Question

Describing the procedure for the serviceability limit state design of a floor slab made of steel fiber reinforced concrete. This article shows how to perform the corresponding design for the SLS by means of the iteratively determined FEA results.

Steel-fiber-reinforced concrete is mainly used nowadays for industrial floors or hall floors, foundation plates with low loads, basement walls, and basement floors. Since the publication in 2010 of the first guideline about steel-fiber-reinforced concrete by the German Committee for Reinforced Concrete (DAfStb), a structural engineer can use standards for the design of the steel fiber-reinforced concrete composite material, which makes the use of fiber-reinforced concrete increasingly popular in construction. This article describes the nonlinear calculation of a foundation plate made of steel fiber-reinforced concrete in the ultimate limit state with the FEA software RFEM.

When modeling a reinforced concrete rib with a masonry wall above, there is the risk that the rib is underdesigned if the structural behavior of the masonry is not correctly considered and the connection between the masonry wall and downstand beam is not modeled sufficiently accurately. This article deals with this issue and shows the possible modeling options of such a structure. In this example, the reinforcement is determined only from the internal forces and without secondary minimum reinforcement.

In RFEM, there is an option to couple surfaces with the stiffness types "Membrane" and "Membrane Orthotropic" with the material models "Isotropic Nonlinear Elastic 2D/3D" and "Isotropic Plastic 2D/3D" (add-on module RF-MAT NL is required).

This functionality enables simulation of the nonlinear strain behavior of ETFE foils, for example.

The following material models are available in RF − MAT NL:

Isotropic Plastic 1D/2D/3D and Isotropic Nonlinear Elastic 1D/2D/3D

You can select three different definition types here:

• Basic (definition of the equivalent stress under which the material plastifies)
• Bilinear (definition of the equivalent stress and strain hardening modulus)
• Diagram:
  • Definition of polygonal stress-strain diagram
  • Option to save / import the diagram
  • Interface with MS Excel

Orthotropic Plastic 2D/3D (Tsai-Wu 2D/3D)

This material model allows the definition of material properties (modulus of elasticity, shear modulus, Poisson's ratio) and ultimate strengths (tension, compression, shear) in two or three axes.

Isotropic Masonry 2D

It is possible to specify the limit tension stresses σ_x,limit and σ_y,limit as well as the hardening factor C_H.

Orthotropic Masonry 2D

The material model Orthotropic Masonry 2D is an elastoplastic model that additionally allows softening of the material, which can be different in the local x- and y-directions of a surface. The material model is suitable for (unreinforced) masonry walls with in-plane loads.

Isotropic Damage 2D/3D

Here, you can define antimetric stress-strain diagrams. The modulus of elasticity is calculated in each step of the stress-strain diagram using E_i = (σ_i -σ_i-1 )/(ε_i -ε_i-1 ).

After the calculation, you can evaluate the results of the individual load steps directly in the module windows or graphically in a structural model.

The results include, for example, deformations, stresses, and internal forces of surfaces, as well as deformations and stresses of solids. It is possible to export the result combinations for each load step to RFEM. You can use these enveloping combinations for further designs in the other RFEM add-on modules.

All input data and results of the add-on module are part of the global RFEM printout report.