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BAZELE PROIECTRII STRUCTURILOR

Conf.dr.ing Adrian CIUTINA

- CURS 4_a -

Principiile proiectrii pe baza strilor limit

Notele de curs pot fi descrcate de pe pagina de webh t t p : / / w w w .ct .u p t .r o / u se rs/ A dr i an Ciu t i n a/
http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/

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Adrian Ciutina, Bazele proiectrii structurilor

o Pentru durata de via intenionat a structurii i pentru gradele desiguran corespunztoare, structura i elementele structuraletrebuie proiectate, executate i meninute de o manier astfel nct:

n reziste aciunilor, influenelor acestora i utilizrii anticipate (SLU);n poat fi folosit pentru toate aciunile gndite la proiectare (SLS);

n posede rezistene structural, de serviciu i de durabilitatecorespunztoare;n ofere rezistena prevzut la incendiu;n nu fie avariat ntr-un mod ireparabil n cazul aciunilor extreme

(explozii, accidente, cutremure) (cerina de robustee).

o n mod tradiional, n concordan cu conceptul fundamental al

strilor limit, se consider c starea unei structuri poate fi clasificatca fiind fie satisftoare (s igu r, n serv ic iu ), fie nesatisftoare(nesecur i ta r , neu t i l i zab i l).

o Condiiile distincte care separ strile satisftoare-nesatisftoareale unei structuri se numesc str i l i m i t.

CAPITOLUL V PRINCIPIILE PROIECTRII PEBAZA STRILOR LIMIT

5.1 Strile limit - principii
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o n general, strile limit definesc acele stri pentru care structura numai satisface criteriile de proiectare. De aceea, fiecare stare limit esteasociat cu o anumit cerin de performan impus pentru structurarespectiv.

OBS: Adesea cerinele strilor limit nu sunt formulate suficient de clar pentru a ineseama de o definiie precis a strii limit corespunztoare.

o n mod tradiional, conceptul strilor limit pentru o anumit structurpresupune c o structur este pe deplin operaional pn la o anumitvaloare a efectelor aciunii E

0, iar peste aceast valoare se presupune c

structura este total nesatisftoare.

5.1 Strile limit - principii
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o n estimarea variaiilor n aciuni, influena mediului i modificrileproprietilor structurale care vor avea loc n timpul perioadei de viade proiectare a structurii, trebuie considerate prin selectarea unorsituaii distincte, reprezentnd un anumit interval de timp cu riscul saucondiiile asociate.

o n acest sens, exist patru situaii de proiectare:n Si tuai i pe rs is ten t e. Acestea se refer la condiiile normale de utilizare i

sunt n general raportate la perioada de via a structurii. Utilizarea normalpoate include i condiii extreme de ncrcare din vnt, zpad, ncrcriimpuse etc.

n Si tuai i t r a n z i en t e . Acestea se refer la condiiile temporare ale structurii, ntermeni de utilizare sau expunere, spre exemplu n timpul construciei saureabilitrii. Aceste situaii sunt raportate la o perioad mult mai scurt dectperioada de via a structurii. n acest caz trebuie definite valori

reprezentative ale aciunilor.n Si tuai i acc ident a le . Ele se refer la condiii excepionale ale structurii sau

expunerii acesteia, spre exemplu la foc, explozii, impact sau cedri locale.Aceasta implic o perioad scurt de timp, dar nu pentru situaiile n care ocedare local rmne nedetectat.

5.2 Situaii de proiectare
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n Si tuai a se ism ic. Se refer la condiiile excepionale aplicabile structurilor

supuse la aciuni seismice.

o Aceste situaii de proiectare trebuie selectate pentru a cuprinde toatecondiiile care pot fi rezonabil previzibile sau vor exista n timpulconstruciei i utilizrii construciei.

OBS: De exemplu, o cldire, dup o situaie de proiectare accidental, cum ar fi ceadin foc sau impact poate avea nevoie de reparaii (pentru o perioad scurt detimp de aproximativ un an), pentru care trebuie considerate situaii de proiectaretranziente.n general, pentru aceast perioad de timp trebuie prevzut un nivel mai szut

al siguranei i factori de siguran mai mici dect cei folosii pentru situaiapersistent de proiectare.

Ca i n situaia proiectrii structurii noi, proiectarea structurii avariate trebuiecut, considernd toate condiiile care vor fi posibile pe timpul noii durate

estimate pentru viaa construciei.

5.2 Situaii de proiectare
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o Pentru a simplifica procedurile de proiectare, sunt recunoscute dou

tipuri fundamental diferite de stri limit:n Sta r ea l im i t u l t im .n Sta r ea l im i t de se rv i c iu .

o St r i l e l i m i t u l t i m e sunt asociate cu cedarea sau forme similare decedare structural.

o Strile limit ultim privesc sigurana utilizatorilor i/sau sigurana

structural.o n anumite cazuri, strile limit ultime privesc i protecia bunurilor

adpostite (de exemplu anumite substane chimice sau nucleare saucapodoperelor unor muzee).

o n aproape toate cazurile care privesc strile limit ultime, primaocuren a unei stri limit este similari asociat cu cedarea.

