beams/Posts for Elevated deck

beams/Posts for Elevated deck

Not a structural engineer, but I've been reading up on concepts. I am in the process of designing a deck. Due to the grading, it's off the first floor but elevated at 8 ft from the ground. Here is the basic design I am trying to accomplish:

My current plan is to use Trex Elevations LGS framing to build the deck as open as possible below and at as high a psf as reasonably achievable. Here is the code report / span tables for Trex Elevations:

My first option is to attach to the house at the ledger planning for 100 psf. I'm trying to size the 17' beam labeled Beam 1, and I'm having trouble understanding what is listed in the span table for the double box beam at 100 psf.

The tributary width carried by that beam is 9 ft (5 ft. towards the house, and 4 ft. towards the second beam – the second beam is 8 ft. from the first beam/house, leaving 2 ft. overhang at the edge of the deck).

If I look at the 100 psf span table, a double box beam can handle a 10 ft. joist span and a 4 ft. overhang – which I would think is equivalent to my 9 ft. tributary area for this beam – at a 12'1" beam span. Using a L/4 beam overhang on each side, I can still cover 17 ft. with a 12'1" beam. But there's a Note 11 at the bottom regarding "intermediate joist span" that says I can't do that? That's what has me confused. What am I missing?

I'm open to using a W8 beam across that span if I have to, but no taller in order to maintain head clearance underneath.

As an aside, also looking to use helical piles for the supports. Here's what I am looking at currently:

Thanks /r/engineering !

Submitted July 16, 2017 at 10:36AM by dr_rex
via reddit

beams/Posts for Elevated deck

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