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metres is about 1 hertz yet measurements on real buildings varied between 0.5 hertz and 2 hertz. If this
variability is used to determine the base shear from the acceleration spectrum response curves in fig. 6
the results can vary between 0.05g and 0.3g. This order of variability in loads exceeds by far that
normally encountered by structural engineers. Fig. 6 shows that there is considerable variability in the
different spectrum response curves in terms of acceleration up to one second. When the displacement
curves are considered in Fig. 7 it is the 3 to 6 second period range that appear to be more variable.
HONG KONG SEISMIC CODE REQUIREMENTS
Analysis, P-delta, time history etc.
Techniques such as push over analysis and non-linear analysis help to identify hinge locations but as
yet these tools rely heavily on the accuracy of the structural modeling and the correct choice of many
variables. Generally the requirement in codes to use time history is not particularly well understood by
practicing engineers and appears to be more of academic than practical value. A significant amount of
judgment is needed and codified guidance is seldom provided to enable practitioners to apply time
history analysis confidently. It may be better to avoid these complex analysis methods for routine
projects in HK.
P-delta, accidental eccentricities, allowable overall and inter-storey drifts and building separation
required to avoid pounding of adjacent buildings will need to be clearly explained in the HK code to
avoid ambiguity or incorrect application by practitioners.
Detailing rules and Ductility
In all the seismic codes confinement reinforcement of potential hinge points is required and one of the
most significant decisions to be made during the schematic design stage is the choice of which specific
detailing rules should be adopted. If options are permitted in the HK code for high, moderate or no
special detailing then it is likely, for the majority of buildings, to be more economical to adopt the least
stringent detailing rules. However it may be better to allow only low levels of ductility detailing for
routine designs.
Large quantities of reinforcement could be saved if nominal ductility reinforcement could be
minimized by not requiring it at all beam column junctions. For instance is it necessary to have
ductility detailing for columns where small beams frame into large columns High-rise structures may
only require ductility detailing of columns for the lowest three stories or so (much like shear walls) and
when the column size reduces or the reinforcement changes. Further high-rise buildings will be
controlled by stiffness due to wind loads rather than by seismic considerations and so a blanket
ductility-detailing requirement would be wasteful of steel reinforcement.
Particular care needs to be taken for items such as transfer floors, soft stories and other locations where
high ductility demand can be expected. Providing additional capacity or higher ductility levels at these
locations may be more economical than increasing the design loads or ductility of the building as a
whole.
Redistribution of moments is allowed by codes. It is probable that where redistribution of moments has
been adopted the clarity of the design will be compromised and the standard of confinement
reinforcement needs to be increased due to an expected higher ductility demand. Redistribution of
moments should be maintained hi codes for elements such as slabs and secondary beams in order to