Resistance Factors for bearing, sliding and overturning per ASCE Sect.12.13.5.2

[LockeBT]

Typically when it comes to stability check for foundations I use service loads (pretty much ASD loads). Without a geotechnical report my presumptive soil capacities (from the IBC or the CBC) such as soil bearing is taken without any factor.

However that section and Table 12.13-1 provides Resistance Factors for Strength Design when it comes to stability check.

For a retaining wall with seismic load there is no point using this. Not only am I getting a huge reduction in my soil capacities, my seismic load is now E instead of 0.7E and my soil loads would be 1.6H. Basically the demanded is factored up while the capacities are reduced.

I'm leaning towards designing everything in LRFD (Strength Design) since the codes are slowly and uniformly moving towards that direction (besides Wood) however that section seems like a double penalty. There is no point checking soil stability in LRFD then?

Anyone has any take on this?

 

[HTURKAK]

I could not get the reason for your thinking as double penalty. The standard allows the use of Allowable Stress Design for Foundation Geotechnical Capacity ( pls look 12.13.6 ) ..

The following doc.( excerpt from FEMA 1051 ) is useful to get the concept for Allowable Stress Design vs Strength Design.

 

[LockeBT]

HTURKAK,

I think I wasn't clear. I don't have issues understanding the difference between ASD and LRFD.

When you check foundation, there is a point where you check for stability, eg. bearing, overturning and sliding. Typically, industry practice will do that in SERVICE LOADS (which is VERY similar to ASD level loads).

Let's say I am checking soil bearing capacity of the footing of a cantilever retaining wall and I do not have a geotechnical report. The soil bearing pressure will be 1500 psf worst case scenario (IBC Table 1806.2).

In ASD:
Capacity: 1500 psf
Demand: Bearing pressure will be derived from Service Load Combinations with 0.7E and H

In LRFD:
Capacity: (resistance factor)*1500 psf = 0.45*1500 psf = 675 psf (resistance factor is from ASCE Table 12.13-1). Now I understand this calculation is wrong because 1500 psf is meant to be used with ALLOWABLE STRESS DESIGN (as stated in IBC section 1806.1)however the code doesn't give you presumptive load bearing values in LRFD. In reality if the codes are consistent the presumptive values will be much higher so it can be used with LRFD.
Demand: Bearing pressure will be derived from LRFD Load Combinations with E and 1.6H

As you can see, I have no way around using LRFD to check for stability unless I have a geotechnical report. Whereas in ASD, I can.

 

[le99]

Hope this article clears your confusion.

https://www.structuremag.org/?p=12898

 

[HTURKAK]

IBC Table 1806.2 provides Presumptive Load-Bearing Values and the presumptive bearing values in the code are allowable
stress values, not strength level values.

The vertical found pressure qall= 1500 psf ( for Clay, sandy clay, silty clay, clayey silt, silt and sandy silt
(CL, ML, MH and CH) material )

In general, the allowable soil stress has a F.S. = 3.0 in this case, the nominal str. Qns= 3*1500=4500 psf. and Allowable foundation load capacities,
Qas= ϕ *Qns =0.45*4500=2025 psf ( this is the value which shall be used with LOAD COMBINATIONS FOR STRENGTH DESIGN )

Pls notice that , for LOAD COMBINATIONS FOR ALLOWABLE STRESS DESIGN with seismic loading, seismic load is multiplied with 0.7
(0.7* Eh ) then the allowable stress increased with a factor of 1.2.

Eventually , All Roads Lead to Rome !!

 

[LockeBT]

HTURKAK,

I agree with everything you say except my trouble is that FoS of 3 to go from ASD to Strength Design. Where do you find that?

I have seen different FoS from geotechnical reports before ranging from 2.5-3. But I don't use 3 with confidence because it's not referenced anywhere.

 

[le99]

In general, I wouldn't mix apple with orange. With respect to the applicability of "the Presumption Soil Pressure" in the tables, the code stated clearly:

1806.1 Load Combinations

The presumptive load-bearing values provided in Table 1806.2 "shall be" used with the allowable stress design load combinations specified in Section 1605.3. The values of vertical foundation pressure and lateral bearing pressure given in Table 1806.2 shall be permitted to be increased by one-third where used with the alternative basic load combinations of Section 1605.3.2 that include wind or earthquake loads.

To me, the words "shall be" mean the matter is "non-negotiable".