6.exemple de reguli de alcatuire(bibl. h.bachmann)

7/30/2019 6.Exemple de Reguli de Alcatuire(Bibl. H.bachmann)

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Basic prin ciples fo r engineers, architects, bu ilding o w ners, and aut ho rit ies

4/2 Sw ay mechanisms are of ten inevitable wi th sof t storey ground

floo rs (Izmit, Turkey 1 99 9).

4/3 Here the front colum ns are incl ined in their w eaker direct ion, the

rear column s have fai led com pletely (Izmit, Turkey 1999 ).

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4/4 This resident ial buildin g is t i l ted as a result of colu m n fai lure

(Taiw an 199 9).

BP 4 Avo id sof t-sto rey groun d f loors!

Avoid sof t -sto rey ground floors!

Basic principles for the seismic design of buildings4

Pro f . H ugo Bachm a nn ibk ETH Zur ich

M any bui ld ing col lapses dur ing earthquakes may beat t r ibut ed to th e fact that the bracing e lem ents, e.g.w al ls, w hich are avai lable in t he up per f loo rs, areom i t ted in the g round f loor and subst itu ted bycolum ns. Thus a ground f loor t hat is sof t in thehorizontal direction is developed (soft storey). Oftenthe colum ns are dam aged by t he cyclic d isp lacem entsbe tw een the m oving so i l and th e upper part o f t hebuilding. The plastic deformations (plastic hinges) atthe top and bo t tom end o f the co lumns lead to adangerous sw ay mechanism (storey m echanism ) with alarge concentrat ion of t he p last ic deform at ions at th eco lumn ends.A collapse is often inevitable.

4/1 This sway mechanism in the groun d f loor of a bui ld ing under

construct ion almost provoked a collapse (Friaul, I taly 1976).


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4/5 The w el l-braced upp er part of the bui ld ing collapsed onto t he

ground f l oor

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

4/8 thanks to resistant colum ns wi th w el l detai led stabi l is ing and

conf in ing reinforcement (Taiwan 1999 ).

4/6 and these are the remains of the lef t edge ground f lour

column (Kobe, Japan 19 95).


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4/10 Likew ise, i t is probable that the slender columns und er the

cladding o f th is exist ing bu i ld ing are too w eak. A few horizontal ly

short reinforced concrete structu ral w alls could help signif icantly

(Switzerland 1998).

4/9 It is feared that exist ing b uilding s such as this one could co llapse

under even a relat ively w eak earthquake (Sw i tzer land 2000 ).


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5/2 In this off ice building also, an upp er storey fai led. The top of the

building has collapsed onto the f loor below, the whole building

rotated and leaned forw ards.

An upper s torey can also be soft in comp ar ison t o t heoth ers i f th e lateral bracing is weakened or o m it ted, or i fth e ho rizontal resistance is stron gly reduced ab ove acertain floo r. The consequence m ay again be a danger-ous sway mechanism.

5/1 In this comm ercial bui ld ing the th i rd f loor has disappeared and

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

BP 5 Avo id sof t-sto rey up pe r floors!

Avoid sof t-sto rey upper f loors!



5/3 This close-up view show s the crushed upper f loor of the of f ice

bui ld ing (Kobe, Japan 19 95).


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5/4 Al l the upp er f loors were too sof t ( Izmi t , Turkey 1999 ).


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6/1 In th is new skeleton b ui ld ing w i th f lat s labs and smal l st ructural

columns designed to carry gravity loads, the only bracing against

horizontal forces and displacements is a reinforced concrete elevator

and stairway shaft, placed very asymmetrically at the corner of the

buildin g. There is a large eccentricity betw een th e centres of m ass

and resistance or st i f fn ess. Tw ist ing in th e plan w il l lead to large

relat ive displacement s in the colum ns furthest aw ay f rom the shaf t

and the danger of punching shear fai lure that this implies. Placing a

slender reinforced concrete w al l, extending the ent i re height o f th e

bui ld ing at each facade in the opposi te corner f rom the shaf t wo uld

be a def in i te improvement . It w ould then be enoug h to const ruct

tw o of the core w al ls in reinforced concrete and th e rest could be f or

examp le in m asonry (Sw i tzerland 19 94).

Asymm etr ic bracing is a f requent cause of b ui ld ingcol lapses dur ing earthq uakes. In the tw o abo ve sketch-es on ly the lateral bracing elemen ts are represent ed(w alls and t russes). The colum ns are not draw nbecause th eir f rame act ion to resist h or izonta l fo rcesand d isplacem ents is sm all. The colum ns, wh ich on lyhave to carry the gravity loads, shou ld ho w ever be ableto f o l low the ho r izont a l d isp lacem ents of the structu rew ithou t loosing th eir load bearing capacity.

