Building a bridge

[buggyman1]

I would like to say: I don't think i'm putting anyone in danger, including myself. The only person's that will ever cross the bridge won't be on anything that will put them in danger (weight wise), except me. It will mostly be walked accrossed. I don't ,(and wouldn't) build bridges for anyone but myself. I would like to thank everyone for their comments, replys, and ideas... That being said...let's move.. ..accross.

 

[LostInTheWoods]

Buggyman,

I must add to the concern about your bridge! Four 6x6s SEEM like they'd be more than enough, but who knows?? And what about those trees near your bridge?? You've obviously given no thought about what would happen if one of THOSE fell on you while crossing the bridge. And fault lines. Did you check for fault lines? Your bridge may be unstable during earthquakes.

In all seriousness, I'd say this is more than sufficient (though I am certainly no structural engineer). If you can apply "Kentucky windage" to bridge construction, I'd say you did a fine job of it. From my limited understanding, MANY factors go into wooden construction of any kind, like the type and grade of the wood. Wood is also rated as to each type's elasticity (how much it will bend before breaking, I think). The duration of the loading is taken into the equation too...its not like a house that is supporting a load all day, every day. Then there are things like compression perpendicular to grain design value. Don't even ask me what that means. People much smarter than Yours Truly figure out that stuff.

I think the well-meaning concern others have for you here is good-natured, but I, personally, would have no fears crossing that bridge with your tractor. (You did check for fault lines, right?)

 

[gerard]

Rebar is used in locations that are loaded in tension; masonry is very strong in compression, and weak as a mouse in tension. You mentioned the Parthenon, look closely at the beams between columns, they span short distances because the underside of a beam is in tension and stone, masonry cant handle tension well.

Guess the trick then is to come up with a design that puts everything in compression (kind of like the romans did with the arches and stuff..........)

 

[Frogmore]

While I am not a structural engineer (and don't even play one on TV), I did download the WWPA Beam/Joist Analysis and Design add in for MS Excel from the WWPA web site (I believe you have to register to get access, but it was free). I used this to check the work of the structural engineer I hired to sign off on the design of my house to make sure he wasn't over engineering it too much.

Assuming that the long span is 7.5 feet, and using a Hem-Fir 6x12 flat (about the same as two 6x6 spiked together), incised for PT and >19% moisture content. It can handle a point load of 1000lb in the center. The total deflection is low at L/613 or 0.15 in. However the bending moment is very high at 89% of the allowable. Now there should be a fair amount of safety factor built in to the calculations so it is probably fine, but I would not say that it is way overbuilt. If you had put the 6x6s on top of each other and laminated them or used a 6x12, the bending moment would have been about half as much and this would be great. But, then I would start to worry about the decking痴 ability to transmit the load to the beam. Since the beam would only be 6 wide it would need to be on the center of the wheels, which would probably be a little bit of a challenge to get correct.

If the wood is actually SPF (South) then the load just exceeds the design values for bending moment.

Having said all that, the bridge does look very sturdy and you have driven your tractor over it. Since your tractor probably isn't going to weigh enough to put 1000lbs on a single wheel, you are probably okay. Pay attention to the noises the wood makes and inspect the beams for cracks regularly.

 

[johng52]

Let's not forget the most probable danger in bridge design...


...

 

[Transit]

Forgmore, your on the beam. Moisture content in your example is 19%. The wood in the bridge is going to have a higher moisture content and will soften the lumber making it sag more.
Many events in nature follow a square law, in this case, if you set two beams side by side the strength will double , take the same two beams, place one on top of the other doubling the depth, the strength will be 4 times greater. Somewhat over simplified, but you get my point.

To see the effect shape has try this:

You’ll need a sheet of paper and two books.

Place the paper on the books to act as a bridge from book to book.
Result, the sheet of paper will sag and be useless as a beam.

Take the same sheet and fan fold it a few times, WWWWW.
Place the sheet of paper on the books.
Result, the paper will now support itself and some additional weight.

Same sheet of paper, just a different shape.
Now for the big words, Cross Sectional Moment of Inertia.
That’s why an I beam is shaped the way it is. The most strength for the material used.

 

[ZetorShaun]

buggyman,
I really love your bridge. It has inspired me to add one to my project list when I get back to the farm. Unless that creek really gets going with a serious 100 year flood it will be there long after we are gone. There is something to all the engineering calculations but I am sure that bridge will hold. Well done.

"Whatever course you decide upon, there is always someone to tell you that you are wrong. There are always difficulties arising which tempt you to believe that your critics are right. To map out a course of action and follow it to an end requires courage." -Ralph Waldo Emerson

 

[BlacknTan]

In a way, this thread points up the reason why I don't post much on internet BBs that much anymore..

If the poster was a friend, and he asked us all to come over and see the new bridge he had constructed, I doubt that many would show up with their engineering library, sliderule, calculator and tape measure to measure it up, calculate the loads, pick it apart and deem it to be safe or unsafe.. We'd just take a look, give a pat on the back and tell our friend how good it looks and what a great job he did!
One the internet however, it seems that folks think it is their God given right to pick things apart and criticize, but maybe it's just a mirror of life in general... It seems that there is always someone to show up at the party with the intention of urinating in the punchbowl..

I wonder why life has not spawned as many experts on any subject as the internet has!

 

[cowboy357]

In a way, this thread points up the reason why I don't post much on internet BBs that much anymore..

If the poster was a friend, and he asked us all to come over and see the new bridge he had constructed, I doubt that many would show up with their engineering library, sliderule, calculator and tape measure to measure it up, calculate the loads, pick it apart and deem it to be safe or unsafe.. We'd just take a look, give a pat on the back and tell our friend how good it looks and what a great job he did!
One the internet however, it seems that folks think it is their God given right to pick things apart and criticize, but maybe it's just a mirror of life in general... It seems that there is always someone to show up at the party with the intention of urinating in the punchbowl..

I wonder why life has not spawned as many experts on any subject as the internet has!

Well said . . Bob

 

[JohnDeere4300]

Remember the Hindenburg fire in Lakehurst in the late 30's? The outer skin was made of a canvas like cloth. To fill the weave to seal of the cloth and pant it, lacquer was mixed with powered Aluminum and Iron Oxide to form a heavy body undercoat. Anyone recognize the formula? Powered Aluminum and Iron Oxide are used in Thermite Welding. The way it works is the Aluminum removes the Oxygen from the Iron, producing tons of heat, and the rest is history. Being that the process was invented in the late 1800's and in Germany, no one noticed? Go figure!

What does this have to do with his bridge ?...

You have criticized his bridge from the start, and i can't wait to see his next bridge that he has planned to build...BUT, i dread to hear what you will have to say

Like i have already said, his bridge is well built from the looks of the pics, and i would cross it with my tractor with my BH on and a load in my FEL.