A few years back (~5) I built a rustic bridge over our seasonal stream by laying deck boards over a couple of tree trunks (5"-6" diameter) that I cut down. 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.

The bridge was a little bouncy, but otherwise seemed structurally sound - that is, until now. I had hoped that by placing the ends of the tree trunks on gravel and some pressure-treated 4x4s it would last for many years, but the trunks have rotted in the middle and begun to crack, and the bridge is no longer safe.

The reason I cut down a couple of trees in the first place, was that even though it apparently exists, it's very hard to get ahold of 20' pressure treated lumber. I had thought that a couple of 4"x6"x20' beams might work well to support the bridge - though I never did any actual load calculations.

I'm not feeling the love for the work involved of cutting another couple of 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 a friend to help move them), 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.

Photos of the current bridge he https://www.flickr.com/gp/7865232@N08/9104G5

-J

On 10/15/2015 12:53 PM, J wrote:
A few years back (~5) I built a rustic bridge over our seasonal stream by
laying deck boards over a couple of tree trunks (5"-6" diameter) that I cut
down. 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.

The bridge was a little bouncy, but otherwise seemed structurally sound -
that is, until now. I had hoped that by placing the ends of the tree trunks
on gravel and some pressure-treated 4x4s it would last for many years, but
the trunks have rotted in the middle and begun to crack, and the bridge is
no longer safe.

The reason I cut down a couple of trees in the first place, was that even
though it apparently exists, it's very hard to get ahold of 20' pressure
treated lumber. I had thought that a couple of 4"x6"x20' beams might work
well to support the bridge - though I never did any actual load
calculations.

I'm not feeling the love for the work involved of cutting another couple of
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 a friend to help move
them), 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! : )

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., ...

--

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...]

On 10/15/2015 3:51 PM, Don Y wrote:
On 10/15/2015 1:41 PM, dpb wrote:
....

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...]

Well, I addressed only the sizing question, but yes (or no ) I'd not
just place the ends on the ground, I'd pour a small pier or similar to
set the ends upon...and, if did that, could more than likely with a
little extra landscaping make 16's work alleviating the joint or
extra-pricey beams.

I'd think it a "feature" as opposed to the maintenance headache of the
culvert (and it's at least a possibility he can't fill that contour,
anyway, given all the restrictions on waterways and all any more.

--

On 10/15/2015 2:00 PM, dpb wrote:
On 10/15/2015 3:51 PM, Don Y wrote:
On 10/15/2015 1:41 PM, dpb wrote:
....

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...]

Well, I addressed only the sizing question, but yes (or no ) I'd not just
place the ends on the ground, I'd pour a small pier or similar to set the ends
upon...and, if did that, could more than likely with a little extra landscaping
make 16's work alleviating the joint or extra-pricey beams.

I'd think it a "feature" as opposed to the maintenance headache of the culvert
(and it's at least a possibility he can't fill that contour, anyway, given all
the restrictions on waterways and all any more.

Dunno. My uncle had a "brook" (no idea as to the technical definitions
of brook, stream, river, etc.) separating his property from neighbor.

Neighbor's property was slightly "uphill" from uncle's. And, brook ran
at a slight grade between the two.

He fabricated a bridge spanning the brook (I doubt it was more than 12? ft
long -- hard to get an accurate picture of childhood memories where everything
always seems so much bigger!)

Brook was not deep -- wade across it with pant legs rolled up. And,
only a couple of feet below the bridge (at the "low" side of the bridge).

But, the bridge kept needing work. Soil erosion on the banks,
frost heave, "weathering" from the sun/elements, railings getting
wobbly, nail pops from the planking, periodic "stain" (wood
preservative) etc.

By contrast, the culvert 20 feet "up river" (up brook??) never saw
*any* maintenance. I.e., the *easier* task would have been to just
drop a length of culvert in place of the bridge (but, it wouldn't
look as "quaint")

A similar situation existed a few doors up from my childhood home.
Eventually, the "brook" was just routed underground for a few
blocks. End of the maintenance issues.

On Thu, 15 Oct 2015 15:41:37 -0500, dpb wrote:


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., ...

Would some sort of pre made floor truss be at all practical?


--
Using Opera's mail client: http://www.opera.com/mail/

On 10/15/2015 3:53 PM, J wrote:

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

Put in a pair of treated 4x4 support posts in the center.

A few years back (~5) I built a rustic bridge over our seasonal stream
by laying deck boards over a couple of tree trunks (5"-6" diameter)
that I cut down. 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.

The bridge was a little bouncy, but otherwise seemed structurally
sound - that is, until now. I had hoped that by placing the ends of
the tree trunks on gravel and some pressure-treated 4x4s it would last
for many years, but the trunks have rotted in the middle and begun to
crack, and the bridge is no longer safe.

The reason I cut down a couple of trees in the first place, was that
even though it apparently exists, it's very hard to get ahold of 20'
pressure treated lumber. I had thought that a couple of 4"x6"x20'
beams might work well to support the bridge - though I never did any
actual load calculations.

I'm not feeling the love for the work involved of cutting another
couple of 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 a friend to help move them), 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?).

For a span of that length, you really need depth to reduce deflection.

If you can find them, 20 foot pressure-treated 2x12's spaced 12" apart
should span your stream nicely. I would add solid blocking every 4 feet or
so to add some lateral stability and prevent the joists from twisting.

If you can't find 20' PT 2x12's, you could probably build box beams with PT
2x4's and PT plywood.

Real lumber yards can usually get longer boards than the home centers.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com

On 10/15/2015 6:07 PM, Dean Hoffman wrote:
On Thu, 15 Oct 2015 15:41:37 -0500, dpb wrote:


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., ...

Would some sort of pre made floor truss be at all practical?

Standard sizes would likely be quite a bit taller and sorta' show their
provenance which might not be desirable. I've no idea cost--have
never actually used one meself, only bought the one house in TN already
in final close-in that used 'em so never had any reason to...

