[ASCE7-10] Where does S_DS = 2/3* S_MS come from? 1

[NTUCE]

In the seismic code ASCE7-10, the MCE spectral value is specified as S_MS, and DBE as S_DS.

Where does the factor 2/3 come from?

I know it's like a reduction factor, but does anyone know how the value be specified?

 

[KootK]

I know, not very intellectually satisfying for something that has such a large impact on design forces.

 

[NTUCE]

Thank you for your response ))
I'm a bit confused. So in UBC code, when MCE occurs, the building designed with DBE will use up all it's ductility and then collapse?

 

[r13]

I think the strength reduction factors in ASD, and load factors ensure the structure won't collapse before MCE is exceeded.

 

[KootK]

I'm a bit confused. So in UBC code, when MCE occurs, the building designed with DBE will use up all it's ductility and then collapse?

That's right. As you've intimated, when we do code seismic design in the US we're really designing for a particular amount of displacement ductility that we hope will correspond to the maximum considered earthquake. If you're versed in the fundamentals of the method, you'll be aware that it's a rough science and quite forgiving, particularly at higher levels of designed in displacement ductility. To the extent that there's a factor of safety in code seismic design, which I suspect is the source of your confusion, I would say that:

1) In a strict sense, no such factor of safety exists.

2) Such a factor of safety sort of exists in:

a) the forgiving nature of the method and;

b) the R-values that we use.

 

[r13]

Our design actually matches MCE demand, for which, if the capability of the structural element to yield is the sole criteria.

ASD - Design capacity (0.6Fy or 0.66Fy)*1.5 ≅ Fy
LRFD - Factored load capacity (≅ 1.5Fy)*2/3 ≅ Fy

The desirable result is a structure with large deflection, that failing serviceability, but not complete collapse.

 

[HTURKAK]

A complimentary explanation is; Usually, the standard building codes define the seismic hazard for a probability of 10% in 50 years,
and that corresponds a return period of 475 years.The ground motion in ASCE 7 based on a maximum considered earthquake
ground motion MCE, with a 2% probability of exceedance within a 50-year period which corresponds a return period of 2,500 years.
The lower bound based on experience is 2/3 .

Some industrial facilities and special having higher risk potential . Regulations for industrial facilities recommend longer return periods using importance factor such as 1.5 .

This picture is copy and paste from FEMA 451B To Seismic Load Analysis 9 - 1.

http://www.ce.memphis.edu/7119/PDFs/FEAM_Notes/Topic09-SeismicLoadAnalysisNotes.pdf

 

[NTUCE]

Thank you guys, I think I have a better understanding of this topic.