Just [load supported]^2/[mass of tower]. Personally, I think the scoring system is a little weird, but I do like that they're trying to put a little more emphasis on holding more weight.
HSO2014 — Astro, Chem Lab (R), Forensics, Geomapping (S), Maglev (R/S), Mission (I), TPS (I)
"A million people can call the mountains a fiction, yet it need not trouble you as you stand atop them." ~xkcd