The problem with building something that will be exposed to the elements is that Mother Nature gets involved. For a 14 foot span, I would use 2x10's. As mentioned, you can get away with 2x8's in residential construction, where the lumber is protected by the elements, but things change faster with wood left out in the elements.
Another thing to take into consideration is that modern pressure treated lumber is saturated with water in order to get enough zinc into it to achieve it's ratings. That massive amount of water remains in each board for a different amount of time, and it also results in swelling, and then over time, shrinking that is impossible to measure. When building a deck for a client, I have seen 12 foot long 2/6's shrink almost an inch in length in one year!!!! That is the worse that I've seen. But the twisting and warping is just as bad.
How are you going to attach a railing to it? Or is it not high enough up in the air to need one?
Have you considered two metal I beams?
What are you using for a footing to attach it too and how will it drain water so the wood is not sitting in a wet spot?
