Building A Bridge

[BTDT]

To me that's a great looking bridge. I hope it serves you well. I got similar negative responses (not from TBN, but from my suppliers) that what I was wanting to do wouldn't work. My bridge (suspension), has served me with no problems for about 5 years now.

 

[Bill Barrett]

As a trucker I have learned if you go real fast over them you'll be fine

LOL +1 and try to do it as smooth as posible!

 

[koop]

To me that's a great looking bridge. I hope it serves you well. I got similar negative responses (not from TBN, but from my suppliers) that what I was wanting to do wouldn't work. My bridge (suspension), has served me with no problems for about 5 years now.

I love suspension bridges! Is it pedestrian use only, or for vehicles as well? I would love to see pictures if you have any.

 

[BTDT]

I love suspension bridges! Is it pedestrian use only, or for vehicles as well? I would love to see pictures if you have any.

Let's see if this link still works.

http://www.tractorbynet.com/forums/photos/90164-bridge-pics.html

 

[RedRocker]

Great bridge koop, I hope you enjoy it.

 

[koop]

Just got the word from the Department of Environmental Quality that no permit will be needed.

My heart goes out to all those affected by the bridge collapse along the Mississippi. This tragic event is particularly sobering for anyone in contact with a steel truss bridge. How does it happen on a state bridge that is frequently inspected?

 

[ktm250rider]

I think that VDOT should have scrapped that bridge before letting a private individual get a hold of it for liability concerns.

Thats the biggest problem today. Who can I blame for my problems. If the bridge fails, its HIS fault. The town shouldnt be liable for it.
Id love a bridge like that. Mines made of a couple of pine logs and left over lumber from building my house. Its not pretty but it works.

 

[Renze]

Renze,

You have seen the picture, I can give you the dimension of every piece of steel you see. Just let me know. I would love to see what your program comes up with.

It would appear I need to reassure everyone that if your program says 40 ton weight limit that I am not going to invite every tandem axle dump truck with 4 pony axles and 23 tons of gravel to come test your results. LOL

Koop,

I am willing to do this in the weekend.
If you really want this, i need dimensions, a kind of grid coordinate from beam joint to beam joint.
Also the dimensions of the beams, is it a standardised profile with a known moment of inertia ?
In Europe we have standardised profiles according to DIN, HE-A, HE-B, HE-M, UNP, IPE where the first character represents the shape of the profile.
I'm not sure if they are the same, but if you have the standard code, someone must be able to find a table of moments of inertia from the internet... Otherwise i have to calculate that as well, and rely on your exact measurements of the profile... ???

 

[Renze]

Koop, when going back and looking at your picture, i see your bridge roughly uses only 3 types of steel profiles. The 4 longitudinal beams in the floor, look like European hot rolled IPE profiles.
The upper beam of the side trusses look like they are composed from sheetmetal by longitudinal welds.
The crossbars look like sets of angle irons, bolted together in the middle by bolts.

It's about an evening of work to calculate the moments of inertia of the non-standardised profiles.
Throwing that stuff into a calculation model isnt that much work at all.

 

[dynasim]

There are a number of people in this thread that think they know more than they actually know.

Rephrased, the question is:

"What is the difference in safety factors between higher speed, higher frequency bridge design versus a low speed, low frequency bridge design."

Condemning someone because they are going to use a well designed, publicly utilized bridge on their own property does not make sense. I am quite sure that there is someone out there that could tell him the facts(The factors are almost certainly in someones structures handbook), without accepting any liability risks(after all, they are objective facts).

I have found that people who say that something complicated can or can't be done based upon "common sense" are almost uniformly mistaken.

The load rating of that sort of bridge is likely well known(they were/are catalog items). Those bridges are very strong(likely stronger than necessary, certainly stronger than a flat bed semi trailer). However, the owner ought to pay a stuctural engineer 4-8 hours to evaluate the bridge, and design the appropriate supports. It will be cheaper than replacing/repairing the concrete after settlement.

