How to compute the resistance of this metallic part (steel angle iron), for a joist fitted in an I-profile beam?

FreeCAD FEM doesn’t support multi-step analyses so you would have to define it manually (by editing the input file). Either using the old-fashioned approach with thermal expansion or just using the *PRE-TENSION SECTION keyword available in CalculiX.

I have just tried to re-open the “plate” file I attached but it won’t open. I get an error message “unknown exception”???
I would appreciate it if someone could let me know if they can open this file.

Cant open “FEM Plate Analysis” file too.

Error:

09:20:25  Pick next point
09:32:19  Traceback (most recent call last):
  File "<string>", line 1, in <module>
<class 'OSError'>: Invalid project file C:/GA-NT/Anwendungen/TS_513538/kann_weg/FEM Plate Analysis.FCStd: ios_base::failbit set: iostream stream error

with

OS: Windows 10 Version 2009
Word size of FreeCAD: 64-bit
Version: 0.20.2.29177 +426 (Git)
Build type: Release
Branch: (HEAD detached from 0.20.2)
Hash: 930dd9a76203a3260b1e6256c70c1c3cad8c5cb8
Python 3.8.10, Qt 5.15.2, Coin 4.0.1, Vtk 8.2.0, OCC 7.6.3
Locale: German/Germany (de_DE)
Installed mods: 
  * BIM 2021.12.0
  * CfdOF 1.24.3
  * CurvedShapes 1.0.4
  * Curves 0.6.8
  * Defeaturing 1.2.0
  * dodo 1.0.0
  * fasteners 0.4.54
  * Manipulator 1.5.0
  * Plot 2022.4.17
  * Reinforcement
  * WorkFeature

Prestressing bolts with ccx, FYI:
https://www.youtube.com/watch?v=ZSDf1bNXo8g

many thanks thomas and NewJoker!
@Laurie can`t open too

I am not sure but I think I created the problem by switching the development and stable app images and saving the original plate with a new name.

I am still having problems with the file so I am going to create a new one.

EDIT: Here is a link to it.
https://www.dropbox.com/scl/fi/yumxkl1ql07hokr89en4k/Rev1_Plate_FEM.FCStd?rlkey=lv9qdojidw6mshzou6lygtg9r&dl=0