Columns with uneven height in same bent

[Imran Structural Engineer]

Hi everyone
I am designing bridges in a project with steep slopes. Overall topography of project is very similar to kicking horse canyon phase 4, British Columbia Canada (picture attached) Due to steep slope and to avoid cutting, both bents cannot be kept at same height.

The issue that I am facing with same configuration while designing on my project is that shorter pier/column on the left fails in shear in seismic design. What is the solution to this problem of shear concentration.
The project in the picture shared above is already completed and opened to traffic. Can someone please help me understand how did they tackle this issue of shear failure with this configuration.

 

[BridgeSmith]

You may be able to make the shorter column a smaller diameter, reduce the stiffness of the restraint between bent and superstructure, increase the reinforcing in the shorter column. There may be other options, but we'd need more information about the overall seismic resisting system and the foundation configuration.

Have you considered columns directly on drilled shafts?

 

[Lomarandil]

Is it not feasible to design the shorter pier to handle the increased seismic load resulting from its stiffness?

 

[hokie66]

Lo is right. You can deal with the difference in stiffness by taking all the load into a single column.

 

[Imran Structural Engineer]

You may be able to make the shorter column a smaller diameter, reduce the stiffness of the restraint between bent and superstructure, increase the reinforcing in the shorter column. There may be other options, but we'd need more information about the overall seismic resisting system and the foundation configuration.

Have you considered columns directly on drilled shafts?

Reducing the diameter of shorter column and reducing thickness of pilecap works, but I was wondering how did they achieve it with apparently same diameter and pile cap.
And Yes, I did consider column directly on drilled shaft but it comes out to be of very large diameter.

Can you please expand on it a little bit?

 

[BridgeSmith]

I did consider column directly on drilled shaft but it comes out to be of very large diameter.

Drilled shafts can provide resistance through end-bearing or side friction. Most of ours use side friction, which makes them have to go deeper in soft or brittle materials, but doesn't require large diameters.

 

[hokie66]

Imran,

We don't know how or even if they dealt with that problem successfully. We were just trying to offer you some advice for your project.

 

[Imran Structural Engineer]

Drilled shafts can provide resistance through end-bearing or side friction. Most of ours use side friction, which makes them have to go deeper in soft or brittle materials, but doesn't require large diameters.

Piles are assumed to be laterally loaded as well (seismic event).

 

[BridgeSmith]

Piles are assumed to be laterally loaded as well (seismic event).

I get that, but presumably part of your problem is that the shorter column is too stiff. You can reduce the overall stiffness, and (assuming the soil and rock substrata are at similar elevations) get it to be more even between the 2 columns, by putting the columns directly on drilled shafts. This has the effect of increasing and more closely matching the effective column lengths, since the drilled shafts become effectively part of the column.

 

[Milliontown]

Caltrans would solve this problem using Isolation Casings. This requires the bents be founded on pile extension columns, using drilled shafts. The shorter column would have an isolation casing drilled larger diameter than the column to extend the column length below ground. This is also more expensive and may present other construction challenges depending on the terrain. If bedrock is near the surface, increasing the length of the drilled shaft may be even more expensive and time intensive. It may be possible to reduce the diameter or length of the column by combining both side friction and end bearing using tip post-grouting, but this would add more cost, and would need to be weighed against the cost of a longer/larger shaft.
[URL unfurl="true"]https://www.concreteconstruction.net/projects/infrastructure/isolation-casings-stand-up-to-earthquakes_o[/url]

See figure 7 as well

There's also handling and installing a long rebar cage, presenting it's own challenges.

Just trying to throw out how others may handle the problem.

 

[tmalik3156]

Different Codes try to deal with seismic effects on unequal column heights differently. There will always be discussion whether the Codes can achieve what they wanted to achieve.

There is a paper available online, in case you are interested.

https://www.seced.org.uk/images/newsletters/TAMANANI, GIAN, AYOUB.pdf

 

[VoyageofDiscovery]

It is too late in the game to be asking this question, consultation with the slope geotechnical engineers would have been helpful during the planning stage to recommend the preferred bridge substructure design. Without any geotechnical/rock mechanics information or guidance though, I would look to optimize this to a single column type with minimum eccentricity.