Design Philosophy with Seismic Foundation Loads 3

[WARose]

The other day an interesting discussion broke among co-workers over treatment of seismic loads at the foundations. As we all know, we (typically) divide the would-be seismic loads our structure sees by the response modification factor (to account for its ability to deform under loading) but we wind up sending that adjusted load to the foundations and a design soil pressure emerges (i.e. like say with spread footing supports). So how can you justify the foundation seeing that load by the same response modification factor that the structure sees when it is a different system? Is it a case where the code writers are counting on some sort of reserve capacity of the soil or is it more along the lines of the soil may not “see it” because of transmissibility (i.e. of dynamic loads)? Granted the code has different requirements for structural components of foundations in certain seismic design categories (and the geotech also has to keep an eye out for soils where liquefaction may be a factor)…….but this has always been puzzling to me. Thoughts?

 

[jittles]

The definition of R is not "ability to deform under loading."

R accounts for energy dissipation within the structure, i.e. the energy is dissipated through various mechanisms (cyclic plastic deformation for example) depending on the LFRS chosen. A value of R=1 does not imply no ability to deform, it means that the structure is designed to remain elastic and no energy would be dissipated internally and all the forces would have to be resolved into the soil.

The soil will in theory never see more load than the seismic loading reduced by the R factor.

 

[jittles]

This is a good article that was posted here previously:

Link

Side note: is it possible to edit a post?

 

[WARose]

Thanks for the info jittles. So its a situation where the energy is "lost" [in some cases] by the damage/deformation the structure gets......ergo the foundation wouldn't see it.

 

[SteelPE]

What I find a little confusing with the process is why are you allowed to reduce the overturning forces on the foundations (ASCE 7-05 section 12.13.4) if you used the ELFA and your structure is not a an inverted pendulum or cantilever column structure?

 

[PicoStruc]

What we do in canada with our Design Capacity Approch for anchored foundation (piles) :

Foundation must be design for the maximum effect of :

1) Reduced Seismic forces (Elastic seismic forces redyced by ductility Rd and overstrengh Ro factors)
2) Nominal or probable capacity of the lateral system depending of the system.

But not higher than the seismic forces corresponding to a ductility of R=1.3.

What is mean is for example,

If you have M_factored=2000, M_Nominal=4000, M_Elastic=6000, you design with 4000 (Capacity of the system)
If you have M_factored=2000, M_Nominal=8000, M_Elastic=6000, you design with 6000 (Elastic response upper bound)

It never happen to me that the reduced forces were used in design ! It rarely controls

 

[PicoStruc]

By M_Elastic, i Meant Elastic Response divide by 1.3 !

 

[DST148]

Ref: Seismic Design of Reinforced Concrete and Masonry Buildings by Paulay & Priestley, 1992, Chapter on Foundation Structures:
At the conceptual stage of design, a clear decision must be made regarding the admissibility of inelastic deformations within the foundation system. Accordingly, the chapter gives separate consideration to elastic and ductile foundation systems.
Ductile Superstructures - To enable ductile superstructure to develop its full strength under the actions of lateral forces, and hence intended ductility, the foundation structure must be capable of transmitting overstrength actions from the superstructure to the supporting soil or piles.
Elastic Superstructures - Foundation systems that support elastic superstructures may be considered in three groups:
(a) Elastic Foundation Systems - The entire structure including the foundation is expected to respond within elasic limits.
(b) Ductile foundation systems - Foundation rather than superstructure is chosen to be the principal source of energy dissipation during the inelastic response. All requirements relevant to ductile performance will be applicable to the design of foundation structure.
(c) Rocking structural systems - The designer may choose rocking of parts or of the entire structure to be the principal mechanism of earthquake resistance.

 

[DST148]

@SteelPE: Ref NEHRP Recommended Provisions, FEMA 450-2/2003 Edition, Part 2: Commentary- In the design of the foundation, the O.M. calculated at the foundation-soil interface may be reduced to 75% of the calculated value. This is appropriate because a slight uplifting of one edge of the foundation during vibration leads to reduction in the O.M. and because such behaviour does not normally cause structural distress.

 

[VoyageofDiscovery]

Doesn't the seismic force come from the ground?