Cooper Class E10 bridge loading

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Cooper Class E10 bridge loading
Posted by Murphy Siding on Sunday, April 22, 2007 10:16 PM

      Something interesting I ran accross in a book-  In the 1880's, an engineer named Theodore Cooper developed a *baseline* of sorts, to uniformly apply design weights when engineering a bridge.  The Cooper Class E10 design is based on the concentrated wheel loads of two Consolidation ( 2-8-0 ) steam locomotives,followed by a loaded train of uniform loading.  The "10" comes from the 10,000# loading on the drive wheels of a consolidation locomotive.  From this basic standard, bridges are designed on multiples of this value.  An E50 loading, for example, would be 5 times the loading of an E10 value.    Apparantly, this system is still used to this day.  If I understand it correctly, modern bridges are designed for E72, if they're concrete, and E80 if they are steel.

     Is anyone familiar with this system of design?  Why is, (or how could) a 125 year old standar, based on equipment loadings, be used and relevent today?  Why the different loadings on concrete verses steel?

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Posted by Anonymous on Sunday, April 22, 2007 10:28 PM

The Cooper rating as you point out is an axle-loading rating based on the loading pattern of double-headed 2-8-0s.  The number in the rating is the axle loading of the driving axle; e.g., E40 = 40,000 lb. maximum weight on the driving axle.  It allows for closely spaced axle loadings to derive a live load that replicates the static weight of the locomotive and train on the bridge.  Additional factors have to be added to the static live load for dynamic loading (acceleration, deceleration, impact, nosing, as well as snow/ice and wind load). 

The Cooper rating was an excellent empirically derived method of estimating live loads on railroad bridges, and it worked well for diesel-electric, too.  Recently AREMA changed recommended practice to include acceleration and deceleration longitudinal loading, which stemmed from the introduction of A.C. locomotives and concern about their high adhesion and tractive effort.  It consists of a simple percentage increase to the Cooper rating.

I don't know where you're finding the difference between steel and concrete, as far as I know we're using E80 for all new construction including culverts.  I'll ask one of our bridge engineers in the morning.

Cooper was a brilliant engineer that greatly advanced the science of bridge engineering in the late 19th century, and while his name lives on in Cooper ratings he was also the consulting engineer for the first Quebec cantilever bridge -- the one that collapsed during construction because the dead load of the bridge was greater than the ability of the bridge to carry it. 

Bridge design is an iterative process and in large bridges the dead load is most of the load. In order to start design the engineer has to make an assumption about the dead load. The engineer uses experience to estimate the weight of the steel in the bridge. From that number he can calculate the stresses on each member, then sizes each member for that stress, and designs the connections.  After the design is complete, if he thinks it's necessary, he tots up all the weight in all the steel in all the members and re-runs the design to see that the original assumptions about the stresses on each of the members were correct.  Back in Cooper's day there wasn't computer support to do this very tedious process automatically -- basically it consists of designing the bridge twice, so engineers used experience and judgement to make assumptions about the dead load, and rarely did the back-check because it was time-consuming and expensive. 

That method was fine so long as bridges got larger very slowly but the Quebec Bridge because of its immensely long main span was a leap into the unknown (only one bridge of comparable size existed, the Forth Bridge in Scotland).  The design budget was insufficient to pay Cooper and Peter Szlapka (the bridge designer) to do the check, with catastrophic consequences.  It was a grave error in professional judgement by Cooper and ended his until-then stellar career. He should not have taken the commission under the terms proffered by the bridge's promoter but apparently his ego got the better of him.

Could I suggest you acquire "Engineers of Dreams," by Henry Petroski?  I think you would find it fascinating.

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Posted by Modelcar on Monday, April 23, 2007 7:42 AM

...Mr. Hadid:

What was done in the rebuild of the failed Quebec bridge center section that changed it to be successful.

Is it possible the bridge you mention over in Scotland {the Forth Bridge}, is one that failed too....while a train was crossing....?

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Posted by Anonymous on Monday, April 23, 2007 9:54 AM

Modelcar:

The "center section" is the suspended truss span between the two cantilever arms.  The failure occurred in the design of the cantilever arms.  They were complete re-engineered (with a proper budget) using stresses that were within the capabilities of steel.  The suspended span of the 2nd bridge fell into the river during lifting into place due to a poor construction detail, not a design detail.  A new suspended span had to be fabricated and lifted into place.

