Bridge Design with Two Unequal Span Lengths 1

[dalauder]

My AASHTO I-Girder concrete bridge requires a minimum of two spans due to fabrication and transportation issues for longer girders. The bridge spans over a rain runoff path and it is not desirable to put a pier at the middle of the runoff due to concerns about permitting and jurisdiction in a potential waterway. Because of this, the bridge can be supported by two abutments and two intermediate piers, with the piers clearing either bank of the potential waterway.

It has been suggested that one of the two piers be removed, leaving two spans of different lengths--approximately 40ft from one abutment to the pier and 100ft from the pier, over the runoff, to the other abutment. This does not seem to be an excessively strange or challenging design, but as I cannot remember a specific example of a bridge that is asymmetrical about the midpoint (outside of bridges with 5+ spans or large inclinations), I would like some input on this idea as well as references or sources about similar designs. I am curious if there are code recommendations regarding bridge symmetry? Thanks.

 

[ishvaaag]

These things were maybe more common in the thirties'.

Such kind of 2D frame was a typical problem for frame analysis by Cross' method in the first course of structural analysis, so reminding everyone of that asymmetry can come handy for some cases.

Of course asymmetry is also somewhat noticeable today in the context of cable-stayed footbridges and bridges to deal with foundations and the economy of the high cost pylons.

http://books.google.es/books?id=gAW...onepage&q=Bridge Elevations Asymmetry&f=false

http://www.kelowna.ca/CityPage/Docs...Panels/Lakeshore Rd Bridge Design Options.pdf

http://discus.steel-sci.org/prizeawards/pdfs/Corus_Student_Guide_to_Steel_Bridge_Design.pdf

 

[bridgebuster]

10 years ago I designed a three-span AASHTO I-Beam bridge, with span lengths of 50-135-50. The finshed structure looks OK. There's no rule against what you want to do. Are aesthetics important?

If you're making the bridge continuous for LL & SDL make sure you check for uplift. Don't forget to account for the temperature force in the pier since you don't have a symetrical layout.

 

[paddingtongreen]

You imply that the girders are precast. I ask why you want to make them continuous, forming the high moment joint looks like an extra, costly, complication over using two simple spans.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dalauder]

Thanks for the input. I don't think aesthetics are of comparable importance to cost, so it should be fine. I'll make sure to check uplift and temperature.

 

[dalauder]

@paddingtongreen (responded while I was posting)--yes, the girders will be precast. We haven't worked out the connection yet (not completely settled on I-Girders), but my boss tends to go with continuous supports.

I suppose the biggest issue is whether the asymmetric layout produces any standardized complications. From what I gather, temperature considerations would be the largest issue.

 

[paddingtongreen]

@dalauder,
My philosophy is to keep it simple. In the simple span condition, the reinforcing in the girders is symmetrical, they can be prestressed or post tensioned. The pier carries minimum moment. There is a minimum of expensive field work.

I have some difficulty in imagining how the continuity will be achieved, how much distance will be required to connect the reinforcing? Will it, of necessity, be moment connected to the pier?



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Qshake]

Please keep in mind that some of these span issues were developed at a time when steel was king and it wasn't common to use continuous construction for prestressed.

As bridgebuster notes it's common these days to find as many arrangements as is conceivable due to the software these days. Back in the days of yore, we were constantly thinking of how to make use of the last bridge we designed or fitting within the constraints of some design charts so that we didn't have to spend hours and hours pouring over hand calcs.

So in short, while these old addages and rules of thumb make for good starts they should be held as the final word.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[paddingtongreen]

I agree that the old rules of thumb should only provide a start, but some things were true, are true and will probably always be true. The old rules of thumb, looking only at primary load paths, provided redundancy; now, with software taking account of all of the load paths, we have to take special measures to provide redundancy, extra girders on bridge spans for example.

My comments on this bridge are not rules of thumb. Fieldwork is more expensive.

I had the stray thought that If this were my bridge, I might build an arch over the central waterway to provide a central support point for the bridge spans, it would look interesting.


Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Qshake]

paddingtongreen - no one is suggesting that your comments are rules of thumb but rather there were rules of thumb used in the industry that may not now be necessary due to the wealth of computer availability and very high processor speeds.

I'm certainly of an age that appreicates the rules of thumbs and not to rely only on software.

Lastly, your stray thought on arches may prove more troublesome than the girder bridge....

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[dalauder]

I thought about arches and like the idea (aesthetically mostly). But realistically, logistics make the use of elaborate cast-in-place work in this somewhat remote location not ideal. Precast Girders on simple piers should be cheaper.

Anyhow...it appears we're going with a symmetrical 3-span bridge. The cost savings weren't great enough over a 2-span and slight curves involved caused a preference for more spans.

Thanks for the help.