Pereti Structurali Din Beton Armat

PERETI STRUCTURALI DIN BETON ARMAT

STRUCTURI DIN ZIDARIE SI BETON Lector ing.Dragos Marcu CURS 2014-2015

La aciunea orizontal datorit flexibilitii pronunate, cadrul poate avea deplasri laterale importante sporim rigiditatea cadrului

Zidria poate rigidiza cadrul pn n momentul n care fisureaz diagonal (dup stadiul de fisurare zidria nu mai este capabil s preia eforturi).


STRUCTURI DIN ZIDARIE SI BETON Lector ing.Dragos Marcu

Dac panourile din zidrie de crmid se nsumeaz , rigiditatea crete: structur n cadru cu panouri active.

Capacitatea cadrului

fisura

Cadru + zidarie cadru

Rigiditatea panourilor de zidrie e limitat => Solutia este peretele structural de beton armat.

CURS 2014-2015

TIPURI DE STRUCTURI DIN PERETI STRUCTURALI

STRUCTURA DIN PERETI STRUCTURALI IN FAGURE (pereti structurali desi, la limita fiecarei incaperi hoteluri, camine, spitale, etc)

Pereti stuct de bordaj cu goluri multiple.

Structura fagure nu permite un parter liber dar e foarte rigid.


STRUCTURA DIN PERETI STRUCTURALI DE TIP CELULAR

Pereii sunt dispui la limita apartamentelor

Pe faad sunt distane mari ntre diafragme grinzi foarte nalte(70 cm.1m) =>stlpi.

Planele pot fi de tip dal, fr grinzi interioare. Parterele la structurile celulare tot nu sunt libere.


STRUCTURA DIN PERETI STRUCTURALI LOCALI


NOIUNI DE CALCUL

pi (uniform distribuita) au o rezultant Ri (compresiune centric)

o din diagram = Ai=Ab +nAa se poate gsi aria n=10-15 bare

Se cunoaste (prin predimensionare) lungimea L se afl grosimea .

AiR

bc ARRs

=cRs

R

Ab=

s=0,150,20

=

=n

iRiR

1

Lb

=


n = nr. de niveluri

Predimensionare la efort unitar tangenial mediu

STRUCTURI DIN ZIDARIE SI BETON Lector ing.Dragos Marcu

t

necper R

SA unde S fora seismic, Rt rezistena la ntindere a betonului

unde G greutatea total a cldirii c coeficientul seismic sau sarcin seismic procentual

GcS =

= gaqT )(c uzual c = 0,15-0,20

Se determin fora seismic se raporteaz la Rt rezult necperA

(care trebuie dispus separat, pentru fiecare direcie n parte) Se propune o dispunere a pereilor se cunoate lungimea din geometrie i grosimea necesar a pereilor:

LAnecper=

CURS 2014-2015

Peretele structural se desprinde de pe teren, talpa fundaiei se mic, > Pat scufundare tasare inegal rupere perete structural

ntinderile la nivelul fundaiei pot fi preluate totui prin fundaii de adncime (piloi, barete).


La nivelul peretelui structural, deasupra fundaiei se admit i ntinderi care sunt preluate de armturi. Sistemele de armare din pereii structurali sunt gndite i la ntindere i la compresiune.

WM

AN= MmmM ba

u += )( izorezistent calculul pereilor structurali

M, R (bare verticale) Q (bare orizontale)

- ncovoiere - compresiune - fora taietoare

M R

Q


Armarea se realizeaz din bare verticale (preiau M i R) i bare orizontale (Q)

nQAa

hMuAa

transv

lungime

=

=)(


-cadru puternic

-puternice, rigide, grinzi mari

-fra grind

grind perete

bulbii pot deveni stlpi la parter

20 40

40

- Parter

La parter bulbul este inclus n stlp

Stlpi parter

Bulb perete structural etaj

GRINDA PERETE


NOIUNI DE CALCUL PENTRU GRINDA PERETE

zon comprimata (eforturi mici) (zon inactiv)

compresiuni+ncovoiere (zona activ)

L

-zona activ are h Intre forele C i I exist braul de prghie z.

L


88

2

max

max

2

max

pLM

zM

I

zIplM

=

=

==

zMI

uu =echilibrat de cuplul C-I

maxMCMu =

OB sau PC, y

u

aIA

=AN

=NA =


SISTEMUL DE ARMARE

limitare a poziiei golurilor peste H activ

(ntinderi puternice) 0,6 H activ Sistem de armare foarte des din plase, complet fara goluri pe h=z. Parter defragmentat pe urm 1,2,3 niveluri nu am nici un fel de gol.

