I would like to construct a very simple footbridge over our creek. The
distance is around 12-14 feet from bank to bank. I figured on using two
16' pieces of lumber as the support (about 18-20 inches apart), with 1x6
decking boards along the top. About 2.5' to 3' wide.

My question is what should I use for the support beams. I don't want it
to get too high off the ground, because we would like to easily ski over
it in the winter.

I was conteplating either using 4x6 lumber or 2x8 lumber. I'm not sure
which will provide a stiffer bridge - which is what I want. How much
weight can I expect such a bridge to hold? Should I be using a third
support beam?


Thanks.

-Jonathan

On Apr 22, 1:47*pm, JJ wrote:
I would like to construct a very simple footbridge over our creek. *The
distance is around 12-14 feet from bank to bank. *I figured on using two
16' pieces of lumber as the support (about 18-20 inches apart), with 1x6
decking boards along the top. *About 2.5' to 3' wide.

My question is what should I use for the support beams. *I don't want it
to get too high off the ground, because we would like to easily ski over
it in the winter.

I was conteplating either using 4x6 lumber or 2x8 lumber. *I'm not sure
which will provide a stiffer bridge - which is what I want. *How much
weight can I expect such a bridge to hold? *Should I be using a third
support beam?

Thanks.

-Jonathan

a 4x4 will hold a truck, a 4x4 will hold a house if it aint got no
wind, Blow a bit, and call back.

On 2008-04-22, ransley wrote:

I would like to construct a very simple footbridge over our creek. The
distance is around 12-14 feet from bank to bank.

a 4x4 will hold a truck

A 4x4 spanning 12-14 feet won't hold a truck.

To the OP, the stiffness of a rectangular beam of depth d and width
varies as bd^3 (that's b*d*d*d). For a 4x6, that's 3.5 * 5.5^3 = 582,
and for a 2x8 that's 1.5 * 7.25^3 = 571. So they are almost
equivalent in stiffness.

Cheers, Wayne

Thanks for the info Wayne,

A friend has a bridge of similar length (but just over a marshy
depression) that is built using 4x4x16s, and that thing would b a bit
too bouncy for comfort when the drop is closer to 2.5'.

I'm not sure what the units are (distance^4 ?) or how the length of the
span figures in, but from the equation, it looks like using 4x6s should
provide almost four times the stiffness of 4x4s (3.5 * 3.5^2 = 150).

Anyone know how to calculate the weight that can be supported if the
weight is centered over a 14' span?

Thanks.

-Jonathan


Wayne Whitney wrote:

A 4x4 spanning 12-14 feet won't hold a truck.

To the OP, the stiffness of a rectangular beam of depth d and width
varies as bd^3 (that's b*d*d*d). For a 4x6, that's 3.5 * 5.5^3 = 582,
and for a 2x8 that's 1.5 * 7.25^3 = 571. So they are almost
equivalent in stiffness.

Cheers, Wayne

On Apr 22, 2:47*pm, JJ wrote:
I would like to construct a very simple footbridge over our creek. *The
distance is around 12-14 feet from bank to bank. *I figured on using two
16' pieces of lumber as the support (about 18-20 inches apart), with 1x6
decking boards along the top. *About 2.5' to 3' wide.

My question is what should I use for the support beams. *I don't want it
to get too high off the ground, because we would like to easily ski over
it in the winter.

I was conteplating either using 4x6 lumber or 2x8 lumber. *I'm not sure
which will provide a stiffer bridge - which is what I want. *How much
weight can I expect such a bridge to hold? *Should I be using a third
support beam?

Thanks.

-Jonathan

At 14' wide, a 16' beam only gives you a 1' overhang on each shore.

Can you safely dig down to the frost line in your neck of the woods
for your posts at a distance of only 1' from the water?

ransley errs again, spectacularly :-)

JJ wrote:

I would like to construct a very simple footbridge over our creek. The
distance is around 12-14 feet from bank to bank. =A0I figured on using
two 16' pieces of lumber as the support (about 18-20 inches apart),
with 1x6 decking boards along the top. About 2.5' to 3' wide.

My question is what should I use for the support beams.

I was conteplating either using 4x6 lumber or 2x8 lumber...

a 4x4 will hold a truck, a 4x4 will hold a house...

A 4x4 with S = bd^2/6 = 7.15 in^4 and M = fs = 7146 in-lb will support
a W = 8M/L = 340 pound distributed load (including the decking boards)
over an L = 14x12" span.

Nick

On Apr 22, 2:47 pm, JJ wrote:
I would like to construct a very simple footbridge over our creek.

I had a neighbor once whose wife wanted about a .25 acre "pond", with
an island, with @ a 25' bridge.

He dug it out, and built the bridge (steel), and I helped set it in
place.

