Meaning of N_cr,TF < N_cr,T in Eurocode

[StructuEng]

Eurocode 3, section 6.3.1.4 says this about torsional and flexural-torsional buckling:

(1) For members with open cross-sections account should be taken of the possibility that the of the member to either torsional or torsional-flexural buckling could be less than its resistance to flexural buckling.

(2) Non dimensional slenderness lambda_T for torsional or torsional-flexural buckling should be taken as:

lambda_T = sqrt(A * f_y / N_cr) for Class 1, 2 and 3 cross sections

lambda_T = sqrt(A_eff * f_y / N_cr) for Class 4 sections

where N_cr = N_cr,TF but N_cr,TF < N_cr,T

where N_cr,TF is the elastic torsional-flexural buckling force
and N_cr,T is the elastic torsional buckling force


My confusion is at the bolded statement. Does anyone have an idea how to interpret that sentence? The critical force N_cr is taken as the torsional-flexural buckling force, but this is less than the pure torsional buckling force..? Does this mean I have to somehow choose my sections such that the flexural-torsional buckling force is critical? Surely that does not make sense, since for doubly symmetrical sections such as common H- and I- sections are not suspectible to flexural-torsional buckling, but are to pure torsional buckling.

I wonder what this inequality means?

 

[HTURKAK]

where N_cr = N_cr,TF but N_cr,TF < N_cr,T

where N_cr,TF is the elastic torsional-flexural buckling force
and N_cr,T is the elastic torsional buckling force


My confusion is at the bolded statement. Does anyone have an idea how to interpret that sentence?

Ncr is the lower of the values Ncr,T and Ncr,TF , but Ncr< Ncr,t

There are some worked examples at the following doc. could be useful for practicing engineers

 

[StructuEng]

Ncr is the lower of the values Ncr,T and Ncr,TF , but Ncr< Ncr,t

There are some worked examples at the following doc. could be useful for practicing engineers

Ah, I mistyped the sentence from Eurocode. It is in fact "but N_cr < N_cr,T", as you have said.

So it just basically means N_cr is the lower of N_cr,TF and N_cr,T. Still I feel Eurocode has a quite strange way of expressing that

Thank you for the guide link, there are some good examples indeed.