follow up on torsional constant j
 

follow up on torsional constant j
there was a previous thread (
calculation of torsional constants is fairly non intuitive, for open re-entrant sections made of plates.
i haven't got a value for u channel, but for an i section that is roughly equivalent i get 672e3, which does not seem unreasonable. typically j for open channels is ~1% of that for similar box sections.
however, using polyakov's spreadsheet off the internet i get a touch less than 3e8, which is frankly unbelievable.
cheers
greg locock
for a channel the correct way to calculate j, as with all open plate sections, is a summation of (1/3)bt^3 for each linear piece of the section. so for your shape which i believe i am interpreting to have two 400mmx12.5mm sides and one 200mmx12.5mm side the j value would be 1/3*2*400mm*12.5mm^3 + 1/3*200mm*12.5mm^3 = 651,042mm.
that would be where i was going wrong then.
i was looking at 'b' and 't' in a global sense. so for the 400mm side sections b=12.5mm and t=400mm. when i should have been using t=12.5mm regardless of orientation.
see this forum, knowledge management at its finest.