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# Portal rafter effective lengths

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## Portal rafter effective lengths

(OP)
Hi everyone

I am doing a design check on a steel portal frame using the Combined Stresses module from Prokon's Frame Analysis. I have tried to replicate an old portal design from a textbook to check if my understanding of the theory is correct, but I have come across some differences when checking the rafter strength under ULS loads.

The 305x102x33 kg/m UB rafter is 12.2m from apex centreline to column centreline. Purlins are equally spaced at 2.031m c/c and kneebraces are provided at the ends of the haunches and at the rafter midspan.
The critical load combination on the rafter causes single curvature Mx moments, ranging from -150kNm at the column to +75kNm at the portal’s apex. I modelled the frame in 2D and I assume lateral loads are carried by by purlins and kneebracing.

The following is my understanding of the effective lengths:
KxL = sloped distance between haunches (effective length between supports about member's strong axis)
KyL = sloped distance between purlins (effective length between supports about member's weak axis)
KzL = sloped distance between haunches (effective length between torsional supports)
KeL = sloped distance between kneebraces (if used) otherwise the sloped distance between haunches (assuming that the haunches will provide some lat. torsional restraint to the bottom flange)

Please see attached screenshot
Is my understanding correct? I'd like to optimize a design rather than using overconservatively large members.

### RE: Portal rafter effective lengths

My recommendations, without knowing the particular construction details of your structure, are listed below.

KxL = somewhat less than sloped distance between haunches due to partial fixity at rafter ends.
KyL = sloped distance between purlins
KzL = sloped distance between haunches and peak (center knee brace should provide torsional restraint).
KeL = sloped distance between knee braces (if used) otherwise the sloped distance between haunches

Knee braces can be installed at any purlin as required to shorten KeL, KzL, and KyL

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

### RE: Portal rafter effective lengths

(OP)
Thanks KootK, much appreciated!

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