When building a deck, installing the beam and attaching it to the post properly is such a critical step. There are a number of ways to accomplish proper beam attachment, but there are also wrong ways to do it also.
I see it all of the time, when the beam is split on each side of the post and attached with bolts. Or worse, sometimes just with screws or nails. This requires the fasteners to carry the weight of the deck. Fasteners are not designed to carry weight loads, they are designed for holding lumber in place.
Not to make this an engineering lesson, but basic framing knowledge needs to be followed when building any structure. The weight of the project needs to travel down the structure, not be supported by fasteners.
With the above method the beam is also a lot weaker by being 2 separate pieces as apposed to being doubled up. Having the double beam fastened together will help prevent sagging and warping of the beam and allow easier fastening of the joists.
The beam needs to be resting on to of the post to carry the weight of the structure. Cutting a notch into the post, or using a metal connector are proper way to attach your beam to the post.
Check out the video below where Jason shows us how to properly iattach a beam to a post.