Clause 7.2.6 specifies the response reduction factor R, which accounts for the overstrength, redundancy, and ductile energy dissipation capacity of the lateral force resisting system. A higher R value implies the structure can sustain larger inelastic deformations and therefore can be designed for lower seismic forces. The R factor is linked to the detailing requirements of IS 13920.
Key Requirements
•R shall be taken from Table 9 based on the lateral force resisting system
•The chosen R value is only valid if the corresponding ductile detailing requirements (IS 13920) are followed
•Dual systems can use the R value for the system providing the primary lateral resistance
•Using a higher R value mandates stricter ductile detailing per IS 13920
•For structures with vertical or plan irregularities, additional restrictions on R may apply per Table 5 and 6
Reference Tables
Table 9 — Response Reduction Factor R for Building Systems (Clause 7.2.6)
Lateral Load Resisting System
R
Ordinary Moment Resisting Frame (OMRF) — RC
3
Special Moment Resisting Frame (SMRF) — RC
5
Ordinary Moment Resisting Frame (OMRF) — Steel
3
Special Moment Resisting Frame (SMRF) — Steel
5
Ordinary Braced Frame — Steel
4
Special Braced Frame — Steel
4.5
Ordinary Shear Wall (SW)
3
Ductile Shear Wall (SW)
4
Ductile Shear Wall with SMRF (SWSF)
5
Ordinary RC Shear Wall with OMRF
3
Ductile RC Shear Wall with SMRF
5
Unreinforced Masonry (URM)
1.5
Reinforced Masonry
3
Confined Masonry
3
OMRF detailing follows IS 456. SMRF detailing follows IS 13920. Using higher R without matching ductile detailing is unsafe.
Practical Notes
✓OMRF (R=3) is the most common for low-rise buildings in Zone II/III where ductile detailing is not mandated
✓SMRF (R=5) reduces design forces by 40% vs OMRF but requires full IS 13920 detailing — cost trade-off favors SMRF for mid/high-rise
✓Shear wall buildings in Zones IV and V almost always use ductile detailing (R=4 or 5) to keep member sizes practical
✓Unreinforced masonry (R=1.5) is prohibited in Zones IV and V
Common Mistakes
⚠Using R = 5 (SMRF) without following IS 13920 ductile detailing — this is unsafe and non-compliant
⚠Confusing R with ductility factor μ from other codes — R includes overstrength and redundancy
⚠Using OMRF R=3 for buildings in Zone IV/V where SMRF or ductile shear walls are mandated
⚠Not recognizing that URM (R=1.5) is not allowed in Zones IV and V