IV CONFERENCIA INTERNACIONAL DE PELIGROSIDAD, RIESGO ... · Universidad de Oriente, Santiago de...

IV CONFERENCIA INTERNACIONAL DE PELIGROSIDAD, RIESGO

GEOLÓGICO E INGENIERÍA SÍSMICA Y DE DESASTRES

8 al 11 de mayo de 2012, Santiago de Cuba. Cuba.

UNIVERSIDAD DE ORIENTE. Facultad de Construcciones

THE INVOLVEMENT OF ARCHITECTURE IN SEISMIC DESIGN

Base Isolation in Italy before and after the 2009 Abruzzo earthquake

Magistral Conference

Prof. Ing. Alberto Parducci

Universidad de Oriente, Santiago de Cuba may 8, 2012 Prof. Ing. Alberto Parducci

1


2

THE INVOLVEMENT OF ARCHITECTURE IN SEISMIC DESIGN

Base Isolation in Italy before and after the 2009 Abruzzo earthquake

Almost every progress in science has been paid for by a sacrifice, for almost

every new intellectual achievement previous positions and conceptions had to

be given up.

Werner Heisenberg, Nobel Prize for Physics, 1932.

FOREWORD

The underlying concepts of earthquake engineering have evolved substantially over recent

years. The objectives have become more ambitious and procedures for building design have

become more complex. Countries subject to seismic risk have updated their standards: some

to a considerable extent, others less so. In any case, greater attention is now focused directly

on the prevention of building collapse by controlling the post-elastic behaviour that

constructions bring into play to defend themselves when attacked by violent earthquakes.

Consequently, design requirements are no longer met by performing only traditional

resistance checks on assigned forces systems, because the checks must now refer to

requirements with predefined performance criteria.

There are now two explicitly expressed performance objectives. The first is an ethical act,

aimed at preventing those disastrous collapses which can injure or kill people. The second is

an economic objective, aimed at reducing building and repair costs in the event of damage,

i.e. aiming for optimal use of available resources. The two objectives are a clear reference to

new earthquake engineering concepts which can only be achieved with the compliance of all

those who design and manufacture buildings, especially the ordinary type, because these are

where most of those exposed to earthquake consequences are to be found. Thus it is the duty

of both the engineering and the architectural (now being offered new openings) sectors to

address these problems and suggest new design prototypes.

When such transformations occur, it is not unusual to find that previously neglected

problems actually become important. In a complex context like seismic design, modification

of reference standards easily begins to challenge consolidated practice.1 The validity of these

practices, however, must be assessed in the light of new knowledge.

1 Thomas S. Kuhn, The Structure of Scientific Revolution, University of Chicago, 1970.


3

Today’s business climate fosters the separation of expertise. In general, the definition of a

building’s morphological aspects is performed by the “architect”, who works by solving

distribution and style issues, using the models learned during their training and which were in

vogue at that time. When the design of the building has been established and the outlines of

the structural system defined, the result is delivered to an “engineer” who may be able to

make only marginal adjustments.2 The engineer is endowed with mathematical skills and

filters everything through complex calculations, performed via automated processes,

undertaking the verifications required by design codes. The calculation methods used are

certainly reliable, but only when solving assigned numerical problems and providing that the

models used do not differ too much from those presumed by the methods applied.

It is probably only excessive confidence based on the (numerical) reliability of the

automatic systems available today that supports the erroneous

opinion that this simple “regularization” will ensure that all

constructions are all equally safe.3 Everyone seems to agree that

this is sufficient, whatever configuration is under examination,

failing to consider that often the bare structure used for the

analysis will later acquire “non-structural” factors affecting the

validity of preceding calculations. For example, the addition of

masonry infill to frame structures may be beneficial if conditions

are static but will be detrimental in the event of an earthquake.

Recourse to this procedure is so widespread that it leaves no room

for other considerations worth dwelling on.

2 Christopher Arnold, Robert Reitherman, Building Configuration and Seismic Design, John Wiley & Sons,

1982. The authors are inspired by Henry Degenkolb and believe that if the initial configuration is mediocre,

the only remedy possible is to put a sticking plaster on it. 3 “There is no more common error than to assume that, because prolonged and accurate mathematical

calculations have been made, the application of the result to some fact of nature is absolutely certain.”

(aphorism attributed to Alfred N. Whitehead).


4

PART 1 - ARCHITECTURE IN SEISMIC DESIGN


5

THE DESIGN IDEA

We intend to support the hypothesis that in a holistic conception of the design process, a

building can be rendered truly anti-seismic only if there is accountable architectural

involvement. Although underpinned by sophisticated numerical analysis, anti-seismic design

must still be regarded as an operation with extensive empirical content. Indeed, it requires a

professionalism that right from the start will visualize an appropriate and seemly design idea.

To support this argument, we should reflect on the topics illustrated below:

• the unpredictable intensity of an earthquake may exceed the resistance capacity of

materials used for a building especially if it is erected according to resistant notions of the

traditional type;

• design standards inevitably have a conventional content.

• for a long time professional training concentrated on the concept of resistance. Eurocodes

introduced the limit state concept, but the situation has changed only in part because often

by habit or learning resistance is still really perceived as the objective that will define

design strategy;

• anti-seismic design, in reality, must be based on the idea of performance from which to

obtain and this is the fundamental point a suitable design idea.

All this is confirmed by the fact that observation of

actual damage produced by earthquakes always shows

that the most disastrous situations can be attributed

primarily to configuration factors. Apart from “non

engineered” constructions, the biggest failures are

triggered by inadequate structural configurations or

morphological choices, rarely by poorly developed

calculations. This is what emerges from site inspections

and examination of reports drawn up by the most

accredited research centres.

OLD CONFIGURATION PARADIGMS

The architectural paradigms that guide design choices do not always correlate with anti-

seismic requirements and some are even detrimental. There is no lack of examples and the

most significant is the pilotis building plan inspired by the “Maison Domino” concept that Le


6

Corbusier proposed at the turn of the 20th

century. It was the first of the five points of

“Nouvelle Architecture.”4 Le Corbusier, however, had no interest in anti-seismic

morphologies and neither did Seismic Engineering.

“Maison Domino” was a truly innovative concept because it associated architectural

composition with the features of a new material: reinforced concrete. The system is rational,

elegant and facilitates configuration aspects, so it became the most frequent design reference.

Nevertheless, as far as seismic episodes are concerned, if they exceed a certain intensity,

reinforced concrete becomes dangerous or, worse still, disastrous. Indeed, it is always

indicated as the primary cause of disasters. The “soft storey effect” it generates is not caused

by pillar resistance (this would be revealed by traditional checks) but by the poor dissipation

potential of the collapse mechanism that occurs when the elastic limits of materials are

exceeded. Result: the upper part of the building may shift significantly and the ensuing “pi-

delta” effect produces high stresses in the columns because of moments caused by upper

weight. The ruinous collapse of ground floors often occurs without serious damage, except

that caused by the actual fall. This vicious circle can also be caused by an architectural factor:

the stiffening effect of masonry infill on upper floors. Yet in earthquake-prone areas this was

and still is a classic building method.

During discussions these arguments are always widely addressed but it cannot be said that

they are given due attention by active designers nor is there adequate research into

composition. This lack of interest is rooted precisely in the difficulties encountered when

proposing new models in milieus where other practices and other concerns are already

entrenched and prevalent.

