wooden bridges

jdkid · Oct 18, 2001

Hi ya's
i have a prob that i wonder if some of the brains on here can help..ok on our farm there is a bridge made from 8x1's nailed together to make the main beams and the top deck is 4x1-6x1's (i'll have to get the right mersures to do this right)all the boards are stacked on there eage so it should be a lot stronger that a thicker beam..if some one can help i can get more info W X H X L etc etc i want to take a dozer and tractors across it (11 tonne dozer D4 and 7-8 tonne tractor)
catch ya
JD Kid
oh yea what about container (20 x 8)bases for bridge building ???any ideas?

TWINKLE_TOES · Oct 18, 2001

JD

Might be able to help if I understood a little better. If the <font color=blue>8x1's</font color=blue> are 8 inch x 1inch, how many are nailed together, the full width of the bridge? Other stuff that would help is kind of wood, what shape its in, distance between vertical supports. Right off hand it don't look to good for the tractor or the dozer.
Al

rmorgan · Oct 18, 2001

Peter--Although I couldn't begin to advise somebody who drives 400 hp tractors around their property about where they could or couldn't take them, I'd sure be interested in seeing a snap of your bridge--as well as your tractor and your farm! That said, it sure sounds like the tractor might end up "down under" the bridge /w3tcompact/icons/laugh.gif . . .

Rick

woodmills1 · Oct 18, 2001

i have some info from state of nh about timber bridges that are built of 2 by stock stood on edge. these bridges are bolted together crossways with iron to draw them together. if yours is nailed 1 by stock i might also be leary of taking the dozer across, though i might try the tractor knowing i had the dozer to rescue it

James "woody" Mills

jdkid · Oct 19, 2001

Hi ya
well the bridge is 20 foot long by 10 foot wide the top is all 4 x 1 's (4 inch by 1 inch) the beams under it are spaced like this 8, 8x1 right on eadge then 58, 8x1 in middle then 8 ,8x1's on the far side prob is where all ya wheels run the main beams are not so even if the beams could hold up the deaking maynot unless i put runners on top of decking.
ok the wood it looks like a harder kind of wood not a fir but i don't realy know
catch ya
JD Kid

TWINKLE_TOES · Oct 19, 2001

That's quite a bridge. I'll see what I can come up with. If you have a rig that you cross safely with now, and know it's weight, measure the midspan deflection with the rig in the center of the bridge. I'm pretty sure there ain't no way I'm going to tell you it's 100% safe. If you were to fall I'm sure I couldn't "catch ya"/w3tcompact/icons/wink.gif

Al

JJT · Oct 19, 2001

JDKID, I designed and built my timberframed house, cutting the frame by hand, (it was an out of control woodworing project). Get me the unsipported span on your bridge, the dimensions on the laminated timbers as well as the type of wood and I'll do the stress analysis for you.

jdkid · Oct 19, 2001

Hi ya
well the unsupported span would be about 17-18 foot the boards are nailed together (stab in the dark there)and wood i don't realy know it dose not look like a fast growing type but not very very fine grain there dose not appare to be many/any knots in it if i had a cam i would get some pics for ya
catch ya
JD Kid

Youare · Oct 19, 2001

I"ve helped build a couple of bridges, one out of wood the other had steel I-beams with a wooden deck. The wooden bridge was over about a 16 foot span, and the steel I-beam bridge was about a 20 foot span.

The wooden bridge has (3) 8-10" diameter hemlock logs where the wheels of a truck would travel and (2) more of the same diameter logs in the middle. There is a 2" thick deck over that and then running the other way another 2" thick 12" wide planked area where the wheel track would be.

I have driven a pick-up loaded with fire wood over the bridge many times, but if I had a dump truck I would not drive it over the bridge loaded.

The steel I-beam bridge was constructed so that a loaded cement truck could pass over it. Which did happen several times. There were two 6X12 I-beams under each wheel track and two 2" decks over that running in opposite directions.

Jd's bridge sounds a little light to be putting all that weight over. If he could shore up underneath it in the middle with some cribbing that would make a world of difference.

