Additional Span or Tall Abutment >12 m 2

[Imran Structural Engineer]

Hello!
I have a query regarding Bridge Planning. Which one of the following options would be more feasible economically and structurally. Providing Tall Abutment (12 or 13 m) or instead provide 40-80 meters of additional span (Prestressed I Girder) to achieve 3-4 meters of abutment height?
Thank You.

 

[Aryal]

We had a kind of similar issue (between MSE Wall and additional bridge span length) and we did cost estimate for both option and decided which one to go. Providing additional 80m span might be costly than providing 3-4m of abutment additional height. I am not sure just my opinion.

 

[BridgeSmith]

Aryal, I think the choice is between a 12-13m abutment height and a 40-80m shorter bridge, and a 3-4m height, so about a 9m difference in abutment height vs. potentially an 80m difference in length.

We usually only use the tall abutment option if we cannot extend the bridge length due to space constraints (intersection too close, etc.). Cost-wise it's not too much different...until you have to figure out how to reduce the soil pressure on the abutment, then all of the sudden, a longer bridge or concrete arch structure starts to look really attractive.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[STrctPono]

100% agree with BridgeSmith. 13m abutment is really tall and will have tremendous forces on it. Once you start working through the numbers, you'll be wishing you added that extra span.

 

[Imran Structural Engineer]

@BridgeSmith
Thank you for your response.
What if to reduce the soil pressure, I provide a hollow type retaining wall(remove most of heel slab) to avoid extra pressure on heel and provide piles for the rest of footing? Would that be a viable solution?

 

[STrctPono]

Not sure what you mean by "hollow type retaining wall" Are you talking about removing the fill behind the retaining wall? (A sketch would really help) Wouldn't that essentially be the same thing as changing this support to a pier and adding an additional girder span? The only way that I can viably see you reducing the load on the abutment is to set your girders on the abutment, support it on deep foundations, and place GRS fill behind the retaining wall. A common practice that we use all the time, however, I have never done it for a 13m tall abutment. I still think an additional span would be cheaper and better if you have that option and are not completely constrained by site conditions. The amount of money spent on the abutment, fill, wingwalls, and any additional retaining walls is going to probably outprice the cost for an additional span.

 

[BridgeSmith]

Sure, you can put a retaining wall behind the abutment, or set the abutment on an MSE retaining wall. If the bridge is over a river, and a wall could be inundated during high water events, special care must be taken to prevent the backfill from washing out. We haven't found those options to be cost-effective under normal circumstances.

There are numerous options; all I'm saying is our experience has been that the most economical solution for us has been to increase the bridge length, rather than use taller abutments. If you have length restraints on the spans of the girders, due to fabrication or girder depth/clearance requirements, the taller abutments can certainly be used.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[SlideRuleEra]

As a former bridge contractor, in the most general terms, superstucture work (bridge span) is significantly more economical than substructure work (abutment).

http://www.SlideRuleEra.net

 

[Imran Structural Engineer]

Thank you everyone for your responses. I going to go with additional spans as per your suggestions as there are no restrictions regarding length of superstructure.
Thanks a lot.