Vacuum limited by actual rafter spacing

[TheEverest]

Dear all,
Can anyone please tell me why we calculate "Vacuum limited by actual rafter spacing" for the rafter design of supported conical roof?

 

[Geoff13]

The thickness of the roof plate is related to the rafter spacing and the external loads. The formula is in clause 5.10.4.4, and the associated loading conditions are found in clause 5.2.2.e. Your program seems to be solving for the allowable Pe based on your specified b and t values, and then outputting a warning when it's less than your specified Pe.