# Best Practices: Pretensioning Setup for Rectangular Membrane/shell Structures

**URL:** <https://prepomax.discourse.group/t/best-practices-pretensioning-setup-for-rectangular-membrane-shell-structures/2923>\
**Category:** General Questions\
**Created:** [August 11, 2025, 12:32pm UTC](https://prepomax.discourse.group/t/best-practices-pretensioning-setup-for-rectangular-membrane-shell-structures/2923 "2025-08-11T12:32:26Z")\
**Posts on this page:** 1\
**Showing post:** 6

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**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\
**Post date:** [August 11, 2025, 6:36pm UTC](https://prepomax.discourse.group/t/best-practices-pretensioning-setup-for-rectangular-membrane-shell-structures/2923/6 "2025-08-11T18:36:16Z")

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> [@Broamb](#):
>
> **My revised understanding:**
> 
> 1. **Step 1** : Gravity + Wind Load (establish baseline loading state)
> 2. **Step 2** : Add pretensioning with **Nlgeom ON** (should reduce displacement from Step 1)

Isn’t pretensioning supposed to happen before operational conditions such as wind load ? The loading sequence should correspond to real life situation.

> [@Broamb](#):
>
> Is **Nlgeom absolutely critical** for thin membrane pretensioning effectiveness?

Nlgeom has to be enabled to get accurate results when there are large strains/displacements, preloads or structural instabilities (buckling). This is nicely explained here: [Flow Chart: Do I need nonlinear geometry? - Enterfea](https://enterfea.com/nonlinear-flow-chart/) and here: [Geometrically nonlinear analysis - how does it work? - Enterfea](https://enterfea.com/geometrically-nonlinear-analysis-introduction/)

> [@Broamb](#):
>
> Should pretensioning forces be applied as **incremental** in Step 2, or as **total values**?

This is explained in point 2 here: [Understanding multistep analyses](https://prepomax.discourse.group/t/understanding-multistep-analyses/1526)

> [@Broamb](#):
>
> **Current issue:** With elastic material and thin geometry, I suspect the membrane isn’t developing sufficient membrane action to benefit from pretensioning - it’s behaving more like a flexible plate in bending.

Keep in mind that PrePoMax and CalculiX have both shell elements and membrane elements. The latter have no bending stiffness.

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