This is a question with no definitive answer.
I guess I would go back and reexamine the premise that you can't put in a seperate foundation. Is it an issue of cost? Are there feelings that a separate foundation will jeopardize the future utility of the building?
As you can see many from the postings some smiths have good luck not having a slab crack. Others cracked almost imedeately.
Factors effecting why slabs crack or don't crack include:
What is the subgrade, the dirt under the floor (sandy, clay, high organic matter) Sand is best, clay and high organic matter worse
Is the subgrade compacted?, Is it tight against the floor. Subgrade can subside leaving the slab in poor contact with the subgrade.
What is the concrete specs and condition?
What reinforcement is present if any?
I doubt if you have any above information other than the general condition of the concrete floor.
Thus it is pretty much the luck of the draw....
Strategies to minimize the potential for cracking include:
1) A Larger base. Construct a base larger than the hammer to spread out the stress more.
2) Resiliant material. Wood, rubber, and similar items will absorb the vibration. Incorporate these into the base as they will reduce the shock transmission to the floor
3) I admit this one is way out in left field. Springs, air bags and similar items can be very effective at dampening out vibration and shock if properly selected. Some are 98% plus effective which is what you need. These present 2 problems: 1)Cost, you can spend hundreds of dollars and more on these. 2) Movement of the power hammer during operation. (it may go up and down as the isolators flex)
Google "Punch Press Vibration Isolators" or "Vibration Isolators"
Let us know how you procede.
Remember there are no garuanties (sp)
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