Lateral restraints can be incredibly significant if we accurately take into account their effect on the effective length of an element. Consider a 10m long Rafter with multiple Purlins connected to its top flange. With the Compression flange restrained, the effective length of the Rafter will be significantly reduced for downwards bending (the Purlins will have no effect on upwards bending). Considering the effect that 4 equally spaced Purlins would have on the effective length of the Rafter is that Kx (effective length factor about the x-x axis) would be 0,25, which makes the effective length of the Rafter about the x-x axis = 2,5m. The effect on the design outcome is very significant.

Considering a standard Portal Frame modelled in SUMO and then exported to Combine using the relevant Design Link:

You will notice that there are multiple nodes along the length of the Rafter. These nodes were created when the Purlins were modelled in will automatically be considered as lateral restraints and will affect the effective length of the Rafters.

Navigating to the Members tab, you will notice that all of the effective length factors are set to 1 but the effective lengths will all be a lot shorter than the actual length of the rafter. That is because the program considers each node along that element as a lateral restraint, until you indicate otherwise. This is applicable and ideal for this specific scenario.

Running through all of the design checks, none of the Rafters, selected as 254x146x31 I-beams, fail. If I now indicate that those internal nodes are not to be considered as member ends (therefore not having any effect on the effective length) the effective lengths now shoot up from a maximum of 0,750m to 5,257m! This resulted in the percentage utilization to jump up to 84% versus 10%. Unless fly-braces have been modelled, this would need to be the case for any upwards forces (wind loading).

To indicate nodes to exclude as member ends, navigate to the Members tab and simply select those nodes to exclude. A table on the righthand side of the window will populate with these nodes, which can be easily removed if need be.

It is very important to model lateral restraints accurately so that their effects are taken into consideration and excluded when applicable.

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