Earthquake risk management of civil infrastructure: integrating soft and hard risks - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 227

2003a, b), structural layout and form, ductility, stiffness, materials and
foundations.

9.2.1 Layout and form

Experience of the performance of buildings in past earthquakes tells us that
certain common-sense ideas are important. For example the elements of a
structure should be well tied together with well-defi ned continuous load
paths with no irregularities. Buildings that are symmetrical perform better
than those that are unsymmetrical. Weak storeys, perhaps required by archi-
tects for open fl oor areas in a building, are a very common reason for col-
lapses. Buildings where the centre of mass and stiffness are not coincident
do not perform well. Plan shapes should be compact. Extensions are prone
to vibrate separately so wide separations are required ( 50 mm per storey)

to allow parts to vibrate separately. Likewise adjacent buildings must be
adequately separated. The distribution of mass within a building is impor-
tant so that high elevations of heavy plant and machinery at the top of a
building should be avoided. Structures should be structurally redundant so
that there is back up if a member fails but this has to be done in a manner
that recognises potential vulnerabilities of form (Agarwal, 2013). Moment
resisting frames with rigid beam column connections can provide good
ductility and redundancy but if poorly designed (e.g. with weak storeys) can
fail catastrophically. Particular attention needs to be paid to the high stress
concentrations around joints. Steel welded joints can be vulnerable and
concrete joints often have congested reinforcement which requires careful
steel fi xing skills and good concreting to ensure proper compaction of the
concrete. Shear walls provide strength and stiffness at low cost and are a
good solution for buildings up to 20 storeys but in taller buildings will prob-
ably need to be combined with other techniques to get suffi cient overall
stability. Special systems to manage the performance of a building during
an earthquake include (a) passive devices to change the period of a struc-
ture or to dampen vibrations, (b) seismic isolation such as rubber bearings,
and (c) active computer control systems to modify behaviour during an
event.

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9.2.2 Ductility

Structures that are brittle fail suddenly - something that no one wants.
Ductile structures absorb energy so that they fail more gradually. Given the
huge uncertainties both in predicting earthquakes and in calculating
response, then the provision of ductility is one of the most effective ways
of preventing loss of life due to earthquakes. Of course, this implies that in
an extreme earthquake some damage to the structure may be inevitable.