I tend to just have one corridor in my drawings but with many baselines and thus one surface for datum, one for finished etc etc.
If you have several corridors in your drawing then simply create the relevant datum/finished surface for each, create a new surface called e.g. Composite Datum and paste all the datum surfaces in to it. You can then use the usual volume tools to determine volumes
neilyj (No connection with Autodesk other than using the products in the real world)
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