Abstract
This paper discusses the application of the Structural Performance factor (SP) within a Direct Displacement-Based Design framework (Direct-DBD). As stated within the New Zealand loadings standard, NZS1170.5:2004 [1], the SP factor is a base shear multiplier (reduction factor) for ductile structures, i.e. as the design ductility increases, the SP factor reduces. The SP factor is intended to acknowledge the better-than-expected structural behaviour of ductile systems (both strength, and ductility capacity) by accounting for attributes of response that designers are unable to reliably estimate. The SP factor also recognizes the less dependable seismic performance of non-ductile structures, by permitting less of a reduction (a larger SP factor) for non-ductile structures. Within a traditional force-based design framework the SP factor can be applied to either the design response spectrum (a seismic hazard/demand multiplier), or as a base shear multiplier at the end of design (structural capacity multiplier) – either of these two approaches will yield an identical design in terms of the required design base shear and computed ULS displacement/drift demands. However, these two approaches yield very different outcomes within a Direct-DBD framework – in particular, if SP is applied to the seismic demand, the design base shear is effectively multiplied by (SP)2 (i.e. a two-fold reduction). This paper presents a “DBD-corrected” SP factor to be applied to the design response spectrum in Direct-DBD in order to achieve the intent of the SP factor as it applies to force-based design. The proposed DBD-corrected SP factor is attractive in that it is identical to the SP relationship applied to the elastic site hazard spectrum C(T) for numerical integration time history method of analysis within NZS 1170.5:2004 [1], SP,DDBD = (1+SP)/2.
Highlights
The SP factor was first introduced within the 1992 loadings standard NZS4203:1992 [2] and later carried over into the seismic loading provisions of NZS1170.5:2004 [1]
This paper provides a consistent approach for the adoption of SP into a Direct Displacement-Based Design (DDBD) framework
Within a Direct Displacement-Based Design framework, the SP factor should be equal to 1.0 - the elastic spectral ordinates (5% damped, typically) are reduced by the damping-ductility relationships inherent within the Direct-DBD procedure, and require no further reduction
Summary
The SP factor was first introduced within the 1992 loadings standard NZS4203:1992 [2] and later carried over into the seismic loading provisions of NZS1170.5:2004 [1]. Through substitution of the linear seismic hazard into Equation 1, the base shear can be expressed as Equation 2 as follows, Vb. where Vb is the design strength, me is the effective mass, TD is the corner period defining the displacement plateau on the design response spectrum, and Sd(TD) is the spectral displacement at TD. As previously discussed, when SP is applied to the design response spectrum the reduction in base shear is proportional to SP squared in Direct-DBD when the displacement spectrum is linear i.e. constant pseudo-spectral velocity. The SP,DDBD factor was determined by back-calculating the spectral multiplier (SP,DDBD) that was required to achieve the same design strength if SP were applied at the end of design as a base shear multiplier This exercise was performed for Soil Category A though to E, with or without the full near-fault factor (N(D,T)), and for a structural ductility of = 1.25 - 6.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Bulletin of the New Zealand Society for Earthquake Engineering
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
