seismic conceptual design of buildings - basic principles

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Seismic Conceptual Design of Buildings Basic principlesfor engineers, architects, building owners,and authorities

H ugo Bachm ann


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Federal D epartm ent of Foreign A ffairs (D FA)

Federal D epartm ent of the Environm ent, Transport, Energy and C om m unications (D ETEC )

Seismic Conceptual Design of Buildings Basic principles

for engineers, architects, building owners,and authorities

H ugo Bachm ann


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Im pressum

Editor: Sw iss Federal O ffice for W ater and G eology

Sw iss Agency for Developm ent and C ooperation

Q uoting: Hugo Bachm ann: Seism ic Conceptual Design of

Buildings Basic principles for engineers, architects,

building ow ners, and authorities (Biel 2002, 81p.)

Available in french and germ an.

This publication is dow nloadable on the internet as

a pdf file at w w w .bw g.adm in.ch

D esign: Brotbeck Corporate Design, Biel

Im pression: 3000e

O rder N um ber: 804.802 e

A dress: BBL, Vertrieb Publikationen, C H -3003 Bern,

Internet: w w w .bbl.adm in.ch/bundespublikationen

Copyright: BW G , Biel, 2003


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The chosen m ethod explains basic principles by m atch-

ing them w ith illustrations, exam ples, and an explana-tory text. The principles, photographs (from the authoror third parties), and the texts are the result of a longresearch and design activity in the challenging andstrongly evolving field of earthquake engineering.The author w ould like to thank, above all, the num er-ous photographs contributors m entioned at the end ofthe booklet, w ho have m ade available the results ofextensive and often dangerous efforts. Thanks are alsoextended to the Federal O ffice for W ater and G eologyand the Sw iss A gency for D evelopm ent and C oopera-tion for editing and carefully printing this docum ent.

Zurich, Decem ber 2002 Prof. Hugo Bachm ann

Authors Preface

For a long tim e earthquake risk w as consideredunavoidable. It w as accepted that buildings w ould bedam aged as a result of an earthquakes ground shak-ing. Preventive m easures for earthquakes w ere there-fore m ostly lim ited to disaster m anagem ent prepared-ness. Although m easures related to constructionm ethods had already been proposed at the beginningof the 20th century, it is only during the last decadesthat im proved and intensified research has revealedhow to effectively reduce the vulnerability of structuresto earthquakes.

The objective of this docum ent is to present recentknow ledge on earthquake protection m easures forbuildings in a sim ple and easy to understand m anner.

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Basic principles for engineers, architects, building ow ners, and authorities

Editors Preface

W orldw ide earthquakes cause regularly large econom iclosses - Kobe in 1995 w ith m ore than 6000 causalities,counted for 100 Billion U S$ of econom ic loss. Earth-quakes are unavoidable. Reducing disaster risk is a toppriority not only for engineers and disaster m anagers,but also for developm ent planners and policy-m akersaround the w orld. D isaster and risk reduction are anessential part of sustainable developm ent.O n D ecem ber 11 2000, the Sw iss Federal C ouncil

approved for federal buildings a seven-point programrunning from 2001 to 2004 for earthquake dam ageprevention. The earthquake resistance of newstructures is a high priority in the C onfederationsseven-point program . The author of this publication,Professor H ugo Bachm ann, has devoted m any years tothe study of seism ic risk and behavior of buildingssubjected to earthquakes. A t the request of theFO W G , w hich expresses its gratitude to him , he agreedto m ake available his extensive scientific know ledge onearthquake resistance of buildings. These guidelinesare designed to contribute to the transfer of research

results into building practice. These results m ust be

taken into account by the design professionals, thusensuring a reasonable earthquake resistance for newstructures at little or no additional cost.

SD C w ould like to contribute to the dissem ination ofknow ledge on seism ic design of buildings by translat-ing this FW O G publication in English and thus extend-ing its readership am ong construction professionals.SD C intends to gather available experience in thedom ains of construction and prevention of naturalhazards and technical risks and to m ake it accessible to

the practitioners in developing and transition countriesin an easy to understand form .

Biel, D ecem ber 2002D r C hristian FurrerD irector of the Federal O fficefor W ater and G eology (FO W G )

Bern, D ecem ber 2002A m bassador W alter FuestD irector of the Sw iss A gency

for D evelopm ent and C ooperation (SD C )


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Table of Contents


O bjectives 6

W hat happens during an earthquake? 7

The m ost im portant natural risk 8

The seism ic risk keeps increasing 9

Insufficient m easures 9

U rgent action is needed 9

BP 1 The architect and the engineer collaborate from the outset! 10

BP 2 Follow the seism ic provisions of the building codes! 11

BP 3 N o significant additional cost thanks to m odern m ethods! 13

BP 4 A void soft-storey ground floors! 15

BP 5 A void soft-storey upper floors! 19

BP 6 A void asym m etric bracing! 21

BP 7 A void bracing offsets! 24

BP 8 D iscontinuities in stiffness and resistance cause problem s! 25

BP 9 Tw o slender reinforced concrete structural w alls in each 26principal direction!

BP 10 A void m ixed system s w ith colum ns and structural m asonry w alls! 28

BP 11 A void bracing of fram es w ith m asonry infills! 29

BP 12 Brace m asonry buildings w ith reinforced concrete structural w alls! 32

BP 13 Reinforce structural m asonry w alls to resist horizontal actions! 34

BP 14 M atch structural and non-structural elem ents! 38

BP 15 In skeleton structures, separate non-structural m asonry w alls by joints! 40

BP 16 A void short colum ns! 42

BP 17 A void partially infilled fram es! 44


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BP 18 D esign diagonal steel bracing carefully! 46

BP 19 D esign steel structures to be ductile! 48

BP 20 Separate adjacent buildings by joints! 50

BP 21 Favour com pact plan configurations! 52

BP 22 U se the slabs to tie in the elem ents and distribute the forces! 53

BP 23 D uctile structures through capacity design! 55

BP 24 U se ductile reinforcing steel w ith: Rm /Re 1.15 and A gt 6 % ! 56

BP 25 U se transverse reinforcem ent w ith 135 hooks and spacedat s 5d in structural w alls and colum ns! 58

BP 26 N o openings or recesses in plastic zones! 60

BP 27 Secure connections in prefabricated buildings! 62

BP 28 Protect foundations through capacity design! 64

BP 29 D evelop a site specific response spectrum ! 65

BP 30A ssess the potential for soil liquefaction! 66

BP 31 Softening m ay be m ore beneficial than strengthening! 68

BP 32 A nchor facade elem ents against horizontal forces! 70

BP 33 A nchor free standing parapets and w alls! 72

BP 34 Fasten suspended ceilings and light fittings! 74

BP 35 Fasten installations and equipm ent! 75

Illustration credits 78

Bibliography 79

C ontacts / Links 80

A ppendix: G lobal Seism ic H azard M AP 81


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The basic principles (BP) are grouped according to the

follow ing subjects:collaboration, building codes and costs (BP 1 to BP 3)lateral bracing and deform ations (BP 4 to BP 20)conceptual design in plan (BP 21 to BP 22)detailing of structural elem ents (BP 23 to BP 27)foundations and soil (BP 28 to BP 31)non-structural elem ents and installations(BP 32 to BP 35)

It is obvious that not all the basic principles are of thesam e im portance, neither in a general context nor inrelation to a particular object. C om prom ises, based on

engineering judgem ent, m ay be adm issible dependingon the hazard level (regional hazard and site effect) andthe characteristics of the structure. O f prim ary im por-tance is the strict adherence to the principles relevant tolife safety, particularly those concerning lateral bracing.O nly principles prim arily intended to reduce m aterialdam age m ay possibly be the subject of concessions.

This docum ent is predom inantly addressed to construc-tion professionals such as civil engineers and architects,but also to building ow ners and authorities. It is suitableboth for self-study and as a basis for university coursesand continued education. The illustrations m ay beobtained from the editor in electronic form at. A ll otherrights, in particular related to the reproduction ofillustrations and text, are reserved.

This docum ent offers a broad outline of the art of

designing earthquake resistant buildings. It describesbasic principles guiding the seism ic design ofstructures. These principles govern prim arily the:

Conceptual design, and theDetailing

of

Structural elementsandNon-structural elements

The conceptual design and the detailing of the structural

elem ents (w alls, colum ns, slabs) and the non-structuralelem ents (partition w alls, faades) plays a central role indeterm ining the structural behaviour (before failure) andthe earthquake vulnerability (sensitivity to dam age) ofbuildings. Errors and defects in the conceptual designcannot be com pensated for in the follow ing calculationsand detailed design of the engineer. A seism icallycorrect conceptual design is furtherm ore necessary inorder to achieve a good earthquake resistance w ithoutincurring significant additional costs.

The outlined principles are thus prim arily applicableto new buildings. H ow ever, it is quite clearthat they m ay also be used for the evaluation andpossible upgrading of existing buildings. Therefore,certain principles are illustrated w ith applications toexisting buildings.

The basic principles are intentionally sim ple. C alculationsand detailed design are only m arginally introduced.A dditional inform ation m ay be found in specialisedliterature (eg. [Ba02]).

The ideas and concepts of the basic principles w eredeveloped w ithin a fram ew ork consisting of num erous

presentations given by the author betw een 1997 and2000, the contents of w hich w ere constantly elaboratedand developed. Each principle is introduced by aschem atic figure (synthesis of the principle), follow ed bya general description. Further illustration is usuallyprovided by photographs of dam age, giving eitherpositive or negative exam ples, and accom panied by aspecific legend.

Objectives



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The effects of an earthquake on a building are prim ari-ly determ ined by the tim e histories of the three groundm otion param eters; ground acceleration (ag), velocity(vg), and displacem ent (dg), w ith their specificfrequency contents. Looking at the exam ple of thelinear horizontal ground m otion chart of an artificiallygenerated Valais Q uake, it is clear that the dom inantfrequencies of acceleration are substantially higherthan those for velocity and m uch higher than those fordisplacem ent.

The ground m otion param eters and other characteris-tic values at a location due to an earthquake of a givenm agnitude m ay vary strongly. They depend onnum erous factors, such as the distance, direction,depth, and m echanism of the fault zone in the earth'scrust (epicentre), as w ell as, in particular, the local soilcharacteristics (layer thickness, shear w ave velocity).In com parison w ith rock, softer soils are particularlyprone to substantial local am plification of the seism icw aves. A s for the response of a building to the groundm otion, it depends on im portant structural charac-teristics (eigenfrequency, type of structure, ductility,

etc).

Buildings m ust therefore be designed to coverconsiderable uncertainties and variations.

In an earthquake, seism ic w aves arise from suddenm ovem ents in a rupture zone (active fault) in theearth's crust. W aves of different types and velocitiestravel different paths before reaching a buildings siteand subjecting the local ground to various m otions.

The ground m oves rapidly back and forth in alldirections, usually m ainly horizontally, but also vertical-ly. W hat is the duration of the ground m otions?For exam ple, an earthquake of average intensity lastsapproxim ately 1020 seconds, a relatively short dura-tion. W hat is the m axim um am plitude of the m otions?For exam ple, for a typical Valais Q uake of anapproxim ate m agnitude of 6 (sim ilar to the earthquakethat caused dam age in the V isp region in 1855), theam plitudes in the various directions of the horizontalplane can reach about 8, 10, or even 12 cm . D uring anearthquake of m agnitude 6.5 or m ore (sim ilar to theBasel Q uake that destroyed m ost of the city of Baseland its surroundings in 1356), ground displacem entscan reach 15-20 cm , and perhaps som ew hat m ore.

W hat happens to the buildings? If the ground m oves

rapidly back and forth, then the foundations of thebuilding are forced to follow these m ovem ents. Theupper part of the building how ever w ould prefer torem ain w here it is because of its m ass of inertia. Thiscauses strong vibrations of the structure w ithresonance phenom ena betw een the structure and theground, and thus large internal forces. This frequentlyresults in plastic deform ation of the structure andsubstantial dam age w ith local failures and, in extrem ecases, collapse.

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What happens during an earthquake?


Rapid g round-mot ion:

Structura l (Building) respo nse:

How long?

How much?

Strong vibrations

Larg e stresses and strains

Local failure

Tot a l failure = Collap se

What ha ppens during a n earthq uake?

Pro f . Hugo Bachmann ibk ETH Zur ich

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Time (s)

Time histo ry of g round m ot ion pa ramet ers

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Pro f . Hugo Bachmann ibk ETH Zurich


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1%7%

47%

Ea rt hq ua kes Wind st o rms Flo o d s Ot hers

28%

7%

Grea t n a tura l ca t a st roph es 1950-1999

45%

M uni ch Re Group, 2000

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Fa ta lit ies: 1.4 mio Eco nomic losses: US$ 960 bn

30%

35%

The most important natural risk

Earthquakes of large m agnitudes can often be classi-fied as great natural catastrophes. That is to say thatthe ability of a region to help itself after such an eventis distinctly overtaxed, m aking interregional orinternational assistance necessary. This is usually thecase w hen thousands of people are killed, hundreds ofthousands are m ade hom eless, or w hen a countrysuffers substantial econom ic losses, depending on theeconom ic circum stances generally prevailing in thatcountry.

The 2001 G ujarat earthquake is a recent exam ple ofsuch a catastrophe. It w as the first m ajor earthquaketo hit an urban area of India in the last 50 years. Itkilled 13'800 people and injured som e 167'000. O ver

230'000 one- and tw o-story m asonry houses collapsed

and 980'000 m ore w ere dam aged. Further, m anylifelines w ere destroyed or severely dam aged and defacto non-functional over a long period of tim e. Thenet direct and indirect econom ic loss due to the dam -age and destruction is estim ated to be about U S$ 5billion. The hum an deaths, destruction of houses anddirect and indirect econom ic losses caused a m ajorsetback in the developm ental process of the State ofG ujarat.From 1950 to 1999, 234 natural catastrophes w erecategorized as great natural catastrophes [M R 00].From these 234, 68 (29% ) w ere earthquakes. The

m ost im portant ones in term s of loss of lives w ere the1976 Tangshan earthquake (C hina), w ith 290'000fatalities and the 1970 C him bote earthquake (Peru),w ith 67'000 fatalities. In term s of econom ic losses, the


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m ost im portant ones w ere the 1995 K obe earthquake

(Japan), w ith U S$ 100 billion, and the 1994 N orthridgeearthquake (U SA ) w ith U S$ 44 billion.In term s of loss of lives and econom ic losses, it can beseen on the figure of page 8 that earthquakesrepresent the m ost im portant risk from natural hazardsw orldw ide. It is tem pting to think that this risk isconcentrated only in areas of high seism icity, but thisreasoning does not hold. In regions of low to m oder-ate seism icity earthquakes can be a predom inant riskas w ell. There, hazard can be seen as relatively low , butvulnerability is very high because of the lack of pre-ventive m easures. This com bined leads to a high risk.

Devastating induced hazards

A part from structural hazards due to ground shaking,extensive loss can be caused by the so-called inducedhazards such as landslides, liquefaction, fire, retainingstructure failures, critical lifeline failures, tsunam is andseiches.For exam ple, the 2001 San Salvador earthquakeinduced 16'000 landslides causing dam age to 200'000houses. In the 1970 C him bote earthquake (Peru), agigantic landslide triggered by the earthquake caused25000 fatalities, m ore than a third of the totalfatalitites. In the 1906 San Francisco earthquake, m ostof the dam age w as caused by uncontrolled fire. In the1995 Kobe earthquake fire w as responsible for 8% ofthe destroyed houses.

The seismic risk keeps increasing

The seism ic risk is equal to the product of the hazard(intensity/probability of occurrence of the event, localsoil characteristics), the exposed value and the vulnera-bility of the building stock. The current building stockis constantly enlarged by the addition of newbuildings, m any w ith significant, or even excessive,earthquake vulnerability. This is above all due to thefact that for new buildings, the basic principles ofearthquake resistant design and also the earthquakespecifications of the building codes, are often notfollow ed. The reason is either unaw areness, conven-

ience or intentional ignorance. A s a result, theearthquake risk continues to increase unnecessarily.

Urgent action is needed

The preceding rem arks clearly illustrate that there is alarge deficit in the structural m easures for seism icprotection in m any parts of the w orld. There is anenorm ous pent up dem and and accordingly a need forurgent action. N ew buildings m ust be designed to bereasonably earthquake resistant to prevent theconstant addition of new vulnerable structures to abuilding stock that is already seriously threatened. Tothis end, the present publication aim s at contributingby spreading the appropriate basic know ledge.


