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2.5.4 Analysis of the Concrete Compression Struts
Analysis of the 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 acting axial force
For ed, the larger value from the quotients of mx/nx and my/ny is applied.
For the analysis of the concrete compression strut, the concrete strut's compression force to be resisted nstrut,+z is compared with 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, it is a shell that is predominantly subjected to bending. The factor fhE for determining 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 of Schlaich/Schäfer (in , page 378). This recommendation can also be found in EN 1992-1-1, clause 6.5.2, which regulates the design of compression struts in framework models.
This value can also be found in the design details (see Figure 2.72).
With it, you can 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 side of the surface is done analogously.