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Re: Site classification - IBC

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Here is my understanding on how the avereraging is done:

1. The IBC formula doesn't use blow count, shear wave velocity, AND undrained shear strength, you (or actually the geotechnical person) use only one of the three. See 1615.1.1. It says that when shear wave velocity data is not known, you can use N values OR undrained shear strength values.

2. See 1615.1.5 for averaging. The top 100 feet is divided in to n layers, for example 20 at 5' each, and the test values at each of these points are averaged.

3. Section 1615.1.5 covers what to do with the rock N values. On page 352, it says N is ... not to exceed 100, as Thomas Hunt said. So you might be averaging in a lot of 100s for the layers of your 100 feet that are in solid rock. In other words, you might have: 5 6 8 7 9 12 13 15 22 33 35 ... 100 100 100 100 100.

Rick Burch
Columbia, SC

John C. Jones wrote:

They're involved, but are curious as well as to how the averaging is done. The formula in IBC uses both blow count, shear wave velocity, and undrained shear strength.. I did get the final results this morning and based on the above I wound up with C, which still makes a big difference. Thanks for everyone's help. IBC is just getting adopted locally and the geotech wasn't completely sure on the averaging either. Between them talking with folks and me talking with you all it wound up getting handled. Thanks again.
John C. Jones, PE
Barnett Associates
Pell City, AL
205-884-0099 (fax)

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