Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 75 L is the developed length of the rafter pair from column to column, taken as span/Cos θ ( θ is the roof slope) I r is the in-plane second moment of area of rafter Factor  cr,r,est This calculation should be carried out if the frame has three or more spans, or if the rafter is horizontal. For frames with rafter slopes not steeper than 1:2 (26°),  cr,r,est may be taken as:     r yr r c cr,r,est tan 2 275 1 55,7 4                                  f I I L h I L D r But where  ≤ 1,  cr,r,est = ∞ where: D is cross-sectional depth of rafter, h L is span of bay h is mean height of column from base to eaves or valley I c is in-plane second moment of area of the column (taken as zero if the column is not rigidly connected to the rafter, or if the rafter is supported on a valley beam) I r is in-plane second moment of area of the rafter f yr is nominal yield strength of the rafters in N/mm 2  r is roof slope if roof is symmetrical, or else  r = tan -1 (2 h r / L ) h r is height of apex of roof above a straight line between the tops of columns  is arching ratio, given by  = W r / W 0 W 0 is value of W r for plastic failure of rafters as a fixed ended beam of span L W r is total factored vertical load on rafters of bay. If the two columns or two rafters of a bay differ, the mean value of I c should be used.