Re: Pratt vs. Howe boxcar trusses


Anthony Thompson <thompson@...>
 

Andy Miller wrote:
. . . if the primary concern with a Howe truss diagonal is buckling (in compression, buckling occurs far before outright material failure), then it should be much less of a problem in a car side than in a bridge, because the car side has all the sheathing attached to the ribs, including the diagonals, and this is a substantial deterrent to buckling, at least in the plane of the car side.
This is exactly right. But the early truss-frame, single-sheath cars were explicitly designed WITHOUT letting the truss sides do any work, and the underframe was beefy enough to carry all the load. This is partly true even as late as the USRA single-sheath cars, whose underframe is substantially overdesigned if the car side trusses were credited as carrying much load.
This VERY conservative approach was decried by those who wished to greatly reduce car weight, in part by reducing underframe sections, and by the first ARA underframes of the early 1920s, that's exactly what happened. Obviously any later car DOES credit the side truss with carrying capacity.
The stouter posts in a Pratt truss help resist outward bulging, and accordingly were advocated as superior to Howe trusses for car sides. But the converse is that Pratt braces (diagonals) are relatively lighter, and those play some role in resisting torsion of the car body. I think if you read the car design literature (which was very active in the 1920s), you will see not only that car designers understood bridges perfectly well, but also that they had concerns about car frame performance which goes beyond bridge issues.
This entire matter is NOT simply understood in terms of bridges, nor of "wood vs. steel" ideas, nor of "conservatism" of railroad car designers (apparently relative to bridge designers, in some accounts).

Tony Thompson Editor, Signature Press, Berkeley, CA
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