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the engineer produces a safe, efficient and econom ical

structure. This is w hy collaboration betw een the architect

and the engineer m ust start at the first design draft!

Serial-design is particularly bad and inefficient. It isnot at all efficient that the architect perform s theconceptual design and selects the types and m aterials ofthe non-structural partition w alls and faade elem entsbefore entrusting the engineer w ith the calculations anddetailed design of the structure. It is also w rong to

consider seism ic loading only after com pleting thegravity load design and selecting the non-structuralelem ents. By then the structure can only be fixed forearthquakes. This w ill often result in an expensive andunsatisfactory patchw ork.

A parallell-design is m uch better and usually substan-tially m ore econom ical. The architect and the engineerdesign together and, taking into account the relevantaesthetic and functional requirem ents, develop a safe,efficient, and econom ical general-purpose structurefor gravity loads and seism ic action. They then togetherselect non-structural partition w alls and facade elem ents

w ith deform ation capacities com patible w ith thedesigned structure. A n optim um result can be obtainedthrough this approach. A close and thoughtfulcollaboration betw een the architect and the engineeris therefore also of interest to the building ow ner.This collaboration cannot w ait for the calculation anddetailed design stage, but m ust start at the earliestconceptual design stage w hen choices are m ade thatare crucial for the seism ic resistance and vulnerability ofthe building.

M any building ow ners and architects are still of them istaken opinion that it is sufficient to include the civilengineer only at the end of the design stage to calcu-late the structure. This is a bad approach that m ayhave serious consequences and cause significant addi-tional costs. Even the cleverest calculations and detaileddesign cannot com pensate for errors and defects inthe conceptual seism ic design of the structure or in theselection of non-structural elem ents, in particularpartition w alls and facade elem ents.

It is im portant that there is a close collaboration betw een

the architect and the engineer from the earliest planning

stage of any building project in order to ensure a good

outcom e, guarantee structural safety, reduce vulnerability,

and lim it costs. By doing so, both partners contribute w ith

different, yet indispensable, expertise. The architect deals

prim arily w ith the aesthetic and functional design, w hile

BP 1 The architect and the engineer collaboratefrom the outset!


Even the cleverest calculations and detailed design cannot

compensate for errors and defects in the conceptual

seismic design of the structural an d no n-structural element s!

Close collabora t ion be tw een a rchitect a nd

civil engineer from the earliest planning stage!

Basic principles fo r the seismic design of buildings

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Wrong:

1. Architect : Conceptua l design

of structure and non -structural

e lements

2. Engineer: Calculations

1. Structure for gra vity loa ds

2. Non-structural eleme nts

3. Structure for seismic action

Much bet ter and more economica l :

The a rchitect a nd en gineer

collabora te

Genera l purpose st ructure

and non-structural elements

Seria l-de sign

Pa rallell-design

Basic principles fo r the seismic design o f b uilding s1/2


The a rchitect an d eng ineer colla bo rat efrom the o utse t !

Architect

Building ow ner

Civil Engineer


1



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The ignorance or disregard of the seism ic provisions of

the building codes, even if only partial, can result in aninferior building [Sc 00]. The reduction in value m ayinclude, am ong other things, the costs of retrofittingm inus the additional costs that w ould have beenincurred to ensure the seism ic resistance of the build-ing at its design and construction stage. The designerscan be responsible for retrofitting costs, as w ell asjointly liable w ith the building ow ners for loss of life ,injury or for any resulting m aterial dam age in the caseof an earthquake. A retrofit generally costs severaltim es m ore than w hat it w ould have cost to ensureadequate seism ic resistance of the new building.

C onsiderable costs m ay also be incurred by disruptionsof the buildings use, such as tem porary evacuationand business interruption. Furtherm ore, determ iningthe responsibility of the architect and engineer cannecessitate lengthy and com plex legal procedures.The building ow ner, the architect, the engineer, andthe authorities therefore have a vested interest inensuring that the seism ic provisions of the buildingcodes are strictly enforced, and that appropriatestructural calculations and verifications are kept w iththe construction docum ents.

In the early 20th century, the first seism ic provisionsin building codes w ere introduced in a few countriesw ith high seism icity. These early seism ic codes havebeen periodically updated w ith increasing know ledgein earthquake engineering. In the 1960's and 1970's,countries w ith m oderate seism icity began to adoptseism ic requirem ents in their building codes.In the sam e period, the better understanding ofdynam ic soil behavior as w ell as inelastic structuralbehavior led to the developm ent of m ore advanced

seism ic codes.

Today, the principles of capacity design together w iththe concepts of ductile behavior allow a safe andcost effective earthquake resistant design. The latestefforts of seism ic code developm ent w ere m ainlyfocused on internationally harm onized standards likeISO 3010, Eurocode 8, and U BC .

U nfortunately, even today, the seism ic provisions of thebuilding codes are not alw ays respected; this is due toeither ignorance, indifference, convenience, ornegligence. M oreover, appropriate official controls and

checks are lacking. Buildings that are very vulnerableand at risk from even a relatively w eak earthquakecontinue to be built today. Investigations of existingbuildings (e.g. [La 02]) show ed how ever, that enforcingthe building code requirem ents m akes it possible tosignificantly reduce the seism ic vulnerability ofbuildings w ith no significant additional costs w hileim proving their resistance against collapse.

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BP 2Follow the seismic provisions of the codes!


Internat ionally ha rmonized standa rds:


2

T. Wenk

ISO 3010International Building Code (IBC)Uniform Building Code (UBC)Eurocode 8

Nationa l s tandards:

SIA 261 (Sw itze rlan d)IS 1893 (India )DIN 4149 (Germany)PS 92 (Fra nce) .

Follow th e seismic provisions of the codes!


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2/2 Buildings in w hich the lateral bracing is m issing or highly eccentric,

or buildings w ith discontinuities, generally do not satisfy the

requirem ents of the current building codes and are therefore likely to

be dam aged or collapse under the effect of even a relatively w eak

earthquake (Sw itzerland 2000).


2/1 Buildings are still built for w hich no verification of adequate

seism ic resistance is conducted in accordance w ith the current build-

ing codes. In the case of this m asonry building, it appears that no

adequate m easures (e.g. reinforced structural concrete w alls) w ere

taken. A n insufficient earthquake resistance m ay cause a significant

reduction in the value of the building, and m ay be the cause of a civil

liability law suit (Sw itzerland, 2001).


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design m ethod nam ed capacity design m ethod.

Thus, structural elem ents such as reinforcedconcrete w alls, w hich are used for w ind bracing,can perform other functions w ithout notable addi-tional cost (e.g. by m odifying the reinforcem ent).Few er additional structural elem ents are thereforerequired in com parison to older m ethods.

Inform ation on the application and advantages ofm odern m ethods can be found in the publication[D 0171]. This docum ent describes the seism ic designof a seven storey residential and com m ercial building.It enables a com parison betw een the deform ation-

oriented capacity design and conventional design(earlier m ethod). The advantages of the m odernm ethod for this exam ple can be sum m arised as follow s(see also page 14):

drastic reduction in the seism ic design forces atultim ate lim it state;

better resistance against collapse;good deform ation control;prevention of dam age for earthquakes up to a

chosen intensity (dam age lim it state earthquake);larger flexibility in case of changes in building use;practically equal costs.

The last three advantages are particularly im portant tothe building ow ner. The larger flexibility w ith respectto the changes in building use results prim arily fromthe fact that the m ajority of the w alls can be m odifiedor even rem oved w ithout any problem .

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3/1 Results of the seism ic design of a seven storey residential and

com m ercial building by different m ethods [D 0171].

The opinion that designing new buildings to be earth-quake resistant w ill cause substantial additionnal costsis still com m on am ong the construction professionnals.In a sw iss survey, estim ates betw een 3 and 17% of thetotal building costs w ere given. This opinion isunfounded. In a country of m oderate seism icity,adequate seism ic resistance of new buildings m ay beachieved at no, or no significant, additional cost.