The bridge you are thinking of in Scotland that failed was the Firth of Tay Bridge, which was conventional truss spans on piers.  The engineer did not take proper consideration of wind loadings and the center spans blew off their piers one night, with an express train inside them.  The construction details were inadequate too, so the entire bridge was torn down and a new bridge constructed.

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Posted by spokyone on Monday, April 23, 2007 10:07 AM
 1435mm wrote:

Could I suggest you acquire "Engineers of Dreams," by Henry Petroski?  I think you would find it fascinating.

S. Hadid 

I just did. Thanks for the tip.
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Posted by MidlandPacific on Monday, April 23, 2007 12:51 PM
 1435mm wrote:

Modelcar:

The "center section" is the suspended truss span between the two cantilever arms.  The failure occurred in the design of the cantilever arms.  They were complete re-engineered (with a proper budget) using stresses that were within the capabilities of steel.  The suspended span of the 2nd bridge fell into the river during lifting into place due to a poor construction detail, not a design detail.  A new suspended span had to be fabricated and lifted into place.

The bridge you are thinking of in Scotland that failed was the Firth of Tay Bridge, which was conventional truss spans on piers.  The engineer did not take proper consideration of wind loadings and the center spans blew off their piers one night, with an express train inside them.  The construction details were inadequate too, so the entire bridge was torn down and a new bridge constructed.

S. Hadid

Actually, although there are still conflicting theories, the most likely cause of the Tay Bridge collapse was probably a combination of poor design and materials choices, metal fatigue, and poor maintenance.  The bridge was very top-heavy at the section that collapsed (the "high girders"), and there was a kink at the south end, which transferred momentum from northbound trains to the bridge.  The piers were basically sets of paired latticework towers, held together by a latticework of thin wrought iron bars that were connected to the main towers by weak cast iron lugs, which suffered repeated damage whenever a train's passage caused the bridge to oscillate.  Over the course of the bridge's life the lugs gradually lost strength, and on the night of the accident, the passage of two fast, heavy passenger trains broke the last connections between two of the towers on a pier, reducing the amount that a pier could move out of center to a mere three feet, and the oscillation (abetted to some degree by the wind) pushed one of the towers (probably the one just behind the train) more than three feet out of its normal position and it toppled over. 

The low girders (or trusses) were reasonably sound, and were removed from their piers, widened, and incorporated into the new bridge, which is still in use today.

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Posted by Murphy Siding on Monday, April 23, 2007 1:33 PM
 1435mm wrote:

The Cooper rating as you point out is an axle-loading rating based on the loading pattern of double-headed 2-8-0s.  The number in the rating is the axle loading of the driving axle; e.g., E40 = 40,000 lb. maximum weight on the driving axle.

I don't know where you're finding the difference between steel and concrete, as far as I know we're using E80 for all new construction including culverts. 

Could I suggest you acquire "Engineers of Dreams," by Henry Petroski?  I think you would find it fascinating.

  The information was in a book Landmarks on the Iron Highway, by Bill Middleton, si I can't say if it's good or bad, ad far as E72/E80 information.  I just didn't understand why there would be a difference.  Thanks for the book recommendation.  I'll add it to my list.  I'm 1/3 through Castles of Steel.  Invincible and Inflexible just pulled into Port Stanley, and there might be some excitement in the near future.Wink [;)]

Using an E80 rating would give you axle loadings of 80,000#, locomotive total weight of 1,112,000# over 100', and a uniform trainload of 80,000# per lineal foot of following train.  I can see where the uniform loading becomes a moot point, after a structure is designed for the concentrated loads.  I was perplexed over the idea of an 1880's rule of thumb still being applicable today, but just using a multiplication factor.  Is it just a case of making the *E* number match the axle loading, and everything else works out, because it would be overbuilt?

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Posted by jchnhtfd on Monday, April 23, 2007 8:15 PM
 Murphy Siding wrote:
 1435mm wrote:

The Cooper rating as you point out is an axle-loading rating based on the loading pattern of double-headed 2-8-0s.  The number in the rating is the axle loading of the driving axle; e.g., E40 = 40,000 lb. maximum weight on the driving axle.

I don't know where you're finding the difference between steel and concrete, as far as I know we're using E80 for all new construction including culverts. 

Could I suggest you acquire "Engineers of Dreams," by Henry Petroski?  I think you would find it fascinating.

