I am covering a topic on welding at university and I am struggling with deriving the second moment area for the structure attached. The desired second moment of area is also given in the attached image.
The first part is particularly confusing to me as I know that the total second moment area for the two vertical rectangles should be d3/6 but it is actually 2d3/3. On wikipedia I found that this is to do with being the moment of inertia for a rectangle collinear with an axis but I am still very confused as to what this means.
Any insight into this would be great,