Seismic Site Class Determination
Seismic Site Class Determination
(OP)
I have a site with soils corresponding to Seismic Site Class E according to IBC 2009. I have 10 feet of soil strata with PI>20, w>40, and su<500 psf.
I am confident that cross-hole shear wave velocity testing will upgrade the Site Class to D. Is is permissible to upgrade the Site Class based on shear wave testing given that the soils meet the three soil profile characteristics for E?
Thank you.
I am confident that cross-hole shear wave velocity testing will upgrade the Site Class to D. Is is permissible to upgrade the Site Class based on shear wave testing given that the soils meet the three soil profile characteristics for E?
Thank you.
RE: Seismic Site Class Determination
In my opinion you can do a shear wave test to determine the site class. Generally these tests consider the soils in the upper 100 feet.
RE: Seismic Site Class Determination
RE: Seismic Site Class Determination
Even by producing an S-waves profile by adequate geophysical investigation, we should reason what it is which governs the site amplification, the overall Vs30 or the soil characteristic described, or both. In the latter 2 cases it would be useless to employ geophysical investigation with the mere purpose of upgrading the soil class.
Also, you describe many firms rule the first opportunity out.
RE: Seismic Site Class Determination
RE: Seismic Site Class Determination
Here in Italy some reliable authors are showing the fallacy of the amplification concept based on the S-wave velocity in the upper 30 m or 100 feet.
RE: Seismic Site Class Determination
RE: Seismic Site Class Determination
what do you mean exactly by dynamic analysis?
Measure of macrotremors or microtremors (HVSR by Nakamura method) ?
Do you guys use something else in your place?
RE: Seismic Site Class Determination
RE: Seismic Site Class Determination
Are you happy with your numerical site response analyses?
I've started to use that method (equivalent-linear numerical simulation )and I'm realizing it can be sometimes off the beam.
The theory is an s-wave coming from the bedrock and travelling upward, wheras the input, a true seismic signal, is mainly made up of surface waves which are subject to multiple reflections (and refractions), with very little of the simulated upward propagating S-wave. The numerous approximations used propagate a final error which is unknown and probably significant.
I see many spurious values (probably function of the input motion values). I see shifted frequencies. I see peaks where they should be and I don't see peaks where they should be. I see pretty high and suspicious values of motion amplification.
My experiences are not always encouraging so far, although I'm going on since the codes reccomend them in some cases. I'm only using 1-D methods, those drawbacks I observed in laterally homogeneous situations.
I don't know if your results are more satisfactory than mine.
RE: Seismic Site Class Determination
The input ground motion and how you apply it is a huge factor. There are a few different opinions out there on what the best way to apply the input ground motion.
RE: Seismic Site Class Determination
What are the other relavant aspects you find critical as far as the application of input motions go (I'm speaking about real EQ motions)?
Here the building code requires 3 to 7 input motions, which must be normalized to meet the code area's spectral features (the design spectrum imposed by the code in those coordinates). This probably takes a little away from reality.
RE: Seismic Site Class Determination
Some of the issues with regard to application of input motions are related to opinions on the use of deconvolution of the bedrock motion, and to the depth at which the bedrock level is set at, particularly if you are modeling a deep soil site. I think there is at least one other issue I can't think of right now. I will try to get my thoughts and references together on this topic.
RE: Seismic Site Class Determination
It's a powerful tool, but it's all too easy to come up with unlikely results.
RE: Seismic Site Class Determination
also, crosshole is a high order geophysical survey. other surveys may be better applied depending on the site conditions. in other words, for large sites, i'd suggest surface performed surveys correlated to a downhole or crosshole survey. i'm not a fan of taking test results from one location and applying it to vast sites. i typically reserve crosshole for very specific locations/foundations due to the high costs. i often perform surface methods and correlate with a select number of downhole surveys.
if you need additional help off the board, feel free to message me at my user name at gmail dot com. good luck.