Randy

jdkid · Oct 19, 2001

Hi ya
well i do know people have taken spray trucks across it all up bout 3-3.2 tonne 6600-7000Lb and from what i'm told it dose not flex with that wieght ....would not fall far but would start a chain smoking,new dacks,loss of face colour kinda reactiononly bout 4-5 foot above little stream
catch ya
JD Kid

JJT · Oct 19, 2001

I need the dimensions of the laminated beams and the quanitity, (2 main support beams?), as well as the dimensions on the laminated deck, (solid 6" x 8' wide?) An 18' span is huge unless there are multiple 8' x ? beams. Let me know, I'll crunch the #'s. Can you put in a middle pier or 2? Even if it's temporary until the spring runoff knocks it out. You'll get 5 - 10 X the max load with only a 6 ' - 8' span..... Even 6x6 posts will do for these piers, (unless it's more than an 8' drop to the streambed).

jdkid · Oct 19, 2001

Hi ya
other thing i could do is get a old shipping container base and put on top my tractor and dozer will fit on 8 foot wide decks any thing wider is lighter and the bridge would hold that ok
catch ya
JD Kid

jdkid · Oct 19, 2001

ok
3 beams 2, 8 by 8 made up of 8 by 1 and a main beam 8 by 58 also made up from 8 by 1..they run full lengh of bridge now top is at right angles made up from 4 by 1 (ie 4 inchs thick decking and 8 inch thick beams )i could put something in the middle and it may come to that
catch ya
JD Kid

TWINKLE_TOES · Oct 20, 2001

JD

I flubbed my dub on calculating the strength of your bridge./w3tcompact/icons/blush.gif I could not convince myself that the spray truck wouldn't go thru the bridge, which of course you know it did not. The difficulty is identifing the modulus of elasticity for this type of construction (nailed 1x8's). Elasticity implies that deformation produced by low stress are completely recoverable after the loads are removed. When loaded to higher stress levels, plastic deformation or failure occurs. Modulus properties are generall derived from parametric (test) data for different types of construction. Adding midspan support will certainly help, but even then, a preload on the vertical supports is advisable to reduce the deflection under load. Sorry for the non help, maybe others on TBN will be more helpful./w3tcompact/icons/smile.gif
Al

JJT · Oct 20, 2001

Asis your bridge will probably will not take a 10 ton load. With a pier in the middle you should be fine. There are too many unknowns to give you a warrantee. Are the individual boards adequately nailed? Rows of 3 20D nails, (top, middle and bottom), every 12" is adequate. Is there any rot? Are there any joints, is every board full length? Are the existing piers solid? The dozer will actual exert less stress on the bridge than the tractor - more of the weight will be distribited to the middle 58x8 laminated beam, and the weight is not concentrated on 4 small foot prints.

You have 2 enemies here, 1) horizontal sheer where the beams meet your piers, a white pine 8x8 is 3000#, your max sheer comes when you enter and leave the bridge, the min sheer comes when you are sitting in the middle, (sheer is equally shared by each end pier), 2) maximum deflection, which will be at a max when you are sitting in the middle of the bridge. I calculate 2" of deflection, figuring 4 8X8 beams, most of the middle boards support -0-.

If your bridge had 8x12 beams supported at midpoint you could drive a 25 ton vehicle across without deflection. My house was built for a 100# snow load, I have 4 8x12 rafters in a 44' run, each capable of taking 50,000#'s with less than .1" deflection.

jdkid · Oct 20, 2001

Hi ya's
i may have to look at putting a crossing in just up from the bridge just thought it would be good if i could use the bridge ,thanks for ya help
JD Kid

jdkid · Oct 20, 2001

thanks for ya help will have to look at other ways of getting across migh put a crossing higher up for tractor and dozer
catch ya
JD Kid

RobertN · Oct 22, 2001

It's an issue with emergency vehicles too. It's really bad when you have an emergency, and the fire engine either snaps the bridge, or the captian refuses to cross it for fear of breaking it and his engine. It has happened in our district.

We've had to back engines long distances to get to a point where a medic rig could pass us and get over a marginal bridge. We've also had crews make long hikes carrying equipment cause they had to leave thier engine.

Of course, in JdKids case, if a D4 can get across, most any emergency vehicle could too /w3tcompact/icons/smile.gif

RobertN in Shingle Springs Calif