Building a bridge: PT or rough cut??

[Diggin It]

todays tree's are not the same as they were 100 plus years ago...

Processing isn't the same either. I'm convinced they knew things back then that have long since been forgotten. It may have been something as simple as only cutting at certain times of the year and letting the wood age/cure naturally.

 

[/pine]

Has acid rain or something changed the molecular structure of the trees/ wood?...Has the fungi or other microscopic organisms gotten more destructive?

 

[jymbee]

Finally getting around to putting a bridge together that will span a small stream. Main supports are two railroad railings that will sit on very large, flat rocks on each end. Wondering what would be the difference between 2x PT as opposed to 2in rough cut with respect to overall life span? It will be used as a crossing for our 4WD garden tractor and my thinking is if the width is equal to the wheelbase of the tractor that either would provide sufficient support. Thoughts?

Interesting/informative responses-- thanks to all for the insights!

The length of the rails is around 20 feet.
John Deere X748 weighs around 1,200 pounds. Add another couple hundred for a 54 inch mower deck
Have bigger tractors, but the JD will be the only one using this particular bridge.

Here's a snapshot

Again, unless I'm missing something, it seems to me that positioning the railings spaced apart at the same width of the tractor wheelbase would mean that whatever wood is involved it wouldn't need to be anything extraordinary. Of course, something like white oak or locust would be great (we have some 50 year old locust fence posts that I still can't break) but not really necessary.

I believe the nearest rough cut mill has a variety-- so Hemlock would be the way to go if that was the option? Any suggestions as to a good way to tie it all together-- metal & wood?

 

[Diggin It]

The length of the rails is around 20 feet.
John Deere X748 weighs around 1,200 pounds. Add another couple hundred for a 54 inch mower deck

So, with the operator, fuel, etc., you're talking close to 2,000 pounds, which means the design needs to be for a minimum of 3,000

Again, unless I'm missing something, it seems to me that positioning the railings spaced

Explain 'railings'. Track rails? Or some sort of bridge sides that have been removed?

 

[Diggin It]

Has acid rain or something changed the molecular structure of the trees/ wood?...Has the fungi or other microscopic organisms gotten more destructive?

Can't quote them, but I seem to recall reading some things about trees not growing as well due to pollution in the air and soil (dropped everywhere by rain).

 

[fried1765]

For that span, with those rails you will most certainly get some deflection.
All things considered, it will probably work for only 1200 lbs. (plus the decking material).
You should consider shortening the effective span however, by using some additional support closer to the edge of the embankment.
The simplest wood to steel attachment for your situation, might just be tubes of a good exterior grade mastic.

You don't need 2X stock for decking, on such a low capacity bridge.
5/4 stock would be fine, and would lessen the deck weight.

 

[oosik]

When we first move here I cut my ancient Ponderosa pines for firewood. I cut a few( big mistake ) that were 38" to 42" on the butt. I soon learned - they were much more valuable for lumber and a 40" tree is a real ***** to manhandle the rounds & split.

Anyhow - it was very noticeable - the last 30 to 40 years of the trees growth had the growth rings much tighter together the the preceding years. Tighter growth rings equal - dryer years - more atmospheric pollution??

Twenty years ago I had the property selectively logged. Got a premium price for the old pines.

JMHO - I would build the bridge out of 8 x 8 timbers and wide enough to accommodate a larger utility tractor. At least 84" wide.

 

[Sawyer Rob]

Has acid rain or something changed the molecular structure of the trees/ wood?...Has the fungi or other microscopic organisms gotten more destructive?

100 year old barns ect., were built from 200 plus year old trees... Those old tree's grew sloooow, and had many years to build up resistance to rot ect..

Then the climate got warmer and the tree's grew much faster, and were cut MUCH younger, so they just aren't as rot resistant as the 300 year old tree's of yesteryear...

We had a OLD barn made of pine that came off this place, it was still solid even though some of it was nearly on the ground, throw a piece of white pine out of todays trees, on the ground and see how long it last!

SR

 

[Jstpssng]

100 year old barns ect., were built from 200 plus year old trees... Those old tree's grew sloooow, and had many years to build up resistance to rot ect..

Then the climate got warmer and the tree's grew much faster, and were cut MUCH younger, so they just aren't as rot resistant as the 300 year old tree's of yesteryear...

We had a OLD barn made of pine that came off this place, it was still solid even though some of it was nearly on the ground, throw a piece of white pine out of todays trees, on the ground and see how long it last!

SR

Not just climate, but human activity affected the growth; we kept (keep) opening them up to make them grow faster, so there's more sapwood. That also makes the wood less strong and the boards warp more.

 

[Industrial Toys]

Not to be smart, But why go with such a temporary material anyway. Why not STEEL? Maybe Wood Decking.

If I needed to build a small bridge, I would buy an old flat deck highway trailer.