Gidday, interesting question, not that I knew the answer
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projectbluebird. Assuming again that 2 8x16's are at least roughly equivalent to a single 16x16, the load rating should be about equal.
Assuming again that 2 8x16's are at least roughly equivalent to a single 16x16, the load rating should be about equal.
If the timber was of the same quality, I would suggest that two 8 x 16's would be more than equivalent to a single 16 x 16.
Cheers, the Bear.
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."
I'd guess that axle loadings include all axles, not just the drivers, and so long as none exceeded 60,000lbs., the bridge would be capable of carrying the load.
Wayne
If anyone is interested, I think I've found a relatively easy solution, in a rather roundabout way.
Looking through some of my old issues of Model Railroader, I found a Bluebook chapter on wooden trestles, giving span/stringer data for various Cooper ratings.
Assuming that the bridge itself can carry at least as much as the approaches, and comparing the approach stringers to the data in the table I find that while there is no entry for (three for each rail, at 16 inches square, as in the plan I posted) There is an entry for six 8x16 stringers at the right span.
According to the table, that would equal a cooper rating of E60. Which if I understand the rating system correctly is safe for an axle loading of 60,000 lbs. or a total weight on drivers of 240,000 lbs. Which seems high to me, but is more than the total weight of the light mikado I've been considering.
I am assuming there are 4 sets of the underside diagonal struts, although I couldn't see that explicitly on the plan. They would likely be in line with the stringers. In some ways that will perform rather like an arch. The clear span is also fairly short so my layman's impression is that it should be able to handle at least a 2-8-0.
The drawing above shows a free-standing bridge--one without abutments. Hence there's a lot of sway bracing below it. If you're using abutments, you can lose everything below the main bridge stringers except the transverse 10 x 14. I would expect you could also lead up to the bridge with a few trestle bents and still do without that bracing.
It looks to me to be easily adequate for modeling a light duty bridge, as noted earlier.
The OP's concern with loading leads me to wonder if he's talking about the weight bearing ability of the model. If that's a concern, I would use a steel bar for each pair of the inner set of stringers (the ones under the rails) instead of wood. In 1/32 scale, that would be 1/2" x 5/8" deep.
Also, if it were my project, I'd test load the bridge before running a piece of equipment over it. If I were being especially brave, I'd use a test weight that was twice whatever I'd run. Then I'd rest happy.
Ed
in general, you would need to determine the load that can be supported in the middle of a span (between two supported ends) which depends on the material strength and its dimensions. In other words, how strong is an I-beam. This problem is similar to spar design in airplanes, my other hobby, which has practical significance. See the section on Material Strength in the following. compositeSpar.html
but you also need to consider the loads on the diagonal beams that support the middle and 1/4 spans of the bridge both from below and above. What is the weakest part of the design that would limit the load?
since the strength of materials like wood and concrete can vary, things like this need to be overbuilt.
this looks like a complicated problem
greg - Philadelphia & Reading / Reading
ndbprrBack when I took some mechanical engineering courses in statics and dynamics the.only thing covered was truss bridges. Never have seen any.data on wood bridge building. Variables include type of wood, taper and jointing, length of span, number pf supports and more. That type of bridge was a road bridge rather then a railroad bridge. Don't recall ever seeing pictures of one in rail service.
They did exist as a steel railroad bridge, one is here in Shreveport (Waddell "A" truss bridges) It was used from 1890 to 1926. I assume this timeframe had much smaller locos traversing this bridge compared to a Big Boy. I would look at UP's examples of bridges back when the BB's were running between Cheyenne and Salt lake.
http://www.flickr.com/photos/28579340@N08/3430559172/
http://www.flickr.com/photos/23869601@N05/3584101114/
"The true sign of intelligence is not knowledge but imagination."-Albert Einstein
http://gearedsteam.blogspot.com/
projectbluebird Apparently the image didn't post. Here is a direct link. http://img.photobucket.com/albums/v607/projectbluebird/bridge_zps5ce52ad5.jpg
Apparently the image didn't post. Here is a direct link.
http://img.photobucket.com/albums/v607/projectbluebird/bridge_zps5ce52ad5.jpg
I'm hoping someone else can give me some pointers on wooden bridge loads. I'm thinking of building a king rod bridge described by Jack work in the April 1960 issue of model railroader to my layout. (a low quality scan is below)
I've tried doing the calculations myself, but frankly, the math involved is beyond me. The same goes for my attempts at calculating cooper loadings. I know that i could probably run a scale big boy across with no worries, but I would like to know what a realistic load limit (or axle loading) would be.
I know it sounds nuts, but then I've begun working in Proto32, so I'm certifiable.