Of course, one could substitute height for thickness on the 4x in a cube
power ratio of additional height to reduction in thickness and maintain
the same vertical stiffness. That is, I ~ bh^3 so to keep same
deflection one needs to raise the height much less than decrease the
distance to keep same overall in the vertical direction.

Well, let's just see numerically

4x6 -- 3.5*5.5^3 = 582
2x8 -- 1.5*7.5^3 = 632

so, in fact, the 2x8 would be ~10% stiffer to the vertical load than the
4x6 but more "wobbly" side-to-side w/o any cross bracing.

--



--

On 10/15/2015 6:51 PM, HerHusband wrote:
....

For a span of that length, you really need depth to reduce deflection.

Yeah, see previous note above--

If you can find them, 20 foot pressure-treated 2x12's spaced 12" apart
should span your stream nicely. I would add solid blocking every 4 feet or
so to add some lateral stability and prevent the joists from twisting.
....

Ya' think!!!!???

Presuming 3-ft wide deck, that'd be 4 2x12 -- compared to the calc's
above that would support roughly 4500 lb mid-span w/ 1/2" deflection.
I don't think there's the intent to be able to drive a car over it...

For a pedestrian bridge of minimal loading a couple 2x8 will be
sufficient load-wise; some cross bracing likely would be desirable to
cut down lateral sway. A third would surely be a major improvement if
OP wasn't trying to be terribly penurious.

Those were, as noted in the other response, based on an actual span of
16-ft, not 20 as the pictures made it pretty clear the extra length was
simply supporting the ends of the logs on the banks and all needed for a
bridge would be some piers at the edges at about 16-ft or perhaps even
closer if went down the bank just a little and left a foot or so
overhang each end...

--

On Thu, 15 Oct 2015 13:51:38 -0700, Don Y
wrote:

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...]


Depending on a lot of factors, putting in a culvert in some
jurisdictions could create a real legal quagmire. Doing construction
in a stream bed, such as for a culvert, can have EPA enforcement
implications. Even a bridge could be an issue if the right
"authorities" were to get wind of it.

On 10/15/2015 3:53 PM, J wrote:
A few years back (~5) I built a rustic bridge over our seasonal stream by laying deck boards over a couple of tree trunks (5"-6" diameter) that I cut down. 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.

The bridge was a little bouncy, but otherwise seemed structurally sound - that is, until now. I had hoped that by placing the ends of the tree trunks on gravel and some pressure-treated 4x4s it would last for many years, but the trunks have rotted in the middle and begun to crack, and the bridge is no longer safe.

The reason I cut down a couple of trees in the first place, was that even though it apparently exists, it's very hard to get ahold of 20' pressure treated lumber. I had thought that a couple of 4"x6"x20' beams might work well to support the bridge - though I never did any actual load calculations.

I'm not feeling the love for the work involved of cutting another couple of 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 a friend to help move them), 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.

Photos of the current bridge he https://www.flickr.com/gp/7865232@N08/9104G5

-J


I see that Dean H suggested a truss. I was thinking the same thing,
made from PT or other weather resistant wood.

Before you determine the length, you have to do an environmental impact
study, get permits from the wildlife commission so a civil engineer can
determine the height and span needed.

I bet you can get the bridge done for under $300,000 if you go with a
modest desigh.

On Thu, 15 Oct 2015 20:07:18 -0500, Ashton Crusher wrote:


Depending on a lot of factors, putting in a culvert in some
jurisdictions could create a real legal quagmire. Doing construction
in a stream bed, such as for a culvert, can have EPA enforcement
implications. Even a bridge could be an issue if the right
"authorities" were to get wind of it.

The EPA wants to expand its power over waterways.
From the Wall Street Journal: http://alturl.com/3z45w



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Using Opera's mail client: http://www.opera.com/mail/

Well, I think the culvert idea is a non-starter. Right now there are just puddles in the stream (it's been pretty dry lately), but in June we had a 4" rain event and the water was less than a foot from the bottom of the bridge, and incredibly intense - jumped the banks at many parts of the stream - though where the bridge is, it's nice and deep. Heck, even the road got partially washed out and that has a huge culvert running under it. There's generally some pretty amazing water in the spring, moving some pretty big rocks along with it - which I think would also destroy any mid-span support that I might be able to place.

I'm not sure I'd be able to really get away with a 16' span - at least not without some really major effort to prevent the banks from collapsing where I'd have to put the pier. I don't think I'd be able to pour a pier at all - I think at best, I would just get some pre-cast concrete footers like they make for decks.

It sounded like 2x8s might have better vertical support than 4x6s - and they'd be about 40% lighter. Though, again, getting the 20' length is difficult. I'll want to figure out my plan before getting a lumber yard to procure this for me. Would they really bother helping me with this for just two or 3 boards?

dpb suggested a 3' flat plate to hold two 10' pieces together at the middle.. I'm not sure exactly what you mean by that - searching HD and Lowes I only saw long narrow metal bars. What do others think of this solution? And if it might work, could someone point me at the appropriate product?

Thanks.

-J

On 10/15/2015 01:53 PM, J wrote:
Trying to keep it as simple as possible. Thanks in advance for any ideas.

Have you considered 20' I beams? You'd probably get away for under $300
and they wouldn't rot any time soon?

On 10/15/2015 9:24 PM, rbowman wrote:
On 10/15/2015 01:53 PM, J wrote:
Trying to keep it as simple as possible. Thanks in advance for any
ideas.

Have you considered 20' I beams? You'd probably get away for under $300
and they wouldn't rot any time soon?

From somewhere else, "I have seen some folks use old mobile home
frames for bridges. Seems to me it would be cheaper and easier to go
that route and be plenty strong."
My idea, a shipping container would make a nice covered bridge, or
make some reinforced concrete beams, sometimes the county here gives the
old bridge beams away when they repair a county bridge. I got one to
take the bounce out of a 2nd story floor.

