House car structure, was RE: Re: Unusual Milwaukee Road boxcar
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Schuyler Larrabee
Tim, if you were a structural engineer, you would know that an opening in the middle of a beam’s web has very little, even no, effect on the bearing capacity of the beam. The web has structural value as you approach the end points, the bearings at the ends of the beam. A simple beam’s structural “action” in the middle of a beam is compression in the top flange, and tension in the bottom flange. In the middle of a beam, the web is just along for the ride.
Box car structure isn’t quite the same because the compression value of the roof isn’t very high, if any. Most of the bearing capacity of the car is in the underframe. So you have a little compression in the roof (not much though) and tension in the underframe. The underframe itself is going through compression along the top of the longitudinal members (at the floor line) and the bottom of those members will be in tension. (The real structural engineers in the audience will forgive me for not getting into the reversal of these forces at the truck bolster . . .)
The ends also don’t provide much longitudinal connectivity between the underframe and the roof so the roof could provide any compression resistance. They mostly provide lateral stability.
The sides? They’re made of thin steel, 0.10”, even less on some cars. That, without lateral bracing, isn’t able to provide much strength to the carrying capacity of the car. Perhaps a little diaphragm strength, but not much.
The only cars where the sides provide strength is the steel angles in SS and DS cars. But in those cars, it’s the truss that’s providing the strength, not the wood sheathing.
Greatly simplified and it’s a whole lot more complicated than that, but it’s the general description of why box cars stand up.
Schuyler |
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is this also true of gondola sides? they don't contribute to the longitudinal strength ? and why do deck and through girder bridges have solid web plates ? seems like they could cut holes in the plates and save a lot of material. Tim, if you were a structural engineer, you would know that an opening in the middle of a beam�s web has very little, even no, effect on the bearing capacity of the beam. The web has structural value as you approach the end points, the bearings at the ends of the beam. A simple beam�s structural �action� in the middle of a beam is compression in the top flange, and tension in the bottom flange. In the middle of a beam, the web is just along for the ride. |
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Tony Thompson
Tim O'Connor wrote:
Or just use steel frame instead of a solid web. You've heard of it, it's called a truss bridge. Tony Thompson Editor, Signature Press, Berkeley, CA 2906 Forest Ave., Berkeley, CA 94705 www.signaturepress.com (510) 540-6538; fax, (510) 540-1937; e-mail, tony@... Publishers of books on railroad history |
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destorzek@...
They can, and sometimes do, but there must be sufficient material around the hole to transmit the stresses. The little windows in the fuselage of an airplane have little effect on the strength of the skin (except if they have square corners). The huge door opening we are talking about leaves little above to transmit the compressive forces along the top edge, and nothing to prevent that little plate from buckling.
Dennis Storzek |
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Back to the question about double door boxcars, since their doors are offset to the left on each side by one door width, the opening on each side is offset by that width. The car side framing for double door cars is carried to where a single door car would have its opening to the right of that opening on each side for a double door car. This helps brace the roof along the eaves line. Still the main strength of any railroad car is in its underframe. Body framing is there for the body to retain its shape and to contain the load within. For house type cars, its to help restrain possible shifting of the load inside. For open top cars like gondolas and hoppers where lading can be loose and spread out its weight, the side and end walls are suitably re-enforced to contain that type of load. In each case, the weight of the load is borne by the framing under the car body. Modern tank cars are about the only 'unit body' or 'frame-less' cars in railway service, where the body and frame are designed as a unit. That design reduces the number of parts and makes a more integrated vehicle. Yet it, also has a down-fall that could increase chances of a tank rupture in the event of a wreck. Here, the tank must absorb all buff and draft forces of the train, as well as bearing weight of the load and its pressure inside. Tanks that were strapped down to underframes did not have to bear buff and draft forces in the past. Ed Bommer |
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Dave Nelson
Not an engineer but of late am spending many days studying and working from mechanical drawings of freight cars…
As others have said, boxcar sides are very thin.