OBS: n anumite cazuri, n care au loc deformaiile excesive care pot fi considerateultime, ele definesc starea limit ultim, chiar dac ele nu au condus la cedristructurale (exemplu: fundaiile utilajelor rotative).

5.3 Stri limit Starea Limit Ultim (SLU)
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o n cadrul proiectrii la SLU, trebuie considerate fenomene:n Pierde rea ech i l i b ru lu i s t ru c tu ra l sau a oricrei pri structurale,

considernd structura ca un corp rigid.n Cedarea s t ru ctu r i i sau a un e i pi a aceste ia , prin rupere, oboseal sau

deformaii excesive.n I n s t a b i l it a t e a st r u c t u r i i sau a unei pri componente.n Transformarea structurii sau a unei pi a acesteia n mecan ism .n Sch imb area neat e p t a t a s i st e m u lu i s t r u c t u r a ln alt sistem.

o SLU a echilibrului static se refer n primul rnd la capacitatea portant astructurii i consider strile limit la:n sturnare.n Portan (prin fora arhimedic).n Alunecare cu efecte de interfa (cum ar fi frecarea).

o n cazurile de echilibru static citate mai sus, structura trebuie privit caun co rp r i g id . n anumite cazuri ns, n cazul sensibilitii la deformaii(efecte de ordinul II), sau la vibraii (efecte de impact), trebuieconsiderate p rop r ie ti l e e las t i ce ale structurii.

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o n cazul SLU, trebuie s se fac distincia dintre dou tipuri de structuri:n To leran t e la degradr i (de tip robust);n Neto le ran t e la degradr i (sensibile la perturbaii minore sau imperfeciuni

structurale).

o n concordan cu tipul structural trebuie considerate efectele tuturortipurilor de mecanisme de deteriorare asupra SLU,.

o Termenul de starea limit ultim a rezistenei este folosit pentru adefini SLU indus de limitarea rezistenei materialului, conform definiiilordin Eurocode 2- Eurocode 9.

o Starea limit ultim a rezistenei definete de asemenea cedrile iniiatede lipsa stabilitii, cum ar fi prin rigiditatea limitat datorat cedrilor deinstabilitate induse de efectele de ordinul II.

OBS: Ca o caracteristic comun a strilor limit ultime este faptul c verificarea estecut la valori de proiectare operaionale mari ale ncrcrilor (cteodat fbaz fizic real).

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o St r i l e Lim i t de Serv ic iu ( SLS) sunt asociate cu condiiile de folosin

normal a structurilor. Ele vizeaz n particular funcionarea structurii saua elementelor structurale, confortul utilizatorilor i aparena lucrrilor deconstrucii.

o Avnd n vedere dependena de tip a efectelor aciunilor, se pot distingedou tipuri de Stri Limit de Serviciu:

n St r i L im i t de Serv ic iu i r evers ib i le sunt

acele stri limit rmn n permanendepite, chiar i n cazul n care aciunile careau cauzat depirea au fost nlturate (deexemplu deformaii permanente sau cedri alereazemelor);

n St r i L im i t de Serv i ciu reve rs ib i l e sunt

acele stri limit care nu vor fi depite ncazul n care aciunile care cauzeaz depireasunt nlturate (de exemplu vibraii excesivesau deformaii elastice);

5.4 Stri limit Starea Limit de Serviciu (SLS)
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o Lund n considerare acceptarea nclcrii limitelor SLS, exist mai multecondiii care pot fi formulate, iar n general exist trei tipuri de SLS:n (1) SLS f depiri;n (2) SLS cu depiri acceptate pentru o anumit durati frecven;n (3) SLS cu depiri acceptate pe durate de termen lung specificate.

o Criteriile de servicibilitate corespunztoare sunt asociate cu ncrcrilecaracteristice, frecvente i cvasi-permanente ale ncrcrilor utile.

o n mod obinuit, pentru verificarea la SLS sunt asociate urmtoarelecombinaii de aciuni corespunztoare celor trei situaii de proiectare:n co m b in ai a ca racte r i s t i c, dac nu sunt acceptate depiri;n co m b in ai a f recven t dac se consider SLS cu depiri acceptate pentru o

anumit durati frecven;

n co m b in ai a cvas i -pe rm anen t dac se consider SLS cu depiri acceptatepe durate de termen lung specificate.


