SEISMIC ENGINEERING: BUILDING EARTHQUAKE-PROOF
VMB
is a Chilean engineering company with a staff of 40 to 50. It specializes in building for earthquakes:
Quake-proof construction. Rodrigo Mujica is clearly proud of the work he and his colleagues.
On May 20, 2010, Rodrigo gave a presentation at the University of Portsmouth.
Foster and Partners had invited
Rodrigo to give a presentation at its headquarters in London, and that is how Rodrigo happened to
be in the neighborhood. Below is a summary.
One of the first things Rodrigo Mujica showed us was a video taken by a surveillance camera on
the 12th floor of a building that was at quite some distance from the epicenter. It was impressive to see people
being propelled from side to side as if on a very rough airplane ride and holding on to some of the building's
fixed structures.
Chile gets a great deal of earthquakes because of the Nazca plate subducting off and
under its coast. In the south of the country, the Nazca plate dips steeply into the mantle;
to the north, its flexure is considerably less.
The strongest earthquake ever recorded occurred
in Chile in 1960. Its magnitude was 9.5 and it appears to have released 54% of all seismic energy released
in the entire 20th century. (Or, 54% of all seismic energy released in the entire 20th century
was released in Chile; that was not entirely clear, but it does not matter much.)
The recent Haiti earthquake's magnitude was 7.0 whereas
the recent
earthquake in Chile had a magnitude of 8.8. The scales are logarithmic, hence much more energy was released in Chile,
but there was much more damage in Haiti (which had no building code, according to information tweeted from
a USGS seminar held
on May 21, 2010) than in Chile. On top of that, what happened in
Chile on 27 February was not one but two earthquakes, according to Rodrigo.
Approximately one minute after the shaking started, an earthquake occurred which
had Concepción as epicenter. Forty to sixty seconds later, the second one occurred at Curicó,
183 kilometers south of Santiago. This complicated the wave pattern and buildings had to take stresses
from more directions than usual.
At Concepción, both the maximum horizontal and the maximum vertical acceleration were
0.65 g (g as in gravitational acceleration). At Santiago, the maximum horizontal acceleration was
about the same as at Concepción the vertical acceleration was 0.28 g which is still substantial.
Chile's buildings - including VMB’s - did pretty well. One - not VMBs - was so badly damaged
that it collapsed. Several - also not VMBs - will need to be demolished, but did not collapse.
(The pictures shown to us showed very little damage to windows, by the way,
which is a result of the use of shear walls). The damage that did occur in Chile was mainly to older
buildings and to adobe houses.
Shear walls
In Chile, earthquake-proof construction of tall buildings starts with the use of shear walls
instead of columns and beams. A shear wall can take a lot of stress and can crack and deform but
will not collapse. All walls in such a building are structural and go all the way through (top to bottom).
Also, there
are many walls inside the building. Together, they usually provide three defenses in all directions.
That is, if it is up to the engineers. They often end up in battles with architects who according
to Rodrigo, prefer beams, columns and lots of open spaces.
Basement isolation
A more sophisticated yet simple technique to be added to the use of shear walls is to place the building
on rubber dampers, quite similar as applied to sensitive scientific equipment
to prevent vibration from traffic and slamming doors.
Large rubber dampers are placed between two plates of steel on top of which the
building is constructed. The rubber dampers absorb most of the shocks in an earthquake. They needed to be
replaced more frequently in the past, but last forty years nowadays. Replacement is easy:
You jack up the building, get the old dampers out and slide new ones in.
The advantages will be obvious. The three disadvantages are: 1) increased construction cost,
2) can only be done with relatively low buildings and 3) it results in a more complex structure.
This was the sort of thing I already expected to hear about at this presentation, but
there are two more techniques, even more beautiful.
Energy dissipation
For taller buildings, they can use a metal plate construction that takes up the building's
movement during a quake. The plates yield and the energy gets dissipated. After a quake,
this structure needs to be replaced because it has yielded.
Two disadvantages: 1) increased construction cost and 2) requires maintenance.
Tuned mass dampers
A tuned mass damper (TMD, also known as active mass damper or harmonic absorber) is
like a pendulum that swings inside the building with the same period, but in the opposite
direction. It is made of suspended heavy concrete blocks, or lead. The design was previously used
to protect high buildings against wind stresses, but not yet against earthquake stresses.
TMDs reduce the acceleration, the velocity and the duration of the shaking. They also
reduce torsion to floors, and damage to non-structural elements.
The Boston Globe has a nice video of the tuned mass damper in Boston's Hancock building (wind protection).
Prior to the installation of the two 300-ton weights on the 58th floor of the Hancock, tenants
complained of motion sickness during high winds.
On wolfram.com, you actually get to
toy with a TMD.
One big advantage of the use of a TMD is that it greatly limits the damage inside the building,
such as computers falling from desks. During the recent quake, only sheets of paper fell off
the desks in buildings protected by TMDs.
There can even be a damper on the TMD - a damper on the damper -
which decreases displacements and duration further.
Its two disadvantages are 1) increased construction cost and 2) it must be controlled
(usually done by computer). High-tech CCTV cameras can be deployed.
It's based on Newtonian laws. You start off with an equation for motion and use
transfer functions to arrive at a solution, in which the period of movement of the
building and the period of movement of the damper cancel each other out.
(I should have taken a photograph of that slide!)
It is a wonderfully elegant idea, this TMD, and the model visualization of an
office building's response with and without TMD - at a factor 10 for clarity's sake -
was very impressive.
Retrofitting
Shear walls can be added to any building. Putting in dampers is more difficult,
but not impossible. Walls may need to be fortified. A building in China is now going to
apply the TMD principle to its top floors where it has all sorts of machinery for the building.
Building code
The presentation did contain a portion that was less impressive but not related to
VMB and only less impressive in one aspect: the time it takes Chile to come up with revision of its
building code is excessively long. Chile's seismic engineering is - obviously - quite
good, but the country does not have that many seismic engineers and not many of those are involved
in devising and revising the building code.
In combination with extensive discussion, this resulted in Chile's first building code
to be issued twelve years after the earthquake of 1960. The 1985 earthquake led to a
second building code in 1996. The first one was mainly based on European practices,
the second one on the U.S. way of building. The next big quake is expected around 2035.
(That said, the northernmost part of the country has been waiting for a big one for quite some time.)
I see career opportunities and bright futures for young Chileans here. Read on.
Market-driven innovation
Believe it or not, many of these developments are market-driven, not motivated by humane considerations.
"People are spoiled.&qupt; They see that their neighbor's house suffered less damage and they
want the same. Mere survival no longer suffices.
Structural damage to other buildings
Finally, below are two images of structural damage to other buildings.
The lower image shows a collapsed floor.
Below, damage to industrial buildings; the lower one was a warehouse.
Disclaimer: Angelina Souren and SmarterScience cannot be held liable for any decisions made on the basis of information
given on this web page.
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