Representation of Bidirectional Ground Motions for Design Spectra in Building Codes

[admin]

Representation of Bidirectional Ground Motions for Design Spectra in Building Codes

The 2009 NEHRP Provisions modified the definition of horizontal ground
motion from the geometric mean of spectral accelerations for two components
to the peak response of a single lumped mass oscillator regardless of direction.
These maximum-direction (MD) ground motions operate under the assumption
that the dynamic properties of the structure (e.g., stiffness, strength) are identical
in all directions. This assumption may be true for some in-plan symmetric structures,
however, the response of most structures is dominated by modes of vibration
along specific axes (e.g., longitudinal and transverse axes in a building),
and often the dynamic properties (especially stiffness) along those axes are distinct.
In order to achieve structural designs consistent with the collapse risk level
given in the NEHRP documents, we argue that design spectra should be compatible
with expected levels of ground motion along those principal response axes.
The use of MD ground motions effectively assumes that the azimuth of maximum
ground motion coincides with the directions of principal structural
response. Because this is unlikely, design ground motions have lower probability
of occurrence than intended, with significant societal costs. We recommend
adjustments to make design ground motions compatible with target risk levels.

Hidden Content

Give reaction or reply to this topic to see the hidden content.

Hidden Content

Give reaction or reply to this topic to see the hidden content.