The LZ would be the rafter length. I would think using 12.948 m or 11.558 m, as appropriate, should be OK. LY would be spacing between purlins or lateral braces if any. The LY and LZ are unsupported lengths used for flexural buckling and would affect the axial capacity. However to specify the laterally unsupported lengths for the compression flanges (LTB), you need to assign UNT or UNB parameters in addition.
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