# Incorrect reaction forces values after changing stiffness **URL:** \ **Category:** General Questions\ **Tags:** results\ **Created:** [June 10, 2025, 9:34am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764 "2025-06-10T09:34:17Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Kraynax](https://avatars.discourse-cdn.com/v4/letter/k/958977/32.png) [@Kraynax](https://prepomax.discourse.group/u/Kraynax)\ **Post date:** [June 10, 2025, 9:34am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/1 "2025-06-10T09:34:17Z")
Hello, I am analyzing the connection of a steel plate anchored to a concrete foundation and I am interested in the resulting forces on the anchors. I modeled the anchors as points attached to the surface of the hole using “Rigid body.” Additionally, point springs are added at the point. I read the forces using “History outputs.” My variable is stiffness K3 (tensile stiffness). ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/1/1dc7cb4c89f535ce6b5e0c0d36f36a82f30cf74e.png) ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/6/6685f28c105c901010a22ca865b5ad545ea0c4c4.png) I have created a model that gives correct results in 90% of cases. I have noticed that when the stiffness is changed, there is sometimes a significant reduction in the results (convergence occurs faster, the resulting forces on the anchors are significantly smaller). I have created a file in which I record the reaction forces on the anchors and the time needed to obtain the results depending on the value of stiffness K3. ![image](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/7/7315717e8a88aff4a172ad7bbfa57e53458d2d41.png) I wonder why such breakdowns occur. Could you help me? I am attaching logs from correct calculations and uncorrect one. [ccx\_output.zip](https://prepomax.discourse.group/uploads/short-url/mRLNZIn1VzwLRCKs6ZUqNIxwKzn.zip) (14.2 KB) ---
**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:** [June 10, 2025, 9:52am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/2 "2025-06-10T09:52:55Z")
Can you share the .inp file from one incorrect calculation ? I could check it in different version of CalculiX, submit in Abaqus and so on. ---
**Author:** ![Kraynax](https://avatars.discourse-cdn.com/v4/letter/k/958977/32.png) [@Kraynax](https://prepomax.discourse.group/u/Kraynax)\ **Post date:** [June 10, 2025, 10:03am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/3 "2025-06-10T10:03:17Z")
Sure, thank you [input.inp](https://prepomax.discourse.group/uploads/short-url/afGDRWEAzoLXvBgUVcajmLd7a9.inp) (61.1 KB) ---
**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:** [June 10, 2025, 10:41am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/4 "2025-06-10T10:41:58Z")
I would need the include file (combinedMesh.inp) to run it too. ---
**Author:** ![Kraynax](https://avatars.discourse-cdn.com/v4/letter/k/958977/32.png) [@Kraynax](https://prepomax.discourse.group/u/Kraynax)\ **Post date:** [June 10, 2025, 10:47am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/5 "2025-06-10T10:47:05Z")
Sorry, you’re right [combinedMesh.inp](https://prepomax.discourse.group/uploads/short-url/5UDL1Ytgu0PC5f7h5jwvJ47sAu9.inp) (668.2 KB) [input.inp](https://prepomax.discourse.group/uploads/short-url/afGDRWEAzoLXvBgUVcajmLd7a9.inp) (61.1 KB) ---
**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:** [June 10, 2025, 12:04pm UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/6 "2025-06-10T12:04:33Z")
Since you can’t get RF from nodes without BCs, I requested spring forces in Abaqus: - anchor1-spring-dof1: 977.6 N - anchor1-spring-dof2: 6878 N - anchor1-spring-dof3: 1780 N - anchor2-spring-dof1: 903.8 N - anchor2-spring-dof2: -221.6 N - anchor2-spring-dof3: 1635 N - anchor3-spring-dof1: 8119 N - anchor3-spring-dof2: 3344 N - anchor3-spring-dof3: 2557 N But you need two-node springs to properly model anchoring (the other node of each SPRING1 element is not available and grounded automatically so the plate is not anchored to the foundation but rather fixed to some imaginary supports floating in the air). Also, your mesh is very coarse and consists of linear tetrahedrons that should be avoided in most cases because they overstiffen the models. ---
**Author:** ![Sound\_Spinning](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/sound_spinning/32/4239_2.png) [@Sound\_Spinning](https://prepomax.discourse.group/u/Sound_Spinning)\ **Post date:** [June 11, 2025, 7:51am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/7 "2025-06-11T07:51:38Z")
As well as the comments above, why is convergence involved at all in this ‘linear’ looking model? Have you tried 1st to solve it linearly and check on sum of reactions? That’s all you need 1st. PS: pls do not use tet4 structural elems, like ever. Switch to tet10 (high-order), always. ---
**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:** [June 11, 2025, 7:59am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/8 "2025-06-11T07:59:02Z")
> [@Sound\_Spinning](#): > > why is convergence involved at all in this ‘linear’ looking model? It actually has all 3 types of nonlinearities - Nlgeom, plasticity and contact. ---
**Author:** ![Kraynax](https://avatars.discourse-cdn.com/v4/letter/k/958977/32.png) [@Kraynax](https://prepomax.discourse.group/u/Kraynax)\ **Post date:** [June 11, 2025, 8:31am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/9 "2025-06-11T08:31:13Z")
Thank you for the answer. These RF from Abaqus are looking good. They are as I would expect. That’s good information. Can I somehow request spring forces in Prepomax or the “history output” option is enough in this case? Also I’ll try with second order elements and maybe then the values will be simmilar. However, is there option to use two-node springs in PrePoMax? ---
**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:** [June 11, 2025, 8:39am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/10 "2025-06-11T08:39:22Z")
> [@Kraynax](#): > > Can I somehow request spring forces in Prepomax or the “history output” option is enough in this case? Unfortunately, there’s no direct way in CalculiX: [How to get SPRING2 element stress? - CalculiX (official versions are on www.calculix.de, the official GitHub repository is at https://github.com/Dhondtguido/CalculiX).](https://calculix.discourse.group/t/how-to-get-spring2-element-stress/1102) Only workarounds like this: [Extracting springs force and elongation - CalculiX (official versions are on www.calculix.de, the official GitHub repository is at https://github.com/Dhondtguido/CalculiX).](https://calculix.discourse.group/t/extracting-springs-force-and-elongation/2212) > [@Kraynax](#): > > However, is there option to use two-node springs in PrePoMax? Only by editing CalculiX keywords: [Spring between two nodes](https://prepomax.discourse.group/t/spring-between-two-nodes/116) ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [June 11, 2025, 10:25am UTC](https://prepomax.discourse.group/t/incorrect-reaction-forces-values-after-changing-stiffness/2764/11 "2025-06-11T10:25:26Z")
> [@Kraynax](#): > > These RF from Abaqus are looking good. Hi Kraynax, I agree with @Sound_Spinning that one first needs to check reactions. And in this sense, I’m curious. ¿How do you know those Reaction forces are ok ( in balance with external forces ) looking at the Springs? I don’t think they are. In fact I think they can’t be. Does your sum of Ri = External Fi ? EDITED: This is why I say looking only springs you can’t check balance. ![imagen](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/f/ff2a0e34cf82c369a5d79b0c0231658cc14fd3f0.jpeg) You have a contact and prying effect. Your bolt reaction can blow up if your plate is not stiff enought. Reaction at the plate contact should be included into the equation.