Initial Forces Control Data

Initial forces, which are defined in Load > Initial Forces > Small Displacement > Initial Element Forces, are added to a specific load case. Or initial member forces are generated from Member forces due to a specific load combination. The initial nodal and element forces resulting from such external load can be checked in Results>Result Tables> Equilibrium Element Nodal Forces / Initial Element Forces.

For the backward stage analysis of a cable stayed bridge, the nodal forces of the completed structure subjected to tension and dead loads are calculated. This makes possible to consider the zero displacement condition while the internal forces exist in the structure.

Also, by considering member forces induced by the self weight of a structure during a pushover analysis, we can avoid creating another time history load case for the self weight.  

The nodal forces are found after multiplying the nodal displacement and the stiffness of the structure, and the member forces are obtained by adding the fixed end forces (moments) to the nodal forces.

From the Main Menu select Load >  Static Load >  Initial Forces > Small > Initial Forces Control Data.

Add Initial Force to Element Force

Check on to add Initial forces, which are defined in Load > Initial Forces > Small Displacement > Initial Element Forces, to a specific Load Case. This function includes the Initial Force input in the selected specific Load Case.  The Initial Force, being an integral part of the Load Case, is included in Load Combinations.

Load Case: Select the load case that reflects the Initial Force.

Initial Force Combination

This function generates Initial Force through linear analysis. Load cases and scale factors are entered to calculate Initial Force for Truss, Beam and Link elements. The generated Initial Force is produced in Initial Element Forces and Equilibrium Element Nodal Forces in Results > Result Tables.

Load Case

Select and enter load cases for which initial axial forces are to be saved or added.

Scale Factor

Enter a scale factor for each load case.

To enter a new or additional load case

Enter the relevant load case and scale factor, and click .

To modify previously entered load case

After selecting the relevant load case, modify the scale factor, and click .

To delete previously entered load case

Select the relevant load case, and click .

Check to Reflect Initial Axial Forces into Geometric Stiffness

Check if Initial Axial Forces are considered.

Note

For reference, in midas Civil, the geometric stiffness consideration due to various types of analysis is as follows: