Re: Pratt vs. Howe boxcar trusses

Anthony Thompson <thompson@...>

Andy Carlson wrote:
I think that the Howe truss designed car was familiar with the RR's back when cars were mostly of all wood construction. The engineering of wood trusses takes advantage of the greater strength of compression for wood- hence the Howe truss. Later, when steel underframes and SS cars became popular, basic familiarity with Howe trusses caused a lot of steel framed boxcars to be built with the Howe truss with it's structural members in compression. As early engineering students quickly learn, steel is much stronger in tension, and bridge trusses take advantage of this property to keep the amount of steel used to the minimum. The enlightened RR engineering departments recognized this when the Pratt truss was selected with it's structural members taking advantage of steel's tensile strength. Not all RR engineering departments were so enlightened- hence the large numbers of Howe steel trussed cars being made.
This is a good summary of the bridge perspective, Andy, but it misses two points. First, steel trusses were essentially universal in bridge building after about 1880, and all their superior features were well recognized, even by car designers. To see this, read the MCB proceedings in the first two decades of the 20th century. Yet Howe truss car framing was still being built. Your idea that somehow the bridge guys were "enlightened" and the poor mechanical engineers designing cars were decades behind, just isn't true.
Second, the car side truss has to do a job not required of the bridge truss (or at least not in the same way): it has to resist sideways and twisting forces. The car-design advocates of the Howe for car sides believed it was superior in that aspect--though their colleagues of the opposite persuasion argued vehemently against them. You can see this in the ARA Mechanical Division proceedings all through the 1920s.
Denny's belief that the Howe is lighter is wrong: in either steel or wood it is heavier, though of course, as Andy observed, it has to be that way in wood to provide the compression strength in the long members. But for bridges, the modest difference in weight is of no real consequence.

Anthony Thompson
Dept. of Materials Science & Engineering
University of California, Berkeley

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