The component response modification factor, rp, represents the ductility which means the energy absorption capability of the component considered in design.
Although the modification factors just described accomplish a transformation of allowable stresses to the earthquake strength limit state, it is not conservative to ignore the provisions in the standard as well as the supplementary provisions in the appendix that deal with design or construction issues that do not appear directly related to computation of equivalent loads, because the specified loads are derived assuming certain levels of damp- ing and ductile behavior. In many instances this behavior is not necessarily delivered by designs conforming to conventional standards, which is why there are so many seemingly “nonload” provisions in this standard and appendix.
This factor is based on judgment with respect to the ductility following benchmark values and used to reduce seismic response in evaluation by modified elastic allowable against plastic collapse or fatal leakage.
rp = 1.0 or 1.5: brittle or buckling failure mode is expected
rp = 2.5: some minimal level of energy dissipation capacity
rp = 3.5: ductile materials and detailing
rp = 4.5: non-ASME B31 conforming piping and tubing with threaded joints and/or mechanical couplings
rp = 6.0: ASME 31 conforming piping and tubing with thread joints and/mechanical couplings
rp = 12.0: highly ductile piping and tubing joined with brazing or butt weld
ASCE 7 Table 13.6-1 Seismic Coefficient for Mechanical and Electrical components
The piping system may have higher ductility than supporting structures. For example, if rp for piping and pipe rack assumes to be 12 and 3.5 respectively in design, the load multiplier is 12/3.5=3.43. Or, both are assumed to be same as 12 in design, the load multiplier is 1.0.
ASME B31E requires ductile piping system refers to a piping system where the piping, fitting,and components are made of materials with a minimum elongation at rupture of 15% at the temperature concurrent with the seismic load. And para 3.4, simplified elastic method is applicable when rp is less or equal to 3.5, which is the minimum ductility expected in piping system in ASME piping.
Otherwise, para. 3.5, the alternative design methods such as fatigue, plastic, or limit load analysis may be applicable. ASME B31E is not mandatory in the code at present.
The gaps in the piping system is another dynamic effect to be considered when the impact load applies to support from piping, and usually it is multiplied by DLF. And DLF for non repeated impact load is 2 as maximum.
In ASME B31.3 301.5.3 Earthquake, the effect of earthquake loading shall be taken into account in the design of piping. The analysis considerations and loads may be as described in ASCE 7, but B31 is not clearly updated in the earthquake design methodology for ductility.