Kerry, I think you nailed it. These structures are far to large and complex to be safely predictable/reliable. I think one of the main reasons we have had so few catastrophic failures in Oz is because we don't usually build unrealistically large/heavy/complex temporary outdoor structures. I heard a story from a reliable source about a big day out stage and ground support system getting hit with a freak gust lifting the thing a half meter and dropping it. The stage/GS/rigging/lighting and sound ALL FINE. A fav little saying of mine: Screw creative, let's go with realistic! Anyway, that's my 2c
