Simultaneous Calculation Of Membrane Or ETFE Foil With Stiff Primary Structure Elements

 Simultaneous calculation of flexible membrane or ETFE foil structures with stiff primary elements presents a unique engineering challenge. Here's why, along with the approaches used to tackle this:

Challenges

  • Nonlinear Behavior: Membranes and ETFE foils exhibit nonlinear stress-strain relationships. Their stiffness changes significantly with the applied load.
  • Large Deformation: These materials undergo large deformations under wind or pressure loads, changing the overall geometry and load distribution of the structure.
  • Interaction: The interaction between the flexible membrane and stiff elements (steel beams, cables, etc.) is complex and iterative.

Calculation Strategies

  1. Specialized FEA Software:

    • Software capable of handling nonlinear geometric and material analysis is essential.
    • Common choices include: Sofistik, Oasys GSA, RFEM, and Easy (form-finding and analysis focused).

  2. Iterative Analysis:

    • Initial Form-Finding: Software determines the equilibrium shape of the membrane structure under pre-tension or pressure loads.
    • Load Application: Wind, snow, and other loads are applied incrementally.
    • Geometric Update: The software recalculates the deformed shape at each load step, adjusting the stiffness of membrane elements accordingly.
    • Iterative Solution: The process repeats until convergence is reached, resulting in an equilibrium state under the applied loads.

  3. Key Considerations:

    • Pre-tension: Accurate modeling of initial pre-tensioning in the membrane is crucial.
    • Boundary Conditions: Properly modeling the supports and connections between the membrane and stiff elements.
    • Wrinkling Analysis: Evaluating the potential for wrinkling of the membrane under specific loads.
    • Dynamic Effects: In some cases, dynamic analysis is needed to assess wind-induced vibrations or other dynamic behavior.

Example Workflow

  1. Define Geometry & Pre-tension: Input the initial geometry of the membrane and primary structure, along with pre-tension values.
  2. Material Properties: Assign appropriate nonlinear material models for the ETFE or membrane and linear models for the stiff elements.
  3. Form-Finding: Run the form-finding process to obtain the equilibrium shape of the membrane.
  4. Apply Loads: Gradually apply external loads (wind, snow, etc.)
  5. Iterative Analysis: The software iteratively solves for stresses, displacements, and updated geometry at each load stage.
  6. Results Evaluation: Analyze stress distributions, deflections, cable forces, and check for stability or wrinkling.

Important Notes:

  • Collaboration is crucial between structural engineers, architects, and possibly membrane/fabrication specialists for successful design.
  • These calculations are complex and require a deep understanding of structural mechanics, nonlinear analysis, and the specialized software being used.

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