Best way to weld these two bits together?

[Trevorleach]

Two lengths of 6 inch channel, one 7.7mtrs long and the other 700mm long.
They need to be one continuous length for a truck body chassis, the channel will sit on edge on top of the truck chassis, then 100x50 box will form the cross members.
Anyway will a simple continuous butt weld both sides do, or will it need an added strengthening plate as well?
The joint will be at the front of the truck chassis, roughly 1 mtr behind the cab, and the truck chassis can and do flex to a degree, so the front 8 feet or so is held to the truck chassis with heavy springs bolted down, rear of the chassis is bolted soldi to the truck chassis.

 

[voipio]

Vee-grooved butt welds should ensure penetration of the full thickness of the steel by the weld, and give the maximum achievable strength. However, those cutouts will be weak points, due to their size. The sharp corners of the holes will concentrate stresses there. Are the cross members going to be fully welded to the main chassis elements, which should help to spread the stress? How close is this design to any existing designs? What will the insurance companies think of this?

 

[Trevorleach]

Vee-grooved butt welds should ensure penetration of the full thickness of the steel by the weld, and give the maximum achievable strength. However, those cutouts will be weak points, due to their size. The sharp corners of the holes will concentrate stresses there. Are the cross members going to be fully welded to the main chassis elements, which should help to spread the stress? How close is this design to any existing designs? What will the insurance companies think of this?

We have a few other trucks running this exact same set up, I check them every 6 weeks and have yet to see a crack of any kind, the cut holes are where the cross raves (100x50 box) are inserted, the x rave ends terminate into a 6 inch piece of folded 4mm c section channel which forms the side rave.
The cross raves are only welded to the heavy C section on the longer sides, this how just about every hgv trailer maker has done them for many years, in fact I would say decades, if you fully seam weld them, then the welds will crack.
Once they are together they are one seriously heavy/rigid bit of kit, they carry stuff from hay/straw right up to mining equipment and everything between.
No idea on insurance, but custom bodies on hgvs are commonplace, and in many cases a given as no off the shelf stuff is available.

I like the idea of some grinder work and a simple butt weld.

 

[Trevorleach]

Pic of last body, you can see cross rave welds, none of it my work as way to neat!