Building A Bridge

[swines]

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.

IMAO - you have no idea what you're talking about. You're NEVER relieved of responsibility for engineering a structure (in this case the abutments). I could tell you long stories about engineering firms being sued for things that they only had a tangential part of. You're "IMHOs" are fine for you - but, I have the feeling you're neither a trial lawyer nor an engineer.

As far as the report goes, it would depend upon how it is worded - whether the firm certified the bridge to a certain load or only rendered an opinion. If it's only an opinion, the firm would not be held as liable - but, any hard engineering like the abutments - not a chance...you're totally liable for the design. Believe me - we have to take risk management courses yearly on just such differences...

That's the exact reason that the inspection engineers would give no more information than that in the report - and why they were "no help." They couldn't be. The report was not current and they can't professionally rely on information that is non-current on a structure that can change because of being exposed to environmental conditions.

 

[shaley]

Can't you just find out who built it from VDOT and get the ultimate load rating from the original plans. I bet CBI must have cranked out a million bridges that looked like that. All the girders are standard shapes.
Hey, Is that bridge in Madison county?

 

[3RRL]

IMAO - you have no idea what you're talking about. You're NEVER relieved of responsibility for engineering a structure (in this case the abutments). I could tell you long stories about engineering firms being sued for things that they only had a tangential part of. You're "IMHOs" are fine for you - but, I have the feeling you're neither a trial lawyer nor an engineer.

As far as the report goes, it would depend upon how it is worded - whether the firm certified the bridge to a certain load or only rendered an opinion. If it's only an opinion, the firm would not be held as liable - but, any hard engineering like the abutments - not a chance...you're totally liable for the design. Believe me - we have to take risk management courses yearly on just such differences...

That's the exact reason that the inspection engineers would give no more information than that in the report - and why they were "no help." They couldn't be. The report was not current and they can't professionally rely on information that is non-current on a structure that can change because of being exposed to environmental conditions.

Geez swines, lol ... I must have pushed a button or something to get a response like that. If you reread my response, my IMHO refers to the owner of the bridge not getting away scott free of responsibility. I think in the event of an accident involving that bridge, regardless if God did the engineering report, any good attorney would file suit on everybody involved with it, including the owner.

So are you saying he (the owner) will not be held liable at all if the engineering company "certifies" the bridge then?

 

[Renze]

Toiyabe, your explanation is from a lawyers point of view:

the explanation from an engineers point of view is:

You can NEVER calculate for an exact maximum load that occurs during its life: the vibrations, jumps, load situation and distribution that occurs in practice over many years has too many variables to calculate it exact.
The safety margin is just to make up for all those variables that you cant measure or predict. No matter how advanced your analysis program is, you'll allways need to use a safety margin to stay ahead of the factors the calculation model misses.

If i'd calculate the bridge for him, i'd calculate the structure on the occurring tension. that's quite accurate.
To evaluate the outcome is another: where fine grain steel like Domex 700 laughs you in the face and will take a swell load for years without fatigue, standard St. 37.2 will snap instantly.

So any advice based on the outcome of the analysis will be based on standard St.37.2
The less accurate the calculation is, the higher the safety margin should be.
According to the engineering standards book, a jump and swell load needs a design safety factor of 1,5 and an alternating load needs a safety margin of 3.

If i give Koop an advice based on lowest grade standard steel 37.2 and a safety factor of 2, this safety factor covers the dynamic peak loads as well as a fatigue margin.

If you want to take it to the next level, you could FEM analyse it with the Von Mises stuff and see if you can lower the safety factor by analysing the dynamic loads etcetera, but thats a lot of work to rise the approved loads just by a few %... I dont think Koop is willing to pay an engineer with a FEM analysis program to get just those extra few % of permitted load...

 

[patrick_g]

Renze, You gave very good info. Curiously, today on NPR (National Public Radio) there was an engineer going on and on about steel fatigue, elastic limits, stress risers and on and on. It was fascinating but I wondered what percentage of the audience stayed awake.

Anyway there is an empirical way short of destructive testing (we intend and fervently hope) to determine the bridges stiffness, its Hooke's law constant well before reaching the elastic limit.

