If you add a column to huge table using ssms you may get this error:
"Unable to modify table. Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding."
Simple solution : Tools > Options -> Designers
Set the time out to some large value in seconds (default is 30 which is half a minute) .I like 600 (6 minutes) for the sizes of data that I deal with.
Do note that you will not be notified of any errors till the end of this duration (e.g. sudden network drops). SSMS will also hang for the entire duration. Be patient and don't over do the value unless you intend to leave it working overnight :)