L6,0z


CLASIFICARE PEREI STRUCTURALI

Perei structurali:

plini

cu goluri

- mici (se calculeaz ca o diafragm plin) - medii - mari


Perei structurali cu goluri mici doar efecte locale bordez golul Apar concentrri de tensiuni n dreptul colurilor

Fiecare gol se va borda cu bare groase din otel. n principiu armarea de bardaj trebuie s echivaleze armtura dislocuit(ntrerupt) de gol.


Perei structurali cu goluri medii (ferestre, ui)

Momentul produs de forele orizontale ridic paletul din stnga i l coboar pe cel din dreapta produce fore tietoare n palei M=M1+M2 Q=Q1+Q2

N centrice verticale excentriciti M din ncrcri orizontale


paleii se comport ca nite perei structurali plini

M

M

Qb compresiune anterioar

l buiandrug p p

buiandrug

B

BB

Bbuiandrugbuiandrug

lM2Q

M2lQ

=

=


Fisurile nseamn articulaii plastice (buiandrugul devine coordonator de deplasare, nu mai are capacitate de preluare a eforturilor).


Perei structurali cu goluri mari

Riglele de cuplare sunt nesemnificative (se rup instantaneu la capete, la cutremur) => Asigur doar coordonarea de deplasare

articulaii plastice la capete paleii= perei structurali plini


(pe ansamblul structurii)= =

transversk

longk

mult mai eficient dect pereii structurali rari, chiar asociai ntre ei

tuburi deschise tuburi nchise

ASOCIEREA PEREILOR STRUCTURALI


centric Structuri tubulare amplasate excentric

Zgrie - nori TUB + CADRE TUB + TUB



w FORM REGULAT N PLAN

DESPRE CONFORMARE


DESPRE CONFORMARE w ASIGURAREA DE RIGIDITATE SUFICIENTA LA ACTIUNI ORIZONTALE

CONFORTUL OCUPANILOR


DESPRE CONFORMARE w NIVELE DE RIGIDITATE CONSTANT (nu mai mult de 10% diferen de rigiditate ntre dou nivele consecutive)

19

Basic principles for engineers, architects, building owners, and authorities

5/2 In this office building also, an upper storey failed. The top of thebuilding has collapsed onto the floor below, the whole buildingrotated and leaned forwards.

An upper storey can also be soft in comparison to theothers if the lateral bracing is weakened or omitted, or ifthe horizontal resistance is strongly reduced above acertain floor. The consequence may again be a danger-ous sway mechanism.

5/1 In this commercial building the third floor has disappeared andthe floors above have collapsed onto it (Kobe, Japan 1995).

BP 5 Avoid soft-storey upper floors!

Avoid sof t-storey upper f loors!

Basic principles for the seismic design of buildings

5

Prof. Hugo Bachmann ibk ETH Zurich

5/3 This close-up view shows the crushed upper floor of the officebuilding (Kobe, Japan 1995).


DESPRE CONFORMARE w EVITAREA PARTERELOR FLEXIBILE

15


4/2 Sway mechanisms are often inevitable w ith soft storey groundfloors (Izmit, Turkey 1999).

4/3 Here the front columns are inclined in their weaker direction, therear columns have failed completely (Izmit, Turkey 1999).

Page 164/4 This residential building is tilted as a result of column failure(Taiwan 1999).

BP 4 Avoid soft-storey ground floors!

Avoid sof t-storey ground floors!


4


Many building collapses during earthquakes may beattributed to the fact that the bracing elements, e.g.walls, which are available in the upper floors, areomitted in the ground floor and substituted bycolumns. Thus a ground floor that is soft in thehorizontal direction is developed (soft storey). Oftenthe columns are damaged by the cyclic displacementsbetween the moving soil and the upper part of thebuilding. The plastic deformations (plastic hinges) atthe top and bottom end of the columns lead to adangerous sway mechanism (storey mechanism) w ith alarge concentration of the plastic deformations at thecolumn ends. A collapse is often inevitable.

4/1 This sway mechanism in the ground floor of a building underconstruction almost provoked a collapse (Friaul, Italy 1976).


