It was suggested that I move this question from the modeling section to the prototype. And so I will:
Along the Columbia River (on the North shore), the BNSF has 4 (maybe 5) old truss bridges. They probably go back to when the main line was built early in the previous century. I've gotta believe they'll be replaced in the near future. I'm wondering what they would be replaced with. They are of various lengths, but the short ones are pretty much the Central Valley 150' bridge. The necessary clearance underneath would seem to negate use of a deck bridge.
So, I'm wondering if they would replace a 150' bridge with another truss, or whether they would go with a through girder. While a 150' through girder is unusual, I believe they're around. Also entering into the decision making is that the girder is more "modern looking". I'm sure BNSF will have to get massive amounts of approvals from all kinds of "stake holders". You'll note that the San Francisco Bay bridge replacement is not another truss.
Just wonderin'
Ed
It's an interesting question to pose, and I appreciate your curiosity. Please consider my answers as information, not commentary, and as general guidelines, not about the specific bridges in your post.
RWM
Do you have any photos or links to photos of any of them ?
Don't be so sure that they'll need to be replaced in the near future. Depending on how conservatively they were designed, the quality of the steel they were built with, how well they've been maintained, and - probably most importantly - what the 'loading history' of the rail traffic over them has been in terms of high axle loads and high stress states for 'fatigue life' stress cycle computations, etc., perhaps only some critical or overstressed members may need to be replaced, strengthened or reinforced such as by 'sistering', etc., which is far cheaper and easier than replacing the entire bridge.
That said, a 150 ft. long span is about the upper limit for a through or deck girder, but not an impossible length - but it's also the lower limit of economy for a through truss bridge. A rule of thumb is that the depth of the girder or truss should be about 1/10 to 1/12 or so of the span, so that would lead to a 12 to 15 ft. high girder or truss, which is probably too tall for a girder but too short for a through truss- so go with a shallower girder and use a thicker or wider flange instead to achieve the required strength.
More importantly - will the new bridge be for 1 track or 2 ? If only 1 track, a through girder could work OK; if 2 tracks, then either the 2 girders would have to be pretty deep/ tall to carry 2 trains at once, or a stronger middle girder would be needed which is more complex, etc., so that would favor using a through truss instead which already has the extra height and strength built-in.
Also, the local conditions for erection and insertion of the replacement span. A through girder doesn't need any temporary falsework underneath, but a truss does. Either one can be built parallel to the existing bridge on one side, and then 'rolled-in' sideways as the old one is rolled out, to minimize the 'downtime' for the mainline, if there is room for that. With the big cranes available today, lifting and moving a through girder would probably be easier than a through truss.
Finally, the floor system - will it be open or a closed/ 'ballasted' deck ? That's not critical, as either type of bridge can accomodate either type of floor, but my sense is that through girders are a little more friendly to ballasted decks, whereas through trusses are better for open decks. If the underclearances are really tight, the through truss can probably have a slightly thinner floor system.
In the past few years, PennDOT has replaced some old PRR - now NS - through-girder bridges on its now single-track "Trenton Cut-Off" line over major highways in the northern Philadelphia, PA suburbs. A pair of through truss bridges of about 250 ft. span each was used at US Rt. 202/ I-76 = "Schuylkill Expressway" in King of Prussia, but a pair of through girder bridges of about 120 ft. span each at PA Rt. 309 in Fort Washington. Here are the coordinates of each, per the ACME Mapper 2.0 application:
Through trusses at US Rt. 202/ I-76 in King of Prussia: N 40.08166 W 75.38917
Through girders at PA Rt. 309 in Fort Washington: N 40.13000 W 75.20201
- Paul North.
P.S. - This post was composed without me being aware of RWM's post. Interesting to see what we treated about the same, and where we went in different directions . . . - PDN.
AMEN! to the above....will just add that if the clearances are acceptable, leave it be and a lot of those older structures were designed to a different structural standard (steel bridge design and metalurgical science were still relatively new) with a much higher factor of safety because of the uncertainties at the time.
I'll confirm the previous two replies, but add some more variables. A Through Truss bridge has three critical parts.
