Longest path in under specific WBS

You can use float path filter. 

To calculate float path:

1) Press F9 (run schedule) then click Options
2) Select Advance Tab then tick calculate multiple float paths
3) Tick Free Float then on Display multiple float paths ending on activity click ... on right side of text box and select the last activity of WBS (say Block A) that wanted to calculate longest path.
4) On Specify the number of float paths to calculate -> change the number to 1
5) Then click Close and click Schedule
6) After running the schedule, create a filter and name it with Float Path (recommended name) Where Float Path equal 1 (criteria)
7) Apply the filter and it will show the longest path of selected WBS

Do the same to other WBS

Rauf,

Each of the 20 buildings should be a separate project. Of course, you need a Global Activity code instead of/as well as the project specific WBS which is such a dissapointment in an 'Enterprise' environment.

The scheduling options allows selecting one float path per portfolio, or one float path per project with a single tick box.

filter

ALL

where wbs equals Block A

and 

where longest path equals yes

Rauf,

Each of the 20 buildings should be a separate project. Of course, you need a Global Activity code instead of/as well as the project specific WBS which is such a dissapointment in an 'Enterprise' environment.

The scheduling options allows selecting one float path per portfolio, or one float path per project with a single tick box.

I am not a P6 user, I do not have the software, but suggest looking at the following link.

How to Display the Critical Path Leading to a "Specific Activity” in P6? - Project Control Academy

Maybe you can create a finish milestone for each individual Block and then find multiple float path, then create a filter for the path(s) number you are interested.

Good Luck