Stiffness analysis in IDEA StatiCa Connection is based on gradual loading of the analyzed member by the defined load effect. In each load step, the rotation of the analyzed member is calculated, and the resulting pairs of bending moment and rotation are plotted as the moment-rotation (M-φ) diagram.
The rotational stiffness of the joint is then derived from this curve. If the load effect includes shear force and normal force in addition to the bending moment (LE2), the rotation and deformation of the joint may develop differently than under bending moment alone (LE1). This changes the shape of the moment-rotation curve and, consequently, the resulting stiffness:
Classification boundaries
According to EN 1993-1-8, these boundaries depend only on the properties of the analyzed member (L, E, I) and on the factor kb, which reflects whether the frame is braced.
They are therefore independent of the load applied to the joint. In the table below, the boundary values Sj,R and Sj,P are identical for all load effects.
As mentioned above, the M-φ curve depends on the combination of internal forces in the load effect. All load components are applied simultaneously and increased proportionally in each load step, so shear and normal force grow together with the bending moment. Moreover, the resistance Mj,Rd decreases. Because Sj,ini is evaluated as the secant stiffness at 2/3 Mj,Rd, Sj,ini is read at a different point on the curve, and the classification can change.
The magnitude of the load, on the other hand, does not affect the result. If the applied loads are too small, they are all multiplied by a common factor until the joint resistance is reached, so only the ratio between the load components matters. This is confirmed by load effect LE3: with a bending moment of 10 kNm instead of 30 kNm, it gives the same Mj,Rd and Sj,ini as LE1.