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o n cadrul proiectrii la SLS, trebuie considerate urmtoarele fenomene(care afecteaz aparena sau folosirea efectiv a structurii):n d e f o r m ai i ex ces ive , dep lasr i , ncovo ier i sau nc l inai i , care pot afecta

aparena i funcionarea structurii, confortul utilizatorilor i pot cauzadegradri ale finisajelor sau ale elementelor nestructurale;

n v i b r ai i exces ive (estimate prin acceleraie, amplitudine i frecven) carepot cauza de exemplu disconfortul utilizatorilor i limita funcionarea

structurii;n degradr i care nu afecteazn mod direct aparena, durabilitatea sau

funciunea structurii (cedri sau fisurri locale).

o n funcie de tipologia structural, cerine adiionale corespunztoareSLS pot fi gsite n normele Eurocode 2 la Eurocode 9. Aceste normeoferi ghidurile adiionale de folosire a procedurii de proiectare la SLS.

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o n cazul structurilor ncrcate cu aciuni fluctuante, cedarea poate avea

loc la nivele care sunt semnificativ mai mici dect cele la care are loccedarea ateptat n mod normal. Atunci cnd o asemenea cedare estedatorat fenomenului de dezvoltare a fisurilor, mecanismul completcuprinde trei faze:n Faza de in ii e re a f i su r i l o r (n care sunt formate microfisurile);n Faza de dezvo l t a re a f isur i l o r (n care fisurile se mresc n fiecare ciclu de

aplicare a ncrcrilor).n Faza de cedar e, n care se produc creteri instabile ale fisurilor, datorate

cedrilor ductile sau fragile ale rupturilor, sau n care seciunile transversalereduse cedeaz datorit curgerii generalizate a materialului.

o n cazul n care, datorit fazei de dezvoltare a fisurilor se formeaz zoneplastice alternante importante, cedarea are loc dup un numr relativ mic

de cicluri, iar mecanismul de cedare se numete low-cycle fatigue(oboseal oligociclic).

5.5 Stri limit Starea Limit de Oboseal
http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/

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o n mod uzual, Starea Limit de Oboseal este separat de Starea LimitUltimi de cea de Serviciu din mai multe motive:n ncrcarea din oboseal este diferit de alte tipuri de ncrcri i depinde de

magnitudinea i de intervalul aciunilor sub condiiile de serviciu, mpreun cuefectul timpului (spre exemplu de numrul de cicluri);

n Efectele oboselii rezid n deteriorarea local a materialului, care conduceulterior la nrutirea condiiilor de ncrcare care accelereaz dezvoltarea

fisurilor;n Atunci cnd sunt ndeplinite anumite condiii (spre exemplu materialul s

posede o duritate suficient), dezvoltarea fisurilor poate fi detectat prininspecii regulate nainte de cedarea efectiv.

OBS: n proiectarea modern la starea limit de oboseal a fost introdus un sistem desiguran care ia n considerare posibilitatea de preavertizare a consecinelor decedare.

o n anumite standarde, Starea Limit la Oboseal este tratat independentde SLS sau SLU.

o Cic lu l de no rm e Eur ocode inc lude Sta r ea Lim i t la Oboseal ncadru l Str i i L im i t e Ul t i m e.

5.5 Stri limit Starea Limit de Oboseal

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o Proiectarea unei structuri include verificri la ambele stri limit: SLS i

SLU, dei natura Strii Limit Ultime difer de natura Strii Limit deServiciu, prin:n (1) n timp ce depirea SLU conduce aproape ntotdeauna la cedarea

structurali la ndeprtarea sau repararea fundamental a structurii,depirea SLS nu conduce la consecine att de fatale pentru structur, iaraceasta poate fi folosit normal dup ndeprtarea ncrcrilor care cauzeazdepirea SLS;

n (2) n timp ce criteriile SLU implic numai parametri structurali i aciunileadecvate, criteriile pentru SLS sunt dependente i de relaiile dintre client iutilizatori (adesea subiective), caracteristicile echipamentelor instalate sau aelementelor nestructurale.

o Diferenele dintre ULS i SLS rezult ntr-o formulare separat acondiiilor de siguran structurali n nivele diferite de siguranstructural, admise n cazul ambelor tipuri de stri limit.

5.6 Stri limit SLS / SLU
http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/http://www.ct.upt.ro/users/AdrianCiutina/

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OBS:(1) Nu toate evenimentele i fenomenele indezirabile pot fi clasificate n SLS sau

SLU. De exemplu, n cazul unui pod metalic de cale ferata, anumite SLS aletablierului pot conduce la deraierea trenului i n consecin la pierderea vieiloromeneti.

(2) Variaia aciunilor, influenele mediului i proprietile structurale care vor

exista n decursul ciclului de via trebuie considerate n procesul de proiectareprin selectarea situaiilor distincte (persistente, tranziente, accidentale iseismice), reprezentnd un anumit interval de timp i riscul asociat.

(3) n cazul ncrcrilor, trebuie considerate un numr realistic de combinaii,pentru a stabili nfurtoarea efectelor aciunilor, care este considerat nproiectare.

5.6 Stri limit SLS / SLU
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