  The information was in a book Landmarks on the Iron Highway, by Bill Middleton, si I can't say if it's good or bad, ad far as E72/E80 information.  I just didn't understand why there would be a difference.  Thanks for the book recommendation.  I'll add it to my list.  I'm 1/3 through Castles of Steel.  Invincible and Inflexible just pulled into Port Stanley, and there might be some excitement in the near future.Wink [;)]

Using an E80 rating would give you axle loadings of 80,000#, locomotive total weight of 1,112,000# over 100', and a uniform trainload of 80,000# per lineal foot of following train.  I can see where the uniform loading becomes a moot point, after a structure is designed for the concentrated loads.  I was perplexed over the idea of an 1880's rule of thumb still being applicable today, but just using a multiplication factor.  Is it just a case of making the *E* number match the axle loading, and everything else works out, because it would be overbuilt?

Oh laddie -- don't knock 1880's rules of thumb!  If it ain't broke, don't fix it, and Cooper's work was sufficiently brilliant that it ain't broke; the physics hasn't changed a bit (except as 1435 pointed out the new AREMA recommendation for longitudinal acceleration).  One might check the resulting stresses for a 'real' case situation for a very very large structure, I suppose, if you wanted to push the limits of the material.  In my opinion, however, a bridge built to the appropriate Cooper loading will be only slightly overbuilt -- and thank goodness railroads prefer slightly overbuilt to slightly underbuilt (your local friendly highway department will go for underbuilt every time, by the way, which is why your interstates are in horrible shape and railroad lines aren't).

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Posted by erikem on Monday, April 23, 2007 11:24 PM
 1435mm wrote:

The Cooper rating as you point out is an axle-loading rating based on the loading pattern of double-headed 2-8-0s.  The number in the rating is the axle loading of the driving axle; e.g., E40 = 40,000 lb. maximum weight on the driving axle.  It allows for closely spaced axle loadings to derive a live load that replicates the static weight of the locomotive and train on the bridge.  Additional factors have to be added to the static live load for dynamic loading (acceleration, deceleration, impact, nosing, as well as snow/ice and wind load). 

The Cooper rating was an excellent empirically derived method of estimating live loads on railroad bridges, and it worked well for diesel-electric, too.  Recently AREMA changed recommended practice to include acceleration and deceleration longitudinal loading, which stemmed from the introduction of A.C. locomotives and concern about their high adhesion and tractive effort.  It consists of a simple percentage increase to the Cooper rating.

I would imagine that seismic loading has also been added since Cooper's time. Of course, new contsruction methods need new earthquakes to properly validate the design - e.g. concrete bridge columns in California (original designs didn't provide enough horizontal rebar to keep the columns intact).

 

 {Reference to Quebec cantilever bridge snipped for brevity}

 Could I suggest you acquire "Engineers of Dreams," by Henry Petroski?  I think you would find it fascinating. 

Petroski's writings came to mind when reading the earlier part of your post - and I also found "Engineers of Dreams" to be a very interesting and informative book. One of his observations was that really major bridge failures occurred about every thirty years and he postulated that was due to  the bridges being designed by engineers with no direct experience in failures of the bridge type they were designing (insufficient paranoia).

A related generational gap is probably why the FAA hasn't been able to replace the Air Traffic Control system. The original system was undoubtedly designed by veterans of the SAGE program and there hasn't been anything since then to provide the necessary design experience. 

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Posted by jchnhtfd on Tuesday, April 24, 2007 7:21 AM
I might add one other thing about the Cooper system -- which applies particularly to the comment on seismic loadings (which is quite correct).  The Cooper system gives you the load, and the distribution of that load.  What it does NOT do is tell you how to hold it up.  That is, there is no prescription for the design and construction (unlike highway work in the US, where there is a handbook which basically says things like 'if the span is 40 feet and you are holding up an interstate, use 6 beams of such and such a shape and construction).  In railway work, once you know what the loading is (Cooper, plus add-ons such as the longitudinal and seismic) it is up to the engineer designing the structure to figure out what members, whether concrete or steel or wood or whatever, arranged and connected in what ways, will hold the load up.  This is not to say that the various railroads don't have standard designs -- they do, and it saves a lot of time.
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Posted by mudchicken on Tuesday, April 24, 2007 9:38 AM
It's not the AC part of the locomotives that adds on the extra safety factor to Cooper's System, it's the "high adhesion tration motors & wheelsets" ...You ought to see what happens on some old timber trestles when there is a predominant direction of tonnage and the stringers start walking off the bents & pile caps!
Mudchicken Nothing is worth taking the risk of losing a life over. Come home tonight in the same condition that you left home this morning in. Safety begins with ME.... cinscocom-west
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Posted by Anonymous on Tuesday, April 24, 2007 11:56 AM

 mudchicken wrote:
It's not the AC part of the locomotives that adds on the extra safety factor to Cooper's System, it's the "high adhesion tration motors & wheelsets" ...You ought to see what happens on some old timber trestles when there is a predominant direction of tonnage and the stringers start walking off the bents & pile caps!

