How to dig a footing for floor joist support???

Hello:
In restoring my wood flooring, I found a rotten spot around the fireplace. I cut out the rotten flooring, subflooring and then found rot in the floor joists that were touching the masonary of the fireplace.
I plan to cut off all the rot, cripple on a 2x10 and connect it to the rim joist with joist hangers on both sides of the fireplace. That will shure up those two joists.
The problem is that there is a cross beam (2 2x10s nailed together) between these two joists so that the other floor joists can be hung. Since there is also rot on this cross beam, I need to replace them also. It has been suggested that I place a support under the new cross member to act as additional support, so I need to build a footing for the support(s).
How should I build this footing pad? Can I just dig a 18 to 20" hole, say 18" deep and fill it with quickqrete? Would that be strong enough?
What about just laying a 2 foot steel plate flat on the ground and placing my supports on these?
Any help would be greatly apppreciated.
Thanks, Richard
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A plate may work or settle or bend , concrete will be best.
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According to a recent episode of Fix this old House, the foundation footing should be 2' x 2' square. 2' deep. They also reccomend bringing it above the surface of the current floor 2" to 4". IIRC they did use quickcrete, but I would double check that because it does not have the compressive (load bearing) strength of other types concrete. Adding a steel plate would be good as it will prevent moisture from wicking up the concrete into the wood. I plan to do this myself for some failing wood supports on an outdoor deck.
Your other alternative would be to use the metal support columns with the screws to adjust the height, $20-40. These seem to be acceptable by code for support as long as they are on concrete and not wood.
--
Nothing Remains the Same-SLQ

"Richard Holliingsworth" < snipped-for-privacy@boeing.com> wrote in message
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