ibc 20002003 response modification factor r for dual system

huangyhg

ibc 2000/2003 response modification factor r for dual system
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
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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.