D5.D.5.3 Clause 6.3.2.3(1) – LTB for rolled sections or equivalent welded section

The UK-NA specifies different values for the λ_LT,0 and β factors to be used in equation 6.57 of BS EN 1993-1-1 for rolled and equivalent welded sections. The current implementation in STAAD.pro does not differentiate between rolled and welded sections and uses the default values in BS EN 1993-1-1 for λLT,0 and β. The values specified in the UK-NA are:

For rolled sections and hot-rolled & cold formed hollow sections:

λ_LT,0 = 0.4

β = 0.75
For welded sections:

λ_LT,0 = 0.2

β = 1.00

The current implementation of STAAD.Pro uses the buckling curves based on Table 6.5 of BS EN 1993-1-1:2005. The UK-NA specifies different limits and buckling curves to be used in this clause as given below:

Table 1. Buckling curves to use with BS EN 1993-1-1:2005
Cross Section	Limits	Buckling Curve
Rolled doubly symmetric I and H sections and hot-finished hollow sections	h/b ≤ 2	b
	2.0 < h/b ≤ 3.1	c
	h/b > 3.1	d
Angles (for moments in the major principle plane)		d
All other hot-rolled sections		d
Welded, doubly symmetric sections and cold-formed hollow sections	h/b ≤ 2	c
	2.0 < h/b ≤ 3.1	d

This table again does not specify which buckling curve is to be used in case of welded doubly symmetric sections with h/b ≥ 3.1 and welded non-doubly symmetric sections. Hence for these cases the new implementation will still use the method specified in the base code as per clause 6.3.2.2(2).