It seems to me that there is a flaw in the establishment of resistance factors for "strength design" of foundations for geotechnical resistance in CH 12 of ASCE, starting w/ 7-16 and unchanged for 7-22.
12.13.5 permits "strength design methods" for geotechnical design of foundations, i.e., using strength (not ASD) load combinations from Ch 2 to determine factored foundation demands and then using the resistance factors in Table 12.13-1 to determine factored geotechnical resistances. According to the commentary (C12.13.5.2), the resistance factors in ASCE 7 were simply adopted from the AASHTO LRFD BDS.
The issue as I see it is that something got lost in translation when the ASCE 7 subcommittee tried to adopt the resistance factors for geotechnical foundation design from AASHTO. They adopted the resistance factors for non-seismic "Strength Limit States" but then specified them for seismic design in ASCE 7 CH 12. The AASHTO LRFD framework distinguishes between non-seismic "Strength Limit State" and seismic "Extreme Event Limit States" as separate cases, and specifies a resistance factor of 1.0 for seismic Extreme Event Limit State design (e.g., AASHTO LRFD BDS 7th Ed., 10.5.5.3.3) with the exception of pile uplift (phi = 0.8). In contrast, the resistance factors for non-seismic Strength Limit States are on the order of 0.45 to 0.5, which is consistent with the value adopted by ASCE 7 (0.45) -- but in the seismic chapter! The correct approach would have been to specify a resistance factor of 1.0 for seismic design in Ch 12 and then separately provide the resistance factors for non-seismic limit states, probably in a chapter other than CH 12.
Would be curious to hear if folks agree or can provide any other commentary on this. The way 7-16 and 7-22 are currently specified makes it disadvantageous to use strength design for foundations, you effectively are penalized in comparison to doing the whole thing in ASD.
12.13.5 permits "strength design methods" for geotechnical design of foundations, i.e., using strength (not ASD) load combinations from Ch 2 to determine factored foundation demands and then using the resistance factors in Table 12.13-1 to determine factored geotechnical resistances. According to the commentary (C12.13.5.2), the resistance factors in ASCE 7 were simply adopted from the AASHTO LRFD BDS.
The issue as I see it is that something got lost in translation when the ASCE 7 subcommittee tried to adopt the resistance factors for geotechnical foundation design from AASHTO. They adopted the resistance factors for non-seismic "Strength Limit States" but then specified them for seismic design in ASCE 7 CH 12. The AASHTO LRFD framework distinguishes between non-seismic "Strength Limit State" and seismic "Extreme Event Limit States" as separate cases, and specifies a resistance factor of 1.0 for seismic Extreme Event Limit State design (e.g., AASHTO LRFD BDS 7th Ed., 10.5.5.3.3) with the exception of pile uplift (phi = 0.8). In contrast, the resistance factors for non-seismic Strength Limit States are on the order of 0.45 to 0.5, which is consistent with the value adopted by ASCE 7 (0.45) -- but in the seismic chapter! The correct approach would have been to specify a resistance factor of 1.0 for seismic design in Ch 12 and then separately provide the resistance factors for non-seismic limit states, probably in a chapter other than CH 12.
Would be curious to hear if folks agree or can provide any other commentary on this. The way 7-16 and 7-22 are currently specified makes it disadvantageous to use strength design for foundations, you effectively are penalized in comparison to doing the whole thing in ASD.
