Downstand Beams, Ribs, T-Beams: Specifics in Design
In the case of combined FEM structures (surface and member elements) as well as folded plate structures, it is possible to attribute a beam structure for the design on a member to a fictitious T-beam cross-section, whose geometry depends on the effective width. When using the 'Rib' member type in RFEM, the stiffness is represented by a slab component (surface element) and a web component (member element). This approach has some design specifics, which are explained in this article.
In the case of bending stress and a compression zone on a slab side, it is assumed that the slab participates in the load bearing effect. The effective width is a geometric size, which can be used for the application of approximately constant, maximum concrete compression stress.
The rib defined in RFEM is transferred to the RF-CONCRETE Members add-on module in such a way that beff,i = bi is set.
If beff,i does not correspond with bi, a user-defined adjustment is possible. Generally, the effective width beff,i is a size that is not determined automatically in the program and thus has to be specified by the user. The effective widths can be adjusted in Window 1.4 of RF-CONCRETE Members after selecting the check box 'Show rib effective widths for determination of internal forces’ (see Figure 2).
Both the integration width and the effective width can only be constant for each member in RFEM. This corresponds to the requirements of EN 1992-1-1 provided in Section 18.104.22.168 (4). If a special accuracy is required for the determination of internal forces, it would be necessary to split the downstand beam into individual members, depending on the different areas of the effective widths.
If a design is carried out for the rib in RF-CONCRETE Members and the structure is additionally designed with RF-CONCRETE Surfaces, the internal forces already taken into account for the member design over the integration width can be neglected in the surface design. The respective setting in the RF-CONCRETE Surfaces design module can be activated in the details (see Figure 3).
The torsional stress must be differentiated according to its cause.
If the torsion in statically indeterminate structures is caused by the compatibility of deformations (compatibility torsion), it is not necessary to perform the torsion design, according to EN 1992-1-1, 6.3.1 (2). Related to this paragraph, MT should not be taken into account for the rib design. You can deactivate MT in Window 1.6 of RF-CONCRETE Members (see Figure 4).
If the torsion is caused by static equilibrium conditions, the torsion design is required, otherwise the structure without torsional stiffness would be unstable (equilibrium torsion).
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Frequently Asked Questions (FAQ)
- When calculating deformations in RF‑CONCRETE Members, I get jumps in the deformation diagram. Why?
- I do not obtain any deformations in the results of the calculation with RF‑CONCRETE Surfaces. What can be the reason?
- I get Error 108 when designing steel stress in RF‑CONCRETE Surfaces. Why?
- When determining internal forces in RFEM, I obtain compression axial forces that do not arise in the design internal forces in the design in RF‑CONCRETE Surfaces. Why, what is the reason?
- Is it also possible to reduce the shear force on a support or perform design with the shear force at a distance d to the support in RF‑CONCRETE Surfaces?
- When performing "manual definition of the reinforcement areas" in RF‑CONCRETE Surfaces, do I have to completely reinforce the entire structural component manually? Or does RF‑CONCRETE Surfaces apply the required reinforcement in the areas where I have not performed the manual definition?
- When I create a user-defined result value, the RFEM solver window opens briefly and the calculation is apparently performed again. Why? I have already performed the calculation before.
- For design with CONCRETE NL, is the creep applied to the entire cross-section, or to the concrete compression zone only?
- Can I design a reinforced concrete structure according to ÖNORM in RFEM?
- Why can I no longer display the intermediate results in RF‑CONCRETE Members?
Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements
Design of reinforced concrete members and surfaces (plates, walls, planar structures, shells)