4 Le Corbusier, Vers une architecture, Edition Crès, 1923.


7

CONVENTIONAL CONTENTS OF DESIGN PROCEDURES

We have already said that design procedures are extremely conventional in content and this

depends on a number of reasons, including the fact that to be applied in design practice a

number of factors of uncertainty must be sidestepped. We do not intend to confute these

standard design codes, because they are the result of extensive studies carried out in the most

accredited research centres: it would be tantamount to refuting shooting competitions where

different weapons (arrows, rifles, darts) are used, each with their own degree of (im)precision,

and not all equally suited to various targets (distant, close, in movement). The irreplaceable

value of the codes lies in their being a corpus and designers must apply their professional

expertise to use them properly each time.

Even without going into detail, it is enough to consider the situations listed below to

acknowledge this and accept the consequences. The analysis is based on the following

statements:

• the earthquake imposes a movement at the base of a construction and transmits a certain

amount of energy to it, which the construction has to manage with the dissipations

associated to its elastic and, if necessary, inelastic oscillations;5

• seismic motion is complicated and unpredictable: it is described by the response spectra

with which rules establish design input;

• spectral intensity and shapes are defined by averaging the recordings obtained in situations

considered to be geologically similar;

• they affect mainly, but not exclusively, horizontal motion components;

• design spectra refer to an elastic response in structures, but the intensity of seismic attacks

can largely overcome the material’s elastic limits.

• the earthquake exhibits characteristics that differ every time and even at the same site can

occur in a different way each time; it is sufficient that an event originate from new sources

for it to determine significant differences in surface motion.

The notes below offer a brief explanation about the uncertainties whose origins lie in these

assumptions.

Conclusion: It is not rational to entrust the delicate problem of seismic safety in buildings

only to automatic numerical calculations based on these assumptions and relinquishing the

need for an appropriate design idea.

---------------------------------------------------------------------------------------------------------------

UNCERTAINTIES IN DESIGN PROCEDURES

The question marks that appear in the figures below indicate the main causes of

uncertainty found in a normal design process. They depend on the nature of its phenomena or

the schematization of calculation models. An overview is given in the notes below.

.• The standards define the design spectra for a conventional damping value, assumed as

viscous and equal to 5% of the critical value. In reality, however, the buildings can

dissipate energy through various mechanisms (plastic, friction, etc) and with very different

capacity.

5 This assumption does not correspond to the concept of stresses induced by systems of “equivalent forces”

that has long governed anti-seismic design.


8

• Design intensity is defined by the spectrum’s PGA Peak Ground Acceleration. This is the

most significant but not the only parameter that governs destructive potential.

• Often soil-structure interaction is not considered, while recordings are obtained in free

soil.

• Spectral representations do not indicate the duration of the earthquake, although this

factor is important for assessing progressive damage preceding collapse.

• Spectral representations will not reveal possible presence of long acceleration pulses

which might shift the oscillatory phenomenon towards impulsive-type behaviour; the

difference may be of importance in studying overturning.

Other aspects are related to the modal analysis used for elastic calculation which, with

appropriate adjustments, constitutes the habitual foundation for design methods.


9

• Modal forms do not always correspond to actual construction deformation, partly because

of variability in mechanical properties of materials, and more so for the interaction of

elements considered non-structural.

• Information is lost in the frequency domain time and practical rules, like the quadratic

type, must be applied to combine modal responses.

• There is no way of predicting how axial forces produced by vertical components of seismic

motion in supporting elements (columns or walls)will combine with the axial forces

associated with rocking effects.

Other problems arise when numerical analysis models, of which several are listed below, are

defined.6

• “Beam” elements reproduce beams and columns, eliminating cross dimensions and

considering a continuous succession of transverse sections. This requires various

arrangements to assess the behaviour of many single points, like frame knots, crucial for

their horizontal resistance.

• The state of cracking is not taken into account and this is physiological in reinforced

concrete, fluctuating with variation in stress intensity.

• Materials, chiefly masonry, do not behave in a linear manner even in the field of small

deformations.

• The non-linear behaviour of plastic deformations, in particular plastic hinges, may only be

loosely reproduced (an aspect that is at the basis of Capacity Design principles).

• Etc, etc.

Some considerations also apply when designing structures outside of seismic zones, but all

have a particular relevance in specific seismic situations, especially when designing buildings

in high-risk areas where earthquakes are expected to be more violent.

---------------------------------------------------------------------------------------------------------------

DEVELOPMENT AND LEGACY OF SEISMIC ENGINEERING

The mechanical sciences evolved in Europe during the Enlightenment period of the

eighteenth century, when it was realized that knowledge of the physical world is obtained by

direct observation of natural phenomena and without preconceived attitudes. Structural

Engineering developed a century later, finding fertile ground in the Theory of Elasticity and

meeting great success because it allowed the development of many important works that

characterized the late nineteenth century. Even today, the Theory of Elasticity is still a reliable

scientific basis with great potential, but (careful!) only within the framework of its

fundamentals: linear elastic behaviour and continuity of deformations. Seismic Engineering

developed later, when these concepts became consolidated and established the basic way of

addressing structural design. It was noted later, however, that Seismic Engineering poses

further problems that fall outside of its scope.

In the early twentieth century, several earthquakes hit intensely built-up cities (1906 in San

6 Structural engineering was defined tongue-in-cheek (Kelsey, Finite Element Method in Civil Engineering) as

“the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyze, so as to

withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect

the extent of our ignorance.”


10

Francisco, USA; 1908, Messina and Reggio Calabria, in Italy; 1923 in Tokyo, the Great

Kanto Earthquake, etc). Consequently standards began to develop, introducing numerical

computation in structural design.

The idea of isolating constructions at their base was already in circulation. After the

Messina earthquake, in Italy, several patents were awarded, but they were simple ideas that

were never applied and did not even attract the interest of academic circles.

Initially there were attempts to copy seismic action by means of horizontal static forces

calculated at about 10% of the weight of the construction. The Elasticity Theory reference led

to structural calculations in the linear field for verification of resistance by evaluation of

“allowable stress”, which was an approach of a static nature with checks replicating normal

working conditions. The implicit but unreliable assumption envisaged that the usual safety

margins would be sufficient to address the

most severe attacks. Moreover, information

was absent for addressing two topics whose

importance was recognized only later: the

dynamic aspect of the problem and the post-

elastic dissipative capacity of structures. The

application of these criteria lasted for some

time with regulatory updates that did not

change the basic approach. This has left a

substantial legacy in the minds of many

designers and to some extent also in learning.

The first step towards a dynamic approach was taken in the middle of the last century,

when the first accelerograms of real earthquakes were recorded. The “response spectrum”7

technique was developed by processing these data. In quantitative terms, this tool brought

forth important information about the actual frequency distribution with which earthquakes

release their energy. Much of this energy reaches the surface in quite a high frequency range,

7 Hudson illustrated this calculation tool in 1956, at the first WCEE (World Conference on Earthquake

Engineering), in San Francisco.


11

over 1 hertz, which includes many

standard constructions.8

The first recordings used to define

design spectra came from El Centro

(California, 1940) and indicated a peak

acceleration of 0.34g (NS component).

For a time it was thought that this value

was indicative of maximum seismic

intensity, but this was far from accurate.

A very important earthquake in the modern history of Seismic Engineering was that of

Imperial Valley (San Fernando, California, 1971). Numerous detection positions were

present, many in the epicentral area, where the peak acceleration registered was in excess of

1g (!). New information and analysis of very serious damage, this time caused by important

new reinforced concrete construction works (buildings and bridges), made a decisive impact

on anti-seismic design. It was clear how important it was to define new criteria in the

approach to and further investigation of the post-elastic behaviour in structures subjected to

repeated cycles of alternating large plastic deformations (plastic fatigue), before collapsing.

Many experimental and theoretical studies were conducted after these events.9 Attention

focused on energy issues and the dissipative capacity of inelastic structural responses. The

Berkeley school was very active in this field, while many experiments were conducted,

mainly in Japan. The innovative concept came from the Christchurch school in New Zealand, 8 This information was only used for practical purposes many years later, to introduce and apply modern base

isolation methods. 9 Experimental research into these aspects was already under way, especially in Japan, but became more

intense in the years to follow.


12

where the principles of Capacity Design were defined in the early 1980s. The expression

describes the design criterion aimed at showing maximum energy dissipation, which then

became the main basis of seismic legislation.

It was at this time that Base Isolation applications began to spread as they had now gone

beyond the pioneering stage.10

Base Isolation was confirmed not only by the theoretical and

experimental evaluations of the effectiveness of the system, based on the dynamic decoupling

of the building from the ground, but by recordings made during the Loma Prieta (Los

Angeles, 1994) and Kobe (Japan, 1995) earthquakes, and later by many other Japanese

recordings.

Together with the previous systems, the use of various types of dissipative devices

(viscous, elasto-plastic, shape memory alloys, etc) has increased and they can also be used to

build Base Isolation systems. In the 1980s and 1990s, the decks of several Italian viaducts

were protected in this way.11

In construction, similar systems can operate in parallel with the

main structure, but it must be flexible enough to withstand the necessary deformation.

We will not be discussing other special methods (tuned masses, active, semi-active and

hybrid systems): they are fascinating but very complex applications and can only be justified

for large-scale constructions of considerable importance, like the Landmark Tower in

Yokohama or the Applause Building in Osaka.

We may conclude this brief summary with Design Capacity and Base Isolation concepts,

thus coming to current notions of anti-seismic design.

PERFORMANCE REQUIREMENTS

Returning to what was said earlier, in high risk areas where demand may exceed the

material’s resistance capacity, the main design requirements are prevention of collapse and

minimization of damage. These are pursued mainly by controlling two quantities:

dissipation capacity and deformations12,13,14,15

.

10

A. Parducci: Seismic isolation: why, where, when: design options for ordinary isolated structures,

International Post-Smirt Conference Seminar on Isolation, Energy Dissipation, Cheju (Korea), August 1999. 11

A significant example is the Coltano viaduct, comprising continuous 450-metre sections across multiple

spans, for a total length of almost 10 kilometres (FIP and ALGA devices by A. Parducci and E. Ciampi,

respectively). 12

E. Elsesser, New ideas for structural configurations, 8th U.S. NCEE, San Francisco (CA), April 2006.


13

We will simplify descriptions to make it easier to explain. Thus, stating that the structure’s

cyclic response can be modelled by bilinear elasto-

plastic behaviour; that various viscous dissipations of

limited effectiveness correspond to the elastic phase,

depending on velocity not on deformation; that

hysteretic plastic dissipations, more dissipative that

the aforementioned, correspond to the plastic phase,

(especially if compliant with Capacity Design

principles), depending on deformation, not velocity.

The purpose of simplification is to highlight the hysteretic contribution associated with

damage to the building. Therefore, let:

Ei be the energy fed into the overall construction;

Ek be the kinetic energy of the moving masses;

Ev be the energy associated with viscous elastic response;

Ei be the potential energy of elastic deformations;

Ei be the hysteretic deformation energy associated with irreversible plastic deformations

(damage).

During an earthquake the building accumulates

energy. If it is able to absorb the energy with

viscous dissipations

during the elastic

phase (this requires

extensive structural

deformability), the

structure oscillates

without being

damaged.9 If this is not enough, it must bring into play the

inelastic deformations of the plastic phase and damage will

occur. If these mechanisms are sufficient, the structure does not

collapse and can be repaired, if this is cost effective. Disastrous

collapse, on the other hand, can be attributed to the inadequacy

of performance, since the latter cannot easily be guaranteed.

-----------------------------------------------------------------------------------------------------------------

AN OVERVIEW OF ITALIAN LEGISLATION

Italian codes have recently been updated to comply with Eurocode directives. The procedures

described below are applicable to design of new buildings. Existing bridges and constructions

are addressed separately. The requirements are expressed in terms of Limit States (LS) and

13

A. Parducci, Nuove concezioni per il progetto sismico - Una sfida per l'architettura e per l'ingegneria, “Eda,

esempi di architettura”, Edizioni Il Prato, special edition, June 2007 English translation available). 14

A. Parducci: Nuovi orizzonti per un'architettura antisismica, Atti del Seminario CNR, Roma, September 2007

(published in “Nuovi Sistemi e Tecnologie Antisismici”, 21° Secolo, Roma, February 2008) 15

M. Mezzi, Deformation vs stiffness motion vs fixity New vision in seismic conceptual design, The 14th

WCEE, October 2008, Beijing, China.

elastic

step

plastic step

residual

damage

force

displacement

cyclic

dissipated energy

Ei

structural

reversible

dissipate

d

+ Ek + Ev Ee + Eh

kinetic elastic

stored ENERGY

viscous hysteretic


14

defined according to performance criteria. In fact, we will consider two kinds of LS:

• LLS = Life-saving Limit State, applied for rare events of high intensity expected to recur

every 500 1,000 years.16

The structure may be damaged, even significantly, but must

retain part of its resistance in vertical and horizontal actions.

• DLS = Damage Limit State, applicable to more frequent events, with recurrence periods of

50 100 years. The construction must be guaranteed immediately usable, even if there is

limited local damage to non-structural elements and plant.

Building design is addressed in two different sections. The first is more general, for

traditional buildings, with “fixed” foundations connected directly to the ground; the other is

for buildings using Base Isolation systems.

In the first case, Capacity Design principles are applied to ensure adequate dissipative

capacity giving significance to a linear calculation of reduced intensity compared to that of

the elastic spectrum. The LLS is met by adjusting calculation results according to practical

rules defined as follows:

• critical areas are taken into consideration, designed for high local ductility, placed in

strategic positions (plastic hinges near frame nodes);

• prevention of inelastic deformation occurring outside of critical areas (upscaling of non-

critical areas);

• promotion of the formation of collapse mechanisms that effectively mobilize envisaged

critical areas, avoiding inadequate poorly dissipative mechanisms like soft storey (weak

beam-strong pillar rule).

These rules serve to ensure the availability of a global dissipative capacity that makes it

reasonable to reduce elastic spectrum values (e.g. 4 5 times more or less) significantly,

according to the configurations of the different structural systems. Morphological

architectural choices are thus involved in this design aspect (!).

DLS requirements, on the other hand, addresses deformations for which limitations are

assigned on the basis of structural type (relative storey drift).

When buildings are provided with Base Isolation the decoupling is required to bring about

a very low spectral input. Consequently, for Base Isolation to make sense, LLS must be

obtained ensuring the structure remains practically in the elastic field, without mobilizing

important plastic dissipation.

----------------------------------------------------------------------------------------------------------------

RESISTANCE AND DEFORMATION CONCEPTS

The resistance paradox: a typical aspect of seismic design. The seismic input received by

a construction depends on the fields in which the two frequency distributions are located: that

of seismic movement, shown by the shape of the response spectrum, and that of the structures

own oscillations. Increasing resistance will lead to an increase in the size of structural

elements, which will bring an increase in rigidity. The oscillation period decreases and in

most cases an increase in demand ensues. Persevering with the path of resistance is like

challenging the earthquake in a battle that will surely be lost. Moreover, horizontal

16

Return periods are assigned according to different reference times conventionally attributed to various

building uses, applying a uniform hazard criterion.


15

accelerations transmitted to each floor will also increase.

Design codes do not address this aspect and do not lay down requirements for limiting

storey accelerations as such, even though it is equally important because it defines the

protection of non-structural parts, plant and content. For

some intended uses, such as hospitals and rooms containing

hazardous or valuable materials, it can become the main

feature.

Earthquake and wind. Both actions stress structures

horizontally but in addition to the different intensities, there

is also a fundamental difference in the way they manifest

themselves. Regardless of the aeroelastic phenomena that

bear no relevance to civil constructions, the airflow around a

building is actually unaffected by structural deformations and

therefore does not alter design input. Wind gust frequencies

are also much slower than those causing building oscillation.

Wind action and effects can therefore be interpreted using

equivalent static action, while seismic action depends on mass inertia forces and is linked to

motion velocity. Consequently, the more slowly the structures oscillate, the smaller seismic

action will be. In addition, hysteretic dissipative capacity mobilized during damage stages

depends directly on the extent of deformations.

Resistance and deformability. Ultimately, the concepts of resistance and deformability

must be properly balanced, giving due attention to the latter, because it can also be beneficial

in the elastic field. So the firmitas indicated by Vitruvius in his famous triad firmitas,

venustas, utilitas (strength, beauty, usefulness), when describing Imperial Rome in the 1st

century BC17

(always quoted in architecture courses), is not entirely true.

PAST EXPERIENCE

Apart from a few some quite important experiences, a culture that will achieve decisive

architectural design committed to seeking morphologies that address seismic issues is still

struggling to establish itself. We could say, simplifying again, that a number of causes may

affect the interest of Architecture as a discipline,18

hoping to stimulate at least some of the

younger architects.19

Several of the following causes also affect behaviour dictated by so-

called common sense.

Vertical perception. Buildings are designed to be in the gravitational field: vertical and

17

Marcus Vitruvius Pollon, De architettura, Giulio Einaudi Editore, Torino 1997 (Latin and Italian texts). 18

Umberto Garimberti states that: “... even science can be psychoanalyzed and subjected to therapy for the

purpose of exposing our intellectual laziness that supports certain conceptual and operational choices, the

subconscious motivations that lead to certain concepts being taken for granted, the practical needs that go in

one direction rather than another, and the stubbornness of insisting on ideas that are proven but lack

prospects …” Paesaggi dell’anima, Oscar Mondadori, Milano, 1996. 19

Max Planck, who won the Nobel Prize for Physics in 1918, said that: “A new scientific truth does not

triumph by convincing its opponents and making them see the light, but rather its opponents eventually die,

and a new generation grows up that is familiar with.” Wege zur physikalischen Erkenntnis, article published

on 9 July 1932.


16

permanent. Often architectural design enhances this feeling. Resistant systems are designed in

a similar way, just as we all conceive space: “... all horizontal directions are equal and form a

plane of unlimited extension. The most elementary model of existential space is a horizontal

plane crossed by a vertical axis.”20

This vision contrasts with the perception that is necessary

for tackling seismic problems.

Static perception. Vitruvian firmitas, which has already been mentioned and which has

always influenced structural design. Collective wisdom perceives buildings as solid and

steady: this is usually what is stated in Faculties of Architecture.21

Compositional elements. Bricks are no longer used in building structures, having been

replaced by a frame that has become the basic construction element. The trilithon of ancient

history became the frame when steel and reinforced concrete building techniques began to be

used for achieving structural continuity with uprights and crossbeams through the knots. If we

examine the behaviour of a mesh

frame, however, it is easy to see that

such a popular element can be

adapted to seismic requirements (for

example, Capacity Design does this),

but intrinsically it does not have the

best attributes required for offsetting

seismic action. The horizontal in-

plain deformability of a spatial frame depends on bending (OK) and shear deformations of its

elements, especially the uprights, associated with storey drift. Maximum moment is found at

the knots (critical areas) where plastic hinges may form. The actual knots are critical, because

their failure makes the whole system unstable. Without Capacity Design22

precautions, a

frame’s dissipation capacity would be inadequate for a high-risk seismic zone?

Intensity of seismic impact. The macroseismic scales used by geophysicists are

20

Norberg and Schulz quoting Rudolf Arnheim in The Dynamics of Architectural Form, University of

California Press, Berkeley, 1977. 21

E. Torroja, Razón y ser de los tipos estructurales, Istituto técnico de la construcción y del cemento, 1960. 22

Even though we are at a high rational level, this argument is somehow reminiscent of C. Arnold’s sticking

plasters (see footnote 2).


17

logarithmic. The statement may seem trivial, but anyone

unaccustomed to using them (like media information

workers) may underestimate the great difference that

exists when magnitude grows by “only” two degrees, for

example. As a guideline, the ratio is 1:100 in terms of

displacement, and 1:1,000 in terms of energy (!).

Conventional value of numerical analyzes. The topic

was discussed above, pointing out the very conventional

aspect of numerical analysis which makes it impossible to base anti-seismic design on a

computational power that does not really exist.

EXAMPLES OF TRADITIONAL ANTI-SEISMIC ARCHITECTURE

We have said that in the past, anti-seismic design was based primarily on the concept of

resistance. When spectral representations showed that demand decreases with the increase of

the oscillation period, building height no longer appeared to be a limiting factor. Thus it may

be interesting to examine how the problem of lateral resistance was addressed in the most

demanding cases, that of skyscrapers, although for these buildings more attention is given to

the problem of hurricanes than that of earthquakes. We are still

referring to the spatial frame already mentioned, but with proper

precautions and additions. Over a certain height, to protect non-

structural parts and plant, lateral deformation must be limited and

there are various solutions.

Technical storey. The height of the frame is broken up into

blocks separated by technical storeys that act as non-flexible

slabs. Pier Luigi Nervi used this principle for the Montreal

skyscraper. The design idea has harmonized many aspects:

• structural, so each block works as an element interlocked

above and below to reduce column bending and transfer total

deflection to axial forces (traction-compression) in the corner

uprights;

• aesthetic, which interprets structural functioning by assigning a

strong architectural value to the corner uprights;


18

• technological, which uses rigid storey to distribute plant along the height.

At the time of the design (1963) dissipation of energy was not considered important.

Schemes of this kind still allowed for balancing design

parameters to achieve optimal performance.

Diagonal stiffening. The concept is similar to that of

triangular geometry used on racing bicycles, which

responds to all the athlete’s demands. Only axial loads

(traction-compression) are mobilized to obtain maximum

strength with minimum weight. This system has been used

to build a number of skyscrapers of which the most famous is the John Hancock Building, in

Chicago (Illinois, USA, 1969). This 100-storey building is 344 metres high and its exterior

tubular structure is slightly

tapered, with large exposed

diagonal stiffened elements.

Analysis of the design showed

that the diagonals play a

significant role in withstanding

vertical loads. The latter

observation sparked the interest

of I. M. Pei, who designed the

amazing Bank of China building

in Hong Kong, surpassing

vertical-horizontal hierarchical

values. The reticular structure

takes both loads to the peripheral

uprights and discharges them onto

the rocky terrain below.

The modern pagoda. The 1972 Transamerica Pyramid, which soars against the San

Francisco skyline, deserves special attention because its movements during the Loma Prieta

earthquake of 1989 (M = 7.2) were recorded. The building vaunts significant lateral

deformability with an oscillation period of 3 seconds. Oscillations of the amplitude of ± 20


19

cm were measured on the 49th

floor (five times greater than at the base), lasting for almost a

minute without causing damage. Dissipations of a “viscose” nature were slowly able to absorb

the energy transmitted by the earthquake, maintaining a sufficiently elastic deformable

structure. The building has an internal metal structure, the façades are prefabricated, and the

base is formed by large interwoven structural elements. The latter leave open spaces free on

several floors without creating a “pilotis effect.” It is possible that some stiffening was

produced by the pre-cast façade elements and that their relative limited sliding contributed to

dissipation effects.

The building has a unique shape and some feel that in general the tapered silhouette, which

is very marked here, is a morphological construction requisite. Moreover, similarity with the

configuration of Japanese pagodas is evident and these are buildings that over the centuries

have withstood strong earthquakes. The figure compares the San Francisco Pyramid with the

Horinji pagoda in Nara (Japan), which is 1˙300 years old. The seismic behaviour of pagodas

was studied by Japanese scholars, who highlighted various effects that should be explored in

depth. At the moment, however, no possible loans to current construction needs have been

tested.23

Critical configurations. The stiffening of frames achieved with shear walls, box-shaped

structures or fascia of high beams can produce undesirable consequences if the resulting

configurations create shear beams in the other elements with which they interfere. This will

foster shear failure, which lacks dissipation, in the shear beams. The situation is particularly

critical in the case of reinforced concrete captive columns exposed to strong shattering.

23

K. Fujita et Al., Earthquake response of ancient five-story pagoda structure of Horyu-Ji temple in Japan, 13th

WCEE, Vancouver, Canada, 2004.


20

PART 2 - BASE ISOLATION


21

BASE ISOLATION

Base Isolation (BI) is a seismic protection system that reduces significantly in the energy

transmitted to a building by the earthquake. The result is obtained by arranging the

construction on supports having conspicuous horizontal flexibility, thus modifying the

oscillation period. The principle is based on the fact that earthquakes carry energy in a fairly

narrow range of frequencies. The highest

response accelerations, amplified with respect

to the ground, are spread over periods of less

than one second. Most low and mid-rise

buildings fall in the same range of elastic

response periods and are therefore exposed to

increased risk. If the period passes this critical

area, demand drops rapidly and becomes very

small when it comes to periods of at least 2

seconds. Reduction in terms of acceleration,

however, corresponds to an increased demand in terms of displacement.

The situation offers two considerations. Firstly, understanding why earthquakes often

distribute their effects in a non-uniform (“leopard-spot”) fashion. Secondly, suggesting a way

to avoid earthquake effects by cunning rather than by force.24

On standards. The figure

shows a typical case of an elastic

response spectrum expressed in a

“capacitive” form (Sa response

accelerations shown vs Sd

displacements) that Italian

standards applied to the city of

L’Aquila (high-risk area). The

spectrum refers to the LLS (Life-

saving Limit State) for a public

building subject to large crowds (rare event, with a 10% probability and reference period of

VR = 712 years). It may be a school located on flat land with medium quality soil (soil C).

Fixed-base design. Italian standards harmonized with Eurocode 8. Designs for the ULS

(Ultimate Limit State), accepting damage estimates even as severe but without collapse. For a

calculation in the elastic range to be significant, the input is evaluated by dividing spectral

accelerations Sa by structure factor q. The greater the reduction in q, the greater the potential

inelastic dissipation capacity of the building, and the more extensive and flexible the plastic

deformation the structure can withstand. Applying the principles of Capacity Design,

standards define q by virtue of:

• the configuration of a resisting system;

24

“Why fight an earthquake? Why not join it and beat it with astuteness?” wrote Frank Lloyd Wright in his

memoirs, remembering that for the Tokyo Imperial Hotel he had created elements to “float” on sludge.

“Rigidity was not the right answer, but flexibility and resilience were.” The building survived the 1923 Great

Kanto Earthquake in Tokyo (M≈8) unscathed.


22

• assigned classes of ductility.

For reinforced concrete frame structures of regular shape, the q factor varies from 3.0 (very

dissipative configuration with low ductility) to 5.85 (dissipative configuration with high

ductility).

Base-isolated building design. The design again uses elastic spectrum Sa, but the q factor

is very small, about 1.5, because the spectral structure reduction is required to be isolated in

the elastic range, at most with slight damage. No special requirements are therefore needed

for ductility.

Isolator displacements are calculated without any reduction of the elastic spectrum. In the

case of Avezzano, ± 35 cm would be needed, plus some marginal guarantee requirements.

Elastomeric isolators. The most

popular isolating devices for these

applications are HDRB high damping

rubber bearings, which are multilayer

rubber-steel devices or LRB, lead

rubber bearings. They are made using

thin layers of silicone rubber (6 10 mm)

alternating with vulcanized metal sheets

(2 3 mm). These devices are very

deformable for horizontal shear

(distortions up to 200%, upper standard

limit). When confined, the crushing of

the rubber is reduced so that the devices deform little in the vertical direction and can be used

as supports. The figure shows the response curve trend obtained with “quasi static” (slow

alternating deformation cycles) force-displacement tests of different amplitudes. The areas

shown in the diagrams measure the energy dissipated in each cycle. Secant stiffness k

depends on deformation amplitude: large (k1) for small amplitudes; small (k2) for large

amplitudes. This behaviour is useful because large displacements are needed for the system to

be effective against violent earthquakes, while more stiffness is needed to respond to normal

operating actions, like that of wind, without significant displacements.

Isolating devices do not have a large dissipation capacity. In LRB devices it is increased

by a lead cylinder fitted inside.

In any case, as will be seen, BI

reduces seismic input by means

of dynamic decoupling rather

than by dissipation. The

equivalent viscous damping can

be evaluated through an equi-

energy criterion.25

With HDRB

25

Within specific limits the relationship =(1/4 )×(WD/WEL) is usde, linking viscose damping to the energy

that the device dissipates in each cycle. The quantities WD and WEL=½kd2 correspond to the areas indicated

in the figure.


23

q

q0

m

m0

isolators

k0, c

0

building

k, c

isolating devices damping is in the order of 10% of the critical value ( ≈ 0.10); for LRB

devices it can reach 25 30%, a value that may be useful for reducing displacements.

Practical considerations. For multi-storey buildings, BI

is best applied when the building has a basement (garage or

cellar). This avoids having to construct an extra floor that

is otherwise unnecessary. The isolators can be placed on

the containment walls and inner columns to insulate the

upper building, leaving the basement set into the ground.

The oscillation period can be increased by fitting sliders

with a low friction coefficient in parallel with the isolating

devices.

-----------------------------------------------------------------------------------------------------------------

THEORETICAL ANALYSIS OVERVIEW

The figure sums up an isolated structure

with a system that has one degree of freedom,

deformable as a shear-type frame, defined by

mass m and stiffness k. Building deformation

is (q - q0). The inclusion of an isolation

system and addition of mass m0 bring a

system with two degrees of freedom. Seismic

input is defined by a history of soil accelerations ag(t) compatible with the design spectrum.

The structure is defined by the following dimensions:

• non-isolated structure: m = building mass,

k = building stiffness,

c = building dissipation coefficient.

• isolation system: m0 = isolated foundation mass,

k0 = elastic stiffness of the isolation system,

c0 = isolation system damping,

• displacements: q0 = displacements by isolator deformation,

q = building displacement (mass m compared to the soil).


24

For an indicative analysis evaluating the system’s elastic response using modal analysis

methods considering the following parameters:

• frequency corresponding to oscillation period TBF of the fixed-base structure:

m

k2 k

mTBF 2

• reference frequency 0 and corresponding oscillation period T0 for the entire isolated

system, considering non-deformable, mobile above isolators:

mm

k

0

020

0

00

k

mm2T

Two dimensionless parameters are assumed: = mass ratio; = period ratio:

mm

m

o

2

0

BF2

0

T

T

It is recognized that is less than 1, but of the same order of magnitude, because the mass m

of the entire building is predominant with respect to m0.

Supposing that the parameter is small ( <1), namely that the reference period T0 is

substantially greater than the TBF of the structure with a fixed base (e.g.: T0≈3TBF, so ≈0.1).

With these assumptions we can write the system of the two equations that describe, in the

linear field, the system motion with two degrees of freedom:

)(

)()(

tamqkqkm

tamqkqkkqm

0

000

Separating variables, proceeding with modal analysis methods and considering small

quantities as negligible infinitesimals, we calculate the characteristics of the two modal

shapes of the isolated system [sample values correspond to = 0.1].

First method period T(1) ≈ T0

modal form u0 = 1

u ≈ 1 +

participating mass )1(m = (m0 + m)(1 - 2) [> 0.99 (m0 + m] (!)

Second method period T(2) ≈ TBF

modal form u0 = 1

u = )( 11

participating mass )2(m = (m0 + m)(1 - ) 2 [<.01] (!)

-----------------------------------------------------------------------------------------------------------------

Compactness requirement. The modal analysis carried out in the previous note leads to an

important result, valid when the ratio = (TBS/T0)2 from fixed-base period to isolated period

is small. This happens when the protected construction is stiff. The first way reproduces

almost completely the entire system response in which most of the construction mass

participates (m+m0). The isolated period in effect corresponds only to isolator deformation

while the building oscillates almost without deformation. The system is hardly sensitive to the

second mode, the one that deforms the structure, because the participating mass is almost null.


25

The result indicates the following

conditions for optimal BI functioning:

• the isolated oscillation period

(immediately assessable TIS≈T0) must

be large because the construction is

certainly located in the low acceleration

response zone (TIS>2, but may well be

more);

• it is better if the structure located on the

isolators is rigid, barely deformable

(T0>3TBF), for dynamic decoupling to be completely effective.

The second condition re-evaluates the use of many traditional building techniques, such as

those of masonry, with a favourable contribution to current bio-architecture problems.

BASE ISOLATION PERFORMANCE

BI has many advantages. The first is to reduce the stresses transmitted by violent seismic

attacks. BI allows an almost elastic response to be obtained without significant damage and

offers the advantages described below.

BI eliminates the resistance paradox because its separates tasks: deformation is entrusted

completely to isolator devices, specially designed and experimented, while the more resistant

upper structure must be rigid so storey dirft is very limited.

Moreover, BI reduces plane accelerations and the damage inside buildings. The images26

below show the effects of the 1994 Northridge earthquake inside Olive View Hospital, which

had been rebuilt after 1971 as a fixed-base structure, with very strong and very rigid, stiffened

walls. Accelerations of 0.8g recorded at the base, and 1.53g at the top, did not damage

structures. Damage to plant and non-structural parts was significant and caused the hospital

huge economic problems.

26

The illustration and information on this topic were provided by Prof. Edoardo Cosenza, of the University of

Naples.


26

BASE ISOLATION AND ARCHITECTURAL DESIGN

Passive seismic protection contrasts with the

firmitas concept. Even Vitruvius would have

defined requirements with the terms motus,

scissio, deformatio (movement, separation,

deformation). The two models in the figure

correspond to a simple BI application and to a

variant known as a “bell-building” whose

fluctuations are controlled by dissipation

devices.27,28

These are not the only models consistent with BI, because developing patterns

like these opens new avenues for experimenting architectural forms that expand freedom of

configuration. More complex configurations, like that shown in the figure, can also become

anti-seismic.

Union House in

Auckland (New Zealand,

1980) is a perfect example

of an architectural layout

that spotlights exactly how

BI works. The foundations

are in “mobile” piles that

cross a quite erratic terrain

and rest on a stiff soil. The piles are bound to the structure by hinges and are housed in a

sleeve of a larger diameter that allows their heads to move. A system of elasto-plastic metal

dissipating devices connected to

a fixed platform at the base of the

building contrasts horizontal

displacements. The elevated

structure is stiffened by diagonal

braces that emphasize the

design’s anti-seismic conception

by stiffening the structure and

passing vertical and horizontal

loads to the isolators. The plastic

threshold of the isolators is

calibrated so that it does not

transmit horizontal forces in

excess of the structure’s

resistance capacity.

27

A. Parducci, Seismic isolation and architectural configuration, Conceptual Conference on the Conceptual

Approach to Structural Design, Singapore, 2001. 28

M. Mezzi, A. Parducci, Aseismic suspended building based on energy dissipation, 10th ECEE, Wien, 1994.


27


28

PART 3 - ITALIAN EXPERIENCES BEFORE THE EARTHQUAKE IN L’AQUILA


29

THE NEW EMERGENCY MANAGEMENT CENTRE FOR CENTRAL ITALY

The initiative was funded by Umbria Regional Government. The building complex is the

result of design research performed through material achievements with a dual purpose. The

first was to circulate BI techniques in a country where they were beginning to spread,

although with some difficulty; secondly, to test the importance of architecture in today’s

seismic problems. The intention was to show that in a holistic conception of the design

process, BI does not limit architectural choices and actually opens doors for the definition of

compositional forms able to mobilize

significant anti-seismic synergies.

The plan shows buildings designed

with differentiated BI solutions, taking

into account their intended use and

functions. In the following descriptions

there is greater focus on the two

achievements considered most

significant, which were designed in

collaboration with the architect Guido

Tommesani and the engineers Alfredo

Marimpietri and Marco Mezzi.

(A) OPERATIONS ROOMS BUILDING.

This is the nerve centre of the complex, shaped like a false cupola, 32 metres in diameter.

The structure rests on 10 HDRB elastomeric isolators with 1000 mm diameter, arranged along

the base perimeter. A double-vault system returns the loads above, exposing a slab floor

composed of ribs that intersect in a complex series of solids and voids. From the first level, by

the supports, 10 reinforced concrete semi-arches rise to meet at the keystone, where a central


30

unit comprising two concentric tubes of pre-stressed reinforced concrete is suspended and

contains the vertical courses. The central tube is extended below the ground surface to contain

the excess lift travel. The floors connect the central unit with the perimeter arches. The

building has a compact structure: inside there are ample spaces without columns and the

ground floor is accessible even though it does not have a pilotis configuration.

Design factors that optimize BI in this building

• High oscillation period (TIS=2.6). A value far from the seismic frequencies that

characterize the site. This ensures a sharp drop in demand which becomes almost

independent of seismic intensity in this field .

The building was designed for PGA=0.49g, indicated by regulations for ULS. It

corresponds to a return period of 950 years. Performance has been extensively verified.

The construction can oscillate slowly, almost undeformed, with displacements of ±40 cm.

• High oscillation period ratio (TIS/TBF > 3). The protected building has more compactness

than resistance. Deformations are controlled by the isolators. Although it is large and

devoid of columns, it has considerably more compactness than a frame structure. It

oscillates horizontally in the first manner, with a 99% participation of the total mass.

• Rocking-effect form. The low centre of mass reduces rocking effects (typical of a tapered

shape). This reduces compression variations on the isolators during seismic oscillations.

• Uniform form. The regular, substantially symmetrical shape meets uniformity

requirements.

• Centrifuging of stiffness. The peripheral layout of the isolators ensures minimal disruption

due to torsional effects.


31

Factors for enhancing strength capacity of reinforced concrete

• Compression of semi-arches. Much of the weight is supported by the core which

discharges the load onto the semi-arches, stressing mainly by compression.

• Centring of compression. In the presence of seismic actions, arch compression centring is

assumed by horizontal connections with the core. The dimensions are required more for

architectural reasons (sunshade elements) than for strength requirements. The calculated

C/D (capacity/demand) factors are almost 2.

• Critical points. The configuration has no critical areas where significant stress

concentrations may arise.


32

Noi abbiamo pensato, abbiamo discusso, abbiamo progettato, abbiamo calcolato;

loro hanno sudato ed hanno sofferto i disagi del lavoro di cantiere.

Sono loro, gli operai, che con le loro mani hanno costruito l'opera.

Awards

• AICAP Award 2010-11. Project Award winner AICAP (Italian Association for pre-

stressed and reinforced concrete).

• The significance of the project has been recognized internationally and is found on the

Earthquake Architecture homepage of the CUREE (Consortium of Universities for

Research in Earthquake Engineering) website.


33

(B) CULTURAL HERITAGE WAREHOUSE.

The project is an example of seismic protection for industrial buildings which cause major

accident risks29,30

in the event of an earthquake and in relation to their processes or contents.

The building has an octagonal plan of 2000 m2 and is about 9 metres high. The perimetral

walls are suspended. The main structure supporting the roof is formed by four steel beams

arranged in a cross, resting on 12 round reinforced concrete columns by means of an isolation

system, of which 8 are peripheral columns at the corners of the octagon, and 4 are central

pillars. The four roof beams are cantilevered to hold a perimeter beam from which the

peripheral closing walls are suspended. The insulation was achieved with LRB devices

arranged on the top of the perimeter columns and by sliding bearings arranged on the inner

columns. The floor is separated from the insulated part and is integral with the terrain. The

isolation system will therefore protect all of the elevated structure: roofs and closing walls.

The decision to suspend the walls not only attained protection of all the building masses,

but also made it possible to avoid the need for foundations for the peripheral walls that would

have had to sustain the seismic actions transmitted by the walls themselves, which are of

considerable height.

The building was designed with the same input as its neighbour and has a very similar

period of oscillation, so performance is also similar.

29

A. Parducci, F. Brancaleoni, Terremoto del 6 maggio 1976 nel Friuli. Considerazioni sul comportamento

degli edifici industriali, Industria Italiana del Cemento, Roma (Italia) 1976. 30

F. Menegotto, La prefabbricazione strutturale: aspetti teorici, Giornate AICAP, Stresa (Italia) 1987.


34

(C) OTHER BUILDINGS IN THE COMPLEX.

These are of no particular interest for our specific topic, because their architecture had

already been defined before deciding to switch to isolation. Necessary adaptations were then

required, for instance in the long single block which the Fire Department uses in part for

accommodation and in part for garages. It was necessary to provide for two different isolator

positions and to maintain roof continuity the two parts were separated with an internal sliding

connection.

(D) THE NEW JOVINE SCHOOL.

In 2002, an earthquake destroyed a school in San Giuliano di Puglia, killing 27 children.

The architectural design of the new school consisted of two adjacent buildings of irregular

shape. Alignment with new codes would have required an estimated 42% increase of the

resistance envisaged, with fallout on

the architecture. The BI option,

commissioned by the Civil Protection

Service, solved the problem and

ensured even better safety than is

offered by fixed-base design.31

This intervention is interesting for

the application of an idea already

mentioned in connection with a

number of Japanese constructions of a

larger size. Instead of isolating the

buildings directly, an entire base

platform is isolated. The same idea,

albeit with a different scope, was

successfully taken up by the C.A.S.E.

Project described below.

31

Design adaptation predefined by P. Clemente and G. Buffarini (ENEA), with M. Dolce and A. Parducci as

external consultants.


35

(E) MASONRY BUILDINGS

ATER Building in Corciano. Masonry buildings today are viewed with great interest by

bio-architecture. In the past, traditional conceptions tended to consider masonry unsuitable for

seismic zones, mainly due to the poor ductility of resistant elements. BI re-evaluates masonry

as an interesting seismic system. A consistent, well-organized configuration that guarantees

proper “box” behaviour, is a simple design option that alone ensures excellent performance

for a base-isolated building, especially for the “compactness” required to achieve dynamic

decoupling on which BI constructs its effectiveness. In 2007, for experimental purposes,

ATER of Perugia built the masonry residential block (seen in the image below) in Corciano, a

high-mid seismic hazard zone (PGA = 0.25 g).

SIRICA 2010 Award. The BI combines in a synergistic way with the energy-saving

demands of bio-architecture. This was highlighted by the Bio-Sisma project, winner of the

2010 Sirica Award (an architectural competition dedicated to this topic),32

in which the author

of these notes participated. Here a single reinforced concrete foundation platform resting on

32

M. Carli (project manager), Residenze BioSisma - Perugia, CNAPPC (National Council of Architects),

“Premio Sirica 2010. Sicurezza nell'abitare”, Di Baio Editore, Milano.


36

insulators was again included, acting as a thermal flywheel. Weights at low levels are not a

problem when BI is applied. The air used for ventilation of the building circulates in the

space below, where the insulators are found.


37


38

PART 4. THE ITALIAN EXPERIENCE AFTER THE AQUILA EARTHQUAKE


39

THE AQUILA EARTHQUAKE

6 April 2009 Earthquake. On 6 April 2009 a violent earthquake (IX-X on the MKS scale)

centred L’Aquila, the capital of the Abruzzo Region, a historic art city of 75˙000 inhabitants.

Not only the age of the buildings, but also the quality of more recent reinforced concrete

constructions, nonetheless designed with outdated regulations and sometimes without

adequate technical applications, rendered the situation

catastrophic. The result was 308 dead, 1500 injured,

20˙000 buildings collapsed or declared uninhabitable,

more than 25˙000 people left homeless.

THE PROJECT C.A.S.E.

The Civil Protection Service intervened promptly

and besides dealing with immediate relief operations,

also planned and implemented the “C.A.S.E.

Project”33

to cover the first phase of reconstruction. In

just over six months, it was able to complete

construction of 185 new buildings applying a new BI

concept. From 29 September 2009 to 19 February 2010, a total of 4450 new, fully-furnished

lodgings were handed over to approximately 15˙000 homeless.

One of the most significant aspects of this operation was the way in which BI was used.

The following description is focused on use

of BI and the central role it played in

completing the project. Descriptions of the

whole operation, complete with

photographic documentation, are available

online in Italian and English, on the Civil

Protection Service website.

33

Acronym for “Complessi Antisismici Sostenibili Ecocompatibili” [sustainable and eco-compatible anti-

seismic complexes]. The scheme was completed but aroused criticism and debate in the political sphere. Here

we describe it in relation to the Base Isolation function used in the project.


40

The project layout. The “isolated

deck” project was partly inspired by the

idea tested in San Giuliano di Puglia

where the entire foundations were

isolated instead of the buildings. Here a

variant was to use the space below,

primarily with a different scope.

A single prototype, consisting of two

parts, was applied to construction of all

the buildings.

The lower part comprises:

• two reinforced concrete plates, 21x57

metres and 50 cm high;

• supporting columns, usually metal, arranged on a 6x6-metre mesh;

• isolation devices of the friction pendulum type, placed above the columns.

The upper part included a three-storey building prototype to be implemented later, over the

upper isolated deck, with a settlement capacity of 70 persons minimum.

Plates and uprights. The design and construction of the two parts, although meant to be a

single unit, was conceived in two independent stages. In fact, a process had to be defined that

would resolve site needs and schedules as quickly as possible.

The lower structures consisted of a

repetitive prototype that was easy to make and

to be used for all buildings. The working

plans were therefore designed to assess

isolation system characteristics using

performance criteria, within the parameters

established for the building above. This

allowed work to start immediately.

Conversely, a preliminary draft was drawn

up for the upper part, based on tenders for

design and construction of the buildings,

which were to fall within parameters

compatible with the isolation system

designed. In this way it was possible to assign

projects and work to numerous qualified

contractors via tenders decided while the lower parts were under construction.

The strategy was compatible with the different characteristics of the construction

companies to be involved quickly and in large numbers. The prototype defined by the

preliminary draft was not shown as an example of an architectural solution, but as a detailed

reference to be interpreted according to performance logic, leaving maximum freedom for

configuration, choice of materials and construction systems.

The size of the concrete plates and the positions of the uprights were established using a

preliminary calculation that took into account distribution factors.


41

A load of approximately 1 MN was calculated for each column. Then a more detailed

sizing was performed, compatible with the idea of different types of building construction

(steel, wood, reinforced concrete, prefabrication) that contractors might present. Similarly, the

building oscillation period was estimated by basic formulas (T≈CS×H0.75

, where H is the

height of the prototype building and Cs0.050 0.085).

The three figures illustrate the flexural stress distribution on the top plate produced by local

lifting needed for any replacement or installation of a device (in some cases, to save time, the

devices were installed after construction of the plate).

The isolation devices. The choice fell to FPS Friction Pendulum System34

devices,

which function allowing relative movement by sliding of an articulated element along the

surfaces of steel spherical caps. Over 7300 devices were installed. The contract allowed for

use of other types of isolators, like rubber-steel. The choice of devices was left to the

contractors and was influenced mainly by the tight turnaround for complying with work

schedules.

The figure shows one of the F-d (force vs displacement) diagrams obtained from laboratory

tests performed on unified radius isolators of R=4 m. The final design of the entire isolated

system was thus performed by assuming horizontal response F force corresponding to the

expression F=(mg) +(mg/R)d, where mg represents the weight of the building; =3% the

friction coefficient; d set displacement. Secant stiffness Keff=14.6 kN/mm corresponding to a

displacement of up to 0.20 m was evaluated as indicated in the chart. Result: isolated period

34

D.M. Fenz, M.C. Constantinou, Behaviour of The Double Concave Friction Pendulum Bearing, Earthquake

Engng Struct. Dyn. June 2006.


42

T=3.29 seconds; damping =20.1%. Seismic tests in the non-linear field were performed

using spectrum-compatible accelerograms with standard spectra derived from records of

known real events (L'Aquila 2009; Imperial Valley 1979; Loma Prieta 1989; Northridge

1994; Kobe 1995; Taiwan 1999).

Housing construction. The figures illustrate some of the building stages. An interesting

solution, adopted in some cases for plant, which was encased in a metal frame suspended

below the isolated top plate. The solution avoids the need for flexible connections.


43


44


45

Environmental sustainability and energy

consumption. A significant aspect, inherent to how

these issues were addressed, should be explored but

is not part of the scope of this presentation, which

focuses on BI.

Conclusion. The entire operation was certainly

costly: a total of 705˙000˙000 €, including

furnishings, utilities, installation of gardens and

initial maintenance, broken down as follows:

• anti-seismic foundations 160

• buildings 424

• furnishings and plant 56

• lifts 10

• urban planning and gardens 55

On the other hand, the cost of maintaining 15˙000 people for a longer duration would be

equally high: an estimated extra 1˙500˙000 € per day for those without a home.

In February 2010, 10 month, after the earthquake, almost 4˙500 fully-furnished dwellings

were completed and delivered. The Emergency Management Service had finished its

commitment and handed over to local authorities. But that is a very different story.

-----------------------------------------------------------------------------------------------------------------

BEFORE CALCULATIONS WERE USED, DESIGN IDEAS WERE REQUIRED

In conclusion, we take a step back: Lisbon earthquake,

1755, a major historical event that was the first occasion

when attempts were made to define a seismic technique with

some engineering content. The idea developed of a gajola

house, also known as a gajola pombalina from the name of

the Marquis of Pombal, who wisely and skilfully managed

reconstruction. The house was based on a structural system

comprising a wooden frame of cross-resistant elements, filled

with inert material. The filling was of mud brick or even

compacted earth. No calculations were performed, but the system was interesting because it

had three advantages First, it was resistant: the wood structure with cross elements was


46

carefully prepared to resist horizontal actions. Second, it was uniform: the structure was

compact and well distributed throughout its height. Third, the capacity to dissipate energy

due to deformation of the inert material. In the most severe seismic attacks this material,

preferably confined to the wooden structure, would play a useful role in dissipation. Perhaps

knowledge was not ripe for considering this important synergistic contribution , but in reality

this did not reduce its efficacy.

It is interesting to note that this building system is still used in Portugal, where it was

invented, to perform structural interventions in existing buildings. A similar construction

technique was also used in Italy during the Bourbon period, after the Calabria Ulteriore

(Southern Italy) earthquake in

1873. There are a few but

insignificant relics. The concept

behind this system is nonetheless

proved by the work of the

architect Ferraresi, published in

works of the time.35

In some

sketches, a wooden double frame

enabled space to be filled by

compacting material inside.

It is interesting to note that in several Latin American countries there are relics (not to be

forgotten) of buildings of historical interest made of wooden frames filled with “tierra

pisada.”

-----------------------------------------------------------------------------------------------------------------

35

Giovanni Vivenzio: Istoria e teoria de’ tremuoti in generale ed in particolare di quelli della Calabria, e di

Messina del 1783, Stamperia Regale, Napoli, 1783.

IV CONFERENCIA INTERNACIONAL DE PELIGROSIDAD, RIESGO ... · Universidad de Oriente, Santiago de...

Documents

Transcript of IV CONFERENCIA INTERNACIONAL DE PELIGROSIDAD, RIESGO ... · Universidad de Oriente, Santiago de...