by Mike-SE » Sun Sep 22, 2013 8:17 am
No factor of safety. Factor of safety is used in stability and strength. (I want you to use the proper terms)
However, the design earthquake force uses the factor "R" to determine the force. R is always greater than 1.0. An R=1.0 translate to a design force equal to 100% of the estimate earthquake force. An R=3.0 translate into an earthquake force of 1/3 of the actual force.
When you check the drift of the building using the force that has been reduced your estimated drift is the drift caused by the reduced force not the actual earthquake.
So, we multiply the calculated drift by the R factor to determine the actual movement of the building.
On a ten story building with an R of 3 and a the calculated drift of 5" the actual drift for checking building separation would be 15" !!!!!! BUT THIS IS NOT A FACTOR OF SAFETY!!! It is reality.