Re: Steel truss bridge

spsalso
 

I am definitely not a bridge expert; but, as we say, I took a couple
of classes. If my assertions are incorrect, I hope someone will
correct me/them.

In a steel truss, there is no reason for the verticals to actually be
vertical. What's happening is that there are a series of triangles
being formed. The fact that there are vertical members is just a
convenience of manufacture and perhaps of aesthetics. An exception
could be for trusses with vertical cables as they can accept no
compressive loads, but I think for the very minor deviation from
vertical for a railroad bridge that that would involve, it wouldn't
matter.

However, the loading of the members will change for a bridge that is
not flat. This is only slight, but if I were designing an inclined
truss bridge, I'd still run the calcs.

Also of interest are the bridge shoes and abutments. The abutments
will probably be built just as if the bridge were level but will be
raised and lowered appropriately. The bridge shoes which normally
"only" take vertical loads will have a horizontal component. For
this reason, I would expect that the rolling bridge shoes will be on
the high side of the bridge--they transmit only vertical loads. The
fixed shoes will however receive the horizontal component of the
bridge and load. I expect it's much better to feed a compressive
load into the abutment and surrounding earth than it is a tension
load. Thus the fixed shoes should be on the downhill side of the
bridge.

All this is descriptive of only a single span bridge. If there were
multi-spans on a grade, it might get a bit sticky. Essentially, the
vertical piers would have to take horizontal loads too. I think this
would result in a much stouter design, especially for masonry.

Edward Sutorik

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