Creative Pultrusions sells structural fiberglass. It is plenty strong (if used wisely), it has a tensile strength of over 30 ksi (30,000 psi) in the longitudinal direction, and about 8 ksi crosswise. Bearing strength is over 20 ksi.
We used pultruded fiberglass for our frame the past 3 years, it's very tough...it won't get bent like aluminum, and it takes a lot to break it.
This year we're trying out plywood...partly because we ran out of the sizes of fiberglass we needed...partly because we want to try something new. Should be interesting!