H ow ever, the expenditure needed to ensure adequateseism ic resistance m ay depend strongly on the

approach selected during the conceptual design phaseand on the relevant design m ethod:

Regarding the conceptual design phase, early col-laboration betw een the architect and civil engineeris crucial (see BP 1). Seism ic protection m ust betaken into consideration in the architectural designof the building as w ell as in the conceptual designof the structure. A bove all, substantial extra costsm ay be incurred if m odifications and additions tothe structure need to be m ade at an advancedstage, since they often require m odifications of the

architectural design also. These m ay be very costly.

C oncerning the design m ethod, it should be statedthat significant progress has been m ade recently.Intensive research has im proved the understandingof the behaviour of a building or structure duringan earthquake and resulted in the developm entof m ore efficient and m odern design m ethods.C om pared to older m ethods, the cost of seism icresistance of a building is reduced and / orthe perform ance during an earthquake is notablyim proved, thus also reducing vulnerability. O f special

im portance are ductile structures and the associated

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BP 3No significant additional costs thanksto modern methods!


No sig nifican t a dditiona l costs tha nksto modern methods !

The costs of ea rthq uake resista nce depen ds on:

p lanning approach a pplied method

Basic principles fo r the seismic design of buildings3



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Seismic conventional design

W est faade Section C Section H

Seismic conceptual design and

capacity designW est faade Section C Section K

W alls, slabs, m ain beam s and colum ns in reinforced concrete to resist gravity loads

Reinforced concrete w alls and fram es to resist earthquake actions

Structural m asonry

4. floor

3. floor

2. floor

1. floor

m ezzaninne

ground floor

1. basem ent

2. basem ent

4. floor

3. floor

2. floor

1. floor

m ezzaninne

ground floor

1. basem ent

2. basem ent

C H

C H

C K

C K


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4/2 Sw ay m echanism s are often inevitable w ith soft storey ground

floors (Izm it, Turkey 1999).

4/3 Here the front colum ns are inclined in their w eaker direction, the

rear colum ns have failed com pletely (Izm it, Turkey 1999).

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4/4 This residential building is tilted as a result of colum n failure

(Taiw an 1999).

BP 4Avoid soft-storey ground floors!

Avoid soft -sto rey g round floo rs!

Basic principles fo r th e seismic design of buildings4


M any building collapses during earthquakes m ay beattributed to the fact that the bracing elem ents, e.g.w alls, w hich are available in the upper floors, areom itted in the ground floor and substituted bycolum ns. Thus a ground floor that is soft in thehorizontal direction is developed (soft storey). O ftenthe colum ns are dam aged by the cyclic displacem entsbetw een the m oving soil and the upper part of thebuilding. The plastic deform ations (plastic hinges) atthe top and bottom end of the colum ns lead to adangerous sw ay m echanism (storey m echanism ) w ith alarge concentration of the plastic deform ations at thecolum n ends.A collapse is often inevitable.

4/1 This sw ay m echanism in the ground floor of a building under

construction alm ost provoked a collapse (Friaul, Italy 1976).


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4/5 The w ell-braced upper part of the building collapsed onto the

ground floor

4/7 This m ulti-storey building escaped collapse by a hairs-breadth

4/8 thanks to resistant colum ns w ith w ell detailed stabilising and

confining reinforcem ent (Taiw an 1999).

4/6 and these are the rem ains of the left edge ground flour

colum n (Kobe, Japan 1995).


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4/10 Likew ise, it is probable that the slender colum ns under the

cladding of this existing building are too w eak. A few horizontally

short reinforced concrete structural w alls could help significantly

(Sw itzerland 1998).

4/9 It is feared that existing buildings such as this one could collapse

under even a relatively w eak earthquake (Sw itzerland 2000).


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5/2 In this office building also, an upper storey failed. The top of the

building has collapsed onto the floor below , the w hole building

rotated and leaned forw ards.

A n upper storey can also be soft in com parison to the

others if the lateral bracing is w eakened or om itted, or if

the horizontal resistance is strongly reduced above a

certain floor. The consequence m ay again be a danger-

ous sw ay m echanism .

5/1 In this com m ercial building the third floor has disappeared and

the floors above have collapsed onto it (Kobe, Japan 1995).

BP 5Avoid soft-storey upper floors!

Avoid soft-storey upper floors!



5/3 This close-up view show s the crushed upper floor of the office

building (Kobe, Japan 1995).


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5/4 A ll the upper floors w ere too soft (Izm it, Turkey 1999).


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6/1 In this new skeleton building w ith flat slabs and sm all structural

colum ns designed to carry gravity loads, the only bracing against

horizontal forces and displacem ents is a reinforced concrete elevator

and stairw ay shaft, placed very asym m etrically at the corner of the

building. There is a large eccentricity betw een the centres of m ass

and resistance or stiffness. Tw isting in the plan w ill lead to large

relative displacem ents in the colum ns furthest aw ay from the shaft

and the danger of punching shear failure that this im plies. Placing a

slender reinforced concrete w all, extending the entire height of the

building at each facade in the opposite corner from the shaft w ould

be a definite im provem ent. It w ould then be enough to construct

tw o of the core w alls in reinforced concrete and the rest could be for

exam ple in m asonry (Sw itzerland 1994).

A sym m etric bracing is a frequent cause of buildingcollapses during earthquakes. In the tw o above sketch-es only the lateral bracing elem ents are represented(w alls and trusses). The colum ns are not draw nbecause their fram e action to resist horizontal forcesand displacem ents is sm all. The colum ns, w hich onlyhave to carry the gravity loads, should how ever be ableto follow the horizontal displacem ents of the structurew ithout loosing their load bearing capacity.

Each building in the sketch has a centre of m ass M(centre of gravity of all the m asses) through w hichthe inertia forces are assum ed to act, a centre of resist-ance W for horizontal forces and a centre of stiffnessS (shear centre). The point W is the centre of gravityof the flexural and fram e resistance of structuralelem ents along the tw o m ajor axes. If the centre ofresistance and the centre of m ass do not coincide,eccentricity and tw isting occur. The building tw ists inthe horizontal plane about the centre of stiffness.In particular, this torsion generates significant relativedisplacem ents betw een the bottom and top of thecolum ns furthest aw ay from the centre of stiffness andthese often fail rapidly. Therefore the centre of resistance

should coincide w ith, or be close to, the centre of m ass,and sufficient torsional resistance should be available.This can be achieved w ith a sym m etric arrangem ent ofthe lateral bracing elem ents. These should be placed,if possible, along the edges of building, or in any casesufficiently far aw ay from the centre of m ass.

BP 6Avoid asymmetric bracing!

M

S W

Avoid asymmet rical horizont al bra cing !

W, S

M



Page 22

6/2 This office building had a continuous fire w all to the right rear

as w ell as m ore eccentric bracing at the back. The building tw isted

significantly, and the front colum ns failed (Kobe, Japan 1995).


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6/5 O riginally, the only horizontal bracing in this 70's auditorium

building at the H nggerberg C am pus of ETH Zurich w ere reinforced

concrete w alls w ith little torsional resistance situated at the rear of

building. Because of the considerable distance betw een the bracing

and the centre of m ass of this large building, it w ould have tw isted

significantly in the plan for even a relatively w eak earthquake

(seism ic zone 1 according to SIA 160). The few highly loaded

reinforced concrete colum ns in the ground floor w ould have experi-

enced substantial displacem ents, particularly in the front of the

building. H ow ever, the colum n detailing w as inadequate for the

required ductility. A dditional steel colum ns w ere therefore built in on

three sides of the building exterior. They form a truss that can

transfer the horizontal seism ic forces to the existing foundations.

This upgrading also fulfilled the need for a strengthening of the

cantilevered structure for gravity loads.

6/6 The incorporation of the new tubular steel truss colum ns is

aesthetically satisfying.

6/3 6/4 In the back, this house share a strong and stiff fire w all w ith

another house. In the front, the facade is substantially softer, so that

the centres of resistance and stiffness w ere situated to the back of

the building. The house tw isted strongly in the horizontal plane, but

did not collapse (U m bria, Italy 1997).

23



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BP 7 Avoid bracing offsets!

7/1 The horizontal offset of the reinforced concrete w all in the

vertical plane causes large additional stresses and deform ations in

the structure during an earthquake. They include large local vertical

forces (from the overturning m om ent), large additional shear forces

in the slabs at offsets, redistribution of the foundation forces, etc.


