Seismic Fa Soil Class C vs D

[hawkaz]

ASCE7-16
For high seismic load areas, does anyone know why Fa is higher for site class C (1.2) than it is for D (1.0)? This is inconsistent with the other site class values and also inconsistent with prior versions on ASCE7.

I don't see this anomaly addressed in the commentary.




Thanks in advance

 

[Aesur]

The commentary specifically talks about this and notes that this is why they added the note that when the site class is "D-Default" you are to use 1.2 minimum rather than the 1.0; because the site class is not based upon a geotechnical investigation and therefore you should use the worst of C or D for Fa. This gets into the "D-Default versus D" discussion. D-default is used without a geotechnical investigation, whereas if an investigation was performed the site could be defined as D which allows for the lower Fa values. I would assume the Fa values were determined through research/testing, maybe one of the geotechs in here could jump in and talk more on that.

 

[hawkaz]

Thanks- Not checking for the default value. Base this on a soils report indicating site class D.
The Fa is 1.0, where it would be 1.2 for site class C.

 

[driftLimiter]

I mean are you just assuming that it should decrease based on your intuition? Have you considered that the soil site class might impact different areas of the spectral curve differently? If you review the Long-Period site coefficient you see a trend like the one you are assuming should be true for short period as well.

Reading the ASCE7-16 Ch 11 commentary yields much insight into how the factors are developed including many further references one could use to gain more understanding on the topic.

Particularly if you look at the figures in C11.4-2 you will note that for Fa the trend is not well covered matched by a linear function. I.E. there are portions where Fa increases and decreases as you increase the shear wave velocity.

I think it is an assumption to call this an anomaly that comes from lack of understanding of what these factors are and how they are developed.

Like I would recommend to any young engineer hung up on a similar problem. Trust the code, focus on what you know and can control.