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.
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.
The first question is, do these bridges need to be replaced? Many bridges that are 100 years old still have plenty of life remaining, or are economically repairable. What usually kills a bridge (absent damage from pier scour, derailment, etc.) is (1) corrosion damage leading to section loss, (2) too many fatigue cycles for its design. As long as neither is excessive, the bridge may not need heavy maintenance or replacement. Obviously the bridges are capable of carrying the gross axle loads put on them.
If a truss was designed in the first place for the stream crossing, there was a good reason why a TPG wouldn’t work then or now. Typically a truss is used because the hydrology and hydraulics of the stream are unfriendly to a pier to subdivide the TPG span, and the amount of steel necessary to construct a TPG that is full-stream width is grossly uneconomic compared to a truss for the same span.
The aesthetics are subjective, but they are also moot. As long as the railroad is replacing the bridge without doing any in-water work, and not leaving its existing right-of-way with a realignment of the line, there’s little requirement for the railroad to subject itself to public agency comment. The permitting problems of the project would probably be much greater to convert the span to TPG than to replace with a truss in kind. Public law doesn’t create a requirement for railroads to subject bridge designs to aesthetic reviews, except in a specific case, below:
Unless federal or state money is being used, the railroad has no obligation to fulfill aesthetic tastes of the public. Even then, the “aesthetic review” component is often non-existent.
The San Francisco Bay replacement bridge is using the design it uses beca
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 mainl
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.
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:
"Bolted or welded plate girders for spans between 50 and 150 ft
Bolted or welded trusses for spans between 150 and 400 ft"
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):
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.
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.
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:
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’
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:
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.
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 certai
