Not sure exactly what you are saying. Are you thinking of bolting your
beam onto a post, mounted on the side?

I would suggest putting the beam directly on top of the posts. You can
purchase post attachments which will hold the beam in place.

I just built an 18' wide deck. My beam was 3 2x12's together, seated
directly on top of 6x6 posts. I trust the beam on posts much better
then the beam bolted to the post. However, I have seen the side mount
thing done.