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2018-02-13

Prestressed Concrete Design in RFEM

Modeling Structure

Structural modeling is the first step of the prestressed component design. In RFEM, you can enter beams, frames, and plates, or model complex buildings and bridges simply and quickly. RFEM 5 includes the environment typical for CAD software, so the program handling is intuitive.

A wide range of typified member cross-sections is available in the program for modeling frame and truss structures. Special cross-sections can be defined in SHAPE-THIN and used for the finite element analysis in RFEM.

The input of loads is facilitated by the implemented load generators, such as for snow load determination. A load combination is created automatically according to the selected standard. It is also possible to define load situations manually, for example in the case of specific construction states. The photorealistic visualization of a model in 3D rendering provides a direct check of the input data.

Tendon Geometry

The definition of the 3D tendon geometry with respect to the shape of the structure can be very time-consuming and difficult. This applies especially to spatially curved structural components. The spatial problem can be simplified to 2D geometry by projecting these components into the XY and XZ planes. In this case, the beam is displayed directly in the XY and XZ planes, and the definition of the tendon geometry is thus greatly simplified.

Another possible difficulty may be the definition of a tendon shape in a slender prestressed beam. This problem may be caused by the fact that beams are usually very long and flat. When displaying the entire beam, the cross-section height is very small and it is very difficult to recognize the tendon distribution in detail. In this case, there is an option in RF-TENDON to scale the prestressed concrete beam vertically and horizontally. Thus, you can display the height of a slender beam as larger so the tendon shape is more recognizable.

The tendon geometry can be generated automatically using the standard shape of the tendon. The program generates the tendon geometry with regard to the existing supports. If necessary, it is possible to adjust the position of the tendon subsequently.

The program creates the 3D geometry of a tendon by merging both projections in the XY and XZ planes. The 3D tendon diagram is generated from both projections of the curved beam. This way, you can obtain the tendon geometry of the curved prestressed concrete beam quickly and easily. For straight prestressed concrete beams, it is not necessary to work in two projections.

Prestress Effects 

The equivalent loads that correspond to the prestress effects are automatically determined as the resulting forces with regard to the eccentricity and the directional change of the tendon. These equivalent loads can be displayed graphically. It is also possible to graphically compare the equivalent loads from the prestress with the external load from the constant and variable load. This way, the prestress effect can be recognized easily in the design in RF-TENDON.

The prestress effect on the design is controlled by the intensity of the prestress forces. According to Prof. Tung-Yen Lin, the quasi-permanent component of the external load should be compensated by the equivalent load from prestress. This "Load Balancing Method" can thus be used for the preliminary design of tendons. Both the external loads and the equivalent loads from the preload can be graphically displayed in RF-TENDON.

The stress in the tendon and the equivalent loads from prestress are calculated with regard to the tendon losses. Anchor slippage and friction losses are determined by direct integration of the strain along the tendon length. The calculation method fully considers all angular changes of the 3D tendon geometry. The stress losses due to the elastic concrete strain are determined using the ideal cross-section properties. The long-term losses are calculated according to the design standard, taking into account the strain losses due to the creep and shrinkage of the concrete subjected to permanent loads, and the stress losses in the prestressing steel due to the relaxation under tension. The load history is considered when determining the creep effect.

Structural Analysis

Prestress effects apply automatically to the structure, and structural analysis using the finite element method can be performed. The prestress effects are calculated with regard to the primary and secondary effects. The internal forces of design strips of the prestressed plate can be integrated into the internal forces of a virtual result member and used subsequently for the cross-section design. The internal forces, including the prestress effects, are calculated in RFEM and exported to RF-TENDON Design.

Design of Prestressed Cross-Sections

The final step in the design of prestressed components is to verify the prestressed concrete cross-section. For this, the RF-TENDON Design add-on module is used. In this program, the design according to Eurocode 2 and SIA 262 is available. Nationally determined parameters and methods can be considered in accordance with eight National Annexes, including DIN EN 1992-1-1/NA and ÖNORM EN 1992-1-1.

Each cross-section shape and reinforcement layout can be checked by calculating the 3D stress distribution in the cross-section. Furthermore, the predefined reinforcement templates are available for typical cross-section shapes. The position of the tendons and ducts in the cross-section are automatically taken from the 3D tendon geometry.

RF-TENDON Design checks whether the cross-section meets SIA or Eurocode 2 standard requirements for both the ultimate and the serviceability limit state designs, including the bridge requirements of EN 1992-2. Moreover, RF-TENDON Design can consider the interactions of all internal forces such as axial forces, bending moments, shear, and torsion. The serviceability limit state values are checked during the design of stresses, crack widths, decompression, and deflection.

Providing the option of an extended analysis with the stress-strain response of the cross-sections, stiffness calculation, and the possibility to display the moment-curvature diagram, RFEM is a powerful software for structural analysis according to Eurocode and SIA. Due to the modern graphical user interface, clear information about the design status, the option to get brief results quickly or to carry out detailed design, as well as a system with warnings and recommendations, the program is extremely user-friendly and universal.


Conclusion

All the necessary steps to perform the prestress design in RFEM 5 can be called by the user. RF-TENDON is an effective, easy-to-understand and user-friendly program for designing prestressed beams and frame structures, as well as plate and complex structures. In addition, the program offers a simple and intuitive input, which allows you to perform quick preliminary designs very efficiently. RF-TENDON and RF-TENDON Design provide a complete solution and support the user when performing the prestress design and dealing with Eurocode 2.

Reference

[1] Navrátil, J.: Prestressed Concrete Structures, 2nd edition. Ostrava: Technical University of Ostrava, Faculty of Civil Engineering, 2014

Author

Mr. Meierhofer is the development leader of programs for concrete structures and is available for the customer support team in case of questions related to reinforced and prestressed concrete design.

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