How do you determine how thick to make a concrete housekeeping pad, or do you just assume that a 4" or 6" thick pad will be sufficient for most cases?

If the pad is just for housekeeping, 4" is adequate. For airhandling units verify that there will be enough height under the unit to trap the cooling coil condensate drain (trap height depends on static pressure in the coil casing & trap configuration depends on if coil is draw thru or blow thru). For pumps,large fans, rotating machinery, provide concrete inertia base per 2003 ASHRAE Handbook, HVAC Applications, Chapter 47 Sound & Vibration Control, Table 42 Selection Guide for Vibration Isolation.

 Tell me when your going to turn that air handler on,
we can check with the National Geologic Event Agency
and see if they record any seismic activity on their equipment.
 Is that for a wind tunnel ? I hope your ductwork
don't colapse and get sucked into the fan.
 Sorry about the jokes, but wow, that's a lot of pressure.
 Improper trapping leads to several problems. If the trap outlet is too short


 the negative pressure created at system start-up will pull water from the trap into the air handler. The "seal" is destroyed, producing the same effect as an untrapped system. If the trap outlet is too tall


 negative pressure will prevent drainage, causing the condensate to back up into the system resulting in property and equipment damage.

Theoretically if you have a fan generating 6