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2.5.4 Analysis of Concrete Compression Struts
Analysis of Concrete Compression Struts
To design the concrete compression strut of a shell, it is divided into three surface layers that are subjected to the design membrane forces.
For shells where the applied moment is relatively large in relation to the applied axial force (ed/h > 0.2), the thickness hE of the two outer layers is reduced to 0.35 ⋅ d. For shells subjected to approximately concentric compression, the surface layer thickness hE is increased to half of the plate thickness h. If the relative eccentricity of the axial force ed/h is between 0 and 0.2, the surface layer thickness is interpolated.
For ed, the greater value of the quotients of mx/nx and my/ny is taken.
For the analysis of the concrete compression strut, the strut's compression force nstrut,+z to be resisted is compared to the resistant axial force of the surface layer nstrut,d.
The resistant axial force nstrut,d depends on the thickness hE of the surface layer and the applied concrete strength fcd,08.
The first step to determine the thickness of the surface layer is to determine the provided load eccentricities in x- and y-direction from the internal forces of the linear plate analysis:
The greater load eccentricity in x-direction is computed as governing. It can be used to determine the relative load eccentricity ed/h.
Since the relative load eccentricity is greater than 0.2, this is regarded as a shell that is predominantly subjected to bending. The factor fhE needed for the determination of the surface layer thickness is 0.35.
Thus, the thickness hE of the surface layer is determined as follows:
The design value of the concrete compressive strength is reduced to 80 % according to the recommendations from SCHLAICH/SCHÄFER (in: Betonkalender 1993/II, page 378). This recommendation can also be found in EN 1992-1-1, clause 6.5.2, regulating the design of compression struts in strut-and-tie models.
This value can also be found in the design details (see Figure 2.72).
With this value, it is possible to determine the resisting force of the concrete compression strut nstrut,d.
- nstrut,d = b ∙ hE ∙ fcd,08 = 100 ∙ 45.15 ∙ 16 = 7,224.00 kN/m
The analysis of the concrete compression strut for the top surface is done similarly.