On Sun, 3 Jun 2018 07:39:08 -0700 (PDT), trader_4
wrote:

On Saturday, June 2, 2018 at 8:53:02 PM UTC-4, Ed Pawlowski wrote:
On 6/2/2018 7:44 PM, david wrote:
I am building a foot bridge over a creek. I don't want to try and sink
footers/posts in the creek itself,Â* I am wondering how long a 2x12 can
span. I am able to find 26 foot treated 2x12's locally.Â* If I sandwiched
two of them
together on each side of a say a 3 foot wide foot bridge would that
work?Â* The
ends of the bridge will be treated 6x6 posts (so 4 total).Â* The span
between
posts will almost be the full 26 feet.Â* My question is:

1. is this safe?
2. is the perfect?
3. is this overkill?

Thanks
David


I don't know for sure, but you can play with some numbers here and get a
rough idea
http://www.awc.org/codes-standards/c...tware/spancalc

According to this, you need some support, but this may be to drive across.
https://www.askthebuilder.com/bridges-joist-sizing/
If you are going to build a small bridge that spans perhaps 8 feet or
less, I would use 2x8's that are placed 16 inches on center. A span of
12 feet would require 2x10's and spans between 14 and 18 feet would
require 2x12's. Anything over 18 feet would require 4x4 or 6x6 posts mid
span to cut the actual span in half.

If you are thinking of building any bridge with a span greater than 20
feet, you better get a structural engineer involved, plain and simple.


Using that span calculator if you build it 3 ft wide, use 12" spacing,
which would be four 2x12, best wood, accept maximum deflection,
minimum loading, it's just barely at the limit at 26 feet. I think
others are right here, there are other better solutions, but they are
likely going to cost more and be more involved. Another factor to
consider is how sound the earth is on either side for the support.

He could always go with a swinging bridge.

http://gfretwell.com/ftp/Ed%20Labrad...20B%20test.jpg