H orizontal bracing offsets, in plane (at the bottom ofthe plan figure) or out of plane (at the top of theplan figure), result w hen the position of the bracingchanges from one storey to another. The bendingm om ents and the shear forces induced by the offsetcannot be fully com pensated, despite substantialadditional costs.The offsets disturb the direct flow of forces, w eakenthe resistance and reduce the ductility (plastic defor-m ation capacity) of the bracing. M oreover, they causelarge additional forces and deform ations in otherstructural elem ents (e.g. slabs and colum ns).C om pared to bracings that are continuous over theheight of the building, bracings w ith offsets increasethe vulnerability of the construction and usuallynoticeably reduce its seism ic resistance. Bracing offsetsm ust therefore be absolutely avoided!

Avoid b racingof fse t !




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8/2 D uring an earthquake, the reinforced concrete cantilever w all

(behind the curtain), w ill induce significant additional stresses in thealready highly loaded colum n on the ground floor (Sw itzerland

2001).

M odifications in the cross section of bracing system sover the height of a building cause discontinuities andlead to sudden variations in the stiffness and resistanceof the building. This can cause irregularities in thedynam ic behaviour and disturb the local flow of forces.A n increase in the stiffness and resistance from thebottom up (left in the elevation figure) is generally lessfavourable than the opposite (right in the elevationfigure). In any case, the calculation of the sectionalforces and the design of the structure as w ell as thedetailing of the discontinuities m ust be conducted verycarefully.

8/1 The transition from a reinforced concrete structural w all to a

fram e structure causes large discontinuities in stiffness and resistance


BP 8 Discontinuities in stiffness andresistance cause problems!

Discont inuities insti ffn ess a nd resista nce

cause prob lems!




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9/1 Such reinforced concrete structural w alls take up only little

space in plan and elevation (Sw itzerland 1994).

9/2 The reinforcem ent of reinforced concrete structural w alls is

relatively sim ple, but it m ust be detailed and laid w ith great care.

The figure show s a capacity designed ductile w all, of rectangular

cross-section, w hich w as added to an existing building (Sw itzerland

1999).

Reinforced concrete structural w alls of rectangularcross-section constitute the m ost suitable bracingsystem against seism ic actions for skeleton structures.The w alls m ay be relatively short in the horizontaldirection e.g. 3 to 6 m or about 1/3 to 1/5 of thebuilding height they m ust, how ever, extend over theentire height of the building. In a zone of m oderateseism icity, in m ost cases tw o slender and capacitydesigned ductile w alls in each m ajor direction aresufficient. The type of non-structural elem ents can alsoinfluence the selection of the dim ensions (stiffness) ofthe bracing system (cf. BP 14). To m inim ise the effectsof torsion, the w alls should be placed sym m etricallyw ith respect to the centre of m ass and as close aspossible to the edges of the building (cf. BP 6).C onsidering seism ic forces transfer to the ground(foundation), corner w alls should preferably be avoid-ed. W hen the w alls have L cross-section (angle w alls)or U crosssections, the lack of sym m etry can m akedetailing for ductility difficult. Reinforced concretew alls w ith rectangular cross-section (standard thickness30 cm ) can be m ade ductile w ith little effort, thusensuring a high seism ic safety [D 0171].

BP 9Two slender reinforced concrete structuralwalls in each principal direction !

Tw o slen de r reinf orced con crete structura lw a lls in ea ch principal direction!




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9/3 This skeleton structure has reinforced concrete structural w alls in

the transverse directions at tw o building corners.

9/4 The structural w alls w ere included as prom inent elem ents in the

architectural concept (Sw itzerland 1994).


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can im pair the building functionally [D 0171]. A consis-

tent design of the structure as a skeleton structure, i.e.colum ns only (no m asonry w alls) w ith som e slenderreinforced concrete structural w alls extending theentire height of the building, is thus also in the long-term interest of the ow ner. A s the interior partitionsare non-structural elem ents, they are easy to refit incase of changes in the buildings use. Extensivestructural m odifications are therefore not necessary.

10/1 This structural stairw ay w all w ill be destroyed by a relatively

w eak earthquake. A total collapse of the building m ay result


M ixed structural system s w ith concrete or steelcolum ns and structural m asonry w alls behave veryunfavourably during earthquakes. The colum ns incom bination w ith the slabs or beam s form fram es,w hich have a substantially sm aller horizontal stiffnessthan the m asonry w alls. The earthquake actions aretherefore carried to a large extent by the m asonryw alls. In addition to the inertia forces from their ow ninfluence zone, the w alls m ust resist those from theparts of the building w ith the colum ns (to the left inthe figure). This results in a seism ic resistance consider-ably less than that of a pure m asonry construction.W hen m asonry w alls fail due to the seism ic actions ordeflections, they can no longer carry the gravity loads,w hich usually leads to a total collapse of the building.M ixed system s of colum ns and structural m asonryw alls m ust therefore be absolutely avoided.

Furtherm ore, such m ixed system s prove to beunfavourable because of their lack of flexibility w ithregard to increasingly frequent building m odificationsrequired by changes in their use. Rem oval of m asonryw alls require heavy structural interventions, w hich arecostly (up to several percent of the building value) and

BP10Avoid mixed systems withcolumns and structural masonry walls!

Avoid mixedsystem s of

columns andstructural

ma son ry w alls!

Reinforcedconcrete fram e

Structuralmasonry wall




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11/1 H ere the colum ns w ere clearly stronger and the m asonry fell

out w hile the fram e rem ained standing (Erzincan, Turkey 1992).

It is still a com m on opinion that filling in fram e struc-tures w ith m asonry w alls im proves the behaviour underhorizontal loads including seism ic actions. This is trueonly for sm all loads, and as long as the m asonry rem ainslargely intact. The com bination of tw o very different andincom patible construction types perform s poorly duringearthquakes. The fram e structure is relatively flexibleand som ew hat ductile, w hile unreinforced m asonry isvery stiff and fragile and m ay explode under theeffect of only sm all deform ations. At the beginning ofan earthquake the m asonry carries m ost of the earth-quake actions but as the shaking intensifies the m asonryfails due to shear or sliding (friction is usually sm all dueto the lack of vertical loads). The appearance ofdiagonal cracks is characteristic of a seism ic failure.

Tw o basic cases can be identified: Either the colum ns arestronger than the m asonry, or vice-versa. W ith strongercolum ns the m asonry is com pletely destroyed and fallsout. W ith w eaker colum ns the m asonry can dam ageand shear the colum ns, w hich often leads to collapse(see also BP 16 and 17).

BP 11Avoid bracing of frames with masonry infills!

Avoid bra cing of fra mesw ith m a son ry infi lls!



Page 30

11/2 In this case the m asonry w as stronger: The colum ns experi-

enced significant dam age and w ere partly sheared; nevertheless, the

fram e is still just standing (M exico 1985).


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11/4 These diagonal cracks are typical of reinforced concrete fram e

m asonry infills (Turkey, Izm it 1999).

11/3 The m asonry w as also stronger in this case; it sheared the

relatively large colum ns (A dana-C eyhan, Turkey 1998).


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12/1 Such and also low er! new m asonry structures, w ithout

bracing reinforced concrete structural w alls, are extrem ely vulnerable

to earthquakes (Sw itzerland 2001).

12/2 This new 3-storey residential building w ith unreinforced

m asonry structural w alls is braced longitudinally by a reinforced

concrete structural w all in each facade, and transversely by an

interior reinforced concrete structural w all (Sw itzerland 2001).

Traditionally in m any countries, houses and sm allercom m ercial buildings are often built w ith unreinforcedm asonry w alls m ade of clay, lim estone or cem entbricks. M asonry is a good construction m aterial interm s of therm al insulation, storage and vertical loadscarrying capacity. For seism ic actions how ever, m asonrystuctures are not w ell suited. O n one hand they arerelatively stiff, so they usually have a high naturalfrequency w ithin the plateau area of the designresponse spectrum and therefore experience largeearthquake actions. O n the other hand unreinforcedm asonry w alls are rather brittle and generally exhibitrelatively little energy dissipation. G enerally, it is notpossible to obtain adequate seism ic resistance (even inregions of low seism icity) and additional m easures aretherefore necessary.