Flexure causes fatigue. Small flexes can be repeated virtually infinitum with no deleterious effect. Large flexures (like bending a coat hinger tightly to 90 degrees and straightening it out again a few times and noticing how easy it is to break it in two)

Somewhere in between the small repeatable ad infinitum flexures and the large clearly damaging flexures there is a point reached where small flexures over enough repetitions will cause failures. Before this was well understood, as well as we do now, (new understandings of metal fatigue has within just the last couple years radically changed the understanding of this phenomenon and the way it is studied in engineering schools strength of materials classes.)

Anyway leaving history, and philosophy behind what do I recommend. I recommend that the bridge be supported where it was designed to be supported and that you instrument the bridge, i.e. put some dial indicators in the obvious locations and then add weights to the bridge to see how much movement you get. Since you don't know the alloys used or how much has been corroded and sand blasted away over the decades it is difficult to estimate the bridges performance. I recommend that instead of having everyone guess at the number of angels dancing on the head of a pin that we get a magnifying glass and count the little buggers!

Support the bridge up off the ground and put some dial gauges on it (like under the center of the span etc.) and load the sucker up and see how much it flexes. It shouldn't cost an arm and a leg to get an engineer to cooperate with this experiment and help you interpret the results. Then you will have some real data instead of a bunch of us guessing at values with no basis in fact for our comments.

I am reminded of the guy who didn't want to pay for an expensive operation so he paid a guy a few bucks to retouch the X-rays and make everything look good.

Bad economy!!! How will you feel if you avoid paying an engineer, save a buck, and have the bridge collapse and injuusr or kill someone. All you need is one lost and confused ready mix cement truck driver drive across your bridge...

I had a lost ready mix driver in my driveway just last week. Luckily my cattle guard was over built for its intended use.

Pat

 

[Toiyabe]

Good god.

DIN St. 37.2 is unheard of in the US. Nearest I can figure it is related to the ASTM 570 series, which is for sheet metal not thick structural steel sections. I presume you use the stuff to build trailers?

The rest of the stuff I guess you picked out of a mechanical engineering textbook. That's nice, but irrelevant to a bridge. I guess you should get a pass on jibberish like "jump and swell load" as english doesn't apper to be your native language.

Safety factors include more than just uncertainties about the future loads and load combinations. They also include uncertainties about material properties and section sizes. They also include acceptable risks, which vary according to application. There's a whole field devoted to the safe design of civil structures - it is called civil engineering. Don't re-invent the wheel.

Patrick_g, the stiffness of the bridge isn't the big issue. The strength is. You can't directly figure out the strength of a structure from its stiffness. For one thing, all common steels all have the just about the same elastic modulus, although strength varies widely. Fatigue damage is related to the ratio of cyclic strain to ultimate strain, not deflection. Bend thin sheet metal and a thick I beam through 90 degrees, the I beam will have much more strain at the extreme fiber.

 

[patrick_g]

Patrick_g, the stiffness of the bridge isn't the big issue. The strength is. You can't directly figure out the strength of a structure from its stiffness. For one thing, all common steels all have the just about the same elastic modulus, although strength varies widely. Fatigue damage is related to the ratio of cyclic strain to ultimate strain, not deflection. Bend thin sheet metal and a thick I beam through 90 degrees, the I beam will have much more strain at the extreme fiber.
+++++++++++++++++++++++++++++++++++++

I will not go into this in great detail as there is a zero WIFM index. If there is doubt about the strength and the bridge for some reason would flex too much for comfort at the intended weights then one simple cure is to put more supports under the bridge so spans are reduced.

The stiffness experiment was not intended to derive the strength of the bridge. It is a cheap, simple, and quick way to attempt to see if you can show the deflection to be so small as to not be an issue with the intended loads and strength of materials used. Who cares what the ACTUAL strength figure is so long as it can be shown to be sufficiently greater than needed?

Irrespective of bridge strength, the supports have to be good enough or the system fails. I hired a PE consulting engineer a few years ago. He brought out a sub contractor and helper with a drilling rig. We bored a 6 inch diameter hole and took samples every 6 inches till we hit solid rock. The PE analyzed the samples in his lab and issued a written report with his PE stamp. He made recommendations for my foundations and drain systems. This was a three story structure with massive amounts of concrete and steel so I wanted to build a foundation from a position of knowledge not good ole boy guess work.

