When I was supervising, one of my guys showed up in shorts and sandals one Friday. I just pointed "Dude?" "Okay, I won't wear them again." "No. You're gonna go change." "Seriously?" "Yup." Didn't come up again.
Most people were polos and khakis. Higher ups would be in dress shirts and pants and/or blazers depending on what was going on. We weren't supposed to wear jeans, but as long as they were in good shape, they were given a pass.