Bent design with two piers having different heights

[Hassaan_16]

Hi! I'm designing a bridge bent with two piers having unbraced heights 15.2m and 8.2m. Originally, the bridge was planned with both piers having equal heights of 15.2m but that is not a feasible option because of too much cut (the terrain is mountainous with closely spaced contours) so I have to change the bent configuration. The bridge has eight spans and the particular bent I'm discussing here (middle span) is on the steepest slope. I am considering two options:
1. To provide a hammerhead pier instead of two circular piers.
2. To go with the circular piers and design them so that their buckilng loads are similar in order to reduce the moments imbalance between both piers.
Most probably the first option will be declined due to site practices in my country so I'll have to go with the second option. Please let me know if my approach is appropriate in the second option, also add what I should keep under consideration if I go with this option.

 

[BridgeSmith]

First, I need to make sure we have our terminology straight. You're talking about a bent that has a cap spanning between 2 columns, correct? On a steep cross slope where the 2 columns will be 7m different in length? And you're considering a hammerhead (single shaft) pier instead? What configuration are the other 7 piers/bents? What options do you have for foundations? Drilled shafts (caissons)? Driven piles?

 

[Hassaan_16]

"You're talking about a bent that has a cap spanning between 2 columns, correct?" Correct. The columns are to be constrained by a single cap in both configurations. I am only thinking of separating their foundation from a single footing for both columns to separate footings at separate levels so I can save the enormous cut (32m height where the slope of 1:3 meets the terrain from the bottom of the pile cap) that needs to be made for the first configuration.

"You're considering a hammerhead (single shaft) pier instead?" Out of consideration now as I mentioned in my query

"What configuration are the other 7 piers/bents?" Other bents (bar one, which has the same issue as this but not as severe) have the same configuration as this one had in the original planning: One bent cap running between two columns, and a combined foundation for both columns, resting on piles.

"What options do you have for foundations? Drilled shafts (caissons)? Driven piles?" Drilled shafts only

 

[Hassaan_16]

Here are the hand made sketches to help illustrate the situation:

 

[HTURKAK]

Eng.Hassaan_16 ,

Your internet country code is PK. Acc. to your sketches , the foundation is piled found. I could not see the reason for the embedment of the ftg. Why you do not raise the ftg level? Frost depth is not issue.
I would consider single ftg , the ht of piers around 12.0 m.


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Hassaan_16]

HTURKAK,
I have also explored raising the footing as shown in the following cad snippet (Please ignore the automatically generated retaining wall and seemingly wrong alignment with the deck):

But there's another issue with this configuration, two piles get exposed 4.8m out of the NSL this way. Consequently, the pile group has different unbraced lengths because the unexposed piles meet the fixity depth 4.8m earlier than the exposed piles. I was reluctant to go this way because of this issue. The "shorter" piles might attract more lateral forces when experiencing earthquake excitations (Earthquake prone region) and fail. To avoid this complication under the foundation, I want to try the route of separate column footings.

 

[HTURKAK]

I do not have any idea for the available local equipments and construction methods in your region. Can you consider single drilled pier/ caisson socketed to rock ?

You may post the soil data , boring to get better responds.




He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Hassaan_16]

Update: I have consulted the lead engineer and he has allowed the 32m cut I was trying to prevent as it only occurs in two bents. This allowance removes all issues with the bent configuration that I was experiencing.
Nevertheless, to answer your question HTURKAK, I couldn't provide a single drilled pier because it exceeded the 2m diameter limitation we have over here.
Thanks HTURKAK and BridgeSmith for responding to my queries. I'll surely be back with more in the future.