The bending moment in the middle of the Stratolaunch's central wing is 800 million inch-pounds, same as an African bull elephant standing on the end of a diving board a mile long.
The wing spars weigh 60,000 lbs each, and there are four of them. The six engines weigh about 9500 lbs each. Which means absent those two components, the whole of the largest wingspan aircraft ever built, even with twin fuselages, weighs barely more than 200,000 lbs.
That is gossamer light.
This looks a bit like Virgin Galactica’s aircraft except for lack of the mid section fuselage. Hopefully no test pilot dies proving this thing like what happened back in 2014 with the Virgin Galactica plane out in the desert.