Why are groups so important if you calculate the buckling lengths?
The buckling strength of a member depends on its buckling length. The buckling length of a member depends on
- the system length
- the end conditions at the start and endpoint of the member. These end conditions can be supports
or an adjacent structure 
.
Since members have to ‘be split’ in order to create a node in Diamonds, the system length and accompanying end conditions are not unequivocally determined. It’s also possible that a certain member is not capable of acting as a buckling support (for example: it is doubtful that a slim L-section will prevent a heavy HEM from buckling).
To explain the principle, consider the green column in the structure above. Assume you want to calculate the buckling length around the y’-axis. Using the corkscrew rule along the y’-axis, the buckling direction of the green columns is to the left or the right.
- if you see bars 1 and 2 as continuous, the system length is 2*L.
- if you see bars 1 and 2 as discontinuous, the system length is somewhere around L depending on the rigidy of the blue bars.
You can already feel that the size of the systemlength is open to interpretation… An interpretation that software cannot and will not make. The solution for this interpretation is defining buckling groups in Diamonds. By doing so, the system length and accompanying end conditions are unequivocally determined. Thus if, you calculate the buckling lengths without defining the buckling groups first, you’ll have no clue what comes out of the buckling length calculations (and the resulting design verification)!
How to define groups for buckling?
Consider the structure below. Focus on the green column. To make (or cancel) a group for calculating the buckling lengths:
- Go to the Geometry configuration
. - Turn on the local coordinate system for bars
. The size of the local coordinate system and fonts can be changed on the right side in the pallet ‘size’.
- Take a solid representation
. - Select ‘buckling around the y’-axis’ in the pallet ‘Show groups’.
- ‘Buckling around the y’-axis’ of the greens bars, means they want to buckle to the left or right. On the left and right side of the green column, we have blue bars. So the blue bars will play a role here.
If you consider the blue bars rigid enough to function as a buckling support, the green columns are discontinuous and bar 1 and 2 should be ungrouped. - Select the green columns and click on
. - If you consider the blue bars not rigid enough to function as a buckling support, the green columns are continuous and bar 1 and 2 should be grouped.
Select the green columns and click on
. - You now defined the buckling groups around the y’-axis, now you do the same thing but for buckling around the z’-axis. Select ‘buckling around the z’-axis’ in the pallet ‘Show groups’.
- Using the corkscrew rule along the z’-axis, the buckling direction of the green columns is to the front or back. There’s nothing blocking the buckling in that direction, both columns have to be grouped.
Select the green columns and click on. - You’ve now defined the buckling groups for the green bars in both directions.
But to be correct, you should define the groups for all bars in the model, for both directions!
