Bowstring bridge

[MIStructE_IRE]

Hi,

I pass this bridge every day and I always wonder about its effective length in terms of why it doesn’t want to buckle sideways.. The arches are not tied together, and me not being a bridge guy expects them to be! What am I missing? Just curious!

 

[Eric C.]

I cannot answer your question but I was on a trip to LA (Lower Alabama) about two weeks ago and saw a very, very similar bridge being constructed in Tuscaloosa. I stopped and spent a few minutes "supervising" as this was the first one of these I had seen under construction. Kind of exciting in an engineering kind of way.

 

[r13]

Compression buckling is possible, but not likely, unless under designed. On top of the end supports, the arch/bow is firmly hold in place by the tension cables, and box shape is strong in torsion, I couldn't image a torsional buckling type failure to occur.

 

[BAretired]

Actually, it is an arch bridge, not a bowstring bridge. There was a discussion of this some time ago on the thread below. I'm not quite so sure of the answer to your question now as I was then.

thread507-273286



BA

 

[MIStructE_IRE]

Thanks all,

BA, yes i recall seeing that thread at the time. In that one though the arches lean toward each other and are connected - that’s what I’d expect to see here.

Not being a bridge guy, if I was taking a stab at this as a structure I’d have braced the arches together. The fact that there does not seem to be an overall out of plane buckling issue just doesn’t make sense to me.

Incidentally, each arch seems to be made up of 4 No. large box sections welded together.

 

[STrctPono]

I will preface my statements by saying that although I am a "bridge guy" I have never designed an arch bridge.

If we are talking about semantics, my belief is that the bridge is not a "bowstring arch" or a tied-arch bridge. 1. The camber in the deck tells me that it is not behaving as a tie. All the tied-arch bridges I see have really flat decks. My belief is that a cambered tension element isn't really doing its job. 2. The arch ribs span from abutment to abutment. It would make more sense to anchor it to the foundation at the abutment and let the soil take the thrust load.

As for why the arch ribs are not tied together to restrain buckling...? Because it looks cooler. Based on that span, I would say ~150ft, there is no real reason to have an arch bridge. You could accomplish the same purpose with a 2-span pretensioned girder bridge and a pier in the center or a single span spliced post-tensioned girder bridge, or even a steel plate girder bridge. Look at the nice formlining on the wingwalls and abutments. The agency obviously wanted this to look good and somewhat grand and iconic. Omitting the bracing from the arch ribs certainly makes it look good and less obtrusive than other through arches. I imagine that a finely tuned 2nd order buckling analysis proved that the arch ribs could span the full length unbraced.

What I want to know is what they are doing with all those dump trucks on top of the bridge? Seeing that the freeway appears to be shut down, it almost looks like they are load testing that bridge.

 

[KootK]

I'm going with one or both of the following:

1) With regard to global, first mode buckling, something a bit magical happens when the arch supports are even slightly lower than the bridge deck. See the sketch below.

2) You'll notice that the lateral stiffness of the arch cross section is greater than it's vertical stiffness. We can use that:

a) to contribute to first mode, global bucking when combined with fixed base connections.

b) to deal with weird, higher mode arch buckling shapes.

c) to make damn sure the section doesn't just flop roll over LTB style locally.

So, yeah, fat box much better than, say, W21x44 strong axis. #1 is the nifty bit though.

 

[LittleInch]

It's difficult to see from this photo but the arch sections may in fact protrude further into the abutments and hence be fixed with some rigidity there.

This is a similar bridge which I really like. They made it in two halves and then welded it together in the middle using a temporary support.

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