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Here is an example of what I am referring to. This bridge is part of the Tom Moreland Interchange in Atlanta, more commonly known as "Spaghetti Junction". This interchange has several flyover bridges, all of which have expansion joints that are not located directly above the piers despite having many piers. But I have noticed that this is relatively common on prestressed concrete box girder bridges, especially ones that have sharp curves such as flyover bridges. This design also seems more prevalent in the western United States. What is the reason for the placement of the expansion joints? Wouldn't it make more sense for them to be directly above the piers?

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  • $\begingroup$ Are we sure it isn't always and not sometimes? Column to beam connection is exactly where I wouldn't want an expansion joint. Putting it in the middle allows both sections of deck to share the joint; attached at one end and floating at the other, would not. And if it wanted to move at the attached end and it couldn't, then the whole thing wracks. Hit me up on SM when that happens. $\endgroup$
    – Mazura
    Dec 3, 2022 at 18:36

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Because the extension of the longer span past the column into a cantilever allows it to enjoy the benefit of the cantilever part's positive moment effect on the reduction of the midspan negative moment. Remember we have a positive moment caused by the cantilever over the post which basically reduces the negative moment at midspan significantly.

And also it creates an inflection point near the post on the longer span reducing its effective length for vibration up to 0.75.

Less moment means a smaller cross-section, less concrete, fewer rebars, and lighter member.

Lighter member with shorter effective length means a much more earthquake-resistant structure.

'

In the figure the black line is the posts and beam, red is the actual moment, and orange is the larger moment if the expansion joint was over the post. cantilever span

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  • $\begingroup$ I think I understand much better, but I am still confused about a few things. You speak of a longer span, but in the example I used each of the spans in each bridge are of similar lengths. Also, I've noticed this design is used in some but not all bridges of this type. Does this mean that concrete box girder bridges where the expansion joints are located on top of the piers require more rebar and concrete? And lastly, do you know of anything that explains this design in detail, such as regulations, engineering manuals, blueprints, etc.? Thanks. $\endgroup$
    – Sagierian
    Dec 3, 2022 at 8:37
  • $\begingroup$ Vibration, eh? Is that why they're always one eighth overhung? Octave $\endgroup$
    – Mazura
    Dec 3, 2022 at 18:43
  • $\begingroup$ @ Octave – Mazura, yes vibration, the natural frequency is a prime factor. The columns are typically short members as compared to the beam, so having an overhang allows the end part of the beam vibrate with a frequency near the frequency of the column. Thus preventing the column to cut like a knife through the beam, as has happened and documented in several major failures with fatalities in earthquakes in California. $\endgroup$
    – kamran
    Dec 3, 2022 at 23:41
  • $\begingroup$ @Sagierian, yes the concrete beam size and amount of rebars are closely related to the moment, shear is of less importance. Also contributing mass in the modes of vibration is directly related to seismic shear and moment. Mass to earthquake load relation is like the size of a battery and its AH capacity. The bigger the mass the more seismic load to contend with. The larger the AH the more charge a battery has. $\endgroup$
    – kamran
    Dec 7, 2022 at 4:24
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A simple answer to your question lies with the spacing of the piers. Expansion joints are typically intended to avoid internal stresses in the bridge due to concrete shrinkage from the curing process or thermal changes. The total shrinkage of a length of bridge girder is proportional to the distance between expansion joints. If the distance between expansion joints is too long, these internal stresses may become large and result in excessive cracking in the bridge girder. If the distance between piers is longer than the desired distance between shrinkage joints, the joints may be located between piers.

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