ASCE Design Spectral Acceleration SD vs Site coefficients & MCER Spectral Response Acceleration

Question

ASCE Design Spectral Acceleration SD vs Site coefficients & MCER Spectral Response Acceleration

CJ255 (Mechanical)

(OP)

2 Jun 22 05:13

Hi,

I cannot wrap my head around the idea of reducing the maximum considered earthquake and to be used for designing structures.
SDS = 2/3*SMS & SD1 = 2/3*SM1

Normally, if we were to talk about safety factors, we would increase the expected loading to be designed. For example a lifting lug would be designed for 100 tonnes to lift a 50 tonnes object.
With this understanding, mainly I have 2 confusions

1. How could we reduce the maximum considered earthquake? Should we not design even above the maximum earthquake to achieve the maximum safety?
2. SDS and SD1 are arrived essentially from S1 and SS. If we can design by reducing the maximum considered earthquake, why did the USGS or other parties not just provide us with a lower S1/SS values?

I had reread this paragraph (Link) multiple times that seems to be explaining my confusion but I still could not grasp the idea of designing not based on the maximum earthquake considered.

Quote (Traditionally, for seismic design, engineers on the U.S. west coast have used a ground acceleration, with a 10% probability of occurrence in 50 years corresponding to a 475-year recurrence interval. In coastal California, a 2500-year earthquake is considered the largest possible earthquake and, it is the considered opinion of the engineering community that a building with proper seismic details designed for an earthquake of 475-year recurrence interval, has a margin of safety of 1.5 against collapse in an MCE event. In other parts of the United States, however, notably in the New Madrid fault area, a 2500-year earthquake may be as much as four-to-five times the 475-year earthquake. Therefore, a building designed in California for a 475-year earthquake has a good chance of not collapsing under a 2500-year earthquake, whereas its counterpart in the New Madrid area may not have this chance. To keep a uniform margin against collapse, the IBC uses a 2500-year earthquake spectral response acceleration for all the areas of the United States. To bring the design up to par with the current practice of designing with a 1.5 margin against collapse, a 2/3 value (the reciprocal of 1.5) of the MCE is used in design. This is the rationale for taking the 2/3 values of SMS and SM1 to arrive at the design response accelerations SDS and SD1.)

Please enlighten me

HTURKAK (Structural) · Answer 1 · 2022-06-02 14:12:59

This subject has been discussed so many times..

I will suggest you to look thread507-479291: Building operational after seismic event and search the site for more threads ..

If you have further questions , or a specific case , you may post ..

driftLimiter (Structural) · Answer 2 · 2022-06-11 00:03:24

Basically there is a bunch of statistics that are used to develop a risk level. The community has established levels of acceptable risk for different types of structures. The 2/3 factor means we are designing for a more frequent earthquake. Its economy, risk tolerance, and statistics that drive this. Don't forget that we design buildings for damage at this level earthquake (generally) energy dissipation and ductility allow us to have buildings that fail safely so its not as simple as a lifting lug and the factor of safety associated with that. In that example you need to realize that we allow for energy dissipation so for the lug we would be designing well into the post-elastic range if you want to make the analogy to earthquakes.

Just a note, the UFC (us military building design standard) does not use the risk targeted earthquake and instead design for the MCE.

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ASCE Design Spectral Acceleration SD vs Site coefficients & MCER Spectral Response Acceleration