IBC 2000/2003 Response Modification Factor R for dual systems

[prsconsultant]

IBC 2000 / IBC 2003 Table 1617.6.2 shows a building response modification factor "R" for a dual system of IMRF (I assume it could be of either steel or concrete
material) with Ordinary Reinforced Concrete Shear Wall as 5.5.

In the same table it shows, R = 5.0 for an Ordinary Reinforced Concrete Shear Wall, R = 5.0 for a concrete IMRF and R = 4.5 for a steel IMRF.

Why is that the R value of the dual system higher than those of the individual systems?

Thanks

 

[WillisV]

Dual systems have more redundancy built in - the moment frames are required to carry a minimum of 25% of the story shear even if their actuall stiffness does not attract that much.

See: thread507-162685