Hi all,

I am analyzing an existing 75 year old bridge for a seismic retrofit. It is a 4 span bridge with concrete superstructure and concrete pier walls on concrete caps and timber piles. When checking the axial capacity of the piles (for both gravity and seismic), several people have told me that I should ignore the bearing capacity of the cap and count only the capacity of the timber piles. I can see this documented in the IBC building code (Chapter 18), but I don't see it anywhere in the AASHTO LRFD Spec or the FHWA Retrofitting Manual.

What is the current standard of practice and where can I find it documented in the AASHTO code?

Thanks...

Jenny,
I don't use any of the documents you have listed, however from an engineering stand point, it is unlikely that the maximum bearing under the pile cap would occur at the same time maximum capacity of the pile was attained. This is due to the different deflection requirements for each type of structure. Pile have a very small displacement v's force compared to a pad footing.   

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This is an example of an unwritten rule of thumb.  There are several sound reasons, albeit engineering common sense, for this.

1.  If the bridge spans a roadway over a creek, the subgrade material is subject to scour and or just plain erosion.
2.  There are typically no requirements for changing the engineering properties of soil beneath a pile cap.  As such the soil may be expected to settle or consolidate and shrink away from the pile cap.  Note that I say typically, driving pile certainly densifies soil immediately near pile and it's possible for seismic retrofitting to use ground improvements such as dynamic compaction.  
3.  Along the lines of what rowingengineer noted and Item 2 above, the engineering properties will still remain disproportionate such that the soil will give and the pile will assume the load.

Please note that most neglect the soil for passive resistance to lateral loads on the side of the pile cap.  Again, the soil cannot be expected to reliably develop passive pressure especially near the surface.

I hope this helps.  

Regards,
Qshake

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Rowingengineer/Qshake -

Thanks so much. It makes so much sense now. Looks like we will need to retrofit the foundation by adding concrete drilled shafts and extending the pile cap. Looks like we will be using 20' deep (preliminary estimate) 2-foot diameter shafts most likely with casing.

Is 13' enough vertical clear space for a drill rig? I will try calling some drilling companies tomorrow, but just wanted to know what you folks thought. Our senior engineer is on vacation and our boss is out of the country right now (we are a small company).

Thanks again...

13' is doable, but check with drillers in the area because they may not have the equipment.

The pile cap bearing capacity should always be ignored in any assessment of the pier capacity. This is due to several reasons, the most important being completly different module of vertical stiffness in between the piles and the underlaying soils, listed as item 3 by Qshake.

I have a great deal of experience with seismic retrofitting of existing foundations and can tell you that most drilled shaft rigs will not fit in 13' of head room.  In the midwest where I'm located the smallest rig requires 21' of head room.  Also check with the drillers as 2' may not be economical whereas a 2.5' or a 3' diameter is more economical.  If cores may be required as part of a QA/QC program it will be a mess trying to core through a 24" shaft with a reinforcing cage.

You might want to consider pin-pile or micropile.  These are very good in low-head room areas.

Regards,
Qshake

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Thanks Everyone!!!!

Qshake - I agree that micropiles are a much better option with that headroom. I think  24" diameter is pushing the limit at that height. I was involved with a project where we had to relocate bridge piers due to a roadway realignment.

We put in 18" diameter piles about 60-65' long then socketed into rock. We had 14 - 16' of headroom.Smaller would have been easier but the owner wanted to reduce the number of piles because of existing utilities.  The DOT specs called for duplex drilling, which generally worked. At some locations the contractor fould it easier to use slurry to keep the hole open then drop the casing in.  

bridgebuster - Thanks for the info, just goes to show that you can learn something new all the time.

With the 21' rigs we used some locations required the contractor to excavate an area down several feet just to track the rig in.  We also used a telescoping drilled shaft method with temporary casing.  The upper casings were large while the bottom ones were narrower.  This can create quite the headache in achieving quality concreting when pulling the casing.  Slurry was called for in the specs but the contractors bid the job in the dry for the most part.  And aside from a few issues no huge problems were encountered.

Regards,
Qshake

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