Hi,
I posted an idea for using plywood sandwiched with foam for a torsion box. The idea was not well received. They were worried the foam would fail under a light load.
I am renovating my laundry room this spring so last fall I mounted my washing machine on 2 3/4" sheets of MDF in between 1 1/2" foam to make a pedestal. I used PL300 foam adhesive to glue the foam and mdf. I bolted it to the floor.
After 5 months and countless loads of laundry, the foam is fine.
If looking to build an easy torsion box or gantry beam, glue plywood around foam. It will be light weight and very strong.
Larry