RF-CONCRETE Members – Online Manual Version 5

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RF-CONCRETE Members – Online Manual Version 5

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2.4.7.1 Material Properties

Material Properties

Method according to EN 1992-1-1, clause 5.7

The design according to EN 1992-1-1, clause 5.7 is based on mean material properties that have been calibrated to realize a global safety factor. The result is a reduced concrete compressive strength that represents a controversial subject due to the distortion of the average characteristic curve for concrete.

Calculational mean values of material strengths
  • Stress-strain curve for steel according to EN 1992-1-1, Figure NA.3.8.1
Table 2.4

fyR = 1.1 ⋅ fyk

ftR = 1.08 ⋅ fyR

Reinforced steel high ductility

ftR = 1.05 ⋅ fyR

Reinforced steel normal ductility

Es = 200 000 N/mm2

Modulus of elasticity for steel

  • Stress-strain curve for concrete according to EN 1992-1-1, Figure 3.2
Table 2.4

fcR = 0.85 ⋅ α ⋅ fck

Ecm

mean modulus of elasticity for concrete (secant)

The following relation between the global safety factor R and the mean material strengths applies:

  • Concrete (γc = 1.5) : 1.5 ⋅ 0.85 = 1.275 ∼ γR = 1.3
  • Reinforcing steel (γs = 1.15) : 1.15 ⋅ 1.1 = 1.265 ∼ γR = 1.3

Figure 2.29 shows how the reduced concrete compressive stress fcR is represented with the calculational mean values in comparison with the concrete's stress-strain diagram. The strong distortion of the characteristic curve for concrete is clearly recognizable. It results in an overestimation of strains, particularly in highly utilized areas, thus leading to overestimated curvatures.

Figure 2.29 Stress-strain relation for internal forces and deformation analysis

Looking at the concrete's characteristic values, we can see the following: Though the theory is based on reduced stresses (0.85 ⋅ α ⋅ fck), according to EN 1992-1-1, clause 3.1.5, the modulus of elasticity corresponds to the mean value.

Method according to EN 1992-1-1, clause 5.8.6

Clause 5.8.6 of the Eurocode standard describes the nonlinear calculation of structural components prone to instability risks. According to EN 1992-1-1, clause 5.8.6 (3), we need to define the stress-strain curves on the basis of design values.

Design values of the material strengths for the calculation of internal forces and deformations, as well as for design on cross-section level

  • Stress-strain curve for steel according to EN 1992-1-1, clause 3.2.7
    • fyd = fyk / γs
    • ftd = k ⋅ fyk / γs
    • Esm = mean modulus of elasticity for steel (200 000 N/mm2)
  • Stress-strain curve for concrete according to EN 1992-1-1, clause 3.1.5
    • fcm = fcd = α ⋅ fck / γc
    • Ec = Ecd = Ecm / γcE