In ASCE 7 it is a factor applied to seismic loads when certain low-redundancy conditions are met.
For ASCE 7-05: For structures assigned to SDC D, E, and F, ρ=1.3 unless one of the following two conditions is met, then ρ=1.0 (12.3.4.2):
1) Each story resisting more than 35% of the base shear in the direction of interest shall comply with Table 12.3-3.
2) Structures that are regular in plan at all levels provided that the seismic force-resisting systems consist of at least two bays of seismic force-resisting perimeter framing on each side of the structure in each orthogonal direction at each story resisting more than 35% of the base shear.
(The number of bays for a shear wall shall be calculated as the length of shear wall divided by the story height or two times the length of shear wall divided by the story height for light-framed construction.)
ASCE 7-10 changed slightly on the redundancy factor but is more or less the same.
Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)
Note that there are also certain parts of the design where the redundancy factor is 1.0:
a. Structures assigned to SDC B or C.
b. Drift calculations and P-delta effects.
c. Design of nonstructural components and design of nonbuilding structures that are not similar to buildings.
d. Design of collector elements, splices, and their connections for which the load combinations with overstrength factor of 12.4.3.2 are used and any member where those overstrength load combinations are required.
e. Diaphragm loads determined using Eq. 12.10-1.
f. Structures with damping systems designed in accordance with section 18.
Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)
