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UWI TODAY
– SUNDAY 9 SEPTEMBER, 2018
EARTHQUAKE
Dr. Richard Clarke
PHOTO: ANEEL KARIM
As we seek to assure appropriate
seismically resilient
communities there is a particular hurdle to be
overcome. This concerns a critical lack of awareness of
what a building code provides to property owners and
end-users in terms of losses. Appropriate seismically
resilient communities are those that implement the
latest scientific and technological advances in the
design and construction of physical infrastructure in
consideration of all the costs involved. Such advances
are passed on to society via the latest building codes
and any property owner or occupant should be
keen to know of same so as to maximize safety and
minimize costs. Hence there is a vital role to be played
by property owners, end-users, and the population as
a whole.
The following statement is invariably shocking at
first and there seems to be no easy way to introduce
it. When the earthquake a building is designed to
resist occurs, there will always be losses in terms of
casualties, damage, and functionality for a period of
time (i.e. downtime).
Therefore for any town, there will always be a
risk that medical and other services or resources
required for recovery operations will be overwhelmed.
However, it is at first believed by property owners
or end-users that not only should there be no losses
but that this is the engineer’s job. Engineers truly
wish this were possible but it is not possible due to
the way nature operates. The issue is this: when a
structure vibrates due to the vibration of the ground,
the damage imparted to the structure depends on the
sum of the forces at each instant of time during the
vibration, and not just the maximum force and it is
literally impossible to predict what that will be (viz.
the technical term is “record-to-record variability”).
So when your engineer designs your structure for
a certain level of force, there is always a chance that
the actual force (hence damage) will be higher. This
is often observed after an earthquake; say there are
10 identical houses on a street , one will collapse and
another will not have a single crack.
This implies that expected losses are to be
considered by the populace who must then decide if
they are acceptable to society. Therefore acceptable
losses represent the basic safety, in monetary terms,
that should be built into the structure. Such acceptable
losses also represent a benchmark that can then be
made part of local design and construction policy.This
benchmark represents basic safety but, of course, a
“The expected number of casualties due
to collapse of concrete buildings or its
components is 50 to 100 persons per 1000
buildings. This is for buildings designed
and built to code. If the buildings are not
designed and built to code, the number of
casualties is about 10 times higher.”
WHAT ISACCEPTABLE LOSS?
B Y R I C H A R D C L A R K E
1000 buildings.This is for buildings designed and built
to code. If the buildings are not designed and built
to code, the number of casualties is about 10 times
higher. The economic cost due to building damage is
about 20 to 30 percent of the replacement value of the
building and roughly 15 times the cost associated with
the casualties for buildings built to code.
Note that these losses are the expected losses given
the latest building codes but the question remains
open as to the acceptability of these and other specific
levels of losses for Trinidad and Tobago. As would be
expected, a higher level of safety hence lower losses
requires providing more resilient buildings. Some
may consider it reasonable that a more developed
country should have lower acceptable losses than a less
developed country for its basic safety provisions. As a
first step the citizenry needs to appreciate the concept
of inevitable losses even if the building is designed
and built to code. Then, in order to provide a policy
on basic safety in the form of acceptable losses, public
consultation is needed. The aforementioned expected
losses are for the region of southern California, USA.
Adecisionwasmade by the Association of Professional
Engineers of Trinidad and Tobago in 1978 to adopt the
building codes in use inCalifornia for local application.
To simplify the decision-making process, the question
can be phrased in terms of what percent of this
benchmark should be adopted for local application.
A level of acceptable loss below 100 percent means
setting a level of basic safety for Trinidad and Tobago
that is higher than that for California and a level above
100 percent means a lower level of safety. Ultimately,
a survey of the public can be conducted and possibly
phrased as follows: “select from the following list,
relative to California, what should be the Trinidad and
Tobago policy on acceptable losses: 80, 90, 100, 110,
120, 130 percent”.
It is envisaged that representatives of residential
districts, commercial, and governmental properties,
will make the final decision. To facilitate the process, an
internet-based questionnaire can be readily prepared
and activated for a certain period andwhen that period
elapses, the responses are analyzed and the results
presented for discussion by these representatives, other
stakeholders, and authorities responsible for policy-
making. Such input will eventually be vital for the
local building code development personnel, practicing
engineers, disaster managers, and development
economists.
Dr. Richard Clarke is a Structural Engineer in the Department of Civil and Environmental Engineering at The UWI St. Augustine Campus.
property owner or end-user can specify a different but
lower level of acceptable loss and instruct the engineer
to design the structure to suit. That is, the engineer is
instructed on the maximum number of casualties, the
maximumrepair cost, andmaximumdowntime for the
building. The owner specifying the acceptable losses,
and the engineer providing a structure to suit these
losses, is the core of the state-of-the-art of building
design, called “performance-based design,” and is the
latest approach to safety under earthquakes.
At this point it is prudent to provide examples
of the extent of losses associated with basic safety
provided by the latest building codes applied to
an earthquake prone region sufficiently similar to
Trinidad and Tobago. For example, the expected
number of casualties due to collapse of concrete
buildings or its components is 50 to 100 persons per
