Interaction Surface
Description
A three dimensional surface can be generated for any design section representing all possible values of combined biaxial bending (Mx and My) and axial (Ax) that satisfy the Ultimate limit state. The limit state criteria are based upon limiting values of compressive and tensile strains in each element which make up the composite section.
This analysis doesn't suffer from the iterative algorithm convergence issues experienced for certain section geometry when using the 2D Interaction Curve analysis.
Section loads defined via the Define Loads form are shown as vectors on the interaction surface. The arrow head on the load vector represents the total load value. If permanent and variable loads are defined then there will be two vectors shown for each section load. Any combination of Mx, My and Ax falling inside the limiting surface may be considered safe. Any load combination which exceeds the limit state is shown in red. The limit state performance ratio can be obtained by either hovering over these arrows or obtaining a report via the Results button. Where the load vector intersects the limiting surface a small sphere is shown. Hovering over this sphere gives the limiting values at this point.
When analysis is complete a 2D plot for an individual loadcase can be shown by selecting the loadcase from the field labelled 'Loadcase'.
There are graphic options available under the General side toolbar.
The Export Surface button creates a spreadsheet with all of the limiting surface values together with the corresponding top and bottom fibre strains.
Interaction Surface Graphic
Form Graphic
Field Help
NA Angle Increments
The surface is generated by rotating the design section 360° about the neutral axis. The number of rotation increments can be specified here. The default value of 74 is equal to 5° increments.
Strain Angle Increments
The distribution of values around the surface from full compression to full tension are based on two parameters.
This field is the strain increment. The default of 50 typically gives a good distribution together with a value of 1.1 for the Geometric progression of strain (set on the Convergence Tolerance form accessed by selecting the Design Section and then right-clicking on the Navigation Pane Menu and picking the Tolerance command).
Ignore Compression Reinforcement
Check this box to exclude all the beneficial effects of reinforcement above the neutral axis. This is particularly useful for biaxial bending problems in complex shaped sections since it is not always evident which bars are in compression, and to be included in the analysis, compression bars must be restrained.
Loadcase
This field contains a drop-down list of all the loadcases that have been defined in the Define Loads form.
Following analysis, either a 3D surface plot can be show by picking Select All (which is always the first option) or a 2D plot for an individual loadcase can be shown by selecting the loadcase from this field.