On Tue, 16 Oct 2012 16:06:56 -0700 (PDT), gary wrote:

4 x 4 redwood posts are bolted to the straps in concrete piers.

When replacing the posts, how can I prevent the bottom of the posts from deteriorating/rotting?

Nothing will be perfect. Wood in the ground won't last forever. However, if
the posts extend beyond (deeper than) the concrete into some gravel, the
bottoms will drain and they won't rot so fast. About the worst thing you can
do is put wood into concrete, without letting it stick through the concrete.