I have build a beam that consists of four longitudinal stringers and 4mm birch plywood.
The beam is 2,4m/8ft long, 15,8cm/6'' high and 5,8cm/2 1/4'' in breadth.
Basically it's an rectangular tube with no bulkheads, You can look inside from one end to the other.
The stringers holds the four plywood sides together.
Two strips above has 20x15mm cross section, the two other 15x15mm.
For assembling I used casein glue and little wood screws.
Weight of the beam is 4128 gram, 1720g/m, 516g/ft
A few hours after construction I have made a load test.
I put the beam with its narrow side onto two wooden strips wich are 2,1m/7ft apart (means span)
Then I put my entire weight (90kg/180 pounds) on it.
The beam bends just 2mm.
Thats amazing, the beam is lighter than the quadratic steel tubes I used for the test frame.
And the steel bends 20mm when I stand on it.