<Edit--I should have read the last quote by Renze. Oops>
Chris

P.S. The supports are very important. Make sure that thermal expansion, and soil bearing load are accounted for. If this were my area of expertise, I would help. It isn't, but I know the calculations are relatively simple and quick.

 

[Toiyabe]

Rephrased, the question is:

"What is the difference in safety factors between higher speed, higher frequency bridge design versus a low speed, low frequency bridge design."
.

And the answer to that question is "there is no difference".

Fatigue was not considered in bridge design until around 1970. Not surprising if you recollect that fatigue wasn't considered in aircraft design before the de Havilland Comet crashes in the early 1950s. Due to the age of the bridge, it almost certainly was designed with no allowance for fatigue.

Dynamic load factors came in around the late 1940's, which may or may not have been before that bridge was designed. If that bridge was designed under modern ASD standards, the DLF would probably be around 15 - 25%, depending on the span. Under modern LRFD standards it would be 33%. But you would be unwise to count on a DLF of even 10%, because you don't know what standard it was designed to. And DLF is not considered to be speed-dependent for design purposes.

Regardless, that would only apply to the bridge as originally designed. And the bridge has deteriorated significantly since it was first built.

Renze wants to plug this bridge into a structural analysis program. Does he know the strength of the steel, or will he assume it based on "common sense", derivied from experiance with steel grades used in Europe in the year 2007? Will that program distribute the dead load for him? If not, how will he calculate and distribute the dead load? How will he model the live load? A single point load in the middle of the bridge? Or will he assume a design vehicle and axle weight distribution? Does he assume full moment transfer at the connections, or assume they are ideal pins, or something in between? Does he assume that failure will occur in the members first, or will he model the connections as well?

The excersize may be fun, but given all these necessary assumptions by someone with little or no domain knowledge, the answer will not be more reliable than a wild guess.

 

[Kendall69]

...The exercise may be fun, but given all these necessary assumptions by someone with little or no domain knowledge, the answer will not be more reliable than a wild guess.

Ahhhhhhhh - Amazing someone who "gets it"

What a breath of fresh air.

Far brighter "brains" "inspected" ( and I use that term loosely ) this bride, and here we have a group of internet "engineers" approving this bridge design.

I don't know why the term blind leading the blind keeps coming to mind?

http://www.cssevents.com/35W.pdf

 

[3RRL]

I'm sorry, but this thread is hilarious.

I have nothing of value to add, other than I'd love to have a bridge and I think it's awesome you're going to use that one.

I'd use it and never look back.

I read this whole thread and find I love this answer the most.

 

[dynasim]

Kendall69

Quote:
<Ahhhhhhhh - Amazing someone who "gets it"

What a breath of fresh air.

Far brighter "brains" "inspected" ( and I use that term loosely ) this bride, and here we have a group of internet "engineers" approving this bridge design.

I don't know why the term blind leading the blind keeps coming to mind?>

Unqote:

Since nothing can be known, we should all go live in caves.

The concept that this bridge(the one shown in this thread) ought to be thrown away because it is deemed "dangerous" by the fellas telling him to throw it away is almost funny. Almost.

I would note that the event in Wisconsin has only one thing in common with this thread, and that is they both involved bridges. Using one to justify a position on the other is not reasonable, and I, personally, am offended by the reference.

The same can be said about the reference to the Tacoma bridge incident, although that was far less tragic.

Chris

 

[firemanpat2910]

its not looking back thats the problem, Dont look down!!!

LOL I agree this thread is useless, Personaly I would not hesitate to use this bridge, I have seen much worse around here, I also agree the foundation is equaly as important as the bridge it self.

 

[swines]

I find Internet "engineering" to be mostly unsubstanstiated opinions - and worth exactly what you've paid for them. I also don't understand the aversion to employing a professional engineer to inspect the bridge and do whatever calculations/verifications are required to calculate the load, AND engineer the abutments for it.

