FoS and Geotechnical Resistance Factor

[ONENGINEER]

Hi

I understand well the concept and application of FoS. However, having gone through the Canadian Foundation Manual, I have a difficulty to understand the geotechnical resistance factor. For example, a resistance factor of 0.5 is recommended for vertical bearing resistance of shallow foundations in CFEM. This means (0.5) x R, where R is the geotechnical ultimate resistance predicted from theoretical analysis using unfactored geotechnical parameters. Isn't that similar to dividing the calculated foundation bearing capacity by a FoS of (2). I.e., R x 0.5 (per CFEM) = R / (2) (per e.g. Navfac)

I am sure there is more into it but looking at some examples of design calculations by engineers, it seems the same outcome is expressed with two types of vocabularies one of them being more trendy. Could someone familiar with CFEM enlighten me. For example, what does one gain by using CFEM instead of traditional allowable load calculations. Thank you.

 

[geotechguy1]

Yes this is a more modern approach.

You compare (geotech ultimate * resistance factor) > (structural ultimate * structural load factor).

Although geotechs tend to still use the FoS approach for slope stability problems.

 

[PEinc]

Remember, if you are doing LRFD design, you multiply different loads by different load factors that are greater than 1 and these factored loads give factored stresses that need to be less than the various material strengths multiplied by their particular resistance factors. The more homogeneous and proven a material's properties are, the higher the resistance factor. The more variable or inconsistent the material's properties, the lower the resistance factor. For example, soil has a low resistance factor; steel a high resistance factor. Well defined dead loads have a lower load factor than traffic live loads. In somewhat simpler terms, for LRFD design with one type of load, the "safety factor" is the load factor for that particular load divided by the resistance factor for the supporting "material."

http://www.PeirceEngineering.com

 

[geotechguy1]

Yeah good point, edited my post to clarify load factors for structural

 

[ONENGINEER]

Thank you. I do not recall having recent dialogues with structural engineers to know how they use the allowable bearing pressures given by a geotechnical engineer. It seems they multiply the allowable bearing pressure by the FoS used by the geotechnical engineer and then apply the LRFD method without necessarily communicating with the geotechnical engineer. In case the structural engineer provides the ULS load during site investigation, does it imply that the structural engineer is looking for factored geotechnical resistance instead of allowable geotechnical pressure/load?

 

[Smoulder]

I am not Canadian but will give my thoughts anyway. We have sim issues in Australia where structures are limit state. Structural loads come in diff types that affect foundations. Sustained loads, short term loads, design ultimate loads, seismic loads. Sustained and short term are for settlement. Ultimate and seismic are for collapse. Easiest for footing design if geotech capacity and comments provided for all of those cases. Say if allowable is governed by clay consolidation then give a required load time for consolidation to happen so struct eng can decide if sustained or short term load is the right one to compare to.

 

[geotechguy1]

A big issue with the type of problem Smoulder is describing is structural engineers don't really know enough about how the ground behaves and geotech engineers don't know enough about how structures behave (and there is a mutual misunderstanding where both sides don't realise that sometimes what is conservative for them might be un-conservative for the other discipline).

Somehow it all seems to work out in the end but I suspect we are going to have a lengthy period of problems as climate change / CO2 gets used increasingly to push for much leaner designs.

 

[GeoEnvGuy]

As per the design example 6.8.1 in the 2023 5th edition for shallow foundations

Do an investigation to get your design parameters
Evaluate consequence factor (0.6-1.4) and understanding factor for bearing capacity (0.35-1.0)
Ultimate limit drained multiplied by consequence and understanding factor
Ultimate limit undrained multiplied by consequence and understanding factor

Evaluate consequence factor (0.6-1.4) and understanding factor for settlement (0.7-1.0)
Serviceable limit multiplied by consequence and understanding factor

For most cases you will end up with a consequence factor of 1.0, a understanding factor of 0.5 for ULS, and an understanding factor of 0.8 for SLS