I built a 20' beam by sandwiching plywood and 2x10s. I alternated 3 layers of 1/2" plywood and 2 2x10s - so plywood, 2x10, plywood, 2x10, plywood. I glued them, clamped them and screwed them all together. I let the glue dry for 24 hours. If I prop up both ends and have two adults stand in the center (about 300 lbs total), it sags less than 1/2 inch. This will be supporting just the ceiling joists, sheetrock and insulation with nothing else. Do you think this is sufficient for a beam?