RF-CONCRETE Surfaces Version 5

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RF-CONCRETE Surfaces Version 5

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2.4.2 Design of Stiffening Moment

Design of Stiffening Moment

After determining the design moments, the program analyzes the concrete compression strut. It checks whether the moments used to stiffen the reinforcement mesh can be resisted by the plate.

In the design details, this check is shown in the Concrete Strut entry:

Figure 2.35 Design of Stiffening Moment

For the determined moments, the program performs a normal bending design at the plate's bottom and top surfaces. However, the design's aim is not to find a reinforcement: Rather, it is to verify that the compression zone of concrete can yield a resulting concrete compressive force that, multiplied by the lever arm of the internal forces, results in a greater moment on the side of the resistance than the acting moment.

The analysis is not verified if the moment on the side of the resistance is smaller than the governing design moment nsstrut even in the case of a maximum allowable bending compressive strain of the concrete and a maximum allowable retraction of an assumed reinforcement.

The current standards regulate the satisfaction of the allowable strains via the limit of the ratio between neutral axis depth x and effective depth d. For this, the stress-strain relationships for concrete and reinforcing steel as well as the limit strains of these standards are used (see the following explanations for EN 1992-1-1).

Stress-strain relations for cross-section design

The parabola-rectangle diagram according to Figure 3.3 of EN 1992-1-1 is used as the calculation value of the stress-strain relation.

Figure 2.36 Stress-strain diagram for concrete under compression

The stress-strain diagram of the reinforcing steel is shown in Figure 3.8 of EN 1992-1-1.

Figure 2.37 Stress-strain relation for reinforcing steel

The allowable limit deformations are shown in Figure 6.1 of EN 1992-1-1.

Figure 2.38 Possible strain distributions in the ultimate limit state

The ultimate limit state is determined through the limit strains: Either the concrete or the reinforcing steel fails, depending on where the limit strain occurs.

  • Failure of concrete, for example C30/37:
  • Limit strain under axial compression:
  • εc2 = -3.5 ‰
  • Failure of reinforcing steel, for example B 500 S (A):
  • Steel strain under maximum load:
  • Simultaneous failure of concrete and reinforcing steel:
  • The limit compressive strains of concrete and steel occur simultaneously.