Commiserations on your bad luck from me too Andy. Not sure how the garage was tethered but I doubt if the oem provision for doing so is sufficient with many of these type of tent structures. Where strong winds are par for the course in parts of the world I've seen relatively small plastic greenhouses strapped down permanently with a number of strops thrown over the top and fixed to the ground with more substantial anchorage. Its what I would do maybe in except for the most sheltered of locations and for a very brief period of use. Maybe make some heavy anchor points for some slung over-the-top strops/ropes with something like postcrete.
