You all seem to be talking about a BLEVE. I had occasion to watch the results of one when a camera crew was interviewing a volunteer fire chief as his crew was battling a fire involving a semi-truck container of liquid natural gas. The camera was recovered after the explosion (the fire crew, camera crew, reporter, fire chief, several police and a number of bystanders were all killed) and the expanding bubble of the vapour fed explosion was clearly evident up until the camera was engulfed.

While such an explosion would cause a lot of fire damage, it would not create a strong pressure wave per se--and certainly not one strong enough to knock out a building support.

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