Each bu ilding in th e sketch has a centre of m ass M(centre of gravity o f a l l the m asses) throug h w hichth e inert ia fo rces are assum ed to act, a centre of resist-ance W for h or izonta l forces and a centre of st i f fnessS (shear centre). The po int W is th e centre of gravityof the f lexural and f ram e resistance of st ructura le lem ents along t he tw o m ajor axes. If th e centre ofresistance and the centre o f m ass do n ot coincide,eccentr ic i ty and t w ist ing occur. The bui ld ing tw ists inthe h or izonta l p lane about the centre of st i f fn ess.In part icular, this to rsion generat es sign if icant relatived isp lacements betw een the bo t tom and top o f thecolum ns furthest away f rom the centre of st i f fness andth ese oft en fail rapidly. Therefo re the cent re of resistance

should coincide w ith, or be close to, t he centre of m ass,and suff icient to rsion al resistan ce sho uld b e available.This can be achieved w ith a sym m etr ic arrangem ent ofthe lateral bracing elements. These should be placed,if possible, along the edges of building, or in any casesuf f ic ient ly far away f rom the centre of m ass.

BP 6 Avo id asym m et ric bracing!

M

S W

Avoid asymmetrical hori zontal bracing!

W, S

M



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6/2 This of f ice bui ld ing had a cont inuous f i re w al l to th e r ight rear

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

signif icantly, and the front columns fai led (Kobe, Japan 1995).


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6/5 Originally, the on ly hor izontal bracing in th is 70' s audi tor ium

bui ld ing at th e Hngg erberg Campu s of ETH Zurich w ere reinforced

concrete w alls w ith l i t t le torsional resistance situated at the rear of

building. Because of the considerable distance between the bracing

and th e cent re of m ass of th is large bui ld ing, i t w ould h ave twisted

signif icantly in the plan for even a relat ively weak earthquake

(seism ic zone 1 according t o SIA 16 0). The few high ly loaded

reinforced concrete columns in the ground f loor w ould have experi-

enced substantial displacements, part icularly in the front of the

bui ld ing. How ever, the column detai l ing w as inadequate for th e

required duct i l i ty. Ad di t ional steel column s were th erefore bui l t in o n

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

transfer the horizontal seismic forces to the exist ing foundations.

This upgrading also fu l f i lled the need fo r a st rength ening of the

canti levered structure for gravity loads.

6/6 The incorporat ion of the new tubu lar steel t russ columns is

aesthet ically sat isfying.

6/3 6/4 In the back, th is house share a st rong and st if f f i re wal l wi th

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

the centres of resistance and st if fness were situated to the back of

the bu i ld ing. The house tw isted st rongly in the h or izontal p lane, but

did not collapse (Umbria, I taly 1997).

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BP 7 Avoid bracing offsets!

7/1 The hor izontal of fset of the reinforced concrete wal l in the

vert ical plane causes large addit ional stresses and d eform ations in

the structure during an earthquake. They include large local vert ical

forces ( f rom t he overturning mo ment ), large addi t ional shear forces

in the slabs at offsets, redistribut ion of the foundation forces, etc.

(Switzerland 2001).

Horizonta l bracing of fsets, in p lane (at the bo t to m ofthe p lan f igu re) o r ou t o f p lane (a t the top o f t heplan f igure), result w hen th e posit ion o f th e bracingchanges f rom o ne storey to ano ther. The bendingm om ents and t he shear forces induced by th e of fsetcannot be f u l ly comp ensated, despite substant ia laddit ional costs.The of fsets d isturb the d irect f low of forces, w eakenthe resistance and reduce the ductil ity (plastic defor-m at ion capacity) of the b racing. M oreover, they causelarge addit ion al forces and defo rm at ions in ot herstructural elements (e.g. slabs and columns).Com pared to bracings that are cont inuou s over theheight of t he bu i ld ing, bracings w ith o f fsets increasethe vulnerabi l ity of the con struct ion and usual lyno ticeably redu ce its seism ic resistance. Bracing off setsm ust therefore be absolute ly avoided!

Avoid bracingoffset!




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8/2 During an earthqu ake, the reinforced concrete cant i lever w al l

(behind t he curtain), w il l indu ce signif icant ad dit ion al stresses in thealready highly loaded column on the grou nd f loor (Sw i tzer land

2001).

