Survival - Spawn Timers

[Azimuth]

I'm not sure if this is the right section to ask about this, but I'll ask anyway. I play a lot of survival and from experience we've noticed that there is a specific pattern to the spawn timers for tank spawns across all maps. Eg. 1st tank spawns at 0:40, second 1:55, third 2:55. I was wondering if anyone knew if there was a file or config that contained the code or information behind this spawn behaviour? And if yes, where this file is located?

Also if anyone knows of the same for L4D1 it would be appreciated (if it's different or not in the pak01_dir equivalent directory).