PGA for liquefaction analysis

[nbr1]

What is the correct PGA for seismic analysis?

1. (Ss/2.5) x Fa as indicated in ASCE 7-02 and 7-05

or

2. Sa/2.5 as indicated in the 2009 NEHRP Seismic Provisions and IBC.

 

[moe333]

I haven't reviewed 2009 NEHRP in depth yet - I know there are changes with regard to risk targeted values/maximum demand but I imagine it depends on which code governs your project.

ASCE 7-05 will be applicible to 2006 IBC and 2009 NEHRP will be applicible to 2012 IBC.

 

[nbr1]

Code requirements aside; which is more justifiable and/or reasonable for liquefaction analysis?

 

[moe333]

As I said, I haven't reviewed 2009 NEHRP in depth yet. The 2009 NEHRP is a more recent code so it may be more justifiable - but either is probably OK. If I have a choice I wouldn't use either. I would run with both probibilistic and deterministic and make an informed decision between the two.

 

[msucog]

I generally use Sds/2.5. I think Sa is intended to mean SdsxFa but I don't recall if it specifically states the period...I'd have to look back through but I'm confident it's typically assumed PGA~Sds/2.5. Keep in mind the seismic spectrum you get from IBC is not what it really looks like. It's a statistical model to emcompass much of what can be expected so the nice and neat plot has limitations. To fine tune the numbers, I'd follow moe333's suggestion and perform site specific seismic assessment. There's other higher end approaches too. And let's not even mention which data you're using to evaluate liquefaction...soil test borings? geophysical surveys? CPT? other? and lab testing? You can get very expensive which is reasonable in some instances/projects but i guess my point is to recognize that there's always ways to refine the data and model. good luck!

 

[nbr1]

I would argue that PGA is better represented by (Ss/2.5) based on what I've researched than the value indicated in the NEHRP 2009 provisions; which is (SDS/2.5).

It seems to me that the NEHRP incorrectly identifies the PGA (amplified for soil effects) for liquefaction as (SDS/2.5) because the SDS value includes the 2/3 seismic margin appropriate for structural design only.

Still unclear on appropriate value, but I've heard that Section 11.8.3 of ASCE/SEI 7-05 correctly identifies the PGA for liquefaction evaluation as (Ss/2.5), which would then be multiplied by the published site coeficient (Fa) to account for soil amplification effects.

 

[nbr1]

Correction:

I should have indicated the following in the initial posting for bullet item 2:

......

2. The IBC and 2009 NEHRP indicate peak ground acceleration to be used in liquefaction analysis as (SDS/2.5); which I assume would be Amax.

 

[moe333]

I would use Sds/2.5 for a project governed by IBC. 11.8.3 states you can use Ss/2.5 in the absence of a site specific study. A site specific study can be very simple if you want it to be and will result in a lower PGA. Ss/2.5 will be very conservative.

 

[nbr1]

Looks like this is addressed in the new ASCE 7-10.