wooden bridge beam size

[jonsstihl]

I need to replace a bridge on my property span is 10 feet tractor is b7800 with loader to aprox. 2800lbs and capacity for another 800 in bucket so say 3800 lbs load with operator

is an 8" X 8" hemlock beam going to do the job.

I found a formula that had this constant for hemlock set at 76


W X H squared X constant
________________________

span



so 8 X 64 X 76 divided by ten gives me 3891.2 Lbs

does this make sense.
thanks

 

[jimmysisson]

What's the bridge design? Do you have two beams or four? What's the surface? How is the tire load spread onto the beams?
I'd be more concerned about hemlock rotting away than about its strength. If the ends are on stone or on conrete, might last a few years, if you keep the dirt away. Got a drawing?
How about PT? If you need it stronger, put more beams in.
Jim

 

[KennyG]

By my tables (assuming 1000 psi) an 8x8 nominal beam, 10 feet long, is rated for 4690 lbs. However, I don't know how you would build a bridge with a single beam.

I'm planning a similar bridge. I will use two beams under the approximate vehicle tracks each made of 2 treated 2x10s, sandwiched with a layer of plywood. Each of these nominal 4 x 10 beams (at 10 foot length) will have a capacity of 3640 lbs. I will probably add a center beam, perhaps a single 2 x 10. This should give me a total safe load of about 9000 lbs, meaning I can drive my truck over it if I need to.

If you add up the costs of 5 treated 2x10s and a little plywood, that's a pretty cheap bridge structure. Your bridge deck and foundations will cost a lot more. What I'm saying is, overbuild the substructure - it doesn't cost much.

 

[LD1]

I need to replace a bridge on my property span is 10 feet tractor is b7800 with loader to aprox. 2800lbs and capacity for another 800 in bucket so say 3800 lbs load with operator

is an 8" X 8" hemlock beam going to do the job.

I found a formula that had this constant for hemlock set at 76


W X H squared X constant
________________________

span



so 8 X 64 X 76 divided by ten gives me 3891.2 Lbs

does this make sense.
thanks

By my tables (assuming 1000 psi) an 8x8 nominal beam, 10 feet long, is rated for 4690 lbs. However, I don't know how you would build a bridge with a single beam.

I'm planning a similar bridge. I will use two beams under the approximate vehicle tracks each made of 2 treated 2x10s, sandwiched with a layer of plywood. Each of these nominal 4 x 10 beams (at 10 foot length) will have a capacity of 3640 lbs. I will probably add a center beam, perhaps a single 2 x 10. This should give me a total safe load of about 9000 lbs, meaning I can drive my truck over it if I need to.

If you add up the costs of 5 treated 2x10s and a little plywood, that's a pretty cheap bridge structure. Your bridge deck and foundations will cost a lot more. What I'm saying is, overbuild the substructure - it doesn't cost much.

My calculations are comming up about right in the middle. But there is more than one way to calculate it, and more than one type of stress involved as well.

The formula I used is (w*d^2)/(9*L) x (k/f)

Where:
w=width of bean in inches
d=depth of beam in inches
L= length of span in feet
(k/f)=same modulus of rupture for hemlock, which is 750

So...(8x8^2)/(9x10) X 750=4266lbs

And that is just a basic point load calculation for a load supported in the middle of the beam. Obviously the tractor isnt going to have ALL its weight at the point at the same time. Simce the load will be distributed a little, that will help.

And I also assume you use 2 beams, so it should handle the tractor fine.

But as others have pointed out, wood deterioriates over time. This will reduce the load bearing capacity. I would look at a GOOD PT beam, or even steel. A couple of steel beams 10' long to support that weight isnt going to cost that much. Maybe $300 at most. And would give added peice of mind. I dont like wood bridges.

 

[jonsstihl]

Sorry about that I guess the obvious stuff is not so obvious when you look at it from another point of view.

The current bridge is two logs half circle in cross section maybe six inches thick with 2X6 decking . it works but one of the logs is broken so I get a lot of deflection when driving over it. luckily the bridge is not high off the stream bed and I think it is bottoming on a rock.

the new bridge will be two beams the appropriate width apart for the track of my tractor and I was thinking of trying to position a rock half way through the span to limit the deflection . the end supports will be those small cement blocks they use in parking lots aprox. 6" X 9" X 72" long. the decking will be 2x6

I was led to believe hemlock was fairly rot resistant.

I completely forgot that there are two beams there so the load is not as borderline as I thought.

thanks

 

[creekbend]

All the other TBN members had sound advice. I built a bridge 32 feet long, 14 foot wide 6 years ago- still standing strong over the creek. I used a mobile home frame with pressure treated 6x6's spaced every three feet and covered with 1-one inch marine plywood with 1x2's on top of the plywood to prevent sliding somewhat. I finished the sides with 2x8's to provide a minute safety feature, but the psychological effect is huge. You may derive some ideas from our posts but the ending decision is yours. It depends on how long the structure is expected to last, amount of construction time, and last but not least how much money you have available. Good Luck.:tractor: By the way,I used lumber that a neighbor of mine had left over from his barn building. This helped in keeping the cost down. The 14x70 mobile home frame cost me $50.00 plus my time transporting it.

 

[beeforty]

I recommend leaving a generous gap between your deck boards. Makes it easier to clean out between the boards and keep them from rotting. Also if you put runners the length to drive on, it spreads the load across more than one deck board.:thumbsup:

 

[KennyG]

Man, we ought to all get together and build something (like my bridge :laughing. I got my numbers from Marks Handbook for Mechanical Engineers, the 1967 edition (shows how far I go back.) The trailer frame is a great idea if you can find one. If you are looking at timbers, keep in mind that height contributes more than width. A 5 x 10 beam is stronger than an 8 x 8. A 3 x 12 is almost as strong as an 8 x 8. You only need enough width to keep the beam from buckling or twisting. I still think treated is the way to go. Under a bridge deck it will last almost forever.

 

[LD1]

If you are looking at timbers, keep in mind that height contributes more than width. A 5 x 10 beam is stronger than an 8 x 8. A 3 x 12 is almost as strong as an 8 x 8. You only need enough width to keep the beam from buckling or twisting. I still think treated is the way to go. Under a bridge deck it will last almost forever.

Not quite.

You are correct that height contributes more than width. But an 8x8 is still a tad stronger than a 5x10. At least in all the tables I looked at and in the formula I used.

And it is significantly stronger than a 3x12.

IF we isolate JUST the beam in the formula I provided, taking out the specif type of wood species and its properties, and the length of the span:

the first part of the formula is JUST based on beam size. W x D^2 (width times depth squared)

So JUST that variable gives an 8x8 beam a # of 512. A 5x10 is 500. And a 3x12 is only 432. And the span tables I looked at confirmed this as well. So basically, an 8x8 is 3% stronger than a 5x10 and 19% stronger than a 3x12.:thumbsup:

So when rough guessing on beam sizing and what is stronger than what, just remember....depth squared times width. Pretty accurate for comparing beams of different widths:thumbsup:

And I do agree that PT would be the way to go. Unless you decide on steel.

 

[Short Game]

I know this much: a square beam will sag quite a bit, even under just its own weight.

I was led to believe hemlock was fairly rot resistant.

I hope it's different from our western hemlock. That stuff is highly prone to rot. However, it takes wood preservative better than most woods, as it is like a sponge. Western hemlock is the hardest firewood to dry out, and the quickest to get wet again. It is extremely popular for pressure treated, ground contact, lumber (great for that). But untreated, it makes compost in a hurry.