Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
(OP)
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...
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...





RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
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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RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
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,
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RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
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...
RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
You might want to consider pin-pile or micropile. These are very good in low-head room areas.
Regards,
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RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
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.
RE: Timber Piles with Concrete Pier Cap - Neglect Cap for Bearing?
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,
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