ASCE7 Site Coefficient F for Site Class D/ Why is it higher than D?

[Sergio75]

We were reviewing the provisions in ASCE7 for a building in Site Class D, with a short period of 1.00. The Fa coefficient is given as 1.1, which is higher than the 0.9 given for site class E, ostentibly a worse soil. This was also observed with short periods greater than 1.25 seconds. This trend contrasts that for site classes A, B, C , where the Fa decreases from Site Class E to Site Class D. This also contrasts the values of site coefficient Fv for all of the periods. Is this a misprint, or can you please provide clarification why this is so.

 

[msquared48]

I don't think it's a misprint. E is a soft clay soil, and I suspect, looking at the progression of the values for Class E in table 11.4-1, that the response of the clay is more for shorter vibrational periods.

Mike McCann
MMC Engineering