Is it very common for the bees to chew away about 1/2" or more of the comb
in the bottom story of a hive?
 
I once made up some entrance-inserts of hardboard (masonite), which when
inserted into the 3/4" high bottom-board entrances would reduce them to
3/8". This also provided a 3-4" "tunnel" into the hive under the lowest set
of frames
(something like the front portion of a slatted rack).
 
Well, the tunnel entrances may well have discouraged gnawing of the bottom
combs, but I can't be sure.  In some colonies the bees built quite a bit of
burr/ladder comb off the bottom of the frames, behind the tunnel portion.
Eventually, the bees ended up chewing at the hardboard anyway, so within
one summer there wasn't much left of the inserts.
 
I am especially curious about this gnawing behavior as it applies to deep
hives such as the Modified Commercial, Jumbo or Dadant. In many of the
photos I've seen of these hives, a considerable portion of the lower brood
combs is non-existant, having been chewed away by the bees.  Therefore, the
actual comb depth is quite a bit shorter than the frames.  It doesn't seem
very effective to use the extra-deep brood frames if you only end up with
9" deep combs in them.
 
So... I wonder what others see in their hives, especially with the deep
frames, and also wonder what can be done about it.   Do the bees sometimes
replace the missing comb?                          Thanks all,          JWG