Bridges normally weigh much more than the live load they are capable of carrying. This design will actually support many times its own weight, without failing.
This 3' model, built entirely from soft, 1/4"x3/4" cedar lath, for example, easily holds both girls even though it's only pinned together with a dozen brads, not properly assembled, and a bit wobbly with 226 lbs. on it. The bridge model weighs 3.5 lbs.