Bridge Design with Two Unequal Span Lengths
Bridge Design with Two Unequal Span Lengths
(OP)
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.
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.






RE: Bridge Design with Two Unequal Span Lengths
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.
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RE: Bridge Design with Two Unequal Span Lengths
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.
RE: Bridge Design with Two Unequal Span Lengths
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Bridge Design with Two Unequal Span Lengths
RE: Bridge Design with Two Unequal Span Lengths
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.
RE: Bridge Design with Two Unequal Span Lengths
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.
RE: Bridge Design with Two Unequal Span Lengths
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,
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RE: Bridge Design with Two Unequal Span Lengths
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.
RE: Bridge Design with Two Unequal Span Lengths
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,
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RE: Bridge Design with Two Unequal Span Lengths
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.