Node equilibrium in the 3D FEM model
The loads in any node in the structural model need to be in equilibrium. Any unbalanced forces are taken by supports. It is recommended to use a load combination instead of an internal forces envelope.
Each node of the 3D FEM model must be in equilibrium. The equilibrium requirement is correct, nevertheless, it is not necessary for the design of simple joints. One member of the joint is always „bearing, “ and the others are connected. If only the connection of connected members is checked, it is not necessary to keep the equilibrium. Thus, there are two modes of load input available:
- Simplified – for this mode, the bearing member is supported (continuous member on both sides), and the load is not defined on the member
- Advanced (exact with equilibrium check) – the bearing member is supported on one end, the loads are applied to all members, and the equilibrium has to be found
The mode can be switched in the ribbon group Loads in equilibrium.
The difference between the modes is shown in the following example of T-connection. The beam is loaded by the end bending moment of 41 kNm. There is also a compressive normal force of 100 kN in the column. In the case of simplified mode, the normal force is not taken into account because the column is supported on both ends. The program shows only the effect of the bending moment of the beam. Effects of normal force are analyzed only in the full mode, and they are shown in the results.
Simplified input: normal force in the column is NOT taken into account
Advanced input: normal force in the column is taken into account
The simplified method is easier for the user, but it can be used only when the user is interested in studying connection items and not the behavior of the whole joint.
For cases where the bearing member is heavily loaded and close to its limit capacity, the advanced mode with respecting all the internal forces in the joint is necessary.