The most visible of course is the pair of TRUSSES. These usually have more than sufficient capacity for even the heavier trains of today. You will find, however, some cases where they were strengthened back in steam days by doubling the trusses. I think one example is at Rock Island, near Wenatchee. CPR did something similar in 1929 with the famous arched bridge at Stoney Creek.
Second are the FLOORBEAMS. These connect the trusses at the bottom chord, and are located between each panel of the truss.
Finally you have the STRINGERS, either two or four, which run from floorbeam to floorbeam and are what the bridge ties (or ballast pan) rest on.
The bridge is only as strong as its weakest link, and from what I have seen the floor system is usually the culprit. I am aware of a number of trusses that have been upgraded by replacing either the stringers or the stringers and floorbeams. While it is not exactly an easy job, it is far easier and cheaper than replacing an entire bridge, and is done panel by panel with short track blocks each time.
A few other comments. A Through Plate Girder has much the same floor system, and older ones may have the same replacement needs. As Paul indicates, with newer steels the 150ft length has become feasible and they can be considered as replacements for the shorter through trusses.
You will also notice I didn't include the top connections of the through truss. These are not load bearing in themselves but keep the trusses in proper alignment. With a Deck Truss of course the bridge ties rest on the trusses themselves and there is no need for the additional complexity of a separate floor system.
John
About a month ago I bought a copy of the new book, Design of Modern Steel Railway Bridges by John F. Unsworth, P.Eng. (CRC Press/ Taylor & Francis Group, Boca Raton, FL, 2010, ISBN 978-1-4200-8217-3). Mr. Unsworth is Manager, Structures Planning & Design for Canadian Pacific Railway where he has been employed since 1987, and is also currently President and Chairman of the Board of Governors of AREMA for 2010-2011.
In section 3.3.2 - Steel Railway Bridge Superstructures at the top of page 77, he notes that Chapter 15 of the AREMA Manual of Railway Engineering recommends:
Obviously, at 150 ft. the subject bridges are right on the 'cusp' or 'inflection point' between truss vs. girder types.
He goes on to note that: "Steel freight railway bridge girder spans can be economically designed with a minimum depth to span ratio of about 1/ 15. Typically, depth to span ratios in the range of 1/ 10 to 1/ 12 are appropriate for modern short- and medium span steel girder freight railway bridges." Based on the first sentence, the girder depth could be as little as 10 ft. for a 150 ft. span bridge, which seems reasonably well-proportioned, at least for a single track.
Still, for a similarly proportioned bridge, take a look at this newly installed massive modern through girder bridge on the UP's Sunset Route at the following links to another thread here, Sunset Route Two-Tracking Updates (photos by K.P. Harrier):
http://cs.trains.com/TRCCS/forums/t/120779.aspx?PageIndex=78 - top 1/4 of page 78, Status Overview as of Saturday, November 6, 2010: - City of Industry to Pomona, CA - Part I (of I-III), Sections A - C (of A-D) - The Industry Up and Over, posted 11-10-2010; and,
http://cs.trains.com/TRCCS/forums/t/120779.aspx?PageIndex=80 - top half of page 80, Sunset Ave. is Now Open! - The 'Up and Over' - The City of Industry, CA - Parts I -VI, posted 11-28-2010.
More interestingly, current train and axle loadings appear to be within the loadings that a lot of those older bridges were designed for - albeit just barely. In section 4.3.1 Static Freight Train Live Load (pgs. 89 - 92), Figure 4.2 on pg. 90 is "Equivalent Cooper's E Loads for Some Modern Railway Freight Locomotives and Equipment on Simply Supported Bridge Spans up to 150 Ft in Length"*. For the 150 ft. spans we're discussing here, the 432,000 lb. 80 ft. long 6-axle locomotives and 315,000 lb. 36 ft. 4-axle cars that are considered in that diagram are about equal to a Cooper's E-72 loading in bending, as compared to moden standards of a Cooper's E-80 loading.
*Citing Dick, S.M., 2006, Estimation of Cycles for Railroad Girder Fatigue Life Assessment, Bridge Structures, Taylor & Francis (pg. 146).