The total cost of the engineer, report, subcontractor, the whole enchilada was less than $1000. This included designing various drain systems and the foundations. My liability DOES have access to his E&O insurance.

This was a bargain. The engineer was a professional in high demand. I got him through recommendations from the civil engineering firm I used. When I asked them to do the work they said it was out of their comfort zone (firm owner was PhD in civil engineering with several staff PE's) and they gave me the contact info for the consulting engineer they used. The point of this is that you can hire good help at reasonable prices. Your insurance company will be pleased if you get a PE to design the bridge supports and test the sub soil so the supports can be tailored to reality and not a one size fits all good ole boy estimate approach.

Everything should be made as simple as possible, but no simpler. -- Albert Einstein

The part that is often forgotten but is of paramount importance is the BUT NO SIMPLER part.

Pat

 

[dullpain]

koop - You should be sued for starting this thread!!! Nice looking bridge , when it's installed give me a call . I'll drive up there and test it out , if it fails I won't sue you.....

 

[Renze]

DIN St. 37.2 is unheard of in the US. Nearest I can figure it is related to the ASTM 570 series, which is for sheet metal not thick structural steel sections. I presume you use the stuff to build trailers?

Yes, in Europe we use DIN, NEN and ISO. Indeed i've used it to build trailers, hospitals, waste processing plants and more. Its the most widely type of steel used in Europe, also in civil engineering, in which i have worked before i got into trailers.

The rest of the stuff I guess you picked out of a mechanical engineering textbook. That's nice, but irrelevant to a bridge. I guess you should get a pass on jibberish like "jump and swell load" as english doesn't apper to be your native language.

Right, my general engineering book says that the common rule for fatigue safety, is 1,5 for a load that amplitudes from 0 to +100% and a factor 3 for a load that amplitudes between a negative and a positive load. (anywhere from -1 to +100%)

Sure i didnt test it myself - these are empirically determined limits commonly accepted through engineering. Or to use your words:

Don't re-invent the wheel.

I was reciting my dutch book, which i translated literally. Did i loose my credibility because my English isnt perfect ?? If you didnt notice that i'm a Dutchman untill this single sentence, i dont have to be embarrased of my English.

Safety factors include more than just uncertainties about the future loads and load combinations. They also include uncertainties about material properties and section sizes. They also include acceptable risks, which vary according to application. There's a whole field devoted to the safe design of civil structures - it is called civil engineering.

Very correct: the reasons i mentioned are only a few. You cannot rule out all factors just by safety margins, the chance of failure just has to be narrowed down to an acceptable level. For exapmple, in case of the dams, dunes and dikes around Holland: a chance of a flood every 10.000 years.

With safety margins to reduce the chance of failure to once in 10.000 years, you're no longer in the field of engineering, but in the field of lawyers.
The same as Koop's bridge with a safety margin of 2: if you want to determine if the risk of failure is acceptable, you'd need a lawyer and a risk analyzer, no longer an engineer.

 

[Toiyabe]

Indeed i've used it to build trailers, hospitals, waste processing plants and more. Its the most widely type of steel used in Europe, also in civil engineering, in which i have worked before i got into trailers.

Maybe for thin cold-rolled members, but not for thick hot-rolled members like those used in this bridge.

Right, my general engineering book says that the common rule for fatigue safety, is 1,5 for a load that amplitudes from 0 to +100% and a factor 3 for a load that amplitudes between a negative and a positive load. (anywhere from -1 to +100%)

This may be usefull in mechanical engineering when the dead load is negligible compared to the live load. Not usefull for civil engineering where the dead load is always significant and often much greater than the live load.

You can't just pick numbers out of a book without knowing what they mean.

I was reciting my dutch book, which i translated literally.

If you gave the dutch equivalent for "jump and swell load" then someone who really cared could figure out what it means. Your english translation of that term does not convey any information, however. If you were an english speaker, that term would peg my BS detector.

The same as Koop's bridge with a safety margin of 2: if you want to determine if the risk of failure is acceptable, you'd need a lawyer and a risk analyzer, no longer an engineer.

Which is why engineers use standard design practices, which specify how to design a structure with an appropriate margin of safety. There are many names for people who make things up as they go along, but engineer isn't one of them.

Anyway, the point is escaping me here. I wanted to explain to you why what you are proposing to do is dangerous. If I haven't convinced you by now, I don't think I ever will.