R values limitations prescribed in ASCE 7-05 Section 12.2.3.1

[sbouvia]

Hello,

According to ASCE 7-05 section 12.2.3.1 the value of R used for design of any story shall not exceed the lowest value of R that is used in the same direction at any story above that story. I take this to mean that I cannot increase the R value as I go down the building.

My question is that it does not say that I cannot decrease the R value as I go down the building. For example if I have a ten story building and the R value for floors 5-10 is 6 and the value of R for floor ground-4 is 3 is that allowed by the code? If this occurs does a two stage equivalent lateral force procedure described after the exceptions have to be performed? Any help would be greatly appreciated. I would also think that if this is allowed that I would still have to analyze the building as having the lower R-value in order to get the higher brace forces in the lower braces.

Thanks,

Scott

 

[frv]

Get a copy of ASCE 7-10. They've explicitly addressed this in 10, and yes, you use the R factors as you describe. However, the forces transferred from the upper system to the lower system must be increased by the ratio of the R values.