If you can find them, 20 foot pressure-treated 2x12's spaced 12"
apart should span your stream nicely. I would add solid blocking
every 4 feet or so to add some lateral stability and prevent the
joists from twisting.

Ya' think!!!!???
Presuming 3-ft wide deck, that'd be 4 2x12 -- compared to the calc's
above that would support roughly 4500 lb mid-span w/ 1/2" deflection.
I don't think there's the intent to be able to drive a car over it...

Hey, you can never be too careful...

The original poster mentioned the current bridge was bouncy. I don't think
the usage was mentioned. If it's just for one or two people to walk across,
yeah, you could downsize the joists and/or spacing. If you need to drive a
couple of quads across the bridge, or push a fully loaded wheelbarrow,
better to play it safe.

You never know if the bride's entire wedding party is going to pose for
pictures on your new bridge. That or a bunch of drunk college kids.

Those were, as noted in the other response, based on an actual span of
16-ft, not 20 as the pictures made it pretty clear the extra length
was simply supporting the ends of the logs on the banks and all needed
for a bridge would be some piers at the edges at about 16-ft or
perhaps even closer if went down the bank just a little and left a
foot or so overhang each end...

I didn't take the time to calculate loads or look up spans in my charts. I
just used an online joist calculator to do a quick estimate. I went with 20
foot joists so there could be a foot of bearing on each end, and an 18 foot
free span. That would give a little extra on each end so the footings
aren't so close the edge of the stream.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com

Well, I think the culvert idea is a non-starter.

Even if you didn't have to worry about environmental rules around a stream,
putting any kind of structure "in" the stream seems like a bad idea.

I'm not sure I'd be able to really get away with a 16' span - at least
not without some really major effort to prevent the banks from
collapsing where I'd have to put the pier. I don't think I'd be able
to pour a pier at all - I think at best, I would just get some
pre-cast concrete footers like they make for decks.

For some reason I was thinking a concrete beam/block on each end, something
like 8"x12"x36". That's about five 60# bags of concrete per beam. Not too
bad if you can get the bags to the site. I would add a couple pieces of
rebar in each block for extra strength before pouring the concrete.

getting the 20' length is difficult.

I had no trouble getting 24' 2x12's for the ceiling joists in our garage or
the rafters in our house. Of course, those were standard framing lumber,
not pressure treated. That might make a difference.

You could always use regular framing lumber and apply a good coat of deck
stain on all surfaces. Then add flashing to the top surfaces before
installing the decking. Just make sure the ends rest on pressure treated
lumber and not directly on the ground/blocks.

I'll want to figure out my plan before getting a lumber yard to
procure this for me. Would they really bother helping me with this
for just two or 3 boards?

The lumber yards I've dealt have been happy to sell me a single board or
box of screws.

Getting 20' boards home will be the real challenge. Mine were delivered as
part of a much larger order. Maybe you could rent a U-haul truck or
something.

dpb suggested a 3' flat plate to hold two 10' pieces together at the
middle. I'm not sure exactly what you mean by that - searching HD and
Lowes I only saw long narrow metal bars. What do others think of this
solution?

I don't know if a 3' plate would be sufficient over that span, especially
if the joint was in the middle.

I mentioned a box beam earlier, but you could probably build a laminated
beam of sorts. Sister a couple of 2x joists together, staggering the seams,
with a good dose of construction adhesive between them. Then drive in a
couple rows of lag bolts every 12" to hold everything together.

Having full length joists would be safer, cheaper, and easier.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com

For the record, yes, the bridge is for light pedestrian (and XC skier) use. This is just a winding path through the woods. I think I cut the deck boards to 30", as 3' just seemed a little too wide and imposing for the setting. If you looked at the pictures, you can see there's no railing either.

For some reason I was thinking a concrete beam/block on each end, something
like 8"x12"x36". That's about five 60# bags of concrete per beam. Not too
bad if you can get the bags to the site. I would add a couple pieces of
rebar in each block for extra strength before pouring the concrete.

I've never actually poured concrete before. I probably could get materials to the site. If I remove the mower deck from my riding mower, it can get there. And I can transport stuff (up to a couple of hundred pounds at a time) in a little trailer that I can tow behind it. However, I'm hoping I can get away with pre-fab 12x12x8 concrete deck blocks they have at Lowes.


getting the 20' length is difficult.

I had no trouble getting 24' 2x12's for the ceiling joists in our garage or
the rafters in our house. Of course, those were standard framing lumber,
not pressure treated. That might make a difference.

You could always use regular framing lumber and apply a good coat of deck
stain on all surfaces. Then add flashing to the top surfaces before
installing the decking. Just make sure the ends rest on pressure treated
lumber and not directly on the ground/blocks.

Great ideas, but too much work for this project at this time.


I'll want to figure out my plan before getting a lumber yard to
procure this for me. Would they really bother helping me with this
for just two or 3 boards?

The lumber yards I've dealt have been happy to sell me a single board or
box of screws.

Oh, I don't doubt they'd be happy to sell me anything they have - just wondering if it would be worth their while to special order such a small quantity of something like this that they don't normally stock. Even the real lumber yard in our area didn't have 20' pressure treated boards on hand (at least not the 4x6 size that I was originally inquiring about)



Getting 20' boards home will be the real challenge. Mine were delivered as
part of a much larger order. Maybe you could rent a U-haul truck or
something.

I previously transported a 16' 4x6 beam (along with some other lumber) on the roof rack of my car (subaru outback at the time, though now I have a RAV-4). I was hoping I could do the same for a couple of 20' boards (2x8). They should weight less than a 4x6x16' - about 90b each according to a table I found online. I'm sure the weight wouldn't be a problem, especially if I did one at a time, and the length shouldn't be an issue either, as our canoe is nearly that long. I wouldn't have to drive very fast. About a 10 mile drive, no big roads.

Would a 2x8x20' board be fine for transport if lain flat on two crossbars about 4' apart (as in the ascii art below)?

