Ss and S1 for 100 years life expectancy 2

[geo321]

Hi
Any ideas how to extract the spectral parameters Ss and S1 for a structure having 100 years design working life ?
It seems that Ss and S1 provided in the asce7-10 are given for 50 years with 2% exceedance probability.
Thanks for the help

 

[JStephen]

The 2% in 50 years is a rate, but doesn't imply a design life of 50 years. In looking through one of my books, I find the author refers to 475-year return intervals and 2,475-year return intervals, corresponding to certain lower probabilities, but that doesn't mean anyone is designing for 2,500 year life-span, either.
Also see Table C.1.3.1b in the commentary.

 

[geo321]

475 corresponds to 10% exceedance in 50 years and 2475 2% in 50 years.
How do you decide which one to use ?? I think eurocode is based on 475 return period and asce7-05 on 2475.
However back to the original question how do u take into account a higher design life span in terms of seismcity?

 

[TLHS]

It depends on your risk profile. What is an acceptable risk? Remember that the chance of exceedance is a life safety level load as well, so the chance of significant damage is higher than that.

If you're doing something with needs other than code level, you have to define the acceptable risk levels, possibly at both life safety and service level.

There is no one right answer to this really broad question. If you're building a fairly conventional structure, it's likely that you can use conventional environmental load levels. You generally reach longer service life lengths by ensuring that you can maintain the structure by providing things like corrosion allowances, more robust sealing systems, maintenance schedules, and similar things.

 

[geo321]

Noted. Thank you.
Does the ASCE provide spectral parameters Ss and S1 based on different exceedance probabilities ? in other words, is there a document stating that for 475 years, the following parameters Ss and S1 apply and for 2475, other parameter values are applicable.

 

[ATSE]

Geo321,
Not sure about ASCE. However, you may find some help regarding variable probability of exceedance --> return period definitions and spectral acceleration mapping for any defined return period in USACE EM 1110-2-6053 (Appendices).
For reference, the 50% exceedance in 100 years (144 year return period) is a common basis for certain load combos for heavy civil structures. 6053 provides a methodology to get the Ss and S1. Includes a couple of helpful examples as well.

 

[chris3eb]

ASCE 7-10 has two seismic levels: maximum considered earthquake and design earthquake. Section 11.4.3 has you calculate S_MS=F_aS_S and S_M1=F_vS₁. These are your MCE spectral response parameters. Section 11.4.4 has you calculate S_DS and S_D1 by multiplying the MCE response parameters by two-thirds. These are your DE spectral response parameters.

I believe that ASCE 7-10 no longer determines the seismic response parameters strictly on a return period basis. The MCE used to correspond to a 2475 year return period and the DE used to correspond to a 475 year return period. It may help to still think about those earthquake levels as having those return periods, and in a lot of cases, they probably do - but there are some areas where there are different return periods.

So to your question, if you wanted to design for an approximate period of 2475 years rather than the DE (approximately 475 years), you could just leave out the two-thirds factor when calculating S_DS and S_D1.

 

[geo321]

Great chris3eb!! Thanks a lot for the info