Building A Bridge

[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.