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IBC Chapter 16 - Determination of Average N value for Seismic Site Class Determination

IBC Chapter 16 - Determination of Average N value for Seismic Site Class Determination

IBC Chapter 16 - Determination of Average N value for Seismic Site Class Determination

I find table 1613.5.2, Equation 16-42 in the 2009 IBC, and the accompanying discussion unsatisfactory. I was hoping someone on eng-tips might be able to provide some additional perspective.

Table 1613.5.2 references the "average" standard penetration resistance in the top 100 feet of the soil profile for the determination of the seismic site class. Evaluating table 1613.5.2 alone, it seems straightforward to me that if I had a site with N=10blows/ft from 0-30 feet and Rock (N=100 blows/ft) from 30 to 100 feet that the average N value should reflect the thickness weighted average N value e.g. 10blows/ft*(30ft/100ft)+100blows/ft*(70ft/100ft) ~ 73blows/ft. Instead the specification presents Equation 16-42 to determine N (bar) which is not the thickness weighted average N value. Equation 16-42 gives 100ft/{(30ft/10blows/ft)+(70ft/100blows/ft)} ~27 blows/ft. A description of what this quantity actually represents eludes me but the value is always lower than the thickness weighted average N value (except in the case of a single layer in the top 100 feet).

Projects that I am working on are generally in an unglaciated portion of the mid-atlantic region of the US and so a relatively shallow residual soil profile (8-18 ft) overlying softer sedimentary rocks is very common. I usually find that the average N value determined using the thickness weighted average N value and the soil profile name in table 1613.5.2 are in better agreement than N (bar) determined with Equation 16-42 and the soil profile name in table 1613.5.2.

I guess if the code council wants to use Equation 16-42 for the determination of N(bar) and use N(bar) to determine the seismic site class, that is fine with me, but either clarify what average is being represented or omit the term average from Table 1613.5.2. Any insight on what equation 16-42 actually represents and why the average is not the thickness weighted average N value would be most appreciated ;)

RE: IBC Chapter 16 - Determination of Average N value for Seismic Site Class Determination

As an update, this discussion with a colleague jogged his memory regarding the difference between the equivalent hydraulic conductivity for horizontal flow through a stratified medium vs equivalent hydraulic conductivity for vertical flow through a stratified medium. Applying this analogy to shear wave propagation, the IBC equation makes sense.

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