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Composite Design

Last modified by StruSoft Developers on 2021/03/22 11:30

Available in: 3D Structure, 3D Frame

In FEM-Design, composite columns can be checked according to the so-called "Simplified method" (EN 1994-1-1 6.7.3), where the cross-sections and their reinforcement are designed based on theoretical interaction surface.



To run composite design for a column, it should be modeled with one of the following cross-sections (highlighted in blue):



  • Cross-section must fulfill EN 1994-1-1: 6.7.1 and requirements.
  • Cross-sections with multiple Steel materials are not allowed.
  • Intermediate section cannot be applied.
  • EN 1994-1-1: 6.7.1. (4) is not applied during the design calculation.
  • EN 1994-1-1: (6) is not implemented

Calculation parameters

The Composite column design functions and settings are available on the Composite design tab.

composite column.png

Beside the parameters used in common RC design calculations the following parameters can be set in Calculation parameter dialog.

  • Method can be chosen for checking combined compression and bending.
  • It can be set whether shear resistance of RC part should be considered.


Check and results

Utilization check

Available checks:

  • Structural steel shear utilization (EN 1993-1-1: 6.2.6)
  • Stirrup utilization (EN 1992-1-1: 6.2, 6.3)
  • Concrete utilization for shear (EN 1992-1-1: 6.2, 6.3)
  • Flexural buckling utilization (EN 1994-1-1:
  • Section utilization (EN 1994-1-1: 6.7.3)
  • Crack width check (EN 1992-1-1: 7.3; for fully encased I sections)
  • Torsional utilization has not been implemented, because EN 1994-1-1 does not discuss the cross-section torsional resistance.
  • Reinforcement definition is not obligatory, but the software assumes that the final reinforcement will be designed following the code.
  • The final utilization value can be 1000%, if some code requirements are not fulfilled (usually the detailing rules).


Manual design

Manual design window is divideded into 3 windows: Cross-section, 3D view and Result.

Reinforcement (longitudinal bars and stirrups) and applied sections can be defined.


The result window can display both the theoretical interaction surface and the utilization check results along the column's length.

αM is not used for the calculation of theoretical interaction surface. Depending on the Calculation parameter settings, the interaction surface used in the checking process can be more restricted.



Based on utilization results, User may change the cross-section from the initial one to a more suitable one (with Applied section).


The manual design (Applied section and Reinforcement) can be approved or rejected.


Applied data can be displayed in model space, too:

  • Reinforcement with "RC Bar, Reinforcement preview" Object layer, and
  • Applied section with  "Composite column, applied objects" Object layer.

Detailed result

The table-based result sheet displays the designed reinforcement, cross-section and material data and the following design checks:

  • Structural steel shear utilization
    • The shear resistance of RC parts can be neglected at Calculation parameters, and so all shear design force will be applied on the steel section.
    • The shear resistance of steel section is calculated according to the EN 1993-1-1: 6.2.
    • The used Moment resistances are preliminary values only for the shear distribution.
      • Mpl,a,Rd is calculated as a summation of the analytically calculated plastic moment resistance of the steel sections without strain control (elongation at failure: 15%).
      • Mpl,Rd is calculated numerically and controlled by steel bar and concrete strains.
      • Ultimate compressive strain in concrete (usually 3.5‰) could limit the composite section plastic resistance, which means the structural steel part could not be fully yielded. In this specific case, Mpl,a,Rd could be greater than Mpl,Rd.

  • Stirrup utilization for shear
    • If reinforcement is not defined in the model, minimal reinforcement quantities are used.
    • This section is not relevant, if the shear resistance of RC parts is neglected.
    • If reinforcement is not defined in the model, VRd,c will be used as the shear resistance of RC parts, which is calculated by using minimal reinforcement quantities.
    • Effective depth is considered the extreme compressed fiber of the concrete part. If reinforcement is not defined, it is calculated as 85% of the height of the concrete part.
    • The data bw is considered as the characteristic width (for example, for an I-shape section: y= concrete width - web thickness, z= concrete height - 2x flange thickness).
  • Concrete utilization for shear (this section is not relevant, if the shear resistance of RC part is neglected)
  • Flexural buckling
    • Ec,eff = Ecm * 1 / (1 + ϕ)  neglects the normal force ratio from the original formula. ϕ is selected from the bar application data by the load combination’s limit state (usually ULS).
  • Section utilization
    • Default settings of Calculation parameters follow the code suggestions for checking the bi-axial design moment into the two principal directions and then the interaction.
    • In FEM-Design, an advanced numerical method is used that allows the general direction examinations.
    • If the shear utilization of structural steel is over 50%, ρ is applied to decrease yield stress of the Av part of the steel section according to the EN 1994-1-1: (3).
  • Crack width
    • Available only for the fully encased I section (because in case of the other available sections steel part protects and encloses the concrete part) if reinforcement is defined.
    • If there is no Load combination with Quasi-Permanent limit state, this section will be not relevant.
    • If reinforcement is not defined, crack width cannot be calculated.

Detailed results can be exported in the following file formats: Microsoft Word, MathML, Mathcad.

The following figure shows an overview of applied reinforcement, applied section and its parts with parameters

detailed result1.png

An example of utilization:

detailed result2.png


Composite design is supplemented by the following tables:

  • Flexural buckling input data,
  • Utilization for Load combination(s), for maximum of load combinations and maximum of load groups.


Parent topic: User Manual