concrete encased hollow circular steel section

huangyhg

concrete encased hollow circular steel section
for a concrete encased hollow circular steel section (compposite section), the eurocode4 says that its section axial resistance may be enhanced to the effect of confining provided the non-dimensional slenderness(which is the ratio of plastic resistance to axial compression to the elastic critical buckling load) is gretaer than 0.5.that is, the resistance need not be enhanced if the non-dimensional slenderness is less than 0.5.
my questions:
1) but, if the non dimensional slenderness is greater than 0.5 , it means that column is all the more slender, right?in that case confining effect should become less, right?
2)then, is'nt the above clause contradictory?

i dont know the code, but i think i know where they are coming from.
say you have a column 10m wide and 5 m tall with uniform stress throughout the concrete core. the stress in the middle of the column will reach failure load before the poissons effect causes any significant restraint force in the steel shell.
you therefore could theoretically get a partial failure in the middle of the column.
then, supposing the column is taller(thus becoming slender) , then does it mean that the effect of restraining due to poissons effect will be there?
plz help!!