1. → How can I select only those beams of the model which have a given material and/or cross-section?
Turn on Property Editor (Window / Property Editor), select a beam with the given material or cross-section, click the line of material or cross-section then click the Filter icon. Selection will be extended to all beams with the same property.
Click the Filter icon on the Selection toolbar. Select Beam from the selection tree and set all other elements unchecked. It ensures that only beams will be selectable. Click the button Property Filtering. Select the desired material and cross-section only and check if no length and reference filtering is on. Close Property Filtering and Filter dialogs. The next selection operation will only include elements matching the filtering criteria.
2. → Is the rib element on a surface edge automatically independent of the surface if the eccentricity is large enough?
The rib element on an edge is never independent of the surface. It is always assumed that the rib and the surface edge has the same deformed shape. If you want to create an independent rib which does not follow the edge deformation use a beam attached to two surface corners by rigid elements.
3. → Why I do need to use "references" to define finite elements?
References are dedicated to allow you to orient the finite elements in the three dimensional space. Please read carefully the corresponding section of the User's Manual.
4. → How can I change the cross section or the material of a group of finite elements?
Click on the row of the tabulated display of you cross-sections or materials that contain the one you want to replace. Click the Library icon, and select the new material or cross-section. Select the Replace radio-button at the bottom of the window, and click OK.