# FoS in a model with different types of structural elements **URL:** \ **Category:** FLAC3D\ **Created:** [July 29, 2025, 2:42pm UTC](https://forum.itascainternational.com/t/fos-in-a-model-with-different-types-of-structural-elements/2184 "2025-07-29T14:42:50Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Zakaria](https://avatars.discourse-cdn.com/v4/letter/z/b5ac83/32.png) [@Zakaria](https://forum.itascainternational.com/u/Zakaria)\ **Post date:** [July 29, 2025, 2:42pm UTC](https://forum.itascainternational.com/t/fos-in-a-model-with-different-types-of-structural-elements/2184/1 "2025-07-29T14:42:50Z")
Hello everyone, I’m trying to conduct a FoS analysis for a quay model (figures below). I tried to reduce cohesion friction for zones, yield tension for cables, plastic moment for piles etc… but the model is always stable even with significant reduction factor. Maybe there is something to which I’m not paying attention… Do you have recommendations on how to trigger global instability in this kind of structure-dominated system? Thanks in advance! ![{79045FCB-BFCD-42B7-8D1E-1C7B9BE251A1}](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/b/be47833977378866395c421f69e871a082c3e3b9.png) ![{28830CE2-DFFA-4413-8EA5-FBAC2DF8044A}](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/2X/5/50a6b21622e2d95358aab6026875726b9b40c4cd.jpeg) ---
**Author:** ![cheng](https://avatars.discourse-cdn.com/v4/letter/c/ecccb3/32.png) [@cheng](https://forum.itascainternational.com/u/cheng)\ **Post date:** [July 30, 2025, 6:48pm UTC](https://forum.itascainternational.com/t/fos-in-a-model-with-different-types-of-structural-elements/2184/2 "2025-07-30T18:48:45Z")
If you run the `model factor-of-safety` command, FLAC3D will automatically calculate the Factor of Safety (FoS) using the built-in strength reduction method. This approach reduces the strength of the zones only, but it provides a useful estimate of the FoS range. This can help guide your selection of an appropriate reduction factor if you are performing manual FoS calculations. **Note:** For models that include both zones and structural elements, the default unbalanced force ratio criterion of `1e-5` may not be sufficiently strict. In such cases, using a smaller value—such as `1e-6` or `1e-7`—is recommended to ensure better convergence and more reliable results. ---
**Author:** ![Zakaria](https://avatars.discourse-cdn.com/v4/letter/z/b5ac83/32.png) [@Zakaria](https://forum.itascainternational.com/u/Zakaria)\ **Post date:** [July 30, 2025, 7:12pm UTC](https://forum.itascainternational.com/t/fos-in-a-model-with-different-types-of-structural-elements/2184/3 "2025-07-30T19:12:20Z")
okay, thank you @cheng