DESPRE CONFORMARE w DISTRIBUIA ECHILIBRAT A RIGIDITILOR N PLAN (CM ~ CR) EVITAREA FENOMENELOR DE TORSIUNE

METROPOLITAN GOVERNMENT BUILDING -TOKYO

nlime: 242,9m 48 etaje 3 subsoluri Suprafa desf.: 196.000 mp Otel, sticla Execuie: 1988-1991


DESPRE CONFORMARE

METROPOLITAN GOVERNMENT BUILDING -TOKYO


21


6/1 In this new skeleton building w ith flat slabs and small structuralcolumns designed to carry gravity loads, the only bracing againsthorizontal forces and displacements is a reinforced concrete elevatorand stairway shaft, placed very asymmetrically at the corner of thebuilding. There is a large eccentricity between the centres of massand resistance or stiffness. Tw isting in the plan w ill lead to largerelative displacements in the columns furthest away from the shaftand the danger of punching shear failure that this implies. Placing aslender reinforced concrete wall, extending the entire height of thebuilding at each facade in the opposite corner from the shaft wouldbe a definite improvement. It would then be enough to constructtwo of the core walls in reinforced concrete and the rest could be forexample in masonry (Sw itzerland 1994).

Asymmetric bracing is a frequent cause of buildingcollapses during earthquakes. In the two above sketch-es only the lateral bracing elements are represented(walls and trusses). The columns are not drawnbecause their frame action to resist horizontal forcesand displacements is small. The columns, which onlyhave to carry the gravity loads, should however be ableto follow the horizontal displacements of the structurew ithout loosing their load bearing capacity.

Each building in the sketch has a centre of mass M(centre of gravity of all the masses) through whichthe inertia forces are assumed to act, a centre of resist-ance W for horizontal forces and a centre of stiffness S (shear centre). The point W is the centre of gravityof the flexural and frame resistance of structuralelements along the two major axes. If the centre ofresistance and the centre of mass 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 relativedisplacements between the bottom and top of thecolumns furthest away from the centre of stiffness andthese often fail rapidly. Therefore the centre of resistanceshould coincide with, or be close to, the centre of mass,and sufficient torsional resistance should be available.This can be achieved w ith a symmetric arrangement ofthe lateral bracing elements. These should be placed, if possible, along the edges of building, or in any casesufficiently far away from the centre of mass.

BP 6 Avoid asymmetric bracing!

MS W

Avoid asymmetrical horizontal bracing!

W, S

M


6


Page 226/2 This office building had a continuous fire wall to the right rearas well as more eccentric bracing at the back. The building tw istedsignificantly, and the front columns failed (Kobe, Japan 1995).

21


6/1 In this new skeleton building w ith flat slabs and small structuralcolumns designed to carry gravity loads, the only bracing againsthorizontal forces and displacements is a reinforced concrete elevatorand stairway shaft, placed very asymmetrically at the corner of thebuilding. There is a large eccentricity between the centres of massand resistance or stiffness. Tw isting in the plan w ill lead to largerelative displacements in the columns furthest away from the shaftand the danger of punching shear failure that this implies. Placing aslender reinforced concrete wall, extending the entire height of thebuilding at each facade in the opposite corner from the shaft wouldbe a definite improvement. It would then be enough to constructtwo of the core walls in reinforced concrete and the rest could be forexample in masonry (Sw itzerland 1994).

Asymmetric bracing is a frequent cause of buildingcollapses during earthquakes. In the two above sketch-es only the lateral bracing elements are represented(walls and trusses). The columns are not drawnbecause their frame action to resist horizontal forcesand displacements is small. The columns, which onlyhave to carry the gravity loads, should however be ableto follow the horizontal displacements of the structurew ithout loosing their load bearing capacity.

Each building in the sketch has a centre of mass M(centre of gravity of all the masses) through whichthe inertia forces are assumed to act, a centre of resist-ance W for horizontal forces and a centre of stiffness S (shear centre). The point W is the centre of gravityof the flexural and frame resistance of structuralelements along the two major axes. If the centre ofresistance and the centre of mass 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 relativedisplacements between the bottom and top of thecolumns furthest away from the centre of stiffness andthese often fail rapidly. Therefore the centre of resistanceshould coincide with, or be close to, the centre of mass,and sufficient torsional resistance should be available.This can be achieved w ith a symmetric arrangement ofthe lateral bracing elements. These should be placed, if possible, along the edges of building, or in any casesufficiently far away from the centre of mass.

BP 6 Avoid asymmetric bracing!

MS W

Avoid asymmetrical horizontal bracing!

W, S

M


6


Page 226/2 This office building had a continuous fire wall to the right rearas well as more eccentric bracing at the back. The building tw istedsignificantly, and the front columns failed (Kobe, Japan 1995).