M odif icat ions in th e cross sect ion of bracing system sover the h eight o f a b ui ld ing cause d iscont inu it ies andlead to sud den variations in th e stif f ness and resistanceof the building. This can cause irregularit ies in thedynam ic behaviour and d isturb t he local f low of f orces.An increase in the stif fness and resistance from thebot tom up ( le f t in the e levat ion f igu re) is general ly lessfavourable than t he opp osite (r ight in the e levat ionfigure). In any case, the calculation of the sectionalforces and t he design of the structure as wel l as thedeta i l ing o f th e d iscont inu it ies m ust be conducted verycarefully.

8/1 The t ransi t ion f rom a reinforced concrete st ructural w al l to a

fram e structure cau ses large discontin uit ies in st i f fness and resistance

(Sw itzerland 200 1).

BP 8 D iscon t inu ities in st iff ne ss an dresistan ce cau se prob lem s!

Discontinuities inst if fness and resistance

cause problems!




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9/1 Such reinforced concrete structural w alls take up only l i t t le

space in plan and elevation (Switzerland 1994).

9/2 The reinforcem ent of reinforced concrete structural walls is

relat ively sim ple, but i t m ust be detailed and laid w ith great care.

The f igure show s a capacity designed d ucti le wall, of rectangu lar

cross-section, w hich w as added t o an exist ing b uilding (Sw itzerland

1999).

Reinfo rced con crete stru ctural w alls of rectang ularcross-section constitute the most suitable bracingsystem ag ainst seism ic actions for skeleton stru ctures.The w al ls may be re lat ive ly short in th e ho r izont a ld i rec t ion e.g . 3 to 6 m or about 1 /3 to 1 /5 o f th ebui ld ing h eight they mu st , how ever, extend o ver theent ire height of th e bui ld ing. In a zone of m oderateseism icity, in m ost cases tw o slender and capacitydesigned d uct i le w al ls in each m ajor d irect ion aresuff icient. The type of non-structural elements can alsoinfluence the selection of the dimensions (st if fness) ofth e bracing system (cf. BP 14 ). To m inim ise th e effectsof torsion, the w al ls shou ld be p laced sym m etr ical lyw ith respect to t he centre of m ass and as close aspo ssible to t he edg es of t he bu ilding (cf. BP 6).Consider ing seism ic forces t ransfer to t he g round(found at ion), corner w al ls shou ld preferably be avoid-ed. W hen t he w alls have L cross-section (angle w alls)or U crosssect ions, the lack o f symm etry can m akedeta i l ing for duct i l i ty d i f f icu lt . Reinforced concretew alls w ith rectang ular cross-section (stan dard th ickness30 cm) can be m ade duct i le w i th l i t t le ef fo r t , thu sensuring a high seismic safety [D0171].

BP 9 Tw o slend er reinf orced con cret e st ructura lw alls in each p rincipal direct ion !

Two slender reinforced concrete st ructu ralwalls in each principal direction!




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

the t ransverse di rect ions at tw o bui ld ing corn ers.

9/4 The structural walls w ere included as prom inent elements in the

architectural concept (Sw itzerland 199 4).


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can impair the bu i ld ing fun ct ional ly [D0171 ]. A consis-

tent design of the structure as a skeleton structure, i.e.colum ns only (no m asonry w al ls) with som e slenderre info rced con crete structura l w al ls extending theent ire height of t he bui ld ing, is thus a lso in t he long-term int erest of t he ow ner. As the in ter ior part i t ionsare non-structural elements, they are easy to refit incase of chang es in t he b uildin gs use. Extensivestructural modif ications are therefore not necessary.

10/1 This structural stairway w all w il l be destroyed by a relat ively

w eak earthquake. A to tal col lapse of t he bu i ld ing m ay result

(Switzerland 2001).

M ixed structura l system s w ith con crete o r steelcolum ns and structura l m asonry w al ls behave veryunf avourably dur ing earthqu akes. The colum ns incom binat ion w ith the slabs or beams form f rames,w hich have a sub stantially smaller horizon tal st if f nessthan the m asonry w al ls. The earthqu ake act ions aretherefore carr ied to a large extent by the m asonryw alls. In addit ion to t he inert ia forces f rom their ow ninf luence zone, the w al ls m ust resist th ose f rom thepar ts o f the bu i ld ing w i th the co lum ns (to the le f t inthe figure). This results in a seismic resistance consider-ably less than that of a pure m asonry construct ion.W hen m asonry w al ls fa il due to the seism ic act ions ordef lect ions, they can n o lon ger carry the gravity loads,w hich usual ly leads to a t ota l co l lapse of the b ui ld ing.M ixed system s of co lum ns and structu ra l masonryw alls mu st t herefore be absolute ly avoided.

