The upper span assembly forms the part the overall assembly that connects the boom to the cross member and then on to the brace. This piece is simple enough a simple T shape with 3 holes that position it at a 45deg to the boom when assembled. However at 11.113mm thick it still can not handle the stresses that it would be subjected to in operation.
The high stresses are concentrated along the beck edge where 2 of the holes are. This area undergoes a stress of 855.099MPa, way in excess of the 240.0MPa strength of the material.
It is at this point we should begin to re-consider the design of this cross member, this is the second part in the assembly that has generated a very unfavourable result when subjected to design loads.
However in this case the overall dimensions reflect a thickness that is at the upper limit of a standard metal thickness. Now this is not to say that you can’t select another thickness you could get up to 12.7mm and still be in a standard size thickness, or you could just design and specify your own that would suffice.
This is an easy solution, however this iterative solution, of just increasing the dimension, means that for every change that happens to one component there is a ripple effect to other parts and components. This may lock the engineer or designer into a loop that produced and un wieldy part or assembly. It could also result in some parts being overdesigned just to ensure a good fit with another part.
Just designing your way around a particular problem would produce a convenient solution for that part but may overlook the overall design. Thus when this particular part produced the result trying to iterate through the solutions meant that the design of other components would have to be changed.
The increased thickness of this part would mean changing the other parts in a very great way, which may be too much or it could point to the fundamental problem of a flawed original design. This I think is the real problem, too many components were made to do the job. From the mounting to the boom then to the main cross-member section then onto the roller rest.
The new design may change the overall geometry of the parts as well not just the number of parts. This must still allow for dis-assembly as this was a critical feature for the original, to allow for dis-assembly for transport.