--------+--+--------

Finally, how does one actually calculate the load bearing capacity of different lumber size options (including the weight of the bridge slats)?

Thanks.

On 10/15/2015 9:00 PM, J wrote:
....

dpb suggested a 3' flat plate to hold two 10' pieces together at the
middle. I'm not sure exactly what you mean by that - searching HD and
Lowes I only saw long narrow metal bars. What do others think of this
solution? And if it might work, could someone point me at the
appropriate product?
....

It's flat plate, obtained from your local ironmonger...make each leg 3
ft was what was intended altho I see as I wrote it, it does sound like
meant overall.

Or, of course, you can sister another tubaX the same way or use exterior
ply for the splice with a cap to hide the exposed edge.

The metal solution would, with only a little extra effort/expense, let
you have the pieces fabricated with an angle so the center could be
higher than the ends that could lend a nice aesthetic touch. Need a
buddy who can weld or have the metal shop do it for you.

I don't recall now precisely what it ran, but we put a 12-ft 1/2" x 8"
plate across the 10-ft span of the barn driveway after we jacked the sag
out of the middle of the span when reroofed and repainted/repaired the
old barn here a few years ago. That piece was under $200 as I recall
new; if the scrap dealer in town had happened to have a piece at the
moment I called it could've been much less. He's (the scrap man, that
is) is really complaining these days that steel prices are way off, so
while I've not priced anything recently, it shouldn't be too bad.
Again, go to an iron/metals monger, not the box stores.

And, two 3/8" are probably overkill; two 1/4 or 5/16" would likely
suffice as they're constrained from flexing sideways by being bolted to
the beams it's the vertical height that is key dimension; same effect as
the 2x8 vis a vis the 4x6 for stiffness in the vertical direction; the
section is still rectangular x-section so same formula for I.

Actually, of course, you could make the same arch construct with the ply
gussets as well. I've never tried it; don't know how well the pressure
treated would bond with resorcinol to do a glue laminated joint; you
might try to do some research and see if anybody has any suggestions on
that. The polyurethanes (Gorilla glue) are waterproof and one part;
again I don't know about the durability with PT material. Be
interesting to look on the PT manufacturers' trade association web site
and see if they have any info on laminating them--I've only done untreated.

And, of course, as somebody (or bodies) have mentioned there's always
the manufactured/engineered beams but you may start talking more monies
quickly for exterior application...

--

On 10/16/2015 8:16 AM, J wrote:
For the record, yes, the bridge is for light pedestrian (and XC
skier) use. This is just a winding path through the woods. I think I cut the
deck boards to 30", as 3' just seemed a little too wide and imposing for
the setting. If you looked at the pictures, you can see there's no
railing either.

For some reason I was thinking a concrete beam/block on each end, something
like 8"x12"x36". That's about five 60# bags of concrete per beam. Not too
bad if you can get the bags to the site. I would add a couple pieces of
rebar in each block for extra strength before pouring the concrete.

I've never actually poured concrete before. I probably could get
materials to the site. If I remove the mower deck from my riding mower,
it can get there. And I can transport stuff (up to a couple of hundred
pounds at a time) in a little trailer that I can tow behind it. However,
I'm hoping I can get away with pre-fab 12x12x8 concrete deck blocks they
have at Lowes.


getting the 20' length is difficult.

I had no trouble getting 24' 2x12's for the ceiling joists in our garage or
the rafters in our house. Of course, those were standard framing lumber,
not pressure treated. That might make a difference.

You could always use regular framing lumber and apply a good coat of deck
stain on all surfaces. Then add flashing to the top surfaces before
installing the decking. Just make sure the ends rest on pressure treated
lumber and not directly on the ground/blocks.

Great ideas, but too much work for this project at this time.


I'll want to figure out my plan before getting a lumber yard to
procure this for me. Would they really bother helping me with this
for just two or 3 boards?

The lumber yards I've dealt have been happy to sell me a single board or
box of screws.

Oh, I don't doubt they'd be happy to sell me anything they have -
just wondering if it would be worth their while to special order such
a small quantity of something like this that they don't normally
stock. Even the real lumber yard in our area didn't have 20' pressure
treated boards on hand (at least not the 4x6 size that I was
originally inquiring about)




Getting 20' boards home will be the real challenge. Mine were delivered as
part of a much larger order. Maybe you could rent a U-haul truck or
something.

I previously transported a 16' 4x6 beam (along with some other
lumber) on the roof rack of my car (subaru outback at the time, though now I
have a RAV-4). I was hoping I could do the same for a couple of 20'
boards (2x8). They should weight less than a 4x6x16' - about 90b each
according to a table I found online. I'm sure the weight wouldn't be a
problem, especially if I did one at a time, and the length shouldn't be
an issue either, as our canoe is nearly that long. I wouldn't have to
drive very fast. About a 10 mile drive, no big roads.

Would a 2x8x20' board be fine for transport if lain flat on two
crossbars about 4' apart (as in the ascii art below)?

--------+--+--------

Finally, how does one actually calculate the load bearing capacity
of different lumber size options (including the weight of the bridge slats)?

Thanks.

It seems to me you're looking at a fair amount of both expense and
effort; yet trying to cut corners to the point much of both may be
wasted in a few short years again. Particularly on the footings, it
would really behoove you to make certain they're set deep enough to not
fail. You've not said where you're located; can you dig easily or is it
exceedingly rocky or is there a solid rock layer or somesuch? What's
the climate and in an area such as this it's going to be subject to
water. It's not difficult to pour premix other than a little effort to
do the mixing and you don't need any exotic forms if you can dig a good
footing hole. They sell pier forms for the purpose that basically are a
large cardboard tube as example of what you'd ideally have.

The yard will, I'm sure, order you most anything you want and you can be
sure they'll make it worth their while! Probably, and you'll want
to check before placing the order, they'll charge you the specific
shipping of the product which may turn out to be as much nearly as it is
depending on their arrangements with suppliers for special ordering.
You may get around some of that if you're willing to wait until it can
be brought in with other materials from the same supplier.