A possible solution consists of bracing unreinforcedm asonry buildings w ith reinforced concrete structuralw alls. H ereby it is possible to lim it the horizontaldeform ations of the m asonry and therefore preserve itsgravity load carrying capacity. The reinforced concretestructural w alls m ust be designed to be sufficientlystiff, the horizontal w all length and the vertical

reinforcem ent ratio being key param eters. They m ustbe able to carry the seism ic actions and to transm itthem to the foundations w hile rem aining elastic, i.e.w ithout notable yielding of the reinforcem ent.The horizontal deflection of the reinforced concretestructural w alls under the design earthquake m ust notexceed the displacem ent capacity of the stiffest, i.e.longest, m asonry w all.

BP 12Brace masonry buildings with reinforced concretestructural walls!

Stiffen m a son ry building s w ith reinfo rcedconcret e structural w a lls!

MasonryStructural

concrete wall Masonry




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12/4 Structural m asonry w alls, reinforced concrete structural w alls

and slabs should respond together w hen subjected to shear,

com pression, and if possible tension (Sw itzerland 2001).

12/3 This new 4-storey m asonry structure is braced by one

reinforced concrete structural w all in each m ajor direction. There

is also a long m asonry w all in both directions that has a horizontal

layer joint reinforcem ent and is anchored to the concrete w all


12/5 This is w hy it is recom m ended to fill in the joints betw een

structural m asonry w alls and reinforced concrete structural w alls

w ith m ortar (Sw itzerland 2001).


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A possible alternative to basic principle 12 for m akingm asonry structures substantially m ore suitable forseism ic actions is to reinforce som e long m asonry w allsand thus stiffen them in the longitudinal direction.In this case, for exam ple, vertical and horizontalm inim um reinforcem ent and stronger vertical rein-forcem ent in the boundary zones m ust be detailed [Ba02]. Thus sliding in the horizontal layer joints can beprevented and a global ductility of up to

~=2 can beachieved. The reinforced w alls can therefore beconsidered as structural m asonry w alls for horizontalactions. The horizontal displacem ent of the reinforcedm asonry w alls for the design earthquake m ust notexceed the ultim ate displacem ent capacity of thestiffest i.e. longest, unreinforced m asonry w all.This is necessary to ensure that the vertical load-bearing capacity of the unreinforced m asonry w alls ispreserved.

BP 13 Reinforce structural masonrywalls to resist horizontal actions!

13/1 13/2 Reinforced m asonry requires special bricks, particularly to

incorporate and coat the vertical reinforcing bars. W orldw ide

developm ents in reinforcing system s and adequate bricks are under

w ay. The tw o pictures show new developm ents in the clay m asonry

industry (Sw itzerland 1998).

Reinforce structuralma sonry w al ls to

resist h orizont a l actions!

Minimum reinforcement

Edge reinforcement




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13/4 13/5 Vertical pre-stressing can also im prove the earthquake

behavior of m asonry w alls by substantially increasing the vertical

force (Sw itzerland 1996).

13/3 This type of vertical reinforcem ent is anchored at the top and

bottom w ith U -shaped bars extending in 2 brick layers. The barsused to anchor the w alls to the slabs or low er w alls are very im por-

tant (Sw itzerland 1998).


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Page 37

13/8 It is also necessary to consider the capacity requirem ent

perpendicular to the w all (out-of-plane). This applies in particular

to gable w alls (cantilever), to other m asonry w alls that are poorly

restrained against horizontal forces and, for stronger earthquakes,

also to w alls supporting slabs. H ere the w alls in the upper floor,

w hich carried only a sm all vertical load, failed out-of-plane (Lom a

Prieta 1989). Reinforcem ent, vertical pre-stressing, or glued on plates

can also prevent such failure.

13/6 The strength and ductility of m asonry w alls in existing

buildings can be im proved w ith carbon fiber or steel plate

reinforcem ents (Sw itzerland 1996).

13/7 The plates m ust be glued on carefully and anchored in the

slabs (Sw itzerland 1997).


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14/1 H ere, the non-structural partition w alls w ere destroyed,

although the fram e structure deform ed only little and is hardly

dam aged. Even the w indow s rem ained intact (A rm enia 1988).

14/2 A nd here, a collapsed partition w all is sim ply rebuilt until the

next earthquake... (A dana-C eyhan, Turkey 1998).

Page 39

14/3 The glass facade of this new m ultistorey building survived a

strong earthquake alm ost w ithout loss, ow ing to special flexible

fastenings for the facade elem ents (Kobe, Japan 1995).

If deform ation-sensitive non-structural partition w allsand facade elem ents (e.g. of m asonry) are incorporat-ed into a horizontally soft structure (e.g. a fram estructure) w ithout using joints, substantial dam agem ay develop even for relatively w eak earthquakes.Experience show s that in such cases a building m ustsom etim es be dem olished, even though the structuresuffered no substantial dam age. A m odern earthquakeresistant design m ust therefore m atch the stiffness ofthe structure and the deform ation capacity of thenon-structural partition w alls and facade elem ents.The interstory drift ratio (i.e. the interstorey drift,,divided by the interstorey height, h) and the vulnerabil-ity of the non-structural elem ents are crucial. Theskillful selection and com bination of structuraland non-structural elem ents can prevent dam ages,even for relatively strong earthquakes.

BP 14Match structural and non-structural elements!

Match s tructural a ndno n-structural element s!

Governing size:

Inter-storey

displacement




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In flexible skeleton structures, it can be beneficial toseparate non-strucutral partition w alls from thestructure by soft joints. This is particularly true forinplane stiff and brittle m asonry w alls.This w ay,dam age occuring even for w eak earthquakes can beprevented. The joints run along colum ns, structuralw alls, and slabs, or beam s and m ust be filled by a veryflexible soundproof m aterial, e.g. boards of softrubber. Styrofoam , cork, etc. are too stiff in this case.The necessary joint thickness (typically 20 to 40 m m )depends on the stiffness of the structure and thedeform ation sensitivity of the partition w alls as w ell asthe desired protection level (dam age lim it stateearthquake < design earthquake) [D 0171]. G enerallythe partition w alls m ust also be secured against out-of-plane actions (plate effect), e.g. by support angles.

BP 15In skeleton structures, separate non-structuralmasonry walls by joints!

Rubber

1040 mm

In skeletonstructures,separate

non-structuralmasonry walls

by joints!



15/1 H ere a vertical joint separates the m asonry w all and the

reinforced concrete colum n, but it is probably m uch too thin



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15/2 The joints thickness here a horizontal joint betw een am asonry w all and a slab and the capacity of the support angles

(bolts) m ust be m atched to the deform ation of the structure and

the capacity dem and for the desired protection level (dam age lim it

state earthquake) (Sw itzerland 1994).

15/3 This joint betw een a m asonry w all and a reinforced concrete

structural w all w as filled by expanded polystyrene boards. But

Styrofoam is too stiff for earthquake displacem ents; soft rubber

w ould be a m ore suitable m aterial (Sw itzerland 1994).


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16/1 The diagonal cracks and shear failures in the short colum ns of

a m ulti-storey car park alm ost caused collapse (N orthridge, California

1994).

The shear failure of so-called short colum ns is afrequent cause of collapse during earthquakes.It concerns squat colum ns, i.e. colum ns that arerelatively thick com pared to their height, and are oftenfixed in strong beam s or slabs. Slender colum ns canbe turned into short colum ns by the addition ofparapet infills in fram e structures (unintentionallyshortened colum ns).C olum ns under horizontal actions in fram e structuresm ay be stressed up to their plastic m om ent capacity(plastification or failure m om ent). In the case of shortcolum ns w ith considerable bending capacity, anenorm ous m om ent gradient and thus a large shearforce results. This often leads to a shear failure beforereaching the plastic m om ent capacity. Short colum nsshould therefore be avoided. A n alternative is todesign and detail the colum ns in accordance w ith therules of capacity design, w hereby the shear capacitym ust be increased to account for the overstrength ofthe vertical reinforcem ent [Ba 02] [PP 92].

BP 16Avoid short columns!

Avoid sho rt column s!

Enormousmoment gradient

shear f ailure!

Mpl

Mpl

l




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16/3 Shear failure in the corner short colum n on the ground floor

led to near-failure of this com m ercial building (Erzican Turkey 1992).

16/2 H ere, the m asonry colum ns in the ground floor of a restaurant

behaved as short colum ns. They w ere highly dam aged by diagonal

cracks (U m bria, Italy 1997).


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17/2 To the left of the destroyed colum n there used to be a w indow

opening sim ilar to the one on the far left of the picture. The already

dem olished m asonry w all under the w indow opening behaved like a

partial infill w all. It m oved to the right, pushed against the colum n

and sheared it off.

17/3 Better transverse reinforcem ent in the colum n (sm all spaced

hoops and ties) w ould probably have prevented the shear failure.

H ow ever, the source of the problem lies in the partial infilling of the

fram e that caused the short colum n phenom enon (lzm it, Turkey

1999).

The infill of parapet w alls into a fram e structure w ithoutthe addition of joints can cause short colum n phenom e-na (see previous basic principle). Shear failure occurs,or in cases of sufficient shear strength a sw aym echanism develops w ith possibly significant secondorder effects (P--Effect).

17/1 In this case, inserting parapet w alls into a fram e led to a short

colum n phenom enon. O w ing to the good confinem ent of the

transverse reinforcem ent, no actual shear failure occurred, but an

equally dangerous sw ay m echanism developed (Friaul, Italy 1976).

BP 17Avoid partially infilled frames!

Avoid partially infilled frames!




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17/6 A possibility to avoid or strongly reduce the unfavourable

effect of infill parapet w alls into fram es, is the addition of joints

betw een the infill w all and colum ns. The joint w as realized correctly,

since it is filled by a soft and therefore strongly com pactible rock

w ool sheet. H ow ever, the w idth only perm its a 1% free lateral drift

ratio of the colum n (Sw itzerland 2001).

17/4 H ere too, inserting m asonry w alls and long w indow openingscaused high additional stresses and colum n failure. The relatively

good behavior of the m assive colum n to the right in the picture con-

tributed to the fact that the building narrow ly escaped collapse.

17/5 This colum n illustrates unsatisfactory detailing (hoops w ith 90

instead of 135hooks, com pare w ith BP 25). W ithout the unfavorable

effect of the infill w alls it w ould how ever have behaved m uch better

(lzm it, Turkey 1999).


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18/1 D iagonal elem ents w ith broad flange cross sections have

buckled about their w eak axis...

18/2 and have broken (Kobe, Japan 1995).

For the bracing of builidings, in particular industrialbuildings, steel truss system s can be used. It m usthow ever, be carefully thought out and designed.The com m on truss bracing w ith centre connectionsand slender diagonal m em bers m ay show a veryunfavourable behavior under cyclic actions. The diago-nals yield under tension, lengthen m ore w ith eachcycle and end up buckling under com pression. U nderrepeated cyclic m ovem ents, the stiffness of the trussbecom es very sm all at the zero deform ation point.This, com bined w ith dynam ic effects, can contribute tothe failure of the structure. Such bracing m ust there-fore only be designed for elastic behaviour, or ifnecessary very low ductility. It is advisable m oreover tocheck com patibility betw een the deform ations of thebracing and those of the other structural andnon-structural elem ents. This can indicate the need form ore stiff bracing or other bracing system s, such asw alls. Steel truss system s w ith eccentric connectionsand com pact m em bers behave m uch better thantrusses w ith centre connections and slender m em bers[Ba 02].

BP 18Design diagonal steel bracing carefully!

Design diago na lsteel bra cing carefully!



18


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18/3 This truss structure also suffered buckling of truss elem ents and

m any local dam ages (Kobe, Japan, 1995).


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19/1 This steel fram e suffered large perm anent deform ations. There

w as probably no lateral bracing and the connection detailing w as

inadequate for cyclic actions (Kobe, Japan 1995).

19/2 The bolts failed in this beam to colum n connection (Kobe,

Japan 1995).

Steel generally possesses a good plastic deform ationcapacity (strain ductility). N evertheless steel m em bersand steel structures m ay show low ductility or evenbrittle behavior under cyclic actions, particularly due tolocal instabilities and failures. For exam ple elem entsw ith broad flanges (colum ns and beam s) m ay buckle inplastic zones or fail at w elds. Therefore, certainrequirem ents m ust be com plied w ith and addtitionalm easures m ust be considered during the conceptualdesign of the structure and the selection of them em bers cross sections [Ba 02] [EC 8].

BP 19Design steel structures to be ductile!

Desig n steelstructures to

be d uctile!Critical zone s


Pro f . Hugo Bachmann ibk ETH Zr ich


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19/5 19/6 There is a w ide crack at the bottom of this m ain fram e

colum n in a m ulti-storey steel building (to the right in the upper

picture). Possible causes include the high cyclic norm al loads, the

high strain rate m aterial defects, w eld defects, and therm al stresses

(Kobe, Japan 1995).

19/3 This picture show s the failure of a typical fram e connection.

The w elding betw een the colum n and the beam failed, resulting in a

w ide crack (Kobe, Japan 1995).

19/4 The rectangular colum n of this 3-storey fram e structure suf-

fered local buckling at its foot. The resulting cracking of the coating

w hite paint is visible (Kobe, Japan 1995).

49



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20/1 The pounding of tw o sim ilar buildings w ith floors at the sam e

levels caused dam age to the faades as w ell as spalling etc. to thestructure (M exico 1985).

Pounding and ham m ering of adjacent buildings cancause substantial dam age, if not collapse. The threatof collapse is greatest w hen the floor slabs of adjacentbuildings are at different levels and hit against thecolum ns of the neighbouring building. In such casesthe joints m ust conform w ith the relevant design rules.This im plies the follow ing:1) the joints m ust have a certain m inim um w idth

(specified in the building codes)2) the joints m ust be em pty (no contact points)In order to enable free oscillations and avoid im pactbetw een adjacent buildings, it is often necessary tohave a substantial joint w idth. A s long as the structuralelem ents do not lose their load bearing capacity atpounding, other solutions are also possible [EC 8].

BP 20Separate adjacent buildings by joints!

Separate adjacentbuildings by joints!




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20/3 The m odern reinforced concrete building to the left collapsed

after pounding against the older very stiff building to the right(M exico 1985).

20/4 The collapsed building w as an extension of the older building

to the left. Either the joint w idth w as insufficient or the buildings

w ere not connected properly. D uring the earthquake, the older

building pounded against the new one and caused its collapse

(Kobe, Japan 1995).

20/2 Substantial dam age resulted from the pounding of these tw o,

very different, buildings (M exico 1985).


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21/1 In order to allow building w ings oriented orthogonally to each

other to oscillate independently, they should be separated by a

sufficiently w ide and com pressible joint.

W hen designing a building, it is im portant to visualisethe dynam ic behaviour of the structure as realisticallyas possible. In this L-shaped building, the stiffnesses ofthe tw o w ings, respective to each principal direction,are very different. The tw o w ings w ill tend to oscillatevery differently but w ill also hinder each other. Thisleads to large additional stresses, particularly at thecorners of the floor slabs and at the end of each w ing,and m ay necessitate heavy structural m easures. Theproblem can be avoided by separating the tw o w ingsby a joint respecting relevant seism ic design rules.The result is tw o com pact rectangular buildings thatare dynam ically independent.

BP 21Favour compact plan configurations!

Favour compact plan configurations!

unfavourable be t t e r




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22/1 A corner area of this building collapsed. The slabs consisted

only of precast elem ents w ithout reinforced concrete cover andw ithout reinforced connections to the vertical load bearing elem ents

(A rm enia 1988).

In m ulti-storey buildings the floor slabs m ust be nearlyrigid diaphragm s. They m ust be properly connected toall the gravity load bearing elem ents to act as sectionshape preservers (diaphragm s). The slabs have toensure that all the vertical elem ents contribute to thelateral resistance. They distribute the seism ic forces anddisplacem ents betw een the various vertical structuralelem ents according to their individual stiffness.Slabs m ade of prefabricated elem ents are not recom -m ended. If this solution is adopted, the floor elem entsm ust be covered w ith adequately cast in placereinforced concrete of sufficient thickness. M onolithicreinforced concrete slabs w ith eventual additionalboundary reinforcem ent bars are m uch better suited toact as diaphragm s.

53

BP 22Use the slabs to tie in the elements anddistribute the forces!


