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Drilled Shaft LRFD

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moe333

Geotechnical
Jul 31, 2003
416
Hi Guys,

I have somehow managed to stay away from LRFD work up until this point in my career, but I now need to deal with it so I’m trying to wrap my head around it.

I have a drilled shaft project for a pedestrian bridge in the Los Angeles area (Caltrans). A complicating issue is there is liquefaction, but no lateral spread. A single shaft at each abutment.

The structural engineer provided the following loads:

Service I Limit State: Total Load and Permanent Load 370 and 315 kips (No Load Factor indicated)
Strength/Construction Limit State: Compression Load 490 kips (Load Factor = 0.7, so 700 kips factored)
Extreme Limit State: Compression Load 315 kips (Load Factor = 1.0)

From my review of AASHTO LRFD, I concluded the following:
Resistance factors for compression for the Service I Limit State and Extreme Limit State = 1.0, so I would use nominal (unfactored) stengths to calculate pier depths for the loads provided. It seems there is no factor of safety for this limit state based on my interpretation?

Resistance factors for compression for the Strength/Construction are provided in AASHTO LRFD. My assumption is I would use either the factored load (700 kips) with the unfactored strengths, or the unfactored load (490 kips) along with the nominal (unfactored) stengths to calculate pier depths, but not both. Is one of these combinations more correct? Is my interpretation correct?

For the Extreme Limit State (liquefaction condition), I would multiply the downdrag load by 1.25 and add it to the compression load to calculate pier depth. Capacity would only be obtained below the liquefied layer. I understand there is also the neutral plane concept that comes into play for this condition, but that is another issue.

For the lateral analysis for the Extreme Limit State (liquefaction condition), I would not add the downdrag load to the compression load. I assume I would only do the lateral analysis for the Extreme Limit State (liquefaction condition)?

I appreciate any comments, Thanks!
 
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Sorry moe - can't be of help. I, too, have had nothing to do with LRFD other than hear about it and wonder why . . . Malaysia is going EuroCode.
 
You may want to try calling Jerry A. DiMaggio of Jerry A. DiMaggio Consulting LLC. Jerry is the former FHWA Geotech guru. He regularly give seminars on LRFD design for walls and foundations. It should not be hard to find him on the internet or on LinkedIn.

 
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