It'd be possible to haul it that way but it'll be flappin' in the breeze
w/ 6 ft overhanging each end. You'll actually probably be better off
with 2 or 3 at a time if you bind their ends tightly together with the
packing strapping tape as that'll stiffen 'em up laterally. What about
eighteens instead of twenties? Think that give you enough span? Would
probably cut cost as well.

As for the deflection and stress calc's, I used

http://sunsetpatios.com.au/beam-deflection-calculator.php

as it had a selector for wood for modulus w/o having to look up a number
and the option to set dimensions ("Use Below" and for a rectangular
cross section leave the flange dimension field blank).

It is somewhat inconveniently for US construction timber in metric
units. Remember to divide the live and distributed loads by the number
of members to get the effective loading on each.

--

For the record, yes, the bridge is for light pedestrian (and XC skier)
use. This is just a winding path through the woods. I think I cut the
deck boards to 30", as 3' just seemed a little too wide and imposing
for the setting. If you looked at the pictures, you can see there's
no railing either.

That's kind of what I figured, but tried to plan for worst case.

I've never actually poured concrete before.
I probably could get materials to the site.

The premix bagged stuff is pretty easy to work with, but getting water to
the site to mix it up might be problematic.

I'm hoping I can get away with pre-fab 12x12x8 concrete
deck blocks they have at Lowes.

You may end up with some settling over time, but it will probably work if
you use a block under each joist (3-4 on each end of the bridge). If I
was going that route I would probably dig out an area 6" deep, 16" wide,
and the width of the bridge. Then I'd fill it with 5/8- gravel and tamp
it down good before placing the deck blocks. It would provide a decent
base for the blocks and provide some drainage underneath.


You could always use regular framing lumber and apply a good coat of
deck stain on all surfaces. Then add flashing to the top surfaces
before installing the decking.
Great ideas, but too much work for this project at this time.

You could probably skip the deck stain if you're in a pinch, and just
cover the tops of the joists with that rubberized flashing they use to
flash windows. You can find rolls of it at Lowes/Depot in the window and
door area.

It probably won't last as long as pressure treated lumber, but the bridge
will be up in the air and not contacting the ground (except at the ends),
so it should hold up several years at least.

Of course, pressure treated would be the better option if you can get it.

Oh, I don't doubt they'd be happy to sell me anything they have - just
wondering if it would be worth their while to special order such a
small quantity of something like this that they don't normally stock.
Even the real lumber yard in our area didn't have 20' pressure
treated boards on hand (at least not the 4x6 size that I was
originally inquiring about)

If they don't have it in stock, they may have minimum quantities for
special orders. Only way to know is to ask.

I would think PT 2X material would be easier to find in long lengths than
4X beams.

I previously transported a 16' 4x6 beam (along with some other lumber)
on the roof rack of my car (subaru outback at the time, though now I
have a RAV-4). I was hoping I could do the same for a couple of 20'
boards (2x8). They should weight less than a 4x6x16' - about 90b each
according to a table I found online. I'm sure the weight wouldn't be
a problem, especially if I did one at a time, and the length shouldn't
be an issue either, as our canoe is nearly that long. I wouldn't have
to drive very fast. About a 10 mile drive, no big roads.
Would a 2x8x20' board be fine for transport if lain flat on two
crossbars about 4' apart (as in the ascii art below)?

Pressure treated lumber can be quite heavy, as it's still green and
pumped full of chemicals. I wouldn't want 20' boards strapped to the roof
of my car, but then I drive a tiny VW Rabbit.

Finally, how does one actually calculate the load bearing capacity of
different lumber size options (including the weight of the bridge
slats)?

I have an old carpentry book that has tables that tell the load
capacities for various joists and beams for a variety of spans and
spacings.

Basically you start with the load you expect to carry, 40 pounds per
sq/ft is typical for residential floors, then multiply that by the span.
For example: 40 pounds x 18ft span = 720 pounds (assumes 40 pounds on
every foot of the beam). Then you find a joist/beam in the chart that can
carry that load over that span. It gets a bit more complicated with wood
species, deflection limits, joist spacing, etc., but that's the
simplistic description.

There are also a variety of calculators online that can give you ballpark
figures too.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com

It seems to me you're looking at a fair amount of both expense and
effort; yet trying to cut corners to the point much of both may be
wasted in a few short years again. Particularly on the footings, it
would really behoove you to make certain they're set deep enough to not
fail. You've not said where you're located; can you dig easily or is it
exceedingly rocky or is there a solid rock layer or somesuch? What's
the climate and in an area such as this it's going to be subject to
water. It's not difficult to pour premix other than a little effort to
do the mixing and you don't need any exotic forms if you can dig a good
footing hole. They sell pier forms for the purpose that basically are a
large cardboard tube as example of what you'd ideally have.

Well, primarily, I'm afraid to do something I haven't done before ;-), but also, I'd rather not leave anything so permanent buried out there in the woods. Digging is probably not too easy - and it would all be by hand with a pick-axe and shovel. I'm in central NY, so it get's cold and snowy. I'm definitely not digging down the 4' or whatever it takes to get below the frost line around here. Bit I'm also not real worried about pressure treated beams on pre-made concrete piers failing, or rotting like the tree trunks I originally used as beams for this bridge. For reference, I built a little woodshed (about 4x8x8) about 15 years ago. It's pressure treated 2x4s and 4x4s, sitting directly on some old railroad ties and/or directly on gravel - not even a hint of rot on that thing and still completely sound.

