I am working for a soil testing firm in Charleston, West Virginia. We have made a lot of recommendations on the choice of HP sections based on ASD. The Factor of Safety (phi value) we used was 0.25.
That is, Design strength = .25 x fy x A
where phi=0.25 fy = yield strength A = cross section of H-pile but this was for ASD.
If it was for LRFD, I want to know what will the phi factor be ?
or is there a completely different approach to obtain the design strength ??? I do have the Manual of Steel Construction (LRFD) with me that suggests phi=0.85 for limit states involving compression buckling but I want to confirm whether or not the "phi" is different for H-piles.
I would appreciate if u will be able to get some references.
You did not say what load factor you apply. If it is, say, 1.4, the 0.5 resistance factor combined with the load factor represents a working stress (WSD) factor of safety of 2.8. A good deal better than the old value of 4.0 (1/0.25) in WSD you reported to be per your “old” practice, but not very impressive. The limit on the structural strength of an H-pile should be no larger than 2.0 (WSD). For LRFD with a load factor of 1.4, this means that the resistance factor should be at least 0.7.
The rational for keeping the resistance factors down is that it is far too common to design only per the structural strength of the pile material and making the assumption that because we wish it so to be, Mother Nature will somehow ensure that the material strength is the aspect governing the pile construction end result. To play it safe, the Code-writing Powers limit the factor on structural strength, cooking the books as it were. It is easier to do this than to make the effort to ensure that the profession gets the training and education necessary for producing safe and economical designs, i.e., using the higher loads where conditions warrant and where indeed the structural strength is the governing aspect.
Bengt H. Fellenius
1. According to AASHTO LRFD 4th ed: 1.1. Resistance factors, phi, for the strength limit state, for axial resistance of piles in compression subject to damage due to severe driving conditions where use of a pile tip is necessary is 0.5 (for H-piles).
1.2. ... under good driving conditions where use of pile tip is not necessary it is 0.6 (for H-piles)
1.3. ... for combined axial and flexural resistance of undamaged piles it is 0.7 (for H-piles).
1.4. Resistance Factors for Geotechnical Resistance of Shallow Foundations at the Strength Limit State shall be taken from Table 10.5.5.2.2-1 of AASHTO LRFD 4th ed.
1.5. ... of Driven Piles from Table 10.5.5.2.3-1 of AASHTO LRFD 4th ed.
1.6. ... of Drilled Shafts from Table 10.5.5.2.4-1 of AASHTO LRFD 4th ed.
2. According to AASHTO Standard Specs: 2.1. Allowable Stress Design limits allowable stress in Steel H-piles at 0.25Fy.
2.2. Spread footings on soil or rock shall be designed for Group 1 loadings using minimum Factor of Safety of 3.0 against a bearing capacity failure.
2.3. For Driven Piles (for ultimate Geotechnical capacity) a Factor of Safety of 2.0 may be used if construction control includes an approved static load test. (Table 184.108.40.206A of AASHTO Standard Specs).