Mixing LRFD and ASD for foundation design

[Okiryu]

Hi, for pile design I am giving axial and lateral capacities calculated using some FOS (piles are end bearing piles), which may be considered as the ASD method from the structural perspective. However, the structural engineer is calculating the loads in the foundation using load combinations using a LRFD approach. My question is, if you are doing a LRFD based design, do you use the "allowable" capacities provided by the geotechnical (means the ultimate capacity affected by a FOS) or do you use the ultimate capacities (capacities non affected by FOS) since your loads are already factored? Thanks!

 

[bridgebuster]

I've worked on a few "mix and match" projects. If the geotech is giving you a pile capacity based on ASD then the load combinations from the SE should be ASD as well.

 

[dik]

The City of Winnipeg now insists on all foundations being designed using Limit States and it has caused an increase of foundation costs by approximately 25%... The geotekkie has to know what the limit states values are.

Dik

 

[Okiryu]

Thanks for your responses. I think that now LRFD is not too common for geotechnical engineers, but I see it is gradually been introduced especially for bridges. Also, what could be my best reference for LRFD and ASD loading requirements, ASCE 7-10?

 

[Lomarandil]

ASCE 7-10 is nice because it addresses both methodologies and is well organized, but it may not be directly applicable to your project (if it's a bridge, for example).

 

[Okiryu]

Lomarandil, I will check ASCE 7-10. My current project is a pier structure...

 

[TLHS]

dik,

In theory, the Canadian Foundation Design Manual has phi factors for geotechnical resistances, so the Winnipeg thing should be do-able. It's the direction we're definitely going. I've run into the issue, though, that geotechnical engineers don't seem to trust it so they don't actually start with ultimate values. They put extra fudge factors in before they apply the phi, because they're afraid of using an ultimate value for design, and end up being more conservative than they would have been in ASD.

The above is more an issue with education and comfort than a problem with the system itself.

There's also, understandable, a lot of confusion over how to deal with serviceability based allowable.

 

[wannabeSE]

I agree with bridgebuster. For me, it is not too much trouble to use a separate set of ASD load combinations for the foundation size.

On a similar note, I thought the soil capacity is quite often governed by settlement (don't know if this is true for piles). More often than not, settlement is a serviceability issue and using service loads (ASD) makes more sense to me.

 

[dik]

TLHS:

No question, it's doable... just inflates the foundation costs, with little or no benefit.

Dik

 

[Okiryu]

Thanks for the responses. I am just keeping my recommendations based on the ASD approach. I agree with wannabeSE, recommendations are based on deformation criteria so ASD may make more sense. I talked with the structural and he will check their load combinations using the ASD method. Thanks again.

 

[BigH]

I do not like the LRFD approach for foundation problems - first, as some have pointed out, the permitted "capacity" as governed by all your load combinations is 95% of the time not the controlling factor as it is a shear based factor. Settlements, total and differential, are almost exclusively the controlling factors so you are not really getting an "allowable bearing capacity" but an "allowable bearing pressure" which is settlement/deformation based. What is the sense of determining capacity if, say, your structure has stringent settlement issues which are even less than the normal 25 to 40 mm of permissible total settlement? I've had projects where 10 mm of settlement is the basis of design/behaviour (and hopefully being lucky enough to be correct on "fuzzy" settlement computations.

Secondly, where does LRFD get the load reduction factors and the property factors? They "correlate" to the old methods - say assuming a SF = 3 for footings. So the question is why?

Thirdly, as far as I have been able to see, page after page is provided for shear capacity - of an isolated footing. I have never seen - and perhaps it is out there - of what happens when you have two footings close by whereby their potential shearing planes (aka prantdl) interact? Of course, for settlement computations we can take this into account by superposition of the vertical stresses of one to the other - but are there procedures for interacting footings? Again, it is the service limits that will govern.

Fourthly, I have never seen, actually in either method, what to do when one has slickensides or other zonal factors in the computations - pre-disposed sliding surfaces. I think this is a bigger issue with the LRFD where things are heavily "codified" and you are damned in you do and damned if you don't.

As one has stated and I have read this elsewhere as well - costs of foundations have gone up due to the LRFD approach . . . so is it really necessary? As Poulos has stated in his State of the Art paper to the Istanbul SSMEFE conference, the old methods work. . . . and, in any event, you need to critically look at your design. My fear is that, especially given the plethora of computer design these days, that the criticality of judgment may end up like the dodo.

My rambling take on the issue. No problem - criticize!

 

[Okiryu]

BigH, thanks. Input from the geotechnical side is what I was also waiting in this thread. I think that sometimes there is a disconnection between the structural and geotechnical. For example, geotechs most of time (if not all) provide recommendations based on some assumptions for loading. We have to do these assumptions since the geotechnical investigations are done typically at the early stages of the project, when sometimes even the building layout is defined. For my projects, I just try to be proactive and follow the design until it's completion, even the project manager tells me "please submit your fee only for the geotechnical investigations. You don't need to be involved until the last stages of the design...". Anyways, I think it is always good to keep the communication with the design team throughout the project's design process.