 y_{k} is a web plan vector of the member which is perpendicular to the member axis
 y_{I} and y_{J} are the vectors at the nodes I and J respectively and parallel to y_{k}

ϕ
_{I} and ϕ
_{I} are the rotations of the vectors y_{I} and y_{J} (see Table 6)
 Assumption of the mean rotation: ϕ
_{k}=(ϕ
_{I} + ϕ
_{J})/2