Site Class Definition

[mjohan]

What is a reasonable amount of due diligence required to determine a site class (IBC 2000)?

As a structural engineer, is it an appropriate request to ask for the shear wave velocity from the soils engineer of record? Can this be estimated?

My experience, utilizing the IBC code, is that borings to determine blow counts are typically performed any where from 0-50 ft. From this determination, a site class is estimated. I have been involved with (4) different jobs (with the IBC 2000 code) all with site class D or E specified. Table 1615.1.1 indicates that this information should be predicted by the top 100 feet, and the shear wave velocity is the desired method. Geotechnical engineers are telling me that a shear wave velocity test is much too expensive.

 

[Zulak]

Site Class is a soils related category, and should be provided by the Geotechnical Engineer. One with experience should be able to interpret the soils information without performing any expensive shear wave velocity tests. This is one of the reasons that I like to get a Geotechnical Engineer from the area that the project is located. He should have plenty of knowledge of the soils in that area even before the borings are performed. Problems arise when he's unsure, and gives you a conservative value, and it drives your seismic forces through the roof.

 

[Focht3]

Shear wave velocity - usually by a cross-hole test - is the preferred method. But most owners won't pay the cost of a really good evaluation, so we are forced to use various correlations.

It's simple economics -



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[mjohan]

Thanks for the responses.

Focht3, do you know the cost of performing such a test?

Like Zulak stated, "Problems arise when he's unsure, and gives you a conservative value, and it drives your seismic forces through the roof." The process of not using a shear wave velocity test will automatically give a seismic site class of "C", at the minimum. On top of that, a low blow count in the soil seems to be driving a site class of "D" by my experience. The code commentary states that NOT using the shear wave test will yield a conservative result. (Commentary FEMA 369)
"However, in recognition of the fact that in many cases the shear wave velocities are not available, alternative definitions of the site categories also are included in the 1994 Provisions. They use the standard penetration resistance for cohesionless soil layers and the undrained shear strength for cohesive soil layers. These alternative definitions are rather conservative since the correlation between site amplification and these geotechnical parameters is more uncertain than that with (shear wave). That is, there will be cases when the values of Fa and Fv will be smaller if the site category is based on (shear wave) rather than on the geotechnical parameters. Also, the reader must not interpret the site category definitions as implying any specific numerical correlation between shear wave velocity on the one hand and standard penetration or shear strength on the other."

I don't think it is out of the ordinary to request an accurate seismic site class. For years Geotechnical Engineers picked a site class based on BOCA which was a description of the soil type and a shear wave velocity. I see now that the shear wave velocity was never used and it was a "soil description" which drove the site class. That is why I have received a report stating if using BOCA, define the site as S1, and if using the IBC, define the site as D. HMMMMMM.

Additionally the code suggests using a very conservative value for building period "T". I can control the T value by performing a rational analysis (if any one has suggestions for performing this analysis, please give a shout).

CONSERVATIVE X CONSERVATIVE X CONSERVATIVE = $$$$$

 

[Focht3]

Cost? Well, it depends -

You should check with local firms for pricing. It will depend on the project size and site conditions. A fee in excess of $5,000 wouldn't surprise me at all -



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[cap4000]

Yes it can be estimated, provided you have a detailed Soils Report. You will need Young's Modulus, soil unit weight, and Poisson's ratio. Obtain the DAS Foundation Engineering Handbook 1990, 2nd edition, it has tables and charts you will need. This may save you money from doing any one of 4 Seismic Tests with the Cross Hole Test the most popular.

 

[Focht3]

Choosing E[sub]Y[/sub] properly requires a lot of experience and local knowledge of the geology and site conditions. Poisson's ratio is a bit of a guess anyway; again, experience counts here.

This isn't a cookbook procedure; be sure that you know what you are doing before you affix you professional engineer's seal to the report...



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