DESPRE CONFORMARE TORSIUNE


DESPRE CONFORMARE

TORSIUNE


DESPRE CONFORMARE - EVITAREA TORSIUNII, RIGIDITATE SUFICIENT

26


9/1 Such reinforced concrete structural walls take up only littlespace in plan and elevation (Sw itzerland 1994).

9/2 The reinforcement of reinforced concrete structural walls isrelatively simple, but it must be detailed and laid w ith great care. The figure shows a capacity designed ductile wall, of rectangularcross-section, which was added to an existing building (Sw itzerland1999).

Reinforced concrete structural walls of rectangularcross-section constitute the most suitable bracingsystem against seismic actions for skeleton structures. The walls may be relatively short in the horizontaldirection e.g. 3 to 6 m or about 1/3 to 1/5 of thebuilding height they must, however, extend over theentire height of the building. In a zone of moderateseismicity, in most cases two slender and capacitydesigned ductile walls in each major direction aresufficient. The type of non-structural elements can alsoinfluence the selection of the dimensions (stiffness) ofthe bracing system (cf. BP 14). To minimise the effectsof torsion, the walls should be placed symmetricallyw ith respect to the centre of mass and as close aspossible to the edges of the building (cf. BP 6).Considering seismic forces transfer to the ground(foundation), corner walls should preferably be avoid-ed. When the walls have L cross-section (angle walls)or U crosssections, the lack of symmetry can makedetailing for ductility difficult. Reinforced concretewalls w ith rectangular cross-section (standard thickness30 cm) can be made ductile w ith little effort, thusensuring a high seismic safety [D0171].

BP 9 Two slender reinforced concrete structural walls in each principal direction !

Tw o slender rein forced concre te structural walls in each principal direct ion!


9


w MINIM DOI PEREI STRUCTURALI AMPLASAI DUP FIECARE DIRECIE PRINCIPAL (de preferat ct mai spre faad)


DESPRE CONFORMARE

CENTRUL COMERCIAL BANKRAS, OLANDA

w DOU SAU MAI MULTE NIVELE DE REZISTEN


DESPRE CONFORMARE - DUCTILITATE

55


Ductile (i.e. w ith large inelastic deformation capacity)structures usually offer substantial advantages in com-parison to similar brittle structures. Most importantly,the required structural resistance can be reducedbringing substantial savings and increased safetyagainst collapse. Whenever possible the structure of abuilding should be designed to be ductile. This is alsoappropriate where the structural resistance for otherreasons is so large that the design earthquake can beaccommodated w ithin the elastic capacity range of thestructure. In this case, it is important because realearthquakes do not read the codes (T. Paulay) andmay be substantially stronger than the design earth-quake and bring the structure in its inelastic domain.

The capacity design method offers a simple andefficient approach to ductile structural design:The structure is told exactly where it can and shouldplastify, and where not. Hence, a favourable plasticmechanism is created. A large and predictable degreeof protection against collapse can be achieved by goodcapacity design [PP 92] [Ba 02].

BP 23 Ductile structures through capacity design!

Duct ile structures through capaci ty design!

Fragile structure

Ductile structure

Failure


23


23/1 Static-cyclic tests on the lower part of 1:2 scale 6-storeyreinforced concrete structural walls have clearly demonstrated theeffectiveness of a ductile design [Da 99]. The capacity designed wallsachieved, at little additional cost, a seismic capacity 3 to 4 timeslarger than that of walls conventionally designed according to theSw iss building code SIA 162.

48


19/1 This steel frame suffered large permanent deformations. Therewas probably no lateral bracing and the connection detailing wasinadequate for cyclic actions (Kobe, Japan 1995).

19/2 The bolts failed in this beam to column connection (Kobe,Japan 1995).

Steel generally possesses a good plastic deformationcapacity (strain ductility). Nevertheless steel membersand steel structures may show low ductility or evenbrittle behavior under cyclic actions, particularly due tolocal instabilities and failures. For example elementsw ith broad flanges (columns and beams) may buckle inplastic zones or fail at welds. Therefore, certainrequirements must be complied w ith and addtitionalmeasures must be considered during the conceptualdesign of the structure and the selection of themembers cross sections [Ba 02] [EC 8].

BP 19 Design steel structures to be ductile!

Design steel structures to

be duct ile!Critical zones


19

Prof. Hugo Bachmann ibk ETH Zrich

DUCTILITATE CUM? - Asigurarea unui mecanism favorabil de

disipare de energie (stlpi puternici-grinzi slabe) (montani puternici-rigle de cuplare slabe).



