Redundancy Factor and Soil Bearing Pressures under SLRS

[LL805]

As a designer, I have always applied rho =1.3 (redundancy factor) when required for the building per ASCE 7-10 12.3.4 to the SLRS foundation design. The foundation design would include both the SLRS foundation itself as well as the soil bearing pressure check. ASCE7-10 12.3.4.1 does not explicitly note foundations as an exception where rho = 1.0 and the ASD load combinations under 12.4.2.3 include rho. I've now been engaged with some other engineers who only apply rho to the foundation design but not the soil bearing pressure check. The argument is that the soil is not part of the SLRS and therefore rho should not be included; ASCE7-10 states that rho is only to be applied to the SLRS system. My counter to this is that the definition to the SLRS system is "that part of the structural system that has been considered in the design to provide the required resistance to the prescribed seismic forces"; the soil resists seismic forces and therefore is part of the SLRS system. In this specific case, the soil bearing pressure has already been doubled for short-term loading.

The more engineers I talk to, the more opinions I get. It seems that more seasoned engineers (10+ years experience) don't think rho should be applied to soils while younger engineers tend to think the code is clear and that you should be applying it. Other articles I've found indicate that it should be applied to "foundations" but do not make a clear distinction between design and soil bearing.

I did a search on this site but didn't find anything on this topic specifically. If this has been discussed previously, please include the link!

Thanks!

 

[WARose]

In most software packages, you would wind up (under the required load combinations) with that included (whether you wanted it or not). It would be difficult to back out of the reaction force anyway.

That being said, and to get to your point, I can see an argument either way. However, I have always taken the redundancy factor as having more to do with the performance of [in terms of probability] structural components (i.e. wood, steel, etc.). Most of the time, when it comes to bearing pressures, I am looking in the geotech report and seeing what safety factor they are using (for allowable pressures), what type of soil it is (i.e. is there a potential for liquefaction?), etc, etc, to see if I am comfortable with the pressures during a seismic event. A factor of 1.3, while significant, typically isn't going to be a make or break thing when it comes to soil pressures. So probably the best advice is: to include it (where required; in an overall model) and consider it an extra safety factor......but keeping in mind: it's probably the least of your concerns when it comes to allowable bearing pressures on some soils in a seismic event.

 

[tstructural]

I would expect that you'll get about a 50/50 split on this one but I do not add Rho to the soil analysis. For the reasons WARose listed above - geotechnical is it's own beast and I let the Geo Engineer use their safety factors to determine my allowable design loads. But - removing the seismic component to get a combined DL + LL + Seismic without Rho from computer models can also be a pain. For simple structures I do it - you can run a 'reaction envelope' without Rho.

 

[LL805]

Thanks for the input. In this case, the plans examiner required the footings (including bearing pressures) to be designed with rho and the footings had to be re-sized accordingly so this really came down to ASCE7-10 code language. I do agree that geotechs rightfully build a lot of conservatism into their allowable bearing pressures. In this case though, the allowable bearing pressure had been doubled for short term loading and the 10% reduction on overturning loads per ASCE 12.13.14 had been applied. Regardless of engineers opinions on the conservatism of geotech reports, a plans examiner is not going to accept a judgment call (nor are all structural engineers qualified to even make that call) and will always fall back to code language.

As far as the modeling goes, depending on the lateral system and program, I typically will set rho=1.0 and then apply it to the load combinations after pulling my lateral forces out of the model.

 

[sandman21]

Rho applies to seismic lateral system, if the structure above has a rho of 1.3 the loads all the way, including the soil should be increased. This is the same in UBC/CBC 97/2001, E was defined to include rho. In CA at least most geotech already have overstrength included in the soil bearing.

 

[LL805]

Thanks for the input. When I posted this question, I concurrently sent out the question to ASCE. The following is their response or "friendly advice" as they are calling it.

Thank you for your question on ASCE 7-10. You question was addressed by one of our subject matter experts. Their friendly advice is:

The rho factor should be included in foundation design, including soil bearing checks, since (1) foundations are not listed in Section 12.3.4 and (2) the basic load combinations in Section 12.4.2.3, for both strength design or allowable stress design, clearly include the redundancy factor. These equations are derived from the load combinations in Chapter 2, with the definition of the earthquake load effect being given in Section 12.4.2 where E = Eh +/- Ev. Rho is introduced in the definition of Eh in Section 12.3.2.1, where Eh = ρQE. If engineers are designing their foundation using the alternative basic load combinations listed in IBC Section 1605.3.2, rho is still required since E is defined per Section 12.4.2 of ASCE 7, which as noted previously, brings in rho.

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