - full height, Build-up h - distance between axes of flanges. If we apply this rule we do not violate any of the code's prescriptions.

[Artur.Kosakowski]

Thank you very much for clarifying this part of the code. I have sent this information to the development team.

[Anonymous]

You are welcome!

[Artur.Kosakowski · Accepted solution]

Corrected in SP4 for RSA 2016.

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