M–N interaction capacity · rectangular section, load factor to the failure envelope

Envelope

stage

p1 · permanent

p2 · + live

Load factor

—

LF = (n3−n1)/(n2−n1),
≥ 1.0 passes

drag p1 and p2 on the plot · the load path is extended to p3 on the envelope

point	n	m	meaning
p1	—	—	permanent load
p2	—	—	permanent + live load
p3	—	—	where the path meets the envelope

Dimensionless axial force n = N / (b h f_c) and moment m = M / (b h² f_c). Failure envelope m = 0.5n − (2/3)n² up to n = 0.75 for the elastic stage II, and m = 0.5n − (β/α)n² up to n = 2β/α for the concrete stage III, symmetric about m = 0. The load path from p1 through p2 is extended to its intersection p3 with that envelope; LF is how far along the permanent-to-live path the section gets before it is reached. Mirrors MN_cap.m.