you idea is a good one but I don't think will work for this application -
elements of the structure will be dictated for each effort.  In other words
the government will supply some of the basic numbering and definitions and we
will expand on that laydown to break the work structure into much finer
detail and add structure where it needs to be added or expanded.  Each effort
will have different primary structure but always based on a maximum 10 place
indenture structure (99.99.99.99.99.99.99...).  For example 1.2 may be
"systems engineering" in one effort  and have five or six levels of breakdown
detail (1.2.1.1.1.15) whereas in the next effort 1.2 may be something like
"config management" with only a couple layers of detail.  Think of it as
trying to index or track the data in 20 different technical manuals all with
different subjects down to the paragraph or sub paragraph level or even
sentence level.  I don't know if that makes any sense but hopefully you get
the idea.  I guess the basic problem is with a 10 level indenture number
system where each layer can be any number between 1 and 99
(99.99.99.99.99.99.99.99.99.99) and each layer establishes a parent child
relationship as you move to the right, what is the best way of tabling it.  
Maybe just a text field where you type in the X.X.X.X.. number is the way to
go but I'm not sure.  Thoughts??