RFEM 6 includes the Form-Finding add-on to determine the equilibrium shapes of surface models subjected to tension and members subjected to axial forces. Activate this add-on in the model's Base Data and use it to find the geometric position in which the prestress of lightweight structures is in equilibrium with the existing boundary conditions.
Until now, the prestress load type had always been an initial prestress in Dlubal Software programs. The defined load magnitude was applied and, depending on the stiffness of the surrounding system, prestress remained more or less as an axial force in the cable.
Friction plays an important role in practice. Without friction, the brakes of cars would be useless, objects on inclined planes would just slide away, and prestressed bolt connections would be impossible.
Prestressed concrete slabs consist of composite, uniaxially stressed hollow plates with a width of about 1.20 m. These elements are prestressed with pre-tension in a precast concrete plant. The precasting is usually done with slipformers. Due to the lesser self‑weight of the non‑solid slab and the existing prestress, these precast prestressed hollow core slabs show a lower deflection than loosely reinforced slabs made of solid concrete.
Numerous nonlinearities can occur in a structural system. The RF-DYNAM Pro - Nonlinear Time History add-on module was developed in order to model them realistically in a dynamic analysis. To explain how the add-on module works, the procedure is described below with an example.
The time-dependent stress losses from creeping, shrinkage, and relaxation have to be considered when designing prestressed concrete components. The consideration of relaxation losses when designing prestressed concrete in RF-TENDON and RF-TENDON Design is discussed in detail in the following text.
The efficient design of prestressed structural components requires a few additional steps that go beyond the standard reinforced concrete design, from modeling tendons to the calculation of equivalent loads to the cross-section resistance design. Therefore, it is important that the software for prestressed concrete design is structured and the navigation is possible in the program. RFEM with two add-on modules RF-TENDON and RF-TENDON Design fulfills these requirements and allows engineers to carry out the complete design of prestressed beams, frames, plates, buildings and bridges according to EN 1992-1-1 with National Annexes and SIA 262.
When modeling surface models, such as a frame joint or similar structures, there is always the question of how to model a prestressed bolt connection. In this case, it is always necessary to find a compromise between the practicable and detailed solution. The following article describes the modeling procedure of such a connection, based on the joint diagram calculation method.
Cable and tensile membrane structures are regarded as very slender and aesthetic building structures. The partly very complex double-curved shapes can be found using suitable form-finding algorithms. One possible solution is to search for the form via the equilibrium between the surface stress (provided prestress and an additional load such as self-weight, pressure, and so on) and the given boundary conditions.
The RF-FORM-FINDING add-on module determines equilibrium shapes of membrane and cable elements in RFEM. In this calculation process, the program searches for such geometric position where the surface stress/prestress of membranes and cables is in equilibrium with natural and geometric boundary conditions. This process is called form-finding (hereinafter referred to as FF). The FF calculation can be activated in RFEM globally in the "General Data" of a model, "Options" tab. After selecting the corresponding option, a new load case or a calculation process called RF-FORM-FINDING is created in RFEM. An additional FF parameter is available for defining surface stress and prestress when entering cables and membranes. By activating the FF option, the program always starts the form-finding process before the pure structural calculation of internal forces, deformation, eigenvalues, etc., and generates a corresponding prestressed model for further analysis.
The form-finding process in RFEM seeks an equilibrium state where the defined prestress of membranes and the prestress or length changes of cable elements with boundary reactions are in equilibrium. For this, the program provides the option to define an isotropic or an orthotropic prestress state for membranes.
The RF‑FORM‑FINDING add‑on module can be activated in the "Edit Model - General Data" window, "Options" tab. By activating the module, a new RF‑FORM‑FINDING load case is created and an additional menu appears in the main program, allowing for the definition of prestress conditions for membrane and cable elements.
The equivalent loads determined in RF-TENDON due to prestress are transferred in RFEM as member loads or as line loads. A member load is used for member types with their own stiffness; a line load is used for member types without their own stiffness. In order to understand which values of the concentrated loads are to be transferred from RF‑TENDON to RFEM, you should use the following display settings: ~ Reference of the loads to the global coordinate system (GCS), ~ Load display: "Point"