Hi Stu,
Take for instance a small A572 gr 50 WF shape column with a web of say 5/16" thick and flanges that are 5/8" thick...but has a 3" thick base plate. The 3" thick base plate would govern how much preheat is required. Remember that the thickest part, at the point of welding, governs....in this case a cold 3" thick base plate would be the heat sink that would suck all of the heat from welding out of that root pass and quench the weld metal too quickly. To counter act this, you would add the proper amount of preheat to the base plate to reduce the cooling rate. And this preheat is verified at a minimum distance of 3" from the joint on the thickest part(in this case the base plate) typically using a tempil stick(a temperature melting crayon).
When I write WPSs for other people, I include the preheat temperatures on the WPS itself, but for our people in my shop, I simply write it saying that the preheat is to be in accordance with AWS D1.1 (current edition)Table 3.2. Auditors have challenged me on this practice in the past, but I teach all of our people how to read a WPS and how to use Table 3.2, so I always welcome them to go out and question any welder on my shop floor, if they have any doubts.