As mudchicken noted above, those old guys were pretty conservative on the safe side due to the limitations of their knowledge and the state-of-the-art and unknown future for the loadings, material properties, analytical techniques, maintenance, etc. One of my college professors - William C. Holstein - had some experience in 'rating' older steel trolley and railroad bridges for reuse as highway bridges, and often said that even allowing for corrosion reduction in the section dimensions and connections, the steel quality, newer loadings, etc., those bridges were about 4 times as strong as were needed.
We find that most bridges designed to an E-60 rating will generally rate out to 286K axle loads, even after some section loss, including the recently added dynamic loading considerations.
Glad to see John Unsworth's book getting some recognition. I have my own copy too. I worked with John for many years. He designed them and I built them. I now always go through a WWJD (that's John, not the other guy) on any plans I am reviewing.
Paul is right that at 150' you are on the cusp. John Unsworth did design the 148' TPG we used to replace a similar length 1905 vintage truss at La Crosse in 1995 on the east channel of the Mississippi River (Tomah Sub Br 283.01). Since then the other 4 similar trusses there have been replaced, but with 5-74' and 2-111' TPG's (John's designs again) with 4 new piers and retrofit on the existing piers. That work was completed in 2001.
I would also highly recommend for you bridge geeks out there, AREMA's Bridge Insepction Handbook. Just go to arema.org to acquire a copy. Also look for the next session of the Bridge Inspection seminar being offered on March 15, 16 and 17 in Newark, NJ. I won't be teaching at this session, but will probably be doing the next one, tentativley scheduled for May.
As the saying goes: "I guess I came to the right place."
Thanks much for the various comments. Here's a couple of pictures of the type of bridge I'm talking about:
http://www.aaroads.com/west/washington006/wa-014_wb_at_rock_cr_02.jpg
http://columbiariverimages.com/Images/BNSF_engine_rock_cove_bridge_2005.jpg
They are certainly in use, as they are on the BNSF mainline from Spokane to Portland. To my awfully untrained eye, the various pieces of the truss match up closely with the Central Valley model truss bridge.
Mr. Pratt probably is wondering what all the fuss is about.
*If it ain't broke, don't fix it! As long as it's serviceable, let it be. The Class 1's tend to be very agressive about their steel bridges. This forum has seen in the past, how absurd some non-bridge experts can be. (CSX at Covington, KY has popped-up on this forum just a few times, the 1929 C&O bridge that replaced an earlier bridge that was the subject of a recent thread)
In my earlier post I referred to the truss bridge at Rock Island, in Washington State. I have now scanned one of the slides, and as you can see, the Through Truss span has become pretty complex. I don't know anything about its history but have assumed that the outer trusses are a retrofit to strengthen the span, probably when heavier steam locomotives appeared in the early years of the last century. Maybe someone else on the forum knows more of its history and can expand or correct.
Calculating the detailed stresses must have been a challenge. Or could the engineer conclude there was so much redundancy that rough approximations were sufficient to confirm the adequacy of the combined trusses?
Oh, my! And I was getting apprehensive about modifying the CV bridge to the prototype. Certainly a possibility, though.
One thing that gets me thinking that these bridges are headed for replacement is that, at least as of July of last year, none (I think) have been repainted. And some of them are looking really tatty. And BNSF has replaced all the signals on the line and added a new siding (an expensive one, I believe). Mind you, I think the bridges, as is, are thoroughly acceptable. To me. Especially if they were repainted. It's hard to beat a nice honest truss for aesthetics.
Signals, sidings, and bridge replacements all come out of different budgets, on different timetables, with different managers, with different philosophies. Also, signal forces and bridge forces each paint their own stuff, and so what one does isn't necessary coupled to what the other does. I wouldn't read a single thing into the observations you've made having any bearing on the future of any bridge.
The CV model strikes me as on the lighter side, like maybe an E-40 design, and the proportions are unusual for a railroad bridge. I've never studied it in detail, but it might be based on a highway bridge.
The GN bridge at Rock Island WA was completed in 1893. I do not know when it was strengthened, but the GN liked to run 2-10-2 locos between Spokane and Wenatchee. That line segment has long 1% ruling grades in both directions, but longer eastward.
I know the work was done before the mid 1960's when I was old enough to begin to pay attention to such things.