"Looks like a million bucks", I said.

"Thanks, I only got about half that in it".
-----

- gpsman

On 2008-04-22, JJ wrote:

I'm not sure what the units are (distance^4 ?)

Yes, the units are distance^4. The relevant formula for the moment of
inertia is actually I = 1/12 * b * d^3, but for comparison the factor
of 1/12 is unimportant.

or how the length of the span figures in

For a point load of fixed weight, deflection at midspan will vary as
L^3. For a distributed load, where the total load increases as the
length increases, deflection will vary as L^4.

but from the equation, it looks like using 4x6s should
provide almost four times the stiffness of 4x4s (3.5 * 3.5^2 = 150).

Yes, the exact ratio is (5.5/3.5)^3 = 3.88.

Anyone know how to calculate the weight that can be supported if the
weight is centered over a 14' span?

I'm afraid answering that question would take a bit longer than space
permits. If you know what you are doing, the span calculators at
www.awc.org are very useful, although they are for distributed loads,
not point loads. You should understand that allowable load may be
limited by deflection (where the deflection criteria is specified as
e.g. L/360, that midspan deflection should be less that 1/360 times
the span) or may be limited by strength.

Cheers, Wayne

JJ writes:

I would like to construct a very simple footbridge over our creek.
The distance is around 12-14 feet from bank to bank. I figured on
using two 16' pieces of lumber as the support (about 18-20 inches
apart), with 1x6 decking boards along the top. About 2.5' to 3' wide.

My question is what should I use for the support beams. I don't want
it to get too high off the ground, because we would like to easily ski
over it in the winter.

I was conteplating either using 4x6 lumber or 2x8 lumber. I'm not
sure which will provide a stiffer bridge - which is what I want. How
much weight can I expect such a bridge to hold? Should I be using a
third support beam?

If you are thinking of spanning the gap with lumber laid straight
across, I'd forget that right away.

You need to construct a triangle shaped support then span the
gap with lumber hung from the triangle:


*
* *
* *
==*===========*==
* *

4x4s (*) and 2x6s (=) would probably do the job.
You need to plant the 4x4s in cement.

Thanks for all the suggestions.

I think I'm going to try the straight across route. It's hard to
measure the actual distance from bank to bank, because the banks are not
straight lines and they slope down to the creek (not a sheer drop). It
could even be as short as 10' in some places - but I'm trying to be
conservative in my estimates so I don't end up with a bridge that's too
short. The actual width of water in the creek is typically about two to
four feet, depending on how much water is flowing. If it's been dry,
there will be just a trickle or no water in it at all for much of the
summer.

I won't be pouring any concrete or digging down to the frostline. I'm
planning to just rest the bridge on top of a couple of 4x4s at each end
- maybe dug half-way into the ground.

Thanks.

-Jonathan

On Apr 23, 10:31*am, JJ wrote:
Thanks for all the suggestions.

I think I'm going to try the straight across route. *It's hard to
measure the actual distance from bank to bank, because the banks are not
straight lines and they slope down to the creek (not a sheer drop). *It
could even be as short as 10' in some places - but I'm trying to be
conservative in my estimates so I don't end up with a bridge that's too
short. *The actual width of water in the creek is typically about two to
four feet, depending on how much water is flowing. *If it's been dry,
there will be just a trickle or no water in it at all for much of the
summer.

I won't be pouring any concrete or digging down to the frostline. *I'm
planning to just rest the bridge on top of a couple of 4x4s at each end
- maybe dug half-way into the ground.

Thanks.

-Jonathan

I won't be pouring any concrete or digging down to the frostline.
I'm planning to just rest the bridge on top of a couple of 4x4s at
each end - maybe dug half-way into the ground.

How often are you planning to rebuild/reset the bridge?

What's going to happen when the ground gets soft from rain or snow
melt? I'd think that things would start to sink and/or shift due to
the weight of both the structure itself and the live-load of the
skiiers/walkers.

How often are you planning to rebuild/reset the bridge?

What's going to happen when the ground gets soft from rain or snow
melt? I'd think that things would start to sink and/or shift due to
the weight of both the structure itself and the live-load of the
skiiers/walkers.

I guess I'll post back in a few years and let you know. The ground is
pretty rocky.

When we moved into our current house (7 years ago), I built a wood shed.
It rests directly on the ground on two adjacent sides, and on a couple
of old railroad ties that were laying around on the other two sides.
The railroad ties are pretty much resting on ground at one corner and
almost fully buried in the ground at the adjacent corners, which created
a level surface for the wood shed on the slightly sloping ground. This
structure has not budged since.

If the bridge shifts a little, I'll adjust it. In fact, if get a few
strong helpers, I could even move it to a different location without too
much trouble.

-JJ