The company I work for does things like this:Big_I

The fly-overs were designed by one of our bridge engineers. My guess is that with an inspection and about 5-6 hours of work he could tell you everything you need to know about the bridge. The abutment structures should be able to be designed for a fixed fee as there are standard abutment designs that are easily site adapted for the structure and conditions.

My advice would be to get an engineering fee estimate - those are FREE. All you need to do is ask the cost for an inspection, report, and design of the abutments from an engineering firm that is regularly engaged in roadway design. They will give you a fixed-fee cost or a not-to-exceed cost. Your insurance company will appreciate it and you will have a professionally engineered bridge system.....and the liability is shifted from you to the engineering firm.

 

[tallyho8]

Looking at that bridge reminds me of a similar bridge that was installed on a county road in Johnson County in the Arkansas Ozarks years ago. It was just a one lane bridge and I first saw it when I was a kid in the 1950s and last saw it around 1980. I went back looking for it a few years ago but it had been replaced with a modern bridge. It was just sitting on the bank on both sides of the creek and was tied to trees on the 4 corners with wire rope. I'll bet that the residents loved it because to this day there are still no bridges in some places where you have to ford the creek on county roads "God willin' and if the creek don't rise".

 

[Renze]

Renze wants to plug this bridge into a structural analysis program. Does he know the strength of the steel, or will he assume it based on "common sense", derivied from experiance with steel grades used in Europe in the year 2007? Will that program distribute the dead load for him? If not, how will he calculate and distribute the dead load? How will he model the live load? A single point load in the middle of the bridge? Or will he assume a design vehicle and axle weight distribution? Does he assume full moment transfer at the connections, or assume they are ideal pins, or something in between? Does he assume that failure will occur in the members first, or will he model the connections as well?

The excersize may be fun, but given all these necessary assumptions by someone with little or no domain knowledge, the answer will not be more reliable than a wild guess.

Ahhhhhhhh - Amazing someone who "gets it"

What a breath of fresh air.

Far brighter "brains" "inspected" ( and I use that term loosely ) this bride, and here we have a group of internet "engineers" approving this bridge design.

I don't know why the term blind leading the blind keeps coming to mind?

http://www.cssevents.com/35W.pdf

It's easy to bash another one's good intentions, but do you dare to stick your head out and make a statement ?... then tell us, what exactly is the purpose of a safety margin ?? do you really have a clue ???

 

[3RRL]

Plus, I don't think this statement is true.
.....and the liability is shifted from you to the engineering firm.

They may now share in the liability but you never get rid of it yourself. Heck, they may have a disclaimer in their report relieving them of all liability? IMHO, as long as it's yours, you will have liability for it.

 

[Toiyabe]

It's easy to bash another one's good intentions, but do you dare to stick your head out and make a statement ?... then tell us, what exactly is the purpose of a safety margin ?? do you really have a clue ???

Sorry about that Renze. I tried to make that point more gently before, but it didn't seem to come across.

There's no shame in being ignorant of bridge design. I am a little less ignorant than you, but still too ignorant to do it myself. Even if I was competent to evaluate this bridge, I still wouldn't "stick my neck out" because I haven't seen the bridge and investigated the site with my own two eyes. To do otherwise is more likely to make things worse than to make things better.

A safety margin is a rather indirect but handy way of expressing an economic equation, with the cost of the structure on one side and the cost of failure multiplied by the probability of failure on the other side. Implicit in that safety margin is the value (in currency units) of a human life. It also reflects uncertainties in the design inputs.

As to what its purpose is, it is to protect the public's interest. The client doesn't enter in to it, except as one individual in a large group of people. In this case, that group of people is everyone who might cross that bridge: The client, the client's family, guests, firemen, the UPS guy, a confused motorist who made a wrong turn, etc. etc. This often annoys clients, as they feel they should be the one to set the acceptable risk level. But an engineer's primary and over-riding obligation is to the public, the client comes second.