wooden bridges

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