Bending Stiffness
As described in Chapter 2.4.1, the calculation is based on constant element-by-element equivalent stiffnesses. For this approach, is important to have a very fine division in zones of significant stiffness changes (tearing, yielding). If the element division is too coarse, the stiffness conditions may be misinterpreted for some parts and stiffnesses may oscillate. In this case, the FE mesh must be refined in RFEM to prevent these effects.
Another important aspect for minimizing a non-convergence of statically indeterminate systems is damping the stiffness change. A change of stiffnesses occurring too abruptly may lead to the iteration "breaking off", especially in cases where the stiffness decreases strongly due to cracking and yielding (slightly reinforced cross-sections).
The following figure illustrates the relation between moment, curvature, and stiffness.
In accordance with the relations described above, the load-dependent secant stiffness arises according to the following equation: