EARTHQUAKE ENGINEERING - A SCIENCE OR AN ART?
Dr. Richard Sharpe
Beca International Consultants Ltd, New Zealand
Is Your Aim to Preserve Monuments?
So you want to preserve your monuments against earthquakes. This is why you are here today.
But against which earthquake? Those of you who have experienced earthquakes know that
whenever the ground or floor we are standing on starts to move, that this may be the start of “the
big one”. Coincidentally I experienced this situation here in Istanbul in mid-September last year
when my colleagues and I were attending a briefing on the Turkish earthquake code at the Civil
Engineering Department of Bogazici University.
Until an earthquake has finished, not one of us can say whether this is the ‘big one’ we were
expecting, or whether this is the pre-cursor to an even bigger one that is going to take place in a
few minutes, days or months time. What we felt may have been generated by a small magnitude
local earthquake, or it might have been shaking transmitted from a large distant earthquake.
I believe that it is appropriate at the beginning of this conference which is considering the
seismic protection of valuable structures to remind ourselves of the unpredictability of
earthquakes. If you think that you have done your job by making sure you have a factor of safety
not less than one for the seismic loads you have chosen, I hope to convince you to think again.
The Choosing of Design Earthquake Loadings
My colleagues and I in New Zealand have been privileged to be trusted with the development of
seismic design codes for the governments of three countries in our highly seismic corner of the
world. I am going to take you through the basic procedures we go through to establish seismic
design criteria. At the end of that, I hope to have convinced you that your emphasis in preserving
important structures should concentrate on :
• making sure that the fundamental mechanisms for resistance of earthquake loads in your
structures are sound, and
• preparing your clients and the public for the day it is demonstrated that your structure is not
earthquake-proof.
Not surprisingly, these are not new ideas. In my country, New Zealand, the Building Act (1991)
governs the way we design structures. With regard to loads (e.g., earthquakes) it says:
Functional Requirement : “Buildings…. shall withstand the combination of loads that they are
likely to experience during construction… throughout their lives” and Performance:
“Buildings…. shall have a low probability of : a) Rupturing, becoming unstable, losing
equilibrium, or collapsing.....throughout their lives,” and (b) Causing loss of amenity through
undue deformation, vibratory response, degradation, or other physical characteristics throughout
their lives…” The first requirement addresses safety, while the second defines acceptable onset
of damage.
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The Code then says that the assumed life of a structure shall be 50 years unless some other length
can be justified. The only other relevant statement it makes is that an acceptable method of
compliance with these criteria is contained in the New Zealand standard for design loadings.
When we go to that Standard, we find that it defines an “acceptable probability of onset of
damage” as, on average, once every 6 to 10 years (an annual probability of exceedance of about
10 %). Do the public understand that the probability is as high as this? You can bet that they do
not.
Ultimate Limit State (Safety) Level : Return Period of 450 years (approximately 10 % probability
of exceedance in 50 years)
As for the onset of collapse, this is set at 10 % probability of exceedance in the life (50 years) of
the structure. Another way of expressing this is to say that it is the shaking which will occur on
average once every 450 years.
Now, if you thought you had a 10 % probability of winning your national lottery in your lifetime,
I suggest you would consider that very good odds. Similarly, if you thought that the aeroplane
that brought you here had similar odds of crashing, you might not have come.
Let us take this analogy a little further. We all know that even the best and latest models of
aircraft sometimes crash - yet we are still prepared to fly in them. We have, from time to time,
been assured that the probability of crashing on a commercial airliner is smaller than crossing a
busy road. Sometimes we make conscious decisions not to fly on some airlines, nor in planes
less than a particular size. We assess the risk.
It is clear that the average member of the public has a better inherent feeling for the risk of flying
than he or she has for living in a building in an earthquake prone area.
So who decides at what level of risk a structure can be considered ‘earthquake-safe’? It seems
that it is rarely the client, or even the public. The engineering profession has made this decision
on behalf of the community by balancing what is reasonably achievable against the cost of this
level. If the day of judgement arrives for us, we can be sure that the public is going to be asking
us to explain why we chose this level.
The Science of Earthquake Hazard Modelling
Having decided on the level of risk we are prepared to accept, the earthquake engineer has to
develop into design loads. The first stage in that process is to model to the best extent possible
the seismicity in the region of the structure. This is earthquake prediction, of course, but it is not
attempting to pick the time and place for the earthquake, but rather set the odds for the lottery.
How is it normally undertaken? Early earthquake codes categorised the susceptibility of regions
to earthquakes on the basis of the past experience of the region's inhabitants. Now, we have the
benefit of :
1. a growing catalogue of recorded earthquakes worldwide, and
2. the development of theories on the potential of mapped faults and
3. their accompanying strain rates to generate earthquakes with some regularity.
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The seismicity modeller depends on some interesting assumptions :
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That (over a long period of time) the frequency of earthquake magnitudes in a particular
region follows a log-normal relationship.
Earthquake Recurrence Relationship
1.0000
0.1000
Records
No of Eq M>=m (/year/1000 km2)
Adopted
0.0100
0.0010
0.0001
0.0000
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Magnitude
Typical Recurrence Relationship for a Particular Area of the World
§
§
That what has been recorded in the past 100 years or so is indicative of what will happen in
the next 50 years.
That the distribution of epicentres within a regional cell in the model is uniform.
To all this we must add the fact that earthquakes originating at different depths may have
different characteristics and travel times to our site of interest. Because of the paucity of data,
the seismicity modeller will turn to any references available to earthquakes occurring over the
last 3000 years ago to add to the list of rare large earthquakes. Professor Ambraseys carried out a
similar interesting study for Turkey and countries to the east, which effectively maps the North
Anatolian Fault.
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Historical Earthquakes in the Middle and Near East 10-1000 AD (Ambraseys, 1970)
The accuracy of the model for these rare large earthquakes is clearly important for the assessment
of risk for buildings and monuments that we want to have a very long life. To complete our
modelling of earthquake hazard at a particular site, we need to know how earthquake shaking
diminishes with distance from the position of energy release. This is described as an attenuation
relationship. Most people understand instinctively that the rate of decay of a vibration is affected
by the medium through which it is travelling, and that different frequencies will be affected
differently.
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Effect of Seismicity Modelling Assumptions on Hazard Calculation
Researchers have sifted through the characteristics of thousands of recorded earthquakes to look
for correlation between the magnitude, depth, distance, site soil conditions and frequency content
to postulate attenuation relationships for regions. Clearly, this information is only available in
sufficient quantity in a few areas of the world. The scatter of the data tends to be high, and so the
relationships have some uncertainty associated with them.
At any one site, the shaking may come from an event anywhere in a 150 to 200 km radius. It is
then relatively straightforward to compute for a particular return period (or probability of
occurrence) the likely response of structures with different natural periods. This involves
systematically examining the effect of all earthquakes likely to occur anywhere in the circle of
influence around the site. This likely response is normally expressed as a response spectrum.
Note that no one individual earthquake is expected to have a response spectrum like that
produced by the process.
It is an idealised version of the response spectrum for a return period corresponding to the
acceptable risk adopted by the code that is commonly stipulated.
Seismic zoning maps for countries can be produced by carrying out a grid of these site hazard
calculations and plotting/contouring the results. There is always plenty of scope to adjust
Structural Seismic Design
So much for the earthquake calculation. Having calculated the seismic hazard that matches our
design expectations for our building site, we need to further modify it for our design process and
intended structure type. Two principal modifications may be necessary.
Our design methods are based on minimum strength characteristics of the materials. In practice,
most materials will be stronger than the specified minimum values by a considerable amount. In
our country, for instance, the distribution of yield stress for reinforcing steel which nominally
yields at 380 MPa looks something like this:
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Distribution of Reinforcing Steel Properties’ Test Results
Similarly, many structures have secondary lateral load resisting elements, which are not
considered in analysis. These give us reasons for decreasing the design loads by a small
percentage to compensate for the conservative approach.
Countering these conservatisms, we have the fact that some structural types and materials
perform in a more brittle fashion than others. Therefore, we may need to factor up design loads
to compensate for the more brittle ones. Modern codes reflect this.
In summary, you should now be fairly confused at the origin of the seismic design forces for your
monument!
However, the message is clear. The calculation of design forces, by reference to a national code
or by site-specific hazard study, has a great deal of uncertainty about it.
How All this Relates to the Checking of the Seismic Resistance of Monuments
Unless you set the design earthquake to be the maximum credible that our earth scientists expect
for the site, the chances are that one day your site will experience an earthquake much stronger
than the design one.
What motion/intensity/time do you want this significant structure to survive? If you are content
to use earthquake loads at similar loads too those you find in building codes, you may be taking a
risk much higher than the public realise.