Use th e slab s to tie inthe elements and distr ibute the forces!

unfavourable be t t e r

Basic principles fo r th e seismic desig n o f b uilding s22



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54


22/2 22/3 In these houses also, the slabs consisted only of precast

elem ents, w hich w ere insufficiently connected betw een each other

and w ith the w alls (A rm enia 1988).


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D uctile (i.e. w ith large inelastic deform ation capacity)structures usually offer substantial advantages in com -parison to sim ilar brittle structures. M ost im portantly,the required structural resistance can be reducedbringing substantial savings and increased safetyagainst collapse. W henever possible the structure of abuilding should be designed to be ductile. This is alsoappropriate w here the structural resistance for otherreasons is so large that the design earthquake can beaccom m odated w ithin the elastic capacity range of thestructure. In this case, it is im portant because realearthquakes do not read the codes (T. Paulay) andm ay be substantially stronger than the design earth-quake and bring the structure in its inelastic dom ain.

The capacity design m ethod offers a sim ple andefficient approach to ductile structural design:The structure is told exactly w here it can and shouldplastify, and w here not. H ence, a favourable plasticm echanism is created. A large and predictable degreeof protection against collapse can be achieved by goodcapacity design [PP 92] [Ba 02].

BP 23Ductile structures through capacity design!

Ductile structures throug h capa city d esig n!

Fragilestructure

Ductilestructure

Failure



23/1 Static-cyclic tests on the low er part of 1:2 scale 6-storey

reinforced concrete structural w alls have clearly dem onstrated theeffectiveness of a ductile design [D a 99]. The capacity designed w alls

achieved, at little additional cost, a seism ic capacity 3 to 4 tim es

larger than that of w alls conventionally designed according to the

Sw iss building code SIA 162.


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In reinforced concrete structures the reinforcing steelm ust enable the developm ent of sufficiently large anddeform able plastic zones. Tw o param eters (ductilityproperties) are crucial to ensure this:strain hardening ratio Rm /Re, i.e. the ratio betw eenthe m axim um tensile stress Rm and the yield stress Re

total elongation at m axim um tensile stress A gtThe strain hardening ratio is also very im portant for thebuckling resistance of reinforcem ent bars in com -pression. The sm aller Rm /Re, the low er the bucklingresistance [TD 01].

In Europe a large part of the reinforcing steel availableon the m arket has insufficient ductility properties, inparticular for the sm aller bars w ith diam eters up to 16m m [BW 98]. In order to ensure that reinforcedconcrete structures reach an m edium ductility, it isnecessary that the reinforcing steel fulfils the follow ingm inim um requirem ents (fractile values):

Rm /Re 1.15A gt 6 %

D esignations such as reinforcing steel in accordancew ith SIA building code 162 or fulfils the building

code requirem ents or ductile or very ductile etc.are insufficient and m isleading because the currentbuilding codes are them selves insufficient. It istherefore highly recom m ended that clear requirem entsare issued at the tim e of the invitation to tender andthat suitable tests are m ade before the purchase andim plem entation of the reinforcing bars.

BP 24Use ductile reinforcing steel with Rm/Re 1.15and Agt 6 %!

Use ductilereinfo rcing stee l

wi th :

Rm/Re 1.15a nd Ag t 6 %!

strain hardening ratio

to tal e longation atmaximum t ensile stress

Elonga tion [%]

Stress[MPa]



Hysteret ic Beha viour o f Stat ic-Cyclic Test Walls

Bendingmoment(kNm)

Bendingmoment(kNm)

Horizontal top deflection (mm)

Horizontal top deflection (mm)

Actuatorforce(kN)

Actuatorforce(kN)

24/1

Pro f . Hugo Bachmann ibk ETH Zrich

24/1 These plastic hysteresis-curves of 2 different 6-storey reinforced

concrete structural w alls w ith (W SH 3) and w ithout (W SH 1) ductile

reinforcing steel clearly illustrate the difference in behaviour. The w all

w ith low ductility barely achieved a displacem ent ductility of =~ 2,

w hile the ductile w all achieved =~6. The ductile w all can therefore

survive an earthquake approxim ately 4 tim es stronger!


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24/2 In this test w all, w ith reinforcem ent bars w ith insufficient strain

hardening ratio R m /Re, the plastic deform ations w ere concentrated at asingle crack (one-crack hinge according to [BW 98]). The reinforce-

m ent bars ruptured inside the w all (x) early in the test. This w eakened

the relevant section and concentrated the subsequent plastic deform a-

tions in it, causing the rupture of bars located at the edge of the w all.

The w all barely reached a displacem ent ductility =~2 after 2 cycles

[D W 99].

24/3 24/4 The failure of the reinforcem ent bars having a relatively

low Rm /Re value w as initiated by their buckling in com pression (left)

follow ed after a load reversal, by rupture in tension (right).

The rupture occurred w here the reinforcem ent bars had experienced

the largest buckling curvature [D W 99].


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25/1 In this colum n of an industrial building m ade of precast

reinforced concrete elem ents, the hoops w ere too w idely spaced and

insufficiently anchored w ith only 90hooks. They consequently

opened, allow ing the vertical reinforcem ent to buckle (A dapazari,

Turkey 1999).

25/2 The hoops anchorage at the foot of this colum n in a fram e

structure also failed because the hoops only had 90 hooks (Turkey,

lzm it 1999).

Page 59

25/3 This transverse reinforcem ent hoops and ties at the edge of

a reinforced concrete structural w all is exem plary concerning anchor-

age w ith 135 hooks. H ow ever, the vertical spacing of the transverse

reinforcem ent is too large, i.e. s = 7.5d instead of s 5d as required

for steel w ith a relatively sm all strain hardening ratio (Rm /Re = 1,15)

[D W 99][TD 01].

W ithin cyclically stressed plastic zones of reinforcedconcrete structural w alls and colum ns, the concretecover spalls w hen the elastic lim it of the reinforcem entis exceeded. In these zones it is therefore necessary tostabilise the vertical bars against buckling and to con-fine the concrete to allow greater com pressive strains.The stabilising and confining transverse reinforcem ent(hoops and ties) m ust be anchored w ith 135 hooks.D am aging earthquakes have repeatedly illustrated that90 hooks are insufficient. The spacing of the trans-verse reinforcem ent m ust be relatively sm all s 5d(d = diam eter of the stabilised bar). This is a conse-quence of the relatively poor ductility properties (sm allstrain hardening ratio Rm /Re) of European reinforcingsteel, w hich result in an unfavourable buckling behav-iour [TD 01].

Sim ilar rules apply to the plastic zones in fram estructures [Ba 02].

W ithin the zones that are to rem ain elastic accordingto the capacity design m ethod it is sufficient to applythe conventional design rules.

BP 25Use transverse reinforcement with 135hooks andspaced at s 5d in structural walls and columns!

Use transversereinforcement

w ith 135hoo ksan d spacedat s 5d in

structural w a llsand columns!




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26/1 This w ell designed earthquake w all has been com pletely

ruined by recesses placed in the form w ork, careless creation of

openings and brutal cutting of the reinforcem ent bars.

26/2 Expensive repair w ork, consisting of refilling the openings w ith

expansive concrete and gluing steel plates restored the designed

ultim ate resistance of the w all. H ow ever, it is alm ost im possible

to fully recover the ductile behaviour obtainable w ith the original

reinforcem ent (Sw itzerland 2001).

O n som e building sites there is a tendency to createrecesses in the structure for services, air ducts etc.,or even larger openings for other purposes, w ithoutconsulting the civil engineer. These recesses andopenings are often inserted into the form w ork ofreinforced concrete elem ents or even jack ham m eredafter concreting. The repercussions are particularlyserious w hen the openings are located in plastic zones.It is necessary to avoid this practice because it can leadto the prem ature failure of carefully designed criticalstructural elem ents and therefore to serious safetyproblem s.

O n the other hand, it is generally possible to placerecesses and even larger openings in the elastic zonesof the structure. The recesses and openings m ust bew ell planned and positioned, and the reinforcem entaround them m ust be strengthened eventually basedon a fram e calculation [D 0171].

BP 26No openings or recesses in plastic zones!

No o pening s orrecesses in pla stic zo ne s!

prohibited!

Basic principles fo r the seismic design o f b uilding s26



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26/3 H ere, an excessively large hole w as created and the reinforce-

m ent w as brutally cut. H ad the engineer b

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