True enough but no one seemed to get excited about it until A.C. locomotives appeared.

I've seen some of those timber bridges.  Exciting!

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Posted by Murphy Siding on Tuesday, April 24, 2007 12:23 PM
 mudchicken wrote:
It's not the AC part of the locomotives that adds on the extra safety factor to Cooper's System, it's the "high adhesion tration motors & wheelsets" ...You ought to see what happens on some old timber trestles when there is a predominant direction of tonnage and the stringers start walking off the bents & pile caps!
Is that due to the wheelsets trying to push( for lack of a better word) the rails toward the back of the train?

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Posted by Anonymous on Tuesday, April 24, 2007 12:32 PM

 Murphy Siding wrote:
 mudchicken wrote:
It's not the AC part of the locomotives that adds on the extra safety factor to Cooper's System, it's the "high adhesion tration motors & wheelsets" ...You ought to see what happens on some old timber trestles when there is a predominant direction of tonnage and the stringers start walking off the bents & pile caps!
Is that due to the wheelsets trying to push( for lack of a better word) the rails toward the back of the train?

Or forward (if it's in dynamic braking at that point).  Think of it as a force applied longitudinally to the rail.  The rail doesn't slide and ball up in a wad somewhere because it is anchored against longitudinal movement, so it resolves the force into the ties, and the ties into the ballast.  The force can cause the track to slide out on the curves or in this case to slide the  stringers off the caps or the caps off the piles.

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Posted by dldance on Tuesday, April 24, 2007 12:34 PM

 Murphy Siding wrote:
 mudchicken wrote:
It's not the AC part of the locomotives that adds on the extra safety factor to Cooper's System, it's the "high adhesion tration motors & wheelsets" ...You ought to see what happens on some old timber trestles when there is a predominant direction of tonnage and the stringers start walking off the bents & pile caps!
Is that due to the wheelsets trying to push( for lack of a better word) the rails toward the back of the train?

For historical accuracy, Golden Spike NHS has a stub switch and the approach rail has no tie plates or rail anchors - just held by spikes.  Three times a day the engine makes run, crossing the switch several times.  Each west bound crossing - the locomotive is accelerating.  Each east bound crossing the locomotive is braking.  After 6 months of this, one rail has crept about 1/2 to 3/4ths of an inch further east.  Now this is one small 30 ton locomotive.  Imagine the reaction forces of the eingines pulling a 10,000 ton train.

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Posted by Anonymous on Tuesday, April 24, 2007 12:44 PM

That's very interesting info, dldance. 

So, for even more historical accuracy is the section gang fed beans and salt pork, and paid $3 a week in gold coin from a paycar?

 

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Posted by dredmann on Tuesday, April 24, 2007 1:28 PM

As pointed out, the Cooper E-rating system is not a rule of thumb for designing bridges; it is a system for describing the axle loadings and spacings on a train. Knowing what train weight ("live load") you have to carry is the first step. Last I knew there was a large binder AREMA bridge design manual that offered suggestions and good practices for designing railroad bridges with adequate strength etc.

As a practical matter, I think that today, an E80 train is supposed to roughly represent a unit coal train, and is the standard design limit for most main lines. Formerly, lower standards were generally used (e.g., E65, E50). But there are exceptions: I have it on good authority that the double-track railroad bridge over the Mississippi River just upriver from New Orleans, origininally built in the 1930's, was designed for two E65 trains (simultaneously) or a single E90 train.

As was aluded to, with a really big bridge, most of the strength of the bridge is used just to hold up its own weight (the "dead load"). For this reason, it is relatively easy to build a large bridge to a higher design load standard.  (E.g., if you want to redesign a culvert to carry twice the axle load, you need to make it almost twice as strong, but if you want to design a long-span bridge to carry twice the axle load, you only need to make it slighly stronger.)

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Posted by dldance on Tuesday, April 24, 2007 2:56 PM
 1435mm wrote:

That's very interesting info, dldance. 

So, for even more historical accuracy is the section gang fed beans and salt pork, and paid $3 a week in gold coin from a paycar?