For construction in the late steam era, one face of a steel L was riveted to the top of the side sill with the other face oriented vertically at the car side.. Several z bars are riveted to the vertical face each getting a very small L at the bottom for side and vertical riveted – essentially a bit more to hold it all in place. plate up at the eaves is the side plate of which one face is vertical along the car side. The individual side sheets are riveted at the bottom to the steel L, at the top to the side plate and at the sides to each other and the vertical posts.. There are also a pair of vertical posts that make up the door frame as well as a structural element across the top. All of these structural steel shapes are just ¼ inch thick... simple L or Z shapes. There are no diagonals anywhere so this is not a truss of any sort. Whatever structural strength it has appears to be due to the monocoque qualities generated by the presence of the sheets that are attached to the structural elements.
Given it is not a truss my hunch is the box of a boxcar is not much more than a light steel equivalent of a canvas covering. Its purpose is to keep moisture and thieves out and only needs to be strong enough to hold itself up. IOW all boxcars are pretty much a decently strong flatcar with a big, lightweight box tossed on top.
I’ll add another hunch: all the variations of trusses used on wood sheathed boxcars 1910-1940 represent the voice of experience in using wood underframes.. IOW there were all over engineered because they either did not really know what the steel underframe could really do and/or all that extra weight from the wood needed a bit more help in holding itself up. But still over engineered.
YMMV.
Dave Nelson |
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exactly .. so why use the web if it only adds weight and cost, and no strength ? Or just use steel frame instead of a solid web. You've heard of it, it's called a truss bridge. |
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destorzek@...
---In STMFC@..., <timboconnor@...> wrote : exactly .. so why use the web if it only adds weight and cost, and no strength ? ================= Tim, If talking about boxcars, to keep the load from falling out through the holes in the truss. If talking about bridges, less expensive fabrication. Dennis Storzek |
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Schuyler Larrabee
Well, Tim, if you want to get into advance structural engineering, there are situations where depth of structure needs to be minimized. The first answer is that if you only had the top flange of a beam and the bottom flange of a beam, they would not have much ability to work in conjunction with each other. Each piece independently would bend. But by having the web, even a thin web, they are held together so they act as a unit.
You can in fact cut holes in the web near the middle of the span. This is often done in buildings to permit piping and ductwork to run through them (though the engineers, both structural and MEP) just hate it when they have to do this to make a building shorter, sometimes required to meet local zoning height limits.
Even more advanced, there are ways to analyze the structural requirements so that the contribution of the top and bottom parts of the web to resisting moment (the tendency to bend) can be calculated. Some parts of the web above and below the neutral axis (vertical midpoint of the beam) then are also in compression and tension respectively. This sometimes requires stiffeners so the web won’t buckle, or thicker web plates. Remember that in deck bridges, there are lateral beams under the deck or track(s); these serve to keep the sides working together of course, but also to keep the side girders upright. Otherwise they have a tendency to try to turn on their sides.
In highway construction, you may see composite beams, built up from plates and angles welded together for long spans or very heavy loading. There are often parts of the “web” of these beams omitted to reduce the overall weight of these beams. There were many of these evident during Boston’s “Big Dig.” 4” thick flanges with web plates 1½ - 2” thick, for a very heavily loaded 60’ span.
And you may see such beams being hauled to the construction site on FREIGHT CARS, including on such cars in the STEAM ERA.
Hey, Sheriff, how’s it goin???
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Schuyler says:
"Hey, Sheriff, how's it goin???" Well, not too bad although I have to say that the discussion about beams, bridges and frt car strength reminds me of the time too many yrs ago when in college, being math majors, a friend and I...feeling a bit arrogant...decided to take as an elective a course in engineering mechanics [ as I recall ]. I knew I was in trouble when the prof began talking about the ground acting up on the legs of a bridge. Mike Brock |
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