Mac McCulloch
cx500 [snip] I don't know anything about its history but have assumed that the outer trusses are a retrofit to strengthen the span, probably when heavier steam locomotives appeared in the early years of the last century. Maybe someone else on the forum knows more of its history and can expand or correct. . . . Calculating the detailed stresses must have been a challenge. Or could the engineer conclude there was so much redundancy that rough approximations were sufficient to confirm the adequacy of the combined trusses?
I too don't know anything about it other than what we see here and what I've seen of it before. it's different and significant enough that it may have been written up in a professional journal such as the AREA Proceedings or the trade press at the time - whenever that was. Nevertheless, I'll venture an answer to your question:
Highly unlikely that the engineer would have settled for a rough approximation - at least not explicitly. Although, as a practical matter the assumptions about the performance and reactions of the 2 interconnected spans to a theoretical train loading - as opposed to those in the real world - do have substantial amounts of unknowns which are covered by various margins or factors of safety, which are essentially an acknowledgment of the existence of a rough approximation.
I can think of 2 ways to analyze the load distribution among the 2 trusses:
1) Assume that each truss carries a certain type of load in proportion to that truss's strength to support that load. For example, if the existing members of the old truss have 40% of the capacity needed to support the bending moment load from the "design train loading" in the center, and the proposed new truss has 60% of the capacity for that load, then assume that said load is distributed commensurately. That principle not only has a obvious logical and 'common-sense' appeal, it is also the basis for distributing loads and analyzing stresses in 'indeterminate' type structures that are often redundant in some ways which prevent a direct solution based simply on the structure's geometry, such as the 'Moment Distribution' method (promulgated by Univ. of Ill. Prof Hardy Cross). In essence, this combined truss structure is an indeterminate structure, so that is also an appropriate rationale.
This approach also illustrates the 'circular' or 'feedback' nature of the challenge of the design and analysis process here - the loads on a member pretty much follow how strong you make the member, so the stronger you make it the more load it picks up and carries, and so the stronger it has to be, and so on. Sometimes the iterative assume/ analyze design process can go on for a while like being on a treadmill, unless you can recognize the 'limit' or 'bound' that the member's load or size is trending towards - like based on past experience and intuition, etc. - and just jump ahead to that.
2) Assume the old truss carries nothing except maybe its own 'dead load' weight, and maybe its floor system is still good enough to transfer the loads across/ out into the new truss, which then carries all of the major loads. But that is wasteful of the existing capacity of the old truss, and the new materials that would be needed in the new truss to duplicate that capacity, so I very much doubt if it was done that way.
Key to any analysis is connecting the two trusses together at each 'panel point' so that they interact and move and share the loads as one. If you imagine 2 long planks - 1 on top of the other - suspended between supports at their ends, and then standing on the top plank, you can envision how as the top plank bows down, it will contact the bottom plank and cause it to bow down too, which will 'mobilize' its strength as well to carry the load. But if the bottom plank is a little wider so that you can stand on it without touching the top plank, then the bottom plank will have to carry all of the load, and so will bend down that much more, which is not desirable. The solution to the latter scenario is to somehow clamp, nail, or screw the 2 planks together so that when under load they act as one, and that was undoubtedly done to this bridge as well.
Thanks for sharing that photo, too.
Wikipedia at - http://en.wikipedia.org/wiki/Rock_Island_Railroad_Bridge_(Columbia_River) (usual disclaimers apply) and the National Register of Historic Places confirm it was built in the 1892 - 1893 time frame; Wikipedia says the 2nd truss was added to strengthen it in 1925, which is plausible.
Link to National Register page for it - not much available 'on-line', it seems - it was added in 1975:
http://nrhp.focus.nps.gov/natregsearchresult.do?fullresult=true&recordid=29
75001842 NRIS (National Register Information System)
There is another good picture of the Rock Island, WA bridge on page 107 of Brian Soloman's book North American Railroad Bridges. It shows it in a 3/4 angle shot with MRL, Soo and BN power. You can get a good look at how the reinforcing trusses are set onto the original trusses.
What are 'fatigue life' stress cycles? How are they measured(?)
Thanks to Chris / CopCarSS for my avatar.
