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

[CJ255]

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.


Please enlighten me

 

[HTURKAK]

This subject has been discussed so many times..

I will suggest you to look thread507-479291 and search the site for more threads ..

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

 

[driftLimiter]

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.