On the 'new Bloor bridge' I was intrigued by the choice of four massive I-beam construction until staring at the other bridges there, their having three supports across the underpass span (new bridge has just one at mid-span) and the *depth* of the bridges to allow sufficient clearance underneath vs. the track grade. The realization of why the four I-beams didn't click until taking a close look at the Queen Subway (underpass) spans:
Note where they get their structural rigidity from: It's *above* the load level, not below it. Also note how many more spans cross at that point now. I'm looking for some definitive history of Toronto's rail bridges from an engineering analysis. A number of those spans shown above have been moved together and apart to allow for other spans to go in. The abutment sills at the west end under the now extant three ex-CP spans were also raised, (about two feet) and this was long prior to the Dufferin re-alignment. (one span new in the last two decades, one as shown above, and one of the same construction added shortly after this pic was taken).
Two things make Googling for the bridge history a very difficult challenge. The name "Queen Street Subway". Guess what swamps that search! And the 'jog' realignment. Any reference to the earlier span realignment is also swamped.
Back to the Bloor Station bridges: Their three supports allowed for much less 'brutish' construction, and at the time, much more *affordable* construction. They were built later than the spans you see above, and ostensibly from cheaper steel that also relied on being filled with concrete to complete the load-bearing rating.
For obvious reasons, it must have been decided not to build the 'new Bloor bridge' that way, but a decision based on aesthetics as well as engineering must have been made for it to be 'shallow section' to fit with the other bridges. But it also had to sit off the intended final alignment and therefore be movable into that final position. It is even within the realm of engineering that no vertical support would be necessary (Paul raised this point earlier) but other factors must have prevailed over that, not the least moving it into places with cranes, and delivering it there by road. A decision must have been dictated by logistics to do it in two spans. As far as visual inspection shows, the two segments are completely unconnected albeit each section presents a tongue flange across their width to allow tying them together with a bond strip. This would provide lateral stiffness, but not vertical rigidity. It would secure alignment.
To allow the span to be in two sections, a centre vertical support would be unavoidable. What I can't yet visualize is how that vertical support structure is to be modified and/or buttressed to allow the jacking over of the approx two feet necessary. I can theorize at least a number of ways, including total removement of the present support structure while temporary support is installed until the final structure is put in place.
If any of you get a chance, check out the new bridge. It lacks the 'ornate' qualities of its senior neighbours, but has *magnitudes* of greater strength. It's a beautiful thing!
Edit to Add: On the 'Ikea style' as per Rejohn...I may have misled on that. I think the *potential* for disassembly might be there, I'd love to read an engineering description of whether that's a moot point or not, and the real thinking nowadays as to using bolt construction instead of rivets. Safety of assembly is obviously a huge one, but there might also be stress factors far better addressed at unions with washer and bolt construction. It may be the case that they *aren't* able to be disassembled, yet one more intriguing aspect of this bridge. What is beautiful to anyone who's done metal work assembly, even chassis work, is the flat head hex socket bolt use and chamfered hole assembly. (From the best visual I can get, they used 90 degrees)
These bolts were just used for the top deck secured (in aggregate) to each of the four I-beams underlying and to the side walkway assembly and wing flanges attached to retain the stone ballast. The actual load frame is all conventional nut and bolt with flat washers, albeit quality hardware.