 

Yes - we had beans last Winter Fest day - cooked on the backhead of the 119.  And I wish we got paid.  Even $3 a week is better than nothing - but I wouldn't trade the experience for anything.  Come visit us - May 10th is just around the corner.

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Posted by Murphy Siding on Tuesday, April 24, 2007 4:46 PM
 dredmann wrote:

As pointed out, the Cooper E-rating system is not a rule of thumb for designing bridges; it is a system for describing the axle loadings and spacings on a train.............

................As a practical matter, I think that today, an E80 train is supposed to roughly represent a unit coal train, and is the standard design limit for most main lines.

Because the Cooper E-rating system was based on axle loadings and spacings of an 1880 consolidation steam locomotive, wouldn't the loading parameters be somewhat different for a 2007 SD-70?  Or is that a non-issue?

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Posted by erikem on Tuesday, April 24, 2007 11:50 PM
 dldance wrote:
 1435mm wrote:

That's very interesting info, dldance. 

So, for even more historical accuracy is the section gang fed beans and salt pork, and paid $3 a week in gold coin from a paycar?

 

Yes - we had beans last Winter Fest day - cooked on the backhead of the 119.  And I wish we got paid.  Even $3 a week is better than nothing - but I wouldn't trade the experience for anything.  Come visit us - May 10th is just around the corner.

Hmmm, I have  a friend who works for ATK-Thiokol near Promontory - may have to come up with a good excuse for a business trip... 

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Posted by Anonymous on Wednesday, April 25, 2007 4:34 AM
 Murphy Siding wrote:
 dredmann wrote:

As pointed out, the Cooper E-rating system is not a rule of thumb for designing bridges; it is a system for describing the axle loadings and spacings on a train.............

................As a practical matter, I think that today, an E80 train is supposed to roughly represent a unit coal train, and is the standard design limit for most main lines.

Because the Cooper E-rating system was based on axle loadings and spacings of an 1880 consolidation steam locomotive, wouldn't the loading parameters be somewhat different for a 2007 SD-70?  Or is that a non-issue?

No no no.  The Cooper System enabled the engineer to plug in ANY axle loading in 10,000 lb. increments (you know, even in that day there were some rather heavy 2-i8-0s!).  And the driving-axle spacing of a 2-8-0 is not much different than a six-axle diesel-electric locomotive.

Today we use Cooper E-80 as standard on every NEW railroad construction project we design and/or specify.  I can't speak to that being a standard on all railroads everywhere in the U.S. but it is certainly a west-of-Chicago standard for Class I railroads or anything that will carry 286K or 315K cars (whatever they might carry) and modern six-axle EMD or GE locomotives.  Overseas heavy-haul has similar axle loadings and uses the same rating.

S. Hadid

 

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Posted by Murphy Siding on Wednesday, April 25, 2007 6:51 AM
 1435mm wrote:

No no no.  The Cooper System enabled the engineer to plug in ANY axle loading in 10,000 lb. increments (you know, even in that day there were some rather heavy 2-i8-0s!).  And the driving-axle spacing of a 2-8-0 is not much different than a six-axle diesel-electric locomotive.

Thank you.  The axle loadings, I can see are compensated for.  The axle spacing comparison, I wasn't so sure of. 

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Posted by CSSHEGEWISCH on Wednesday, April 25, 2007 6:52 AM
I'm not absolutely sure on this, but I believe that the Pilbara ore haulers in Western Australia have bridges and culverts built to an E90 rating or higher.
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Posted by dknelson on Wednesday, April 25, 2007 7:48 AM

I have read that certain bridges designed in the early 1950s were based on the assumption that axle loads would be lighter -- assuming F7s and GP7s in other words, plus 40 and 50 foot freight cars -- into the future than in the last days of steam.  I am not sure where I read this but the practical effect was that some older bridges were better able to stand up to today's heavy locomotives and freight cars than were bridges designed during the transition era.

Here by the way is a very interesting website on bridge failures

 http://www.uwlax.edu/globalengineer/draft/Problem%20bridges/index.problems.html

The Quebec bridge, before and after

Quebec.jpg (294647 bytes) queb3.jpg (167065 bytes)

 

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Posted by Modelcar on Wednesday, April 25, 2007 8:22 AM

....dknelson:  Remember seeing pic's similar of the mammoth bridge at Quebec.  Great views.  The structure sure does look like the structural steel is so big and heavy one wonders how it holds up it's own weight.

