Horizontal seismic coefficient reduction
Horizontal seismic coefficient reduction
(OP)
It is my understanding that "full" values of the horizontal seismic coefficient (Kh) are rarely applied in practice when using any pseudostatic approach available (Mononobe/Okabe, Seed/Whitman, etc). A geotech professor told me there are journals that justify the reduction of Kh for design purposes by means of claiming mass energy distribution effects (by Makdisi I believe) and some other phenomena. Does anybody knows of additional journal/reference/book that define/allow other reduction effects on Kh? The reason behind this is because having a range of 0.3 to 0.7 for Kh seems completely ridiculous, even taking into account California's high seismic activity. Thanks in advance.





RE: Horizontal seismic coefficient reduction
Kh = 1.66 (Am) (Am/d)^0.24
Am = (1.45 - A)(A)
where d is allowable permanent deformation in millimeters.
RE: Horizontal seismic coefficient reduction
Thanks,
DRG
RE: Horizontal seismic coefficient reduction
But in general, if you have a safety factor of about 1.15, your slope deformations should be less than a foot.
RE: Horizontal seismic coefficient reduction
I think this is a case where Code requirements and Reality don't mesh well. We are forced to use Kh in our analysis, but by using it it make all our models fail. Therefore, we come up with "conventions" to convince ourselves that only using a fraction of the Kh is acceptable. While at the end, we are only designing to mitigate for a smaller earthquake.
RE: Horizontal seismic coefficient reduction
RE: Horizontal seismic coefficient reduction
it's sort of like many soils--the strength testing suggest the use of say 8 ksf bearing pressure however when you run a settlement analysis, it indicates that 3 inches of settlement may occur. but then at 3ksf, you're down to 1/2 inch. if the structure can handle 3 inches of settlement, then go for it...if it cannot, then use the more appropriate lower value. here's some good links to nchrp 611 which is the most recent publication i've seen (the documentation in the links below is in line with the asce7 proposed revisions that i have seen):
http://onl
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http://o
RE: Horizontal seismic coefficient reduction
A new study coming out from Sitar at UC Berkeley shows how seismic earth pressures are significantly lower than those predicted by Mononobe-Okabe.
RE: Horizontal seismic coefficient reduction
Thanks,
DRG
RE: Horizontal seismic coefficient reduction
The summary I read said the measured load distribution was triangular, not inverted triangular.
RE: Horizontal seismic coefficient reduction
The 'right' value of Kh has been quite of an issue recently in Italy due to the new seismic laws issued.
If Base shear= kh*weight force of structure, It has been observed that kh is never equal to Ag/g, i.e: PGA, rather a part of it.
probably the Japanese came out with a first approximation by Noda's relationship (1976)
Later, kh in slopes has been most frequently related to 0.5, and in Duncan & Wright's book, as howardoark says, a table of values ranging from 0.17 to 0.75 (or so) are reported.
It is true that pseudostatic analyses are only raw approximations, it is also true though that dynamic analyses are more complex, require a design seismic signal, yield as an output a displacement which is often awkward to handle (compared to a traditional factor of safety).
Recent research in the Italian territory has shown that the 'real' kh ranges from 0.18 to 0.28 in function of PGA, and soil type.
Analyses have been based on a comparison of newmark-type analyses and pseudostatic analyses, on slopes and on retaining walls.
It has always been evident that pseudostatic analyses of slopes and walls yielded overconservative results.
Especially walls turned out to be un-designable by the standard, traditional budgets allowed...
RE: Horizontal seismic coefficient reduction
RE: Horizontal seismic coefficient reduction
As you say, it's a function of many things among which structure ductility in buildings and soil ductility in slopes and walls.
The 'right' value I was referring to was a 'reference' value stemming from dynamic analyses