A.Christopher DeBlois responds: Determining whether or not a slab or footing has adequate capacity to safely support a point load is a two-step process. First, determine the design load carried by the column. Based on the tributary area from the framing at roof, attic, and floors, collect the loads from each level and add them up through the building. Typical column loads in residential construction (not counting giant mansions with football fields for great rooms) can be as little as a few thousand pounds and as much as 10 tons. In the former case, no steel column is even needed and the load could have been supported directly on a slab. Even a 4-inch slab-on-grade can support up to 5,000 pounds without a footing if you put a decent base plate on the column (1/2x9x9 inch, for example). In the case of 10 tons, however, only a good footing designed for the necessary load will do. In a retrofit, plan on cutting out whatever concrete is there and installing a new footing to be certain you have adequate capacity.
The gray area in between is where all those gray hairs come from. If the standard footing is 2x2 feet, then the load capacity of the footing should be at least 10,000 pounds (2 ft. x 2 ft. x 2,500 pounds per square foot), and perhaps even more if the soil has a bearing capacity in excess of 2,500 psf. You can probably support a little bit more...
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