Establishing R (or Rw) and Cd Factors for Building Seismic Provisions

This paper derives the basic formulas for establishing the response modification factor R and the displacement amplification factor Cd used in the National Earthquake Hazards Reduction Program (NEHRP) recommended provisions. An extension of the approach is used to derive the system modification factor Rw used in the 1988 Uniform Building Code. These terms are primarily functions of both the structural overstrength and structural ductility factors. Because of the empirical nature of the NEHRP recommended values for these factors, it is difficult to justify the relative values between R and Cd factors for most of the building systems. The structural overstrength factor, which can be determined from analytical studies, depends on structural redundancy, story drift limitations, multiple load combinations, strain hardening, participation of nonstructural elements, and other parameters. Although the ductility factor of an individual structural member can be determined experimentally, there is no general agreement within the profession of how the concept of ductility factor should be applied at the structural system level.