If this effort lasts 15 years, I'll be totally happy. This is a rustic path through the woods, so I'm trying to avoid overkill. I'm not building a monument for the ages, just trying to make sure I get something strong enough to support the weight of a couple of people walking (or skiing) across from time to time - no wedding parties ;-). I'm also trying to avoid getting into the hundreds of dollars for this project. Even though not available around here, Lowes advertises 2x8x20 PT for $35 each (2x10x20 for$42). I'll probably need some more wood for lateral stabilization and some screws, but the rest of the bridge is pre-existing and can be reused. Other than that, I just need the concrete blocks. Lowes sells those fancy deck blocks with grooves for 2-by lumber - which run $6.50 each - but it might be fine just getting 4 rectangular concrete blocks (4x8x16") at $1.30 each


The yard will, I'm sure, order you most anything you want and you can be
sure they'll make it worth their while! Probably, and you'll want
to check before placing the order, they'll charge you the specific
shipping of the product which may turn out to be as much nearly as it is
depending on their arrangements with suppliers for special ordering.
You may get around some of that if you're willing to wait until it can
be brought in with other materials from the same supplier.

It'd be possible to haul it that way but it'll be flappin' in the breeze
w/ 6 ft overhanging each end. You'll actually probably be better off
with 2 or 3 at a time if you bind their ends tightly together with the
packing strapping tape as that'll stiffen 'em up laterally. What about
eighteens instead of twenties? Think that give you enough span? Would
probably cut cost as well.

I called the local semi-real lumber company and they said it would be no problem getting the lumber I asked about 2x8x20 PT. I didn't realize x18' was a thing. I'll definitely measure carefully before placing the order and see what I can get away with. I didn't get a price - because the person on the phone seemed to be in a hurry to do something else and as I wasn't going to place an order right then, I just let it drop. Hopefully, not too bad though.


As for the deflection and stress calc's, I used

http://sunsetpatios.com.au/beam-deflection-calculator.php

as it had a selector for wood for modulus w/o having to look up a number
and the option to set dimensions ("Use Below" and for a rectangular
cross section leave the flange dimension field blank).

It is somewhat inconveniently for US construction timber in metric
units. Remember to divide the live and distributed loads by the number
of members to get the effective loading on each.


Thanks. I think I can see how to use that page to determine deflection - just gotta convert to kg and mm :-).

I was just looking at another web page that gives a simple formula for constructing "box bridges" on a hiking website http://www.nynjtc.org/content/simple...s#comment-5047 though, as they point out, the formula they give for determining beam size a gross measure and not certified in any way.. I wasn't too thrilled with their mixing of units (ft, inches) and their use of what appears to be a magic number without proper explanation, but the final results are probably sound. Interestingly, they say that their formula "uses strength as the benchmark, not deflection" because "... moderate deflection is acceptable for foot bridges." At least that makes me feel a little better about the bounce in the old bridge. Of course, they also don't define "moderate". Is that 1/2" of deflection you've mentioned a couple of times now a kind of standard for certain kinds of construction?

So now, I'm a little unclear on what the best metric to go by is. I think it's fine to have a little bounce in the bridge (makes it feel more woodsy), as long as it's not going to be at risk for actually breaking.

Thanks.

-J

....[snip for brevity]...

If this effort lasts 15 years, I'll be totally happy. This is a
rustic path through the woods, so I'm trying to avoid overkill....
....
I'm also trying to avoid getting into the hundreds of dollars for this
project. Even though not available around here, Lowes advertises 2x8x20
PT for $35 each (2x10x20 for$42). I'll probably need some more wood for
lateral stabilization and some screws, but the rest of the bridge is
pre-existing and can be reused. Other than that, I just need the
concrete blocks. Lowes sells those fancy deck blocks with grooves for
2-by lumber - which run $6.50 each - but it might be fine just getting 4
rectangular concrete blocks (4x8x16") at $1.30 each

OK, as long as you're content with the possible ramifications and can
live with it, it's your call

....

I called the local semi-real lumber company and they said it would
be no problem getting the lumber I asked about 2x8x20 PT. I didn't realize
x18' was a thing. I'll definitely measure carefully ...

They're available, particularly in fir; not so positive about PT,
though. Twenties and sixteens are certainly the most common choices but
"it never hurts to ask"


As for the deflection and stress calc's, I used

http://sunsetpatios.com.au/beam-deflection-calculator.php

as it had a selector for wood for modulus w/o having to look up a number
and the option to set dimensions ("Use Below" and for a rectangular
cross section leave the flange dimension field blank).

It is somewhat inconveniently for US construction timber in metric
units. Remember to divide the live and distributed loads by the number
of members to get the effective loading on each.


Thanks. I think I can see how to use that page to determine
deflection - just gotta convert to kg and mm :-).

I was just looking at another web page that gives a simple formula
for constructing "box bridges" on a hiking website
http://www.nynjtc.org/content/simple...s#comment-5047 though, as
they point out, the formula they give for determining beam size a gross
measure and not certified in any way. I wasn't too thrilled with their
mixing of units (ft, inches) and their use of what appears to be a magic
number without proper explanation, but the final results are probably
sound. Interestingly, they say that their formula "uses strength as the
benchmark, not deflection" because "... moderate deflection is
acceptable for foot bridges." At least that makes me feel a little
better about the bounce in the old bridge. Of course, they also don't
define "moderate". Is that 1/2" of deflection you've mentioned a couple
of times now a kind of standard for certain kinds of construction?

So now, I'm a little unclear on what the best metric to go by is. I
think it's fine to have a little bounce in the bridge (makes it feel
more woodsy), as long as it's not going to be at risk for actually breaking.

Wood is pretty forgiving before breaking, given it is basically sound.
I just looked at the deflection number as a first cut on what would be a
reasonable target size of member inferring (pretty accurately as it
turns out by your later description ) your intended purpose from the
picture and initial query and having been satisfied with the original
before it failed.

The half-inch isn't at all a design standard, no, but it comes about as
a result of the building code 1/360 allowable deflection rule for joists
and beams. 15-ft/360*12"/ft -- 0.5" exactly, in fact. So, 1/2" in a
16-ft span is less than that. Being within that pretty much assures
that stresses won't be excessive for structural timbers.

