How are the internal forces for downstand and upstand beams (ribs) determined in the program?
For modeling downstand or updstand beams, use the member type "Rib." First, the beam must be defined as a rectangular cross-section. In the dialog box for the details of the beam, specify an effective width. From the rectangular beam and the effective width, a resulting T-shaped section is created.
For 2D models, there are no axial forces in the members and surfaces, but only the internal forces Vz, MT, My or mx, my, mxy, vx, and vy.
For this to apply for ribs, another calculation method than the 3D model is used. The T-beam (defined from member cross-section and effective widths) is placed with its centroidal axis in the surface axis.
Now, if you change the effective width, the stiffness of the system changes too!
In 3D, the same specifications are necessary. You need to define the rectangular cross-section and the effective width. The internal modeling, however, differs fundamentally. The eccentricity is calculated from halve the plate thickness and halve the member height.
For 3D models, the rectangular cross-section defined as rib is indeed connected eccentrically to the surface.
The eccentricity is determined from halve the plate thickness and halve the member height.
Thus, the stiffness of the system is defined and does not change with a change of the effective width (deformation of the system remains the same, not the internal forces).
The axial forces of the T-beam are determined from the sum of the member axial forces and the surface axial forces integrated over the effective widths.
To determine the bending moments, the axial forces as well as moments of the T-beam are integrated over the effective width. The sum of the integral of the surface moment with the member moments yield a part of the T-beam moment. Also added are the integral of the T-beam axial forces and the member axial force, that are multiplied by the corresponding lever arm relative to the center of gravity of the T-beam.
The result is a beam moment relative to the T-shaped cross-section of the entire T-beam.
With the rib internal forces, you can carry out a reinforced concrete design for the T-beam.
In the Display navigator, under "Results --> Ribs," you can switch between rib internal forces and member internal forces.
The advantage of the more complex 3D rib model is that you can use it to consider the stiffnesses and internal forces more exactly.