Building A Bridge

[koop]

I have recently purchased a steal pony truss bridge from the state to be used for a private road entrance. I have started preparing the bridge for welding and painting. I hope to set it on new abutments in a couple months.

My problem is engineers. The age of the bridge means the state no longer has good records on the bridge. I do have an inspection report done a two years ago detailing the bridges condition. When I contacted the inspection company (an engineering group) they would offer no additional information other than what was in the report.

What I would like to know is what is a safe load to cross the bridge with every once in a while. It was posted at three tons to limit heavy traffic in a residential neighborhood. The inspectors said it could be posted at seven tons given its condition at the time. I am repairing the badly rusted areas and setting it on new abutments. It also will no longer see 300 - 400 vehicles per day.

My question is simply: A state bridge that can handle 7 tons all day everyday can handle how much weight a few times a year? Without spending thousands, engineers seem unwilling to help. I turn to the tractor community for some good common sense help.

[Greyfields]

As a Civil Engineer, I will not comment.

[tommu56]

I don't mean to be smart but

Lets put it in My terms (electrical)

If you have a wire rated for 15 amps continuously
and you only want to use once a day for 1 minuet how much will it carry safely?

The answer is 15 amps!

tommu56

[Kendall69]

I was never good at math, so I asked my 10 year old daughter for help on this one.

Her answer...7 tons all day everyday can handle 7 Tons few times a year.

These word math problems make my head hurt.

I did mean to be sarcastic to make a point. You are dealing with human life here, yours, your family, and WHOEVER crosses that bridge. You don’t want to spend thousands on certified engineers to answer your question is in my opinion a travesty. To save money you are jeopardizing, the safety of your family, by using old data, or trying to get by on the cheap. Here’s a clue, if the state sold the bridge, they had a reason, and I’m thinking it’s past it’s prime.

Do me a favor don’t ask me over to your house on Thanksgiving, because I will NEVER cross that bridge.

I apologize up front, but someone has to wake you up.

[gotrocks]

koop, I guess I'm a little more reckless that the other responders.
My decision would be based on how far down into the abyss and the span.
I would like a short 20 foot bridge across a wet weather stream on the back of my property, if it collapsed the drop would be less than 3 feet.
And I don't believe the "drop" would be instant, it would probably
s a g s l o w l y into the sandy bottom.

Watch out for Billy Goat Gruff!

[HappyCPE]

Originally Posted by koop

The inspectors said it could be posted at seven tons given its condition at the time. I am repairing the badly rusted areas and setting it on new abutments. It also will no longer see 300 - 400 vehicles per day.

My question is simply: A state bridge that can handle 7 tons all day everyday can handle how much weight a few times a year? Without spending thousands, engineers seem unwilling to help. I turn to the tractor community for some good common sense help.

Not to pile on, but you got your answers. The bridge will handle 7 tons. After you repair the rusted areas [are you just removing the rust or replacing metal components with new?] and put it on new, engineered abutments then you can HIRE the engineer to tell you how much weight it can carry. There's a lot of common sense help on this board but you get what you pay for.

FWIW I'm assuming the bridge is historic which is why they sold it, instead of scrapping it. It is a good thing you're doing by saving the old stuff. I did get a piece of advice [free] from an engineer - Don't bounce. Cross the bridge slowly because the impact load from a bouncing vehicle can exceed the capacity.

[BTDT]

Will this be the only way into/out of your property? Will firetrucks and ambulances have to come across this bridge to get to you in an emergency? How much weight do you think you need to cross it with "a few times a year"?

Good luck with your project.

[D7E]

If we need a short bridge we just bury flat bed semi trailers in the ground ,If officials dissagree a good size culvert pipe covered with clay and gravel , no weight issues at all .

[Timber]

Remind me to never ask you folks a dumb Question. Nice job driving a New Member away. Nobody will ever accuse you of being over kind. What is wrong with just answereing the poor guys Question.

[schmism]

he's asking about duration factor.

For a given time a load is on a structure, you can multiply the design values by a factor.

in my world (trusses)
in some cases you have to take a deduction .9DF for permant dead loads.
for wind loads which are assumed to be a load applied to the truss for only an extreamly short period of time, we use 1.6DF

But my world, its all statics. With bridge loads there is a dynamic load you have to account for. Many times youll see a speed limit listed for small bridges for heavy loads, due to the dynamics that the bridge has to support.

Your right that the bridge will likely support more than the 7 tons, but to determine what the safe operational load is, a detailed site inspection will have to be done.

In short, you wont get a "it will support xyz" answer over the internet!