In compliance with DIN 1054, the allowable soil pressure "Sigma_all" has been entered for the soil contact stress analysis. RF‑FOUNDATION Pro requires a "characteristic value" to be entered. How are both approaches of DIN 1054 and EN 1997‑1 compatible?

Answer

This discrepancy is often caused by changing DIN 1054 to EN 1997.


In the "old" DIN 1054, the design was performed by using the characteristic values on the action side and the allowable stress on the resistance side. The actions were used without partial factors and compared with certain allowable stress. In this case, the "eta" resistance was completely included in the allowable stress.

In the predecessor module of RF‑FOUNDATION Pro, which performed the calculations according to DIN 1054, there was a special tab "Ground Failure Analysis (Service Loads)" for this purpose.

In the Eurocode, the ground failure design is performed in a different way.
Here, a partial factor is applied to the action and the resistance side. Thus, the loading is increased by the factor of 1.35 or 1.5, and the resistance is reduced by the factor of 1.4.

With regard to the "old standard," the "eta" resistance is completely included in the "allowable stress sigma_all."

Under the Downloads link below, you can find a model file for RFEM or RSTAB, which clarifies the problem in RF‑/FOUNDATION or RF‑/FOUNDATION Pro. Here, the design has not been performed by using the user-defined entry of the soil pressure, but using the allowable stresses from the standard case tables. There should be the same soil with approximately the same foundation dimensions resulting or the Eurocode and for the old standard.

The following assumptions for the foundation have been made in both add-on modules:
  • Cohesive soil
  • Pure silt - UL
  • Stiff consistency
  • Embedment depth of the foundation t = 4.92 ft
When using the all. soil resistance according to DIN EN 1997‑1 from the standard case tables, the factor of 1.4 is already included. The base values Sigma‑R,d(B) of the soil resistance also differ by the factor of 1.4 compared to the allowable soil pressure "sigma_all" (DIN 1054).

Results of the comparison:

Ground failure design according to DIN 1054 in RF‑/FOUNDATION (old):

Ground failure design according to EN 1997‑1 in RF‑/FOUNDATION Pro:

In spite of different input values, the results from DIN 1054 and EN 1997‑1 are comparable.

If you want to recalculate the foundation in RF‑/FOUNDATION Pro, which has already been designed with RF‑/FOUNDATION (old), you would have to apply twice the soil pressure Sigma_R,k:

Sigma_R,k (input in RF‑/FOUNDATION Pro) = 1.40 (partial factor for ground failure) x 1.35 (resistance on the load side in the example) x Sigma_all (from RF‑/FOUNDATION (old)).

In the attached file, this has been done in CA2 in both add‑on modules. In this case, the allowable soil pressure has been entered as 4,595 kip/ft² in RF‑/FOUNDATION. In RF‑/FOUNDATION Pro, 8,688 kip/ft² has been entered.


Keywords

Foundation Ground failure Soil contact stress Soil pressure Characteristic Allowable

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