Unusual precast bridges on I-90 in South Dakota

[Tom958]

Yesterday I was Google surfing around South Dakota when I came upon this bridge in Spearfish. As you can see, it's a mostly-normal AASHTO-style precast concrete bridge except that there's a nonstandard precast component about twenty feet long that rests on the center bent and supports the spans over the roadways. The center component has large haunches to support the roadway-span beams, but there are metal fittings, apparently with tie rods, that span every connection. I don't think the fittings were original equipment; perhaps they were retrofitted for seismic reasons. Per http://bridgereports.com/1492794, it was built in 1971 and its condition is 7/9 in every category.

It's not at all clear why they would've done this, or, for that matter, done anything unusual here at all. The median is only sixty feet and the skew angle nil, so unusually long span lengths weren't required. I don't think aesthetics was a factor, either. I'm stumped. 

By way of comparison, there are a few like this in Hawaii. Here, though, the center component is cast in place integrally with the center bent, and the spans are unusually long.

There's also a pair of similar bridges about six miles east of Spearfish, built at the same time. I didn't thoroughly scour the state looking for more, but I had a pretty good look around and didn't see any others.

Any idea why they would've built such a thing? 

[Duke87]

Note that between your first and second examples, the overpass at exit 14 used to be the same deal if you tick Street View back to 2013. This overpass was replaced to convert that interchange into a SPUI and allow four thru lanes of traffic across it.

So that's three consecutive overpasses that were built with the same design, probably all built as part of the same contract.

As for what's up with that design, well, the gaps where the outer beams rest in notches on the center piece allow for thermal expansion - though, interestingly, the bridge deck itself only has expansion joints at the ends, not in the middle where those gaps in the beams are.

[Dirt Roads]

Yesterday I was Google surfing around South Dakota when I came upon this bridge in Spearfish. As you can see, it's a mostly-normal AASHTO-style precast concrete bridge except that there's a nonstandard precast component about twenty feet long that rests on the center bent and supports the spans over the roadways. The center component has large haunches to support the roadway-span beams, but there are metal fittings, apparently with tie rods, that span every connection. I don't think the fittings were original equipment; perhaps they were retrofitted for seismic reasons. Per http://bridgereports.com/1492794, it was built in 1971 and its condition is 7/9 in every category.

It's not at all clear why they would've done this, or, for that matter, done anything unusual here at all. The median is only sixty feet and the skew angle nil, so unusually long span lengths weren't required. I don't think aesthetics was a factor, either. I'm stumped. 

Any idea why they would've built such a thing? 

I'm still waiting for a bridge expert to respond to this, but I think I have a general idea.  Bridge support columns need to be sized both for load bearing and frequency dampening (stiffening).  The support columns for this bridge appear to be identical in length and circumference.  I'm wondering if a longer center column was going to need to be resized for stiffening and it was deemed less expensive to utilize a different style of (whatever that haunched bearing cap is called) in order to use standard length concrete stringers.