1. The link provided above is not valid for sharing with everyone who has it.

2. Weld calculations use national standards. I once used the British Standard for allowable stress. Please do some research and find the appropriate national standard for your country for welding.

3. You have to choose the approach you plan to take since four methods were given in the first video.

4. I assume your CAD model is a solid.  I suggest you start by creating a Midsurface model. If you Midsurface in SpaceClaim, it will pass the thickness on to Mechanical, however you can Midsurface in SolidWorks and manually enter the wall thickness. Split the welded edge of each channel along the length of the beam at the start and end of each weld segment. Then you can apply Fixed Joints (or Weld features shown in one of the videos) to extract the force going through each weld segment.