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.
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.
RWM
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.
- Paul North.
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
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.
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
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.
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.
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
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