Weird bridge in Eastern Maine (Eastern Maine RR / NBSR)

PS: The book also has a pic of the bridge as originally built on pg 57. The two center spans were originally a "conventional" through-truss design...

One other thought, as the bridge is at the begining of the yard, the "Arch" may have been a way to stiffen the bridge but also give extra head room for the switching crews that were riding the tops of the cars, back then some most of the brake wheels were "stem winders" so the crews were on top of the cars.

Peter

Here is a photo from the NERAIL Photo website, with a NB Southern Train on the bridge.

http://photos.nerail.org/s/?p=90410

I took the liberty of posing the question re: the significance / structural usefulness of the arches to some engineer friends. One posed the question to her father, who early in his career worked as an engineer designing railroad bridges for NYSDOT:

This appears to be a through-truss bridge.

The bowed members are to stabilize the trusses on each side.

It is likely that the span is short enough that very tall trusses were
not necessary, but from the age of this structure, note the stonework an
each side of the abutment, curved beams would have been a distinct
rarity. The height of the bow gives necessary clearance for the trains
passing through.

One other friend (not a practicing PE, but he plays one on TV or something to that effect) reached the same conclusion, albeit somewhat more colorfully, and also gave the lowly software guy (me) a bit of a lesson in fundamentals of engineering:

Pretty pictures....

> Can anyone tell if those arches might have any structural significance?

Yes. No. Depends on what you mean...

They aren't well shaped. If you put anything heavy on top of them they would
tend to splay the trusses out sideways. Splaying trusses out sideways is
bad. So I don't think they carry a load. Loads are heavy, but...

They are probably stiff enough that they could help keep the trusses from
splaying out sideways, acting as stiffeners. The tops of the trusses under
load will be in compression and want to buckle and could maybe benefit
from lateral bracing and ... bracing forces are much (how much?
nobody will ever tell me) lower than load carrying (sorry, grammar's going
all to pot).

Well, if they're lateral bracing, what keeps both sides from falling over
at the same time? Normally you'd do it with diagonals right across the track
where trains go. Oh. Trains. The connection between the truss and the brace
needs to essentially prevent rotation at the top. There's torque there but
we call it a moment. Wide (left to right) arch member solidly connected can
provide a moment. Again, it doesn't have to be that strong.

Why is it arch shaped? One piece is nice. These days the curve would be
expensive but mayn't have been back in the day. It's pretty both in itself
and in echoing the shape of the trains. It has to be tall because, there's
trains.

Whatever it was originally, whatever changes may have taken place, the bridge does seem to have worked. Not only is it weird, but it does also seem to be unique. More than a bit of googling on truss-bridge engineering showed no others like it. Usually in through-truss construction, the upper chord mimics the sides by having a series of triangles in the form of struts and lateral bracing, but to dissipate lateral loading instead of vertical. One site showed that wind-loading was a concern met by the struts and lateral bracing for the upper chord, but -- given the relatively short length of the spans -- wind-loading was probably less of a concern than the lateral loading of car-rock. There seems to be no evidence in the form of holes or attachment plates that there ever was lateral bracing for the arches, the analog of the struts. Again, the relatively short length of the span may have been cause for abandoning the usual practice of lateral bracing and relying on the intrinsic strength of the arch to dissipate lateral loading.
However unique the bridge in question might be, it isn't currently listed as either a national or Maine civil-engineering landmark.

Some engineer mistakenly had gotten his hands on engineering drawings for the Conestoga wagons!!!

Heres a bridge of similar design in Australia. Not my photo. http://rrpicturearchives.net/showPictur ... id=1773244

From what I can see, those arches appear to be steel "I" beams and if so, absolutely would be for structural support. It's possible that they may have increased the weight load on the bridge and served as a support for a canopy as well. I would imagine that in the wintertime that snow would bunch up on a structure like that without a covering, a "snow shed" if you will.

"I would imagine that in the wintertime that snow would bunch up on a structure like that without a covering, a "snow shed" if you will."

With respect that doesn't make sense. The arches "cover" only the through truss arrangement. Any problematic snow accumulation would occur in the plate girder section... which has no arches.

A valid idea, though I find it odd that these are one piece steel I-beams. These aren't your regular off the shelf iron. These have been specially made in a foundry at twice the cost. The only reason that I can think off for their necessary round contour would be to facilitate a fabric cover which could be slung over the top and secured at the sides. The round contour would keep the fabric from tearing in the wind. Summertime it could be removed as we see it when it wasn't necessary to expose it to the elements. The only other explanation that I can think of is that they might represent an unfinished project that was halted mid stream (no pun intended), perhaps due to a road going belly-up.

A bit of time on both Gearth and Google and a Google link to YouTube show the Bathurst, New South Wales, bridge was built in 1876 and is now OOS, having been replaced by a poured-on-site concrete structure. The top-chord placement for the New South Wales bridge was less frequent than for the Maine bridge. Snow is not unknown in New South Wales, but probably not a frequent or dramatic enough occurrence to have been a factor in design. The body of the old bridge is a fairly tight lattice, so protection against horizontal wind loading may have been the reason for the top chords. Since the in-photo caption in the shot provided by 3rdrail indicates a speed of 10 kmh, the old bridge may have been in rough shape by 2002, top chords to the contrary.