Obviously, an excessively heavy load will overload and overstress a bridge member, and cause it to fail by either breaking, cracking, or stretching ="yielding". What's not so obvious but nevertheless often true is that lesser loads - still within the allowable loads for the bridge - also cause damage, which accumulate over the life of the structure, and can eventually lead to much the same kinds of failures. The common analogy is bending a paper clip - bend it a little bit, or even for a large angle - just 1 time, and it likely won't break. Bend it through a large angle many times, and it will eventually break - that's because each bend is a complete "stress cycle" and a reversal from compression to tension or vice-versa on one side or the other of the paper clip. Bend it only a little bit, and it will still break eventually, but it will last for many more repetitions of the bending than it did when bending it through a much larger range or angle. That kind of failure is a "fatigue failure", although the "strain" (motion) range is extreme - no bridge member is ever bent that far out of line.
The challenge is to be able to figure out how much of a load is the threshold for starting to cause that kind of damage, and how many of them can be tolerated over the life of the bridge ? That analysis is complicated because in a general freight train of mixed loads*, there will be: 1) a bunch of empties*; 2) some moderate loads; 3) a lot of near-max. carload capacity loads*; and, 4) maybe a few more that are close to the bridge's maximum allowable load. The first 2 categories can be ignored - it's the last 2 that are of concern. There are mathematical/ statistical ways to essentially 'equalize' those different loads to a common basis, which are then often called "Constant Amplitude".
*Unit trains or "fixed consists" such as multi-level auto-rack cars are relatively easy - all loads of a certain weight range in one direction, and all empties in the other.
Once that is done, the "fatigue life" of a bridge member can be assessed (for existing bridges) in light of its past loading history (Good luck with that ! - finding those detailed records, though) or predicted or design loads(for proposed bridges). That fatigue life is usually measured in terms of the "Stress Range Cycles" for the Constant Amplitude as defined above.
Another complicating factor is that shorter bridges will experience more cycles than longer ones - the shorter bridges are more sensitive to the loads from individual wheels and cars or locos, whereas longer bridges are affected more by the weight of the total train. As a result, the AREMA Manual indicates that the number of Variable Amplitude Stress Range Cycles per Train ranges from about 3 cycles per train for spans over 100 ft. to as many as 110 cycles per train for bridges shorter than 50 ft. So one way to deal with that is to make the members of shorter bridges much stronger, so that they don't experience near as high a stress over their many more cycles.
Accordingly, AREMA recommends that members and their 'details' be designed for different stress ranges and nuymber of cycles, depending on their nature and how critical they are to the bridge, and how long the bridge span is. The number of cycles ranges from 1.8 million to 22 million. For reference, 60 trains per day for a design life of 80 years is 1.75 million, so you can see that the number of cycles per train ranges from about 1 to about 12.
Much of this post above is taken from Unsworth's book, Section 5.3.2.2.2 - Fatigue Design Criteria, which includes 5.3.2.2.2.1 - Railway Fatigue Loading, and 5.3.2.2.2.2 - Fatigue Strength of Railway Bridges, (pgs. 212 - 224 inclusive), to which I refer you for a much more thorough treatment of the theory and literature, analysis, and discussion.
Link to a 3/4 angle photo of the Rock Island (Washington) Bridge by Steve Carter, who sometimes posts here as "ottercove" if I recall correctly:
http://www.railpictures.net/viewphoto.php?id=254428
The apparent pattern of the principal members in the middle of the 2nd/ outer/ lower truss is sure unusual . . .
Back to the original bridge question here -
Most rational and prudent railroad bridge owners have those bridges inspected at least once every 1 - 2 years, and then make repairs as necessary, so it's unlikely that these have any unknown problems with them, or any issues at all, if the trains arre running at full 'track speed' over them.
Further, I should have noted previously that last year the FRA adopted new Bridge Safety Standards, effective Sept. 13, 2010, at 49 CFR Part 237, which can be found at (29 pages, approx. 206 KB in size):
http://www.fra.dot.gov/downloads/safety/BridgeFinalRule2010.pdf
In brief, that regulation requires each Class I railroad to create and adopt a "Bridge Management Program" by March 14, 2011, which includes annual inspections, etc. So again it's not likely that any problems will last long.
