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Caltrans Seismic Design Criteria & ARS curves

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KSRAO

KSRAO

Structural
Nov 11, 2002
9
As many of you may know, in California state, design of standard bridges for seismic loading is done based on "Siesmic Design Criteria (SDC)" developed by CALTRANS. Essentially, the procedure it uses is based on deterministic ARS approach and a set of standard ARS curves to be used to calculate the seismic laoding (for different soil profile types (soil profile type B to E)and Earthquake magnitudes) are given in "APPENDIX B" of the SDC.

My question is:

In case of soil profile types C,D&E, Why 2 peack ground acceleration values (one within the parentheses and one outside) are given for each ARS curve?

A note in these graphs says that the peak ground acceleration (PGA) value given outside parentheses is for soil type B and the value inside the brackets is for the given soil profile (C, D or E as the case may be).

Still, it's not clear to me why 2 PGA values are given. Wonder if anyone could through some light.

Many thanks in advance.
PS: Sorry, if my question was ambiguous.

-thanks.



 
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THROWING light.
Earth quakes can be measured using two scales: intensity and magnitude.

The magnitude of an earthquake is a measure of the total energy released by an earthquake. Not a very good measurement for engineering design.

The intensity of an earthquake is the measure of seismic effect at a particular local. A very good measurement for engineering design. The biggest determinant of an earthquake's effect for a given location is the type of soil your sitting on--i.e. what type of motion will be produced.

In sum, you need to look beyond acceleration. Good luck.
 
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koodi, thanks for your response.

As you rightly said, yes, ARS value depends on EQ magnitude, PGA and soil profile type (A/B/C/D/E or F). And hence there are different sets of ARS curves corresponding to different soil profiles.

But my question was: why is there a mention of soil type B in reading the ARS value for soil profiles C, D and E from their correspoding ARS curve?

Hoping to hear more comments, especially from those who are familiar with CALTRANS procedure.

Thanks again.

-Rao.
 
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Rock is the soil type that has the least localized effect on an earthquake's motion. The other ARS curves are adjusted for the localized soil type. All of this analysis is highly subjective. Caltrans is just giving you a value to compare the effect of soil types. If you'll note the soil values "not in parentheses" are repetitions of previous ARS curves for soil type B.

So far as Caltrans workers responding, don't hold your breath. Not many Caltrans employees can write in English--regardless of whether they are native English speakers.

Or maybe that has just been my experience--anyways, it felt good to vent.

adios, ciao, bis bald, au revoir, and good night
 
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