DUCTILITATE CUM? - Utilizarea unei armturi ductile Rm/Re>1,15; Alungire> 7,5% - Armare transversal deas - Detaliere corect a armrii (ciocuri la 135 deg)

56


In reinforced concrete structures the reinforcing steelmust enable the development of sufficiently large anddeformable plastic zones. Two parameters (ductilityproperties) are crucial to ensure this: strain hardening ratio Rm/Re, i.e. the ratio between

the maximum tensile stress Rm and the yield stress Re total elongation at maximum tensile stress AgtThe strain hardening ratio is also very important for thebuckling resistance of reinforcement bars in com-pression. The smaller Rm/Re, the lower the bucklingresistance [TD 01].

In Europe a large part of the reinforcing steel availableon the market has insufficient ductility properties, inparticular for the smaller bars w ith diameters up to 16mm [BW98]. In order to ensure that reinforcedconcrete structures reach an medium ductility, it isnecessary that the reinforcing steel fulfils the follow ingminimum requirements (fractile values):

Rm/Re 1.15 Agt 6 %

Designations such as reinforcing steel in accordancew ith SIA building code 162 or fulfils the buildingcode requirements or ductile or very ductile etc.are insufficient and misleading because the currentbuilding codes are themselves insufficient. It istherefore highly recommended that clear requirementsare issued at the time of the invitation to tender andthat suitable tests are made before the purchase andimplementation of the reinforcing bars.

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

Use duct ile

rein forcing steel with:

Rm/Re 1.15 and A gt 6 %!

strain hardening ratiototal elongation at maximum tensile stress

Elongation [%]

Stre

ss [

MPa

]


24


Hysteretic Behaviour of Static-Cyclic Test Walls

Ben

ding

mom

ent (

kNm

)B

endi

ng m

omen

t (kN

m)

Horizontal top deflection (mm)

Horizontal top deflection (mm)

Act

uato

r fo

rce

(kN

)Act

uato

r fo

rce

(kN

)

24/1

Prof. Hugo Bachmann ibk ETH Zrich

24/1 These plastic hysteresis-curves of 2 different 6-storey reinforcedconcrete structural walls w ith (WSH3) and w ithout (WSH1) ductilereinforcing steel clearly illustrate the difference in behaviour. The wallw ith low ductility barely achieved a displacement ductility of =~ 2,while the ductile wall achieved =~ 6. The ductile wall can thereforesurvive an earthquake approximately 4 times stronger!

58


25/1 In this column of an industrial building made of precastreinforced concrete elements, the hoops were too w idely spaced andinsufficiently anchored w ith only 90hooks. They consequentlyopened, allow ing the vertical reinforcement to buckle (Adapazari,Turkey 1999).

25/2 The hoops anchorage at the foot of this column in a framestructure also failed because the hoops only had 90 hooks (Turkey,lzmit 1999).

Page 5925/3 This transverse reinforcement hoops and ties at the edge ofa reinforced concrete structural wall is exemplary concerning anchor-age w ith 135 hooks. However, the vertical spacing of the transversereinforcement is too large, i.e. s = 7.5d instead of s 5d as requiredfor steel w ith a relatively small strain hardening ratio (Rm/Re = 1,15)[DW 99][TD 01].

W ithin cyclically stressed plastic zones of reinforcedconcrete structural walls and columns, the concretecover spalls when the elastic limit of the reinforcementis exceeded. In these zones it is therefore necessary tostabilise the vertical bars against buckling and to con-fine the concrete to allow greater compressive strains.The stabilising and confining transverse reinforcement(hoops and ties) must be anchored w ith 135 hooks.Damaging earthquakes have repeatedly illustrated that90 hooks are insufficient. The spacing of the trans-verse reinforcement must be relatively small s 5d (d = diameter of the stabilised bar). This is a conse-quence of the relatively poor ductility properties (smallstrain hardening ratio Rm/Re) of European reinforcingsteel, which result in an unfavourable buckling behav-iour [TD 01].

Similar rules apply to the plastic zones in framestructures [Ba 02].

W ithin the zones that are to remain elastic accordingto the capacity design method it is sufficient to applythe conventional design rules.

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

Use transverse rein forcement

with 135 hooks and spaced at s 5d in

structural walls and columns!


25




DUCTILITATE CUM?

58













25


58













25




DUCTILITATE CUM?


The Basic Safety Performance Objective Building Performance Level EQ Ground Immediate Structural Motion Operational Occupancy Life Safety Stability Serviceability

EQ (SE)

Design EQ (DE)

Maximum EQ

(ME)

Performance Based Design

Sfrit !


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Pereti Structurali Din Beton Armat