Who knows - the reason they haven't been painted recently may be related to anything from temporary economy-based budget cutbacks for non-essential and discretionary work, to obtaining permits and contractors to properly manage and contain the inevitable lead paint flakes from the older paint on them when being sandblasted from getting into the river, to more extensive work on them being planned soon anyway - so why spend money on paint that's only going to be replaced or marred soon anyway ?
Thanks, Paul, for locating a much better picture of the Rock Island bridge. I only passed through the area once, briefly, and the sun angle was terrible. Furthermore, it was in the era when the motive power was what I concentrated on. Now that I am older and wiser, I see all sorts of opportunities overlooked in the background of my older slides!
Hi Steve. I trust that all is well with you, except your memory. I was the designer of Tomah Sub Br 283.01, 5-74' and 2-111' TPG's and the 4 new piers. Geez. John writes a book and then he gets credit for everything!
Take care.
Kevin
Paul_D_North_Jr Still, for a similarly proportioned bridge, take a look at this newly installed massive modern through girder bridge on the UP's Sunset Route at the following links to another thread here, Sunset Route Two-Tracking Updates (photos by K.P. Harrier): More interestingly, current train and axle loadings appear to be within the loadings that a lot of those older bridges were designed for - albeit just barely. In section 4.3.1 Static Freight Train Live Load (pgs. 89 - 92), Figure 4.2 on pg. 90 is "Equivalent Cooper's E Loads for Some Modern Railway Freight Locomotives and Equipment on Simply Supported Bridge Spans up to 150 Ft in Length"*. For the 150 ft. spans we're discussing here, the 432,000 lb. 80 ft. long 6-axle locomotives and 315,000 lb. 36 ft. 4-axle cars that are considered in that diagram are about equal to a Cooper's E-72 loading in bending, as compared to moden standards of a Cooper's E-80 loading. - Paul North.
PDN: What is the load ability for a Cooper's E-80 loading? What comes to mind is that both UP and BNSF o operate steam trains on a irregular basis. Both the present UP locos and #4449 are heavy locos. Also the high wide heavy loads that come at irregular intervals is a point. I am thinking about the 1M# transformer that CP recently handled. So what kind of Cooper's loading is needed for these occasional loads?
Another slightly off topic thought is ---- Did this give NS have another reason to end their steam program because of bridge loading on the probable steam routes? I seem to remember that much of SOU RR never had very heavy load ability? Also maybe CSX may have loading problems and do not want heavy steam locos ?
Cooper's E-80 loading is based on a theoretical double-headed pair of "Consolidation"-type steam locomotives - 2-8-0's - followed by a train. Each loco has 80,000 lbs. on each of 4 indivdual axles at 5' spacing (60" max. diameter, more likely about 56") apart (320,000 lbs. over 4 x 5' = 20 ft. overall = 16,000 lbs./ ft. average), with 40,000 lbs. on the pilot truck 8 ft. ahead, and 52,000 lbs. on each of the 4 tender axles at varying spacings. Altogether, each loco is 568,000 lbs. over a 48' wheelbase, or about 11,800 lbs. per ft. Since there's an 8' space between their axles, the two locos together for a Cooper's E-80 loading have a 104' long wheelbase and 1,136,000 lbs., or about 10,920 lbs. per ft. There's also a space of 5' between the trailing loco and the following train, which is a continuous load of 8,000 lbs. per ft. - no individual axles are considered.
Since 80,000 lbs. per axle is about the highest axle load that has ever been accepted for general use on the U.S. railroad network, any 4-8-4 (such as the 844 or the BNSF's locos) would not have more than 80K on its 4 driving axles - the same number of axles as Cooper's 2-8-0's. Also, those steamers have 80" diameter drivers so that overall wheelbase is at least 26.7', as compared to the 20' for Cooper's. Likewise, the 3985 is a 4-6+6-4, but it has only 12 heavy axles, not the 16 of Cooper's, though at 69" diam. they are a little closer together - but still not as close or 'dense' as Cooper's 2-8-0's.
Per UP's webpage on the 844 at -
http://www.up.com/aboutup/special_trains/steam/locomotives/844.shtml
- the total engine plus tender weight is 908,000 lbs. over its 114.2' length/ 98.4' wheelbase or 9,230 lbs. per ft. The 4 driving axles have 266,500 lbs. on them or 66,625 lbs. per axle, so you can see that by any measure the Cooper's E-80 loading is considerably heavier than the 844.
