Buildings may shake, but won’t fall: engineers

The fact that structures easily withstood last month’s brief
earthquake is a testament to the high standards of construction in Cayman, according
to engineers.

The fresh images of the devastating
12 January earthquake in Haiti that flattened its capital Port au Prince prompted
many in Cayman to race out of their buildings in last month’s seconds-long
tremor, fearing the structures would collapse around them.

But construction experts say that
the likelihood of buildings collapsing, as seen in Haiti, is unlikely here.

Emerson Piercy, chief building
control officer at the Planning Department’s Building Control Unit, said
Cayman’s buildings and structures were constructed to, and usually above, the
local minimum requirements to withstand earthquakes, as set out in the Building
Code.

He said Cayman is considered in a
moderate earthquake zone.  By comparison, parts of California and Japan
are considered to be in high earthquake zones.

While measuring the size and
magnitude of an earthquake is simple, measuring the pattern of shaking, or
ground acceleration, is more complex. Predicting how and to what extent the
ground will shake is vital in designing structures that can withstand a quake,
and local building codes need to take into account the number of faults in an
area and their distance from a location; the types of faults; how often an
earthquake is likely to recur; and the predicted magnitudes.

The near surface geology and
construction materials and techniques are also taken into account when drawing
up a building code.

Mr. Piercy said it was not just
offices and homes that were built to withstand earthquakes locally but also
transmission towers and hydraulic structures.

“The code does not address design
criteria for specific magnitude earthquakes; rather it gives design
coefficients for buildings based on regional seismic zones, seismic hazard
exposure groups and seismic performance categories.  One such coefficient
that is used is the peak ground acceleration, which is a measure of earthquake
acceleration on the ground. 

“The PGA it is not a measure of the
total size of the earthquake, but rather how hard the earth shakes in a given
geographic area,” he said.

Mr. Piercy said most people go
above and beyond the minimum requirements of the Building Code, adding that it
was unlikely Cayman would, in an earthquake, experience the extent of damage
and collapsed buildings seen in Haiti.

“In most cases, buildings here
supersede the minimum that the code requires… historically, even in
structures that are exempt, those have been built to a higher than minimum
standard. You’ll notice that in this recent 5.8-magnitude quake, there were no
reports of any significant damage to structures.

“Even in 2004, with the 6.8
magnitude earthquake, we didn’t hear of anyone having any serious issues,” he
said.

He said most structures in Cayman
are of reinforced concrete. “Looking at the photos of the rubble in Haiti, you
don’t see any reinforcing steel or any concrete-filled block cores,” Mr. Piercy
said.

Although a large sinkhole appeared
in South Sound in the quake, there were no other reports of damage following
the 19 January 5.9-magnitude earthquake in Cayman.

Predicting the frequency of
recurrences of earthquakes in a particular area is a notoriously inexact
science, but a US-based firm, ABS Consulting Engineers, commissioned by a local
engineering company to formulate an equation for Cayman, has come up with
figures that suggest that Cayman is likely to experience an earthquake similar
to last month’s once every 36 years.

David Champoux, senior engineer of
Apec Consulting Engineers, which hired ABS to do the seismic risk assessment
study, said: “The seismologists who looked at this particular event said this
occurrence is once every six years within the Caribbean faults and once every
36 years for Cayman.

“It does not mean that every six
years it would necessarily happen close to home. Quantitatively, we’re in about
one sixth of the whole area, so if it happens in the region every six years, we
can expect it here every 36 years.”

According to the study, the maximum
possible earthquake magnitude for Cayman would be 7.75 and an earthquake of
that size would be estimated to occur on average every 2,870 years.

Mr. Champoux said that the planning
regulations and building code in Cayman, along with the higher standard of materials
used here, meant it was unlikely buildings in Cayman would collapse in the
event of a high magnitude earthquake.

The study by ABS showed that the
acceleration forces felt in George Town in the 2004 6.8-magnitude earthquake in
Cayman were 75 per cent of design accelerations required by the Cayman Islands
Building Code.

It also showed that the likely
average frequency of recurrence of a 6.8 quake like that in 2004 was 440 years
for the Swan Islands Fault Caribbean wide.

Ian Washbrook, principal engineer
at Halcrow Yolles International Engineers, agreed that the way buildings are constructed
in Cayman means collapse is unlikely.

 “We are in a fairly moderate zone of
seismicity, not a high zone like California, or Mississippi valley or Missouri.
Grand
Cayman is relatively close to the North American/Caribbean transform fault but
not located right on top,” he said.

“Even if we had 7.0-magnitude, we
would not see collapsed buildings like we saw in Haiti. There would be damage,
but not necessarily collapses because we have better quality structures when it
comes to construction and design,” he added.

TOPhaitiearthquakeSTORY

A woman walks among debris in Port-au-Prince 14 January. Buildings collapsed because of poor construction.
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