frame-title

I set the frame-title-format to this:
(set frame-title-format '(:eval (if (buffer-file-name) 
(abbreviate-file-name (buffer-file-name)) "%b")))
This works nice for buffers, but not so nice for directories.
Can anyone show me, what to do, to show the (abbreviate-file-name 
dired-directory) in a similar way.
So that the frame-title works for files and directories.

Thanks fo any help.
Dieter