Q.: In high-rise construction we never remove our shores until we're satisfied by tests that the floor has achieved sufficient strength. In recent years we've switched from cylinder tests to cast-in-place cylinders that are removed from the floor at the time of testing. Nevertheless, being aware of structural failures, I'd like to know whether there is any other precaution we could adopt that would make me feel just a little more secure against a catastrophic failure.
A.: One contractor with 40 years experience tells us that when he strips the lowest set of shores he always leaves about 25 percent of them in place--just as the kind of precaution you're inquiring about. This means that he has the equivalent of 2 1/4 floors of shoring instead of 2, but he always feels safer. When he strips the next floor he takes out 75 percent of the shores and goes down to the lowest shored floor and removes the last 25 percent. It's a little bit of a nuisance to work on two floors instead of one, but it makes him and his crew feel better, and it may pay off in preventing a failure. In case it is expected that loads will be applied to any part of the top floor he also carries some point shoring down three levels below that load.