Interesting web site too on failed bridges....Remember seeing the twisting from side to side until failure of the suspension bridge out west on the "news reel" in a theater at the time right after it's failure....

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Posted by CSSHEGEWISCH on Wednesday, April 25, 2007 10:22 AM
The pictures of the Pont du Quebec show the fall of the suspended truss section, which was a failure of construction technique, not a bridge failure.  The cantilever sections did not fail and in fact did not require any rebuilding after the additional stresses put on them when the truss section slipped out of the stirrups and fell.
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Posted by Redwood Chopper on Wednesday, April 25, 2007 10:34 AM

Interesting topic, this. I recently bought a copy of Prior's Construction & Maintenance of Railway Roadbed & Track (1907) to learn some of these basics. Frederick J. Prior, at the time the guide was published, was with the Engineering Department of the Pennsylvania Railroad System. He does not directly mention the Cooper standard, although I have seen it referenced elsewhere in more recent books. Instead, Prior cites PRR's c.1907 strain standard for bridges was "to meet a load of two locomotives weighing 187 tons each and a train load of two and one-half tons per lineal foot...the actual weight of our H-6-A [2-8-0] locomotive now in use is 168 1/2 tons."

relative to the stresses and strains of steel specified for bridges, Prior mentions Cooper respectfully and briefly on page 408:..." 'Mr. Theodore Cooper, one of the most eminent bridge engineers in the country, has published a general form of specification, and has printed on the cover of same a sentence which is probably the most important than any...between the covers of his or any other specifications...'The most perfect system of rules to insure success must be interpreted upon the broad grounds of professional intelligence and common sense.' "

I see in Trains to Yosemite (Burgess, 2004) that California's Merced Irrigation District initially specified in its preliminary engineering studies a Cooper E-40 loading in planning the Yosemite Valley Railroad new steel and concrete bridges over the large agricultural reservoir from the dammed Merced River, "the latter typical of the lightest railroad bridges built around 1910." The YV operated nothing heavier than 120,270 lb (weight on drivers) Baldwin 2-6-0s.

 

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Posted by Anonymous on Wednesday, April 25, 2007 10:39 AM

It figures that the PRR would use its own standard.  Note that the YV 2-6-0s have an axle loading on drivers of 40,000 lbs -- or E40.

Quentin -- the photos inserted above show the SECOND Quebec bridge, which was vastly heavier than the first (failed) bridge.  As noted by Paul, the center suspended truss section fell while it was being lifted off barges to its final position due to a construction error.  Also, the narrative at the linked site is an account of the collapse of the first bridge (even though it pictures the second), which isn't very helpful.

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Posted by Redwood Chopper on Wednesday, April 25, 2007 10:48 AM

PRR - "The Standard Railroad of the World" - I always thought this promotional phrase the company employed was just self-aggrandizement, and surely it was, but I have to guess it came semi-deservedly in that the Pennsy had to tote a great deal of weight in its many daily freight trains of coke, coals, steel and iron along with a host of general merchandise. But that's grist for other topic threads...

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Posted by Redwood Chopper on Wednesday, April 25, 2007 11:09 AM

This whole thread catches my interest on a narrower and historic scale. I am presently working on a history of the Union Lumber Company of Fort Bragg, CA and its two railroads - one being the well-known California Western (CWR). I have yet to gain access to CWR's records, and have not yet seen anything specific in Union Lumber's surviving archived papers relative to the timber trestle and timber or steel bridge designs the company used between 1885 and 1970.

Initially, CWR and its immediate forebear (Fort Bragg Railroad) used redwood A-frame (or modified King) through bridges of a single span with longer trestled approaches to cross creek and river gulches where spring freshet floods might do the most damage. Two particularly bad Noyo River crossings (before 1926) required Howe deck trusses with trestled approaches. By the 1920s as heavier steam power and train loadings became the norm, the railroad began to abandon the use of A-frame bridges ad settled into using steel plate deck bridges on reinforced concrete piers, often with redwood timber trestled approaches. Still, through the late 1940s, steam power had to be separated (mid-train  helpers) over most of the CWR since double-headed locomotives exceeded the A-frame bridge loading restrictions. I think the last A-frame bridge was removed and replaced with a steel deck bridge about 1964.

I haven't yet been able to determine if the ULCo/CWR used the Cooper standard in planning its timber bridges and trestles, or whether Cooper was or is used for anything other than steel bridges on steel or concrete/masonry piers.

~Kevin

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