If you really want to understand how this is done and the criteria used,
the following link is very thorough on the details but readable in
defining terms for the non-engineer--

http://www.wwpa.org/TECHGUIDE/DesignValues/tabid/855/Default.aspx

There's a link to properties/design values at

http://www.wwpa.org/Portals/9/docs/pdf/beam.pdf

Since this is from the Western Woods Assoc, it's mostly for the western
woods but there's an entry for "Pine...(South)" that'll be good enough
although you could go double check from the treated products group
website, too.

The most critical item in that table is that while the modulus doesn't
change much for graded structural to #1 and #2, the bending stress limit
reduction is almost a factor of 2X reduction for #1 vis a vis select
structural.

Does your other tool mention anything about the actual materials in its
stress limitation computation?

--

On 10/16/2015 1:10 PM, dpb wrote:
...[snip for brevity]...

....

The half-inch isn't at all a design standard, no, but it comes about as
a result of the building code 1/360 allowable deflection rule for joists
and beams. 15-ft/360*12"/ft -- 0.5" exactly, in fact. So, 1/2" in a
16-ft span is less than that. Being within that pretty much assures that
stresses won't be excessive for structural timbers.

If you really want to understand how this is done and the criteria used,
the following link is very thorough on the details but readable in
defining terms for the non-engineer--

http://www.wwpa.org/TECHGUIDE/DesignValues/tabid/855/Default.aspx

There's a link to properties/design values at

http://www.wwpa.org/Portals/9/docs/pdf/beam.pdf

Since this is from the Western Woods Assoc, it's mostly for the western
woods but there's an entry for "Pine...(South)" that'll be good enough
although you could go double check from the treated products group
website, too.

The most critical item in that table is that while the modulus doesn't
change much for graded structural to #1 and #2, the bending stress limit
reduction is almost a factor of 2X reduction for #1 vis a vis select
structural.

Does your other tool mention anything about the actual materials in its
stress limitation computation?

Oh, one last point...the pricing for PT you've gotten (cheaper by quite
a lot than I figured would have been, but confirmed here that's not out
of line of what is available pricing locally) is for #2 so the above
derating would be in play if you were to try to design to those
criteria. Your choices are, of course, either go w/ the higher grade
for more confidence w/ smaller members or instead of two use three (or
four instead of three, obviously). Can choose whatever is the most
cost-effective solution at whatever level of comfort you wish as to
conservatism in sizing for an anticipated load/usage.

--

On 10/16/2015 5:16 PM, dpb wrote:
On 10/16/2015 1:10 PM, dpb wrote:
...[snip for brevity]...

...

The half-inch isn't at all a design standard, no, but it comes about as
a result of the building code 1/360 allowable deflection rule for joists
and beams. 15-ft/360*12"/ft -- 0.5" exactly, in fact. So, 1/2" in a
16-ft span is less than that. Being within that pretty much assures that
stresses won't be excessive for structural timbers.

If you really want to understand how this is done and the criteria used,
the following link is very thorough on the details but readable in
defining terms for the non-engineer--

http://www.wwpa.org/TECHGUIDE/DesignValues/tabid/855/Default.aspx

There's a link to properties/design values at

http://www.wwpa.org/Portals/9/docs/pdf/beam.pdf

Since this is from the Western Woods Assoc, it's mostly for the western
woods but there's an entry for "Pine...(South)" that'll be good enough
although you could go double check from the treated products group
website, too.

The most critical item in that table is that while the modulus doesn't
change much for graded structural to #1 and #2, the bending stress limit
reduction is almost a factor of 2X reduction for #1 vis a vis select
structural.

Does your other tool mention anything about the actual materials in its
stress limitation computation?

Oh, one last point...the pricing for PT you've gotten (cheaper by quite
a lot than I figured would have been, but confirmed here that's not out
of line of what is available pricing locally) is for #2 so the above
derating would be in play if you were to try to design to those
criteria. Your choices are, of course, either go w/ the higher grade for
more confidence w/ smaller members or instead of two use three (or four
instead of three, obviously). Can choose whatever is the most
cost-effective solution at whatever level of comfort you wish as to
conservatism in sizing for an anticipated load/usage.

Oh, and I just saw a typo I missed first time...

... the bending stress limit reduction is almost a factor of 2X
reduction for #1 vis a vis select structural.

should be for #2, not #1.

--

It's pressure treated 2x4s and 4x4s, sitting directly on some old
railroad ties and/or directly on gravel - not even a hint of rot
on that thing and still completely sound.

We went for a hike today and saw several small bridges on the trails that
reminded me of your situation. They just had pressure treated timbers (6x6
I think) semi-buried in the ground on each end and the bridges rested on
those (no concrete piers or anything). They just move up and down with the
frosts, no biggy.

I am not completely sure, but it looked like they had just used standard
lumber for the bridge itself, not pressure treated.

These are in a city park used by thousands of people every day.

Lowes advertises 2x8x20 PT for $35 each (2x10x20 for$42).

I would opt for the 2x10's. It would only add $28 to your total bill and
the extra stiffness would be noticeable for sure.

I didn't realize x18' was a thing.

I've seen 8's, 10's, 12's, 16's, 20's, and 24's.

Can't say I've seen 14's or 18's.

Good luck!

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com

I just wanted to thank everyone again who responded and post an update. Thanks to the mild weather through December, I found some time to do this project.

Even though the 16' timbers that I ended up using seem marginal in length, that is what I went with for simplicity (easier to acquire and transport - I went with 2x10s). The bridge rests on either end on pressure treated 4x4s embedded in the ground. They 2x10x16s are spaced ~20" apart (on center) and there are five 2x10 cross-members (roughly every 3') for lateral stiffness. I reused the deck boards from the original rustic bridge, which were conveniently held together in 4' lengths by the 2x4 edge railings. I split the extra 4' length of deck boards into two and used it for ramps on either side of the bridge (supported by 2x4s bolted to the main bridge) so that we can easily ski across (when/if we get enough snow). The new bridge can be viewed he https://flic.kr/s/aHskriAbFc (some pics are before the addition of the ramps at each end)

I'm not too worried about the bridge rotting where it comes into contact with the ground. There is some gravel there to help out with drainage, and the 4x4s that were there supporting the original beams (which were just felled trees) seem practically as good as new after 5 years. I suspect erosion and undercutting of the banks will cause the demise of this bridge long before rot. But I'm hoping that won't happen too soon.