Furth ermo re, such m ixed system s prove to b eunf avourable because of their lack of f lexib i l ity w ithregard to increasingly f requent bu i ld ing m odif icat ionsrequired b y changes in their use. Remo val of m asonryw alls require heavy structu ral int ervention s, w hich arecostly (up to several percent of the building value) and

BP10 Avoid m ixed system s w ithcolum ns an d stru ctu ral m ason ry w alls!

Avoid mixedsystems of

columns andstructural

masonry w alls!

Reinforcedconcrete frame

Structuralmasonry wall




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11/1 Here the columns w ere clearly st ronger and t he m asonry fe l l

out w hi le the f rame remained standing (Erzincan, Turkey 199 2).

It is st i l l a com m on o pinion t hat f i l l ing in fram e struc-tures w ith m asonry w alls imp roves the behaviour un derhorizontal loads including seismic actions. This is trueonly fo r sm all loads, and as long as the m asonry rem ainslargely intact. The combination of two very different andincompatible construction types performs poorly duringearthquakes. The frame structure is relatively flexibleand som ew hat ductile, w hile unreinfo rced m asonry isvery st if f and f ragile and m ay explode u nder th eef fect o f on ly sm all deform at ions. At the beginning o fan earthqu ake the m asonry carries m ost of t he earth-quake actions but as the shaking int ensif ies the m asonryfails due to shear or sliding (friction is usually small dueto the lack of vertical loads). The appearance ofdiago nal cracks is characteristic of a seism ic failure.

Tw o b asic cases can be iden tif ied: Eith er the colu m ns arestrong er than the m asonry, or vice-versa. W ith strong ercolum ns the m asonry is com pletely destroyed and fallsout . W ith w eaker colum ns the masonry can dam ageand shear the column s, w hich oft en leads to collapse(see also BP 16 and 17 ).

BP 11 Avo id bracing of fram es w ith m asonry inf i lls!

Avoid bracing of frameswi th masonry inf ills!



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

enced sign if icant dam age and w ere part ly sheared; nevertheless, the

fram e is st i l l just stand ing (M exico 1985 ).


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

masonr y inf i l ls (Turkey, Izm it 1 999 ).

11 /3 The masonry w as also strong er in this case; it sheared the

relat ively large columns (Adana-Ceyhan, Turkey 1998).


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12/1 Such and also low er! new m asonry st ructures, wi tho ut

bracing reinfo rced concrete struct ural w alls, are extremely vulnerable

to earthquakes (Sw i tzerland 20 01).

12/2 This new 3-storey resident ia l bui ld ing w i th unreinforced

masonr y struct ural w alls is braced lon gitu dinally by a reinfo rced

concrete structu ral w all in each facade, and tran sversely by an

interior reinforced concrete structural wall (Switzerland 2001).

Tradit io nally in m any coun tries, ho uses and sm allercom m ercia l bui ld ings are of ten bu i l t w ith unreinforcedm asonry w al ls made of c lay, l imeston e or cem entbr icks. M asonry is a good construct ion m ater ial interm s of th ermal insulat ion, storage and vert ica l loadscarrying capacity. For seism ic actions ho w ever, m ason rystuctu res are not w el l su ited. On one hand they arerelatively st if f , so t hey usually have a high n atu ralf requency w ith in th e p lateau area of th e designresponse spectrum and therefore experience largeearthquake act ions. On t he oth er hand un reinfo rcedm ason ry w alls are rather brit t le and generally exhibitrelatively lit t le en ergy d issipat ion. Generally, it is no tpo ssible to ob tain ad equat e seism ic resistance (even inregion s of lo w seism icity) and ad dit io nal m easures aretherefore necessary.

A possible solution consists of bracing unreinforcedm asonry bu i ld ings with re info rced con crete structura lw al ls. Hereby i t is possib le to l imit the h or izonta ldeform at ions of th e m asonry and t herefore preserve i tsgravity load carrying capacity. The reinfo rced concretestructura l w al ls must be d esigned t o b e suf f ic ient lyst i f f , the ho r izont a l wal l lengt h and t he vert ica l

re info rcem ent rat io being k ey param eters. They mustbe able to carry the seismic actions and to transmitthem to th e foun dat ions w hi le remain ing e last ic, i.e .w i thou t no tab le y ie ld ing o f the re in fo rcem ent .The hor izonta l def lect ion o f th e re info rced concretestructura l w al ls und er the design earthqu ake mu st no texceed the displacement capacity of the stif fest, i.e.longest , masonry w al l.

BP 12 Brace m ason ry bu ildings w ith reinf orced con cretest ructura l w alls!

Sti ff en masonry bui ldings wit h reinf orcedconcrete st ructu ral walls!

MasonryStructural

concrete w all Masonry



6.exemple de reguli de alcatuire(bibl. h.bachmann)

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