At this point of the decision-making, it is necessary to decide how much we are prepared to put
the future of the structure in the hands of the seismicity scientists, and how much in the hands of
the structural engineer (the artist?).
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If the risk of failure from earthquake is to be reduced to a very low level, there are two clear
choices - and both require intervention.
Firstly, the structure can be strengthened to resist satisfactory design loads, which represent
something in excess of the maximum credible shaking that earth scientists believe possible for
the site.
Secondly, the structure can have its dynamic response to ground shaking predictably modified to
such an extent that the risk of serious damage is very low.
This is what I mean by “science or art?”.
My experience tells me that the latter option is the one to promote. Of course, it requires a little
input from the former approach as well.
The combined uncertainties of in-place material properties, actual paths of load resistance and
earthquake prediction are high.
These uncertainties can be bounded by introducing devices and systems, which limit the response
of the structure to be protected. The fundamental requirement of these systems is that they are
simple and robust. That immediately excludes electromechanical and precision-made devices
requiring maintenance and power sources.
Designers of essential facilities in earthquake prone regions are increasingly turning to baseisolation systems for high levels of certainty in protection. If the desire to give an existing
structure a far lower risk of failure from earthquake than its original designer considered is
paramount, a base-isolation solution that retains the essence of the visible structure is achievable.
Base Isolation is the technique of introducing a horizontal sliding joint at the foundation level
that effectively allows the ground to shake under the building, the building staying still in space.
Historic buildings in my country mean those as young as 100 - 120 years - presumably much
younger than the majority you are interested in. However, the seismic hazard is very high and we
wish to retain these buildings as part of our heritage.
I will conclude by showing some views of some of these buildings which have been baseisolated, along with a few that may become monuments of the future.
Most of these structures incorporate lead -rubber bearings which were developed in Wellington,
New Zealand some 25 years go. Their principal inventor, Bill Robinson, continues to develop
even more simple devices for seismic protection along the same principles of simplicity and
reliability. In a paper at the 12th World Conference on Earthquake Engineering earlier this year,
he presented his work on what he calls a Ro-ball - a solid rubber ball a little bigger than a fist,
which has potential as a base-isolator and a buffer. Each ball can support in excess of 20 kN.
How more simple could base-isolation be than a building sitting on a number of these?
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Example of a Ro-Ball
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Characteristics of a Ro-Ball
The message from all these examples is that these devices can be unobtrusive and simple.
How far could we take this? What about, for example, the securing of Nepal’s famous
Bhaktapur Palace of 66 Windows, of timber and sun-dried bricks. It was re-built after severe
damage in the 1934 earthquake. In its present state, collapse is inevitable again in moderate
shaking. It is understood that proposals to secure it are stuck on the level of intervention
(introducing additional elements in non-traditional materials) required. Base isolation seems to
be a most suitable compromise. In principle, the existing structure could have a below-groundlevel foundation slab constructed in place. A simple base isolation system similar to those used
in the previous examples would allow the preservation and repair of the structure to be
undertaken in traditional materials without compromise to the superstructure. The floors could
be could still be earthern. The price to be paid would be a perimeter movement gap in the
ground that could be easily disguised. The solution is low technology, low maintenance. The
protection it would afford is unrivalled.
Here is another example :
The Cinili Kiosk here in Turkey could perhaps be modified like this :
A Possible Base Isolation Scheme for the Cinili Kiosk?
The technology is proven – both in its incorporation in new buildings during their construction,
and in retrofitted examples. Here are some examples :
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Hearst Mining Building Retrofit, University of California, Berkeley
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Parliament Buildings, Wellington New Zealand – Successfully base-isolated as a Retro-Fit
General Assembly Library, Wellington – Successfully Base-Isolated by Retro-Fit
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Maritime Museum, New Zealand – Base-Isolated on Lead-Rubber Bearings by Retro-Fit.
In Summary
Without destroying your confidence in the processes, which are used as the basis of seismic
design codes around the world, I hope you now leave this international forum with a clearer
understanding of the proper place of design codes in the long-term preservation of our historical
assets.
To intervene or not structurally to protect a building from future earthquakes comes down to a
simple decision as to what risk of damage or destruction is considered acceptable by those
responsible for making such a decision.
The examples shown of a particular technology for base-isolating such historical monuments
provides what may be an acceptable level of intervention. The simple calculations and pragmatic
nature of base isolation devices I have shown today give confidence that there is an acceptable
way to provide long-term protection to heritage structures.
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