LRFD Geotechnical/Foundation Design

[anominal]

A soil report we received reads:

...we recommend over excavating 18-inches and using the following Maximum Factored Bearing Resistances:

Service Limit State – 3.0 ksf
Strength Limit State – 6.0 ksf

Are the two limit states derived from a common Ultimate Bearing Capacity? Or is their no correlation between the values?

Also, does anyone have a reference/glossary of terms for structural engineers? In everyday office conversations I keep interchanging nominal with ultimate and girder with stringer etc... it's bad.

 

[SLTA]

Strength limit state comes from the factored loads and is used to design the bending, shear, etc in the footing. Service limit state is from the unfactored loads and is used to size the footing's plan area.

Amazon has several civil engineering dictionaries available, but I might recommend just doing a google search on specific terms, as you'd likely get better information that way.

 

[vandede427]

I think it's bad wording, but they're probably trying to tell you there's a 2.0 factor of safety. I'd use 3 ksf for design.

 

[SpecialEddie]

Is this a transportation project? "Service" and "Strength" limit states are specific limit states defined in the AASHTO LRFD Bridge Design Specifications. There is one other specific limit state, the "Extreme Event" limit state which is to be used for seismic design. The fact that the geotech did not describe this limit state leads me to believe that this project is not subject to seismic design requirements or is a buried structure, which does not require seismic analysis per the AASHTO LRFD code.

You need to get a copy of the AASHTO LRFD code and look at Chapter 3. That is where the limit states and relevant load combinations are defined. If your project is transportation related in any way, you better check that the design code you should be using is AASHTO LRFD. If it is, you should find someone who is familiar with it to do the design. If not, then you should ask the geotech to reword his recommendations in "building lingo" so that there is absolutely no confusion on your part when doing structural design. While the others above are on the right track, there are subtle differences between AASHTO and IBC geotech analysis, and you should ask the geotech to adjust his wording accordingly - you should not guess his intent. Also, the AASHTO load combinations to be used with the "strength" limit analysis have different load and reduction factors than the IBC load combinations. I find it strange that the geotech would be using AASHTO terminology on a non-transportation project. Usually it is the other way around (not many are familiar with AASHTO unless you are in the bridge design world) and we always ask them to convert to AASHTO terminology (eg. "service" and "strength" level recommendations). If they refuse to do so or don't understand what we are talking about, we suggest to the owner that they find another geotech who knows what they are doing.

Good luck.

 

[anominal]

Thanks for that Eddie;

This IS in fact a transportation project. And our office does have a copy of the AASHTO LRFD; it was my mistake for not adding any context to my question, I forget not everyone works transportation.

Soils/geotechnical engineering is not one of my strengths, I was only asking if the two "allowable" bearing pressures are related.

e.g.:
Service Limit State = (f1)(f2)(f3)(qu)
Strength Limit State = (f1)(f2)(f3)(qu)

While skimming through AASHTO it did not become apparent to me.

 

[GalileoG]

Actually, I came across the exact same wording on a geotech report for a retail building. I understood it the same way Vandede427 did, in that you can use factored loads and check against ultimate bearing capacity or service loads and check against service bearing capacity.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[SpecialEddie]

I do believe that the various bearing pressures for the different limit states are derived from a single value, and I think it is the ultimate bearing capacity. I believe Chapter 10 of the AASHTO LRFD goes into detail of how geotechs determine the different factors to derive "service," "limit," and "extreme event" bearing capacity recommendations. Hopefully a geotech will stumble across this thread and provide you with more specific resources that can shed more detailed light on your question.

 

[apsix]

The best action when there is something you don't understand in a report is to ask the author.

 

[anominal]

apsix; I agree. Would you believe this question stems from an E-Mail from the author?

Something to the effect of:
"I think I may have flip-flopped the words for the service and strength bearing pressures"

Luckily, it was a lack of understanding on the authors part, and they had not made such an egregious error in a final report.