Rune shards can be used on any AW if you know how ,,when the shard bar is full you DO NOT have to place it on that actual AW select any AW you are in the process of adding runes to just place it in that AW .
SHARDS are just rune building blocks.
Also if you have an AW that is complete but not yet placed and it has a ton of extra shards .
if you dont intend on placing it in your city at any time ,just place it ie build it and then sell it right away and all them extra shards are now available to build runes with,, when you have a full rune complete just select another AW you want to use and place the rune for that AW.
It is good way to make use of rune shards that would just sit around for ever and a day .
hope this helps
Keith