Defining Symmetry Beams
On the right-hand edge of the model, in the Y-direction, we wish to assign a 'symmetry' beam (a beam element to represent half of a 'normal' beam lying along an axis of symmetry), as shown below:
N.B. Beams on defined lines of symmetry need careful treatment. If the full beam section is used it will implicitly increase the degree of support provided to the slab along this line. To compensate for this it is necessary to reduce the effective size of the beam. This is done by defining a special beam with flange width and web thickness which are half of the actual values.
To do this:
1. The "Properties | Beam Sections" dialogue box should already be open following the last round of section-type allocations. If this is not the case, reopen this window.
2. Choose the "Add" tab to create a new section type.
3. In the 'Number' drop-down box, the number '4' is automatically generated.
4. Select the 'Type', 'Family' and dimensions of the desired beam. In this instance, to create the 'symmetry' beam, we choose 'I Section', 'Custom' and enter the required dimensions as shown (i.e. the exact dimensions as for the 356 x 127 x 39 UB section, but with half the web thickness):
5. Select the number of (FE) divisions to use in the beam (in this instance, 5 in both the local X and Y directions is sufficient).
6. Click the 'Add' button to enter the new section type on to the database.
7. Click the "Attach" tab to assign the new section type to the required elements.
8. Select the required section type (i.e. '4') from the drop down list.
9. Choose the members that require the 'symmetric' cross-section by clicking them on the main screen.
10. Click the 'attach' button.
N.B. When a section (number) is selected in the 'attach' mode, all of the members currently assigned that section size are indicated in red, providing a useful check on data entry.