ASCE 7-16/22 12.13.5 Strength Design for Foundation Geotech Capacity, Resistance Factors

[geoman77]

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.

 

[HTURKAK]

I am not sure if penalized for every case . IMO, the use of strength design for foundations is not a mandatory requirement and the designer may choose allowable stress design ( 12.13.6 Allowable Stress Design for Foundation Geotechnical Capacity ) .

When you look ACI 318, The code allows either the strength design method (as per ASCE 7 Section 12.13.5 ) , or allowable stress design..


ACI 318 Clause R13.2.6.1 Permissible soil pressures or permissible deep foundation strengths are determined by principles of soil mechanics and in accordance with the general building code. The size of the base area of a footing on soil or the number and
arrangement of deep foundation members are established by using allowable geotechnical strength and service-level load
combinations or by using nominal geotechnical strength with resistance factor and factored load combinations...

EDIT : I will suggest you to look CH 7 FOUNDATİON of the following doc.

https://www.fema.gov/media-collecti...sions-new-buildings-and-other-structures-2015