The 3985 is of course a little different, but per its webpage at -
http://www.up.com/aboutup/special_trains/steam/locomotives/3985.shtml
the total engine + tender weight is 1,074,000 lbs. over its 121.9' wheelbase or 8,811 lbs. per ft. The 6 driving axles have 404,000 lbs. on them or 67,333 lbs. per axle, so the Cooper's E-80 loading is again considerably heavier than the 3985 !
I presume that the N&W's J 4-8-4 611 and A 2-6+6-4 1218 are similar. (The 611 is 872,600 lbs. total over 109.2' = 8,000 lbs. per ft., 288,000 lbs. on the 70" diam. drivers = 72,000 lbs. per axle, per - http://en.wikipedia.org/wiki/Norfolk_and_Western_611 The 1218 is 951,600 lbs. total over 121.9' = 7,810 lbs. per ft., 433,350 lbs. on the 6 axles also with 70" diam. drivers = 72,225 lbs. per axle, per - http://en.wikipedia.org/wiki/Norfolk_%26_Western_1218 and http://www.vmt.org/Loops-Collections/Steam-locomotive-Loop/Class-A-Steam-Locomotive-1218.html So the N&W engines are a little lighter in total weight and per ft. of track than UP's, but had higher axle loadings - but still about 10% less than Cooper's E-80 axle loadings, though !)
I never heard or saw that the bridges were a reason for NS to end its steam program - that wasn't a concern when they were running, either. If the bridges could handle modern 6-axle diesels which weigh up to 400,000 to 420,000 lbs. each = 67,000 to 70,000 lbs. per axle, as a practical matter the weights of the steamers aren't too much different from the loads of the trailing truck of 1 diesel and the leading truck of the next following one.
The 'overweight' heavy loads are not addressed in Unworth's book - a surprise, considering that Cp moves them,as you note. Nevertheless, they're not usually a problem. The builders of the Schnabel and other heavy-haul flatcars are usually careful to make them with a long enough wheelbase and enough trucks and axles under them that the weight per ft. of track and the axles loads are all within Cooper's E-80 or another acceptable lesser loading. For example, a 1 million lb. gross weight (load + railcar) isn't near as heavy as Cooper's double-headed 2-8-0's, at 1.136 million lbs. over 104 ft. Alternatively, at an allowable 8,000 lbs. per ft., that mega-load needs to be on a car (or cars) that are at least 125 ft. long together. And at 80,000 lbs. per axle, those cars need to have at least 12.5 axles carrying everything. It would be pushing things a bit to use either 2 - 6 axle flat cars or 3 - 4-axle flats, but it could be done, esp. if the speeds over the bridges were limited to 10 MPH or less to reduce the impact factor from the moving load. A more prudent approach would be to use a pair of 8-axle flatcars - 2-2+2-2 - or a Schnabel car with a similar number of axles, for a total of 16 axles. That would reduce the axle loading for a 1 million lb. load to about 62,500 lbs., which is consistent with a 250,000 lb. or 263,000 lb. standard freight car on 4 axles, which is a nominal 100-ton payload or so.
7j43k Oh, my! And I was getting apprehensive about modifying the CV bridge to the prototype. Certainly a possibility, though. [snip]
http://www.shop.cvmw.com/150ftHOscaleTrussBridgeKit-1902.htm
"The prototype bridge that the kit was copied from is a Southern Pacific bridge at Piru, CA (2 spans). It was manufactured by the N. Y. Bridge Company and was erected in 1902. It is a very common and typical design that is used throughout North America and even other parts of the world. Many are still in use and many subtle variations are possible. Modifications in bracing, rail, guard-rails, walkways and railings, telegraph arms attached, and different portals are common. They are most often painted black, but there are examples of silver, gray, red oxide and green that I have seen."
