Behnam Heydari
Senior structural engineer - Citadel Sazeh
The redundancy factor (ρ) is used in relation to seismic load combinations specified in ASCE 7, Section 12.4. It is denoted as ρ and is used in the equation Eh = ρ QE , where Eh represents the effect of horizontal seismic forces and QE is a component or connection force that results from application of horizontal loads.
The value of the redundancy factor depends on the seismic design category (SDC) and structural configuration. It can be either 1.0 or 1.3, and may differ in the two horizontal directions. If a redundancy factor of 1.3 is determined in one direction, it applies to all connections and components designed to resist seismic loads in that direction. However, for drift and p-delta calculations, ρ can be taken as 1.0.
In Structures within SDCs B and C, ρ is taken as 1.0 in each direction. However, for moment frames in SDCs D, E, and F, the allowable story drifts must be divided by ρ. It’s important to note that for structures in SDCs D, E, and F, the redundancy factor is initially 1.3 but may be reduced to 1.0 if it passes either a configuration test or a calculation test.
The configuration test requires the structure to have no horizontal structural irregularities and at least two bays of perimeter seismic force-resisting elements on each side of the building for each story that resists over 35% of the seismic base shear. The number of bays for a shear wall is calculated as the total length of the wall divided by the story height, or two times the total length of the wall divided by the story height for light-frame construction.
In the calculation test, a lateral load-resisting element or connection is removed from the structure to determine if its removal causes an extreme torsional irregularity or a reduction in the lateral strength of the structure by more than 33%. If neither occurs, the redundancy factor can be taken as 1.0. However, if the structure already had an extreme torsional irregularity before the component or connection was removed, the torsional irregularity test fails and ρ must be taken as 1.3.
