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1) Zone factor / Seismology
Generally structural engineers do not need much understanding of seismology other than when a time
history analysis is required. Even topics such as attenuation, frequency, shear wave velocity, peak
ground acceleration etc are concepts practitioners need to know little about provided they are given an
easily defined structural spectrum from which they can carry out the design.
The zone factor Z is the equivalent peak ground acceleration (pga) on rock for the location under
consideration. In most codes the benchmark is the 10% probability that an earthquake of this size will
occur in 50 years (equivalent to a return period of 475 years). The generally accepted range for the pga
on rock in Hong Kong varies between O.OTg and 0.12g. This value tends towards a low level of
earthquake risk rather than a medium level.
Recently the Chinese map shows Kowloon and New Territories as 0.1 g and Hong Kong Island as
0.15g. see Fig. 1 (Gao Mengtan 2002). A finer zoning would be more appropriate in order to reduce
the difference between the two areas see Fig. 2 (Pun et al.2002). However "Drawing lines on maps, in
social conduct, in religion, in law itself, has not proved one of humankind's greatest skills. Misery,
expense and death are the usual outcomes," (Bill Mantle Canberra Times 15/8/98). It may be more
pragmatic to adopt a single value for Hong Kong. In the authors opinion because of the many
multiplying factors of this basic parameter it would be best to adopt a low value of say 0.8 for HK.
Fig 1 (Gao Mengtan 2002)
Illustration of seismic hazard in Hong Kong
Fig 2 (Pun et al.2002)
10% in 50-year pga bedrock contours
2) Importance factor
The Importance factor I can vary between 0.8 and 1.5. There may be a case to use this factor to
provide a higher level of protection to properties on Hong Kong Island as compared to the New
Territories for economic reasons rather than to use variable pga or Z factors in HK.
3) Dynamic factor
The Dynamic factor C is a function of the structural period and the frequency characteristics of the
earthquake. The latter are highly dependent on the soils at the site; soft sites tend to amplify longer
period motions, while on rock sites most of the energy is at short periods. C is therefore a function of
the soil characteristics at the site, as well as the period of the structure being considered. C varies
similar to the structural response of the building discussed under the building period & structural
response spectra section.