On 10/15/2015 1:41 PM, dpb wrote:
On 10/15/2015 3:00 PM, Don Y wrote:
On 10/15/2015 12:53 PM, J wrote:
A few years back (~5) I built a rustic bridge over our seasonal stream by
....
...The stream channel is about 16' from bank to bank and ~4' deep. I
cut the tree trunks to about 20', so about 2' of contact with the
ground on either end.
....

trees every 5 years, so I'm looking for a better - and hopefully simple
solution. I don't really have the energy or skill for something fancy. If
I could get the 4"x6"x20' pressure treated beams ...
... and if they would work in this application, I would do that. But
failing that - is there another way to solve this problem? (like maybe
nailing together some shorter overlapping 2"x6" boards?).

Trying to keep it as simple as possible. Thanks in advance for any ideas.

What sort of *traffic* are you trying to accommodate? How accessible
is this area? What sort of maximum water flow do you have to accommodate
(obviously, "0" is your pictured low end! : )

I did a quick beam deflection calculation for SYP 4x6 on 16-ft spacing with a
dead center load of 150 lb plus distributed 3.5 lb/ft. Since that's one beam,
the actual load for two would be 300 lb which ought to cover the kind of
traffic I'd think this would bear given what it looks like would be from the
picture...

Anyway, center max deflection for that loading is roughly 0.5" which indicates
it would be more than adequate for the purpose.

As for using something other than 20-footers which undoubtedly will be pricey,
a couple 10's with a (say) 3/8" flat plate of 3-ft length bolted thru with 1/2"
bolts would work and be reasonably easy to do with only a couple of folks and a
temporary bracing to hold the ends up while making the connections. Pre-drill
the plates and then bore the holes in the beams in place. Could get a little
fancy and have somebody fabricate them with a little arch in 'em and have some
rise to the center for some aesthetics, too...

Gets as involved from there as may care to be including adding a center riser
with a tension rod/cable from each end to stiffen 'er up, etc., etc., etc., ...

Any lumber in that environment is going to rot, mold, splinter (from freezes)
etc. over time. (pressure treated lumber?)

I'd, instead, consider placing some culvert sections in the streambed
and covering them with rock and soil. Given the apparent amount of
debris in the area, something of relatively large diameter may be in
order. And, from time to time, it may be necessary to "unclog"
portions. But, not prohibitively difficult (any moreso than something
coming downstream and taking out a "bridge"

[Hard to get a grasp on the sort of grade that's involved...]