Among the benefits of the new bridge are that it is much more sturdy (much less deflection than the original rustic bridge - in fact, hardly noticeable) and that I was able to level it.

I was able to construct the new bridge almost entirely by myself - I had a little help from my 9yo daughter. Though afterwards, my next door neighbor complained about not being asked (invited?) to help. He also said, that he preferred the "Indiana Jones" feel of the old bridge :-). However, it's a good thing no one got hurt on the old bridge, as it actually fell into the creek when I was removing the deck boards. I think the pinching together of the 2x4 railings as the old bridge sagged, was actually what was holding it up! This past spring before I realized how unsafe the bridge was becoming, we actually crossed it during a local flood event. The creek was a torrent, with water level about one foot or less below the bottom of the bridge. You could hear the rocks being moved along the bottom of the creek. Had someone fallen in then, they would probably not have survived. I think next time, I won't be stupid enough to cross the bridge under those conditions, even if I think the bridge is perfectly sound.

Cheers everyone, and happy new year.

On Tue, 5 Jan 2016 12:12:59 -0800 (PST), J wrote:

The new bridge can be viewed he https://flic.kr/s/aHskriAbFc (some pics are before the addition of the ramps at each end)

No flowing water stream before, now you do. Good for you.

Any trolls under that bridge, little feller??

Looks like it should be good for 10+ years unless the supports wash out. You could put some large stones/boulders along the banks up-river of the support beams to slow down any washout of the banks.

On Tuesday, January 5, 2016 at 3:13:05 PM UTC-5, J wrote:
I just wanted to thank everyone again who responded and post an update. Thanks to the mild weather through December, I found some time to do this project.

Even though the 16' timbers that I ended up using seem marginal in length, that is what I went with for simplicity (easier to acquire and transport - I went with 2x10s). The bridge rests on either end on pressure treated 4x4s embedded in the ground. They 2x10x16s are spaced ~20" apart (on center) and there are five 2x10 cross-members (roughly every 3') for lateral stiffness. I reused the deck boards from the original rustic bridge, which were conveniently held together in 4' lengths by the 2x4 edge railings. I split the extra 4' length of deck boards into two and used it for ramps on either side of the bridge (supported by 2x4s bolted to the main bridge) so that we can easily ski across (when/if we get enough snow). The new bridge can be viewed he https://flic.kr/s/aHskriAbFc (some pics are before the addition of the ramps at each end)

I'm not too worried about the bridge rotting where it comes into contact with the ground. There is some gravel there to help out with drainage, and the 4x4s that were there supporting the original beams (which were just felled trees) seem practically as good as new after 5 years. I suspect erosion and undercutting of the banks will cause the demise of this bridge long before rot. But I'm hoping that won't happen too soon.

Nice job but I'd like to suggest a minor improvement, if for nothing else but longevity.

In the spring, why not extend the beams farther up on to the banks? You could sister some
2 x 10's, maybe 6' long, onto the beams for 2 to 3 feet. That would extend the bridge 3 to 4
feet onto the solid ground on each side. If the bank eroded, the bridge would still be supported.

If nothing else, that would give you time to get some big boulders or something else to
rebuild the bank while still having use of the bridge.

That looks like it was a fun project.



Among the benefits of the new bridge are that it is much more sturdy (much less deflection than the original rustic bridge - in fact, hardly noticeable) and that I was able to level it.

I was able to construct the new bridge almost entirely by myself - I had a little help from my 9yo daughter. Though afterwards, my next door neighbor complained about not being asked (invited?) to help. He also said, that he preferred the "Indiana Jones" feel of the old bridge :-). However, it's a good thing no one got hurt on the old bridge, as it actually fell into the creek when I was removing the deck boards. I think the pinching together of the 2x4 railings as the old bridge sagged, was actually what was holding it up! This past spring before I realized how unsafe the bridge was becoming, we actually crossed it during a local flood event. The creek was a torrent, with water level about one foot or less below the bottom of the bridge. You could hear the rocks being moved along the bottom of the creek.. Had someone fallen in then, they would probably not have survived. I think next time, I won't be stupid enough to cross the bridge under those conditions, even if I think the bridge is perfectly sound.

Cheers everyone, and happy new year.

Among the benefits of the new bridge are that it is much more sturdy (much less deflection than the original rustic bridge - in fact, hardly noticeable) and that I was able to level it.

I was able to construct the new bridge almost entirely by myself -

Looks nice. Is the snow from the old bridge deck too?

On Tue, 5 Jan 2016 12:12:59 -0800 (PST), J wrote:

Congratulations.

I was able to construct the new bridge almost entirely by myself - I had a little help from my 9yo daughter. Though afterwards, my next door neighbor complained about not being asked (invited?) to help. He also said, that he preferred the "Indiana Jones" feel of the old bridge :-).

I can see why you didnt' invite him, or ask him.

However, it's a good thing no one got hurt on the old bridge, as it actually fell into the creek when I was removing the deck boards. I think the pinching together of the 2x4 railings as the old bridge sagged, was actually what was holding it up! This past spring before I realized how unsafe the bridge was becoming, we actually crossed it during a local flood event.

What's the difference between a flood event and a flood?

The creek was a torrent, with water level about one foot or less below the bottom of the bridge. You could hear the rocks being moved along the bottom of the creek. Had someone fallen in then, they would probably not have survived. I think next time, I won't be stupid enough to cross the bridge under those conditions, even if I think
the bridge is perfectly sound.

Cheers everyone, and happy new year.