The bridge is apparently still standing, even though the rails have been removed, and it may have a walkway, path, or trail across it, as it is just east of "Warring Park" in Piru. Piru is about 40 miles NW of Los Angeles, and about 10 miles W-SW of I-5 on Rt. 126 from just north of Santa Clarita. The bridge is at these Lat./ long. coords., per the "ACME Mapper 2.0" application: N 34.41672 W 118.78900
Here are also a couple links to nice photos (not mine) of it:
http://www.flickr.com/photos/80651083@N00/368418590/sizes/l/in/photostream/
http://www.flickr.com/photos/sir_baldilocks_of_the_glen/2534969217/sizes/l/in/photostream/
A couple years ago a UP freight derailed and took out a similar former SP bridge near Palisade, Nevada. Here are some links to, for, or related to that:
http://cs.trains.com/trccs/forums/p/144348/1604384.aspx - thread here
http://www.railpictures.net/viewphoto.php?id=63671
http://www.railpictures.net/viewphoto.php?id=266734
http://en.wikipedia.org/wiki/File:PalisadeNV.jpg
http://www.rgusrail.com/nvpalisade.html
http://www.somewherewest.com/SP%20and%20The%20Overland%20Route/Palisade%20Canyon,%20Nevada.htm
Here's a picture of 4449 about to cross one of "my" bridges, though this one has two more panels than the "Central Valley" bridge:
[EDITED] Thanks for sharing that link.
The 2 additional panels are not the only difference, aside from the general 'heavier'/ larger/ stronger appearance, which might also be due to the added length. In particular, look at the lower chord - notice how it's thinner/ smaller/ lighter under the middle 4 panels, and thicker/ larger/ heavier under the 2 panels at each end ? Why might that be ? Especially since the usual case is that the largest 'bending moment' = load is at the middle of the bridge's span . . . ? But that's not true of the top chord . . .
Also, the verticals in "your bridge" are built-up with the 'laced' webs, and the middle 2 panels also have built-up diagonals that are much thicker/ larger/ heavier than on the SP/ CV bridge, even though the diagonals in the adjoining panels are also comparatively thin eyebars. It's hard to tell from this photo's angle, but all of the diagonals appear to slant down towards the middle of the bridge, and there aren't any panels with an 'X' shaped pattern of diagonals - correct ?
The short answer is that with that specific arrangement of the diagonals, the designers might have thought that the bottom chord at the ends could be put into compression with certain loads at certain locations and so needs to be bigger and stiffer to prevent buckling. However, that's highly unusual - normally all of those lower members of a Pratt truss are in tension only. Coincidentally, the Unsworth book considers exactly that in an example of "influence lines" in Example 5.5 on pgs. 168 - 171 for a very similar 156.38 ft. long, 8-panel Pratt through truss, member L1 - L2 and L3 - L4 in Figures E5.5c and E5.5e, respectively. And in the middle the bottom chord will always be in tension - like a cable, so it can be thinner - just enough to resist the 'pull'.
See also this link to a commenrtary on the similar 9-span, 1,291 ft. long ex-SP Salt River Bridge in Tempe, Arizona:
http://www.tempe.gov/museum/Tempe_history/properties/hps228.htm
(Darn - I was in that area less than a month ago, and had no idea - it would have been worth a side trip to see & photograph it. Maybe next time . . . )
Must be bridge season...or the engineers like playing cards with each other...
CN has started the process to replace the swing bridge over the Fox River in Oshkosh, WI. It was erected in 1899 and is structurally strong...but other bridge related machanical failures. Currently the bridge is a truss bridge: three sections with the center pivoting to allow marine traffic clearance. The replacement seems likely to be a TPG in four sections with #3 being a bascule, pivoting up to 60 degrees. Any idea why the RR is likely to change from one type to the other here?
Dan
Same or wider horizontal opening for the boat traffic, with less mechanical complexity - a simple horizontal axis, instead of essentially a turntable out in the middle of a river = more reliability. Also, less weight and length of structure to move, and easier to access from that end for maintenance and repair instead of out at the 'island'. (I presume the existing swing bridge is symmetrical, with the pivot in the middle of the center truss, and not on one end, which is rare.) Also, the center-pivot type creates 2 channels of the same width, but if only 1 is needed for the traffic volume, then that's a redundancy that costs money to build, maintain, and move with each opening. Both types have unlimited vertical clearance - contrast with a vertical lift bridge - so that's not likely the reason.
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