The twist factor kt depends upon the flange restraint conditions and the cross section shape. If the critical flange switches from top to bottom within the segment, the critical flange thickness is assumed to be the thickness of the flange at the end of the segment. For AS4100 and NZS3404, kt is calculated from table 5.6.3(1), while for AS1250, SABS0162, BS5950, HK CP2011, IS800, EUROCODE 3 and AS3990 it is taken as 1.0.