.7 Reduction to Cs Value for ASD Approach

[theclipper]

If the seismic response coefficient (Cs) is calculated per ASCE 7 12.8.1.1, where is it allowed by code to take a .7 reduction if utilizing an ASD approach?

And if I do utilize that reduction, am I to input the reduced seismic load into the load combinations for ASD in 12.4.2.3, which reduce it again by .7, or .525?

I've never had a good understanding of where that .7 came from, so have never utilized it for ASD design. I'm questioning now if that is overly conservative. Perhaps some reference literature on the code derivation of the Cs value would be helpful, but I can't find one at the moment.

Thanks for any help or direction.

 

[OHIOMatt]

The loads, as calculated by the formulas in the code generate factored loads for use in LRFD. The .7 takes the "factor" out to a service level. You only get to do it once.

 

[JoshPlumSE]

Just to clarify:

The purpose of the 0.7 fact is just as OhioMatt suggests. It is to reduce the factored seismic loads down to a service level load for use with ASD design.

However, in the cases where the code ASD load combinations show a 0.525 factor on the earthquake load combinations this is a combination of TWO factors. You've still got the 0.7 factor to get the seismic force down to ASD levels. But, there is an additional 0.75 factor which accounts for the low probability that two transient loads will act at the their maximums at the same time. This is why you will only see the 0.525 = 07*0.75 on load combinations that include seismic + Live load.