Wind Loading – Local Member Wind Diagonal Values

Date: 02/11/2020

Versions: 2018.03 +

Program: Wind Loading Analysis (in British Standard)

Description

In the British Standard on wind analysis (BS 6399 Pt2) reference is made to wind diagonals. Usually these wind diagonals are the bracing members within a frame which see a reduction in applied wind pressure as they are taking wind loading from the overall frame rather than local higher winds.

However, wind reduction factors can also be applied to local wind on selected members which take wind from a large area, for example portal rafter members where the wind area could be (say) 20m long by 7m wide giving a local ‘a’ diagonal value of 21.2m, rather than the default value of 5m.

This can be taken care of using the ‘Applies to local wind on member only’ option.

Explanation

Normally when you set up wind diagonal groups they are for bracing members in a

structure and the wind load they carry is for the overall building size, hence the

diagonal value is for a whole face of a building. The ‘Applies to local….’ box would

remain unchecked. In this approach the wind pressure is what gets reduced by the

wind diagonal reduction factor, and we have multiple load cases for different wind

diagonals to produce the appropriate force sets.

Now take the case of a portal frame rafter or column. The wind load on these

members is applied directly to them and their diagonal value would be a more local

value of the portal frame. In this case you would check this box to apply the diagonal

reduction to only the members selected in that wind diagonal group. In this approach

the actual wind member load is reduced by the wind diagonal reduction factor, hence

the multiple diagonal load cases are not required. Hence, wind diagonal groups of

this type do not participate in the generation of a separate wind load case with its

own “a” value.

Off course in any frame you may have a mix of wind diagonal groups where this

setting is on and off.

Regards,

MasterSeries Team