114458688 Bond Anchorage and Development Length

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Lecture on

CE 4014

Design of Concrete Structures

Yangon Technological University

Department of Civil Engineering

Dr. Khin Than YuProfessor and Head

(Bond, Anchorage and Development Length)

Part (I)

20-3-2008


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Department of CivilEngineering, YTU 2

Design of Concrete Structures

Text and Reference


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FUNDAMENTALS OF FLEXURAL BOND

In reinforced concrete beamsit is assumed that strain in theembedded reinforcing bar isthe same as that in thesurrounding concrete.

Therefore, it is essential thatbond force is developed on theinterface between concreteand steel to prevent significantslip from occurring at theinterface.


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Source of bond strength

Weak chemical adhesion Mechanical friction between

steel and concrete Slip induced interlocking of

natural roughness of the bar

with concrete End anchorage, hooks :

providing tie arch actioneven for bond broken beam.

Force in the steel,T = Mmax / z

Deformed bar: providing bondforce via the shoulders of theprojecting ribs bear on thesurrounding concrete.


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a. Bond Stress Based on Simple

Cracked Section Analysis

dT = dM / jdFor local equi l ibr ium ,

change in

bar force = bon d force

at the contact

surface

u o dx = dT,

u= dT/

o dx

= dM / o

jd dx

= dV / ojd

u = local average unit bond stress

o = sum of the per imeter of al l bars

Jd = internal lever arm between tensile

and compressive force resultants

dx = sho rt piece of length o f beam


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b. Actual Distribution of Flexural Bond

Stress

Pure bending case Concrete fails to resist tensile

stresses only where the actualcrack is located. Steel T ismaximum and

Tmax = M / jd . Between cracks , concrete does

resist moderate amount of tensionintroduced by bond.

uis proportional to the rate ofchange of bar force, and highestwhere the slope of the steel forcecurve is greatest.

Very high local bond stress

adjacent to the crack.


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Beam under transverse loads,

According to simple cracksectional theory, T isproportional to the momentdiagram and u is proportionalto shear force diagram.

In actual, T is less than thesimple analysis predictioneverywhere except at theactual cracks.

Similarly, u is equal withsimple analysis prediction only

at the location where slopes ofthe steel force diagrams areequals .If the slope is greaterthan assumed, bond stress isgreater; if the slope is lessbond stress is less.


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ULTIMATE BOND STRENGTH AND

DEVELOPMENT LENGTH

Types of bond failure

Direct pulloutof bars(small diameter bars are

used with sufficientlylarge concrete coverdistances and barspacing)

Splitting of theconcretealong the bar

(cover or bar spacing isinsufficient to resist thelateral concrete tensionresulting from thewedging effect of bardeformations)


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a. Ultimate Bond Strength

Direct pull out For sufficiently confined bar, adhesive bond and friction are overcome as the

tensile force on the bar is increased. Concrete eventually crushes locally ahead ofthe bar deformation and bar pullout results. When pull out resistance is overcome or when splitting has spread all the way to the

end of an unanchored bar, complete bond failure occurs.

Splitting Splitting comes from wedging action when the ribs of the deformed bars bear

against the concrete. Splitting in vertical plane Splitting in horizontal plane: frequently begins at a diagonal crack in connection with

dowel action. Shear and bond failures are often interrelated.

Local bond failure Large local variation of bond stress caused by flexural and diagonal cracks

immediately adjacent to cracks leads to this failure below the failure load of thebeam. Results small slip and some widening of cracks and increase of deflections. Harmless as long as the failure does not propagate all along the bar.

Providing end anchorage, hooks or extended length of straight bar(development length concept)


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b. Development Length

Development lengthis the length of embedment necessary to developthe full tensile strength of bar, controlled by either pullout or splitting.

In Fig., let

maximum M at a and zero at support fs at aT = Abfs _

Development length concepttotal tension force must betransferred from the bar to the concrete in the distance l bybond stress on the surface.

To fully develop the strength T = Abfy

ld , development length

Safety against bond failure: the length of the bar from any point ofgiven steel stress to its nearby end must be at least equal to itsdevelopment length. If the length is inadequate, special anchorage canbe provided.


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c. Factors influencing Development

Length

Tensile strength of concrete

Cover distance

Bar spacing

Lateral reinforcement

Vertical bar location relative to beam depth

Epoxy coated bars or not

Excess reinforcement

Bar diameter


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ACI CODE PROVISION FOR DEVELOPMENT

OF TENSION REINFORCEMENT

Limit (c + k

tr) / d

b= 2.5 for

pullout case

fc are not to begreater than 100 psi.


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For two cases of practical importance, using (c + ktr) /

db= 1.5,


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Example:


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Continue:


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Continue:


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ANCHORAGE OF TENSION BARS BY

HOOKS

In the event that the desired tensile stress in a bar can not

be developed by bond alone, it is necessary to provide

special anchorage at the end of the bar.


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b. Development Length and Modification

Factors for Hooked Bars


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Example


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ANCHORAGE REQUIREMENTS FOR WEB

REINFORCEMENT


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DEVELOPMENT OF BARS IN

COMPRESSION

Reinforcement may berequired to develop itscompressive strength by

embedment under variouscircumstances.

ACI basic developmentlength in compression

ldb= 0.02dbfy/fc


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BAR CUTOFF AND BEND POINTS IN BEAMS

Theoretical points of cutoff orbend

T = As fs = M/z

T = function of (M)

ACI Code: uniformly loaded,continuous beam of fairly regularspan may be designed usingmoment coefficients.


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b. Practical Considerations and ACI Code

Requirements


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If cutoff points are in tension zone (to preventformation of premature flexural and diagonal

tension cracks) no flexural bar shall be terminatedunless the following conditions are specified.


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Standard Cutoff

and BendPoints

For not more

than 50% oftensile steel isto be cutoff orbent

S i l R i h P i


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c. Special Requirements near the Point

of Zero Moment

It is necessary to consider whenever the moments over thedevelopment length are greater than those corresponding toa linear reduction to zero.

Bond force per unit length , u = dT / dx = dM / zdx,

proportional to the slope of the moment diagram. Maximum bond forces u would occur at point of inflection

and pullout resistance is required.

Slope of M diagram at any point = V at that point

Let Mn = nominal flexuralstrength provided by those

bars extend to the

point of inflection.


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For assumed (conservatively) uniformed slope of momentdiagram Vutowards the positive moment region, length aatM = Mn

a= Mn/Vu

Thus a must be greater than or equal to ld

ACI Code

Simply support case


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d. Structural Integrity Provisions

For major supporting elements, such as columns, totalcollapse can be prevented through relatively minorchanges in bar detailing owing to accidental or abnormalloading.

If some reinforcement properly confined is carriedcontinuously through a support catenary action of beamcan prevent from total collapse even if the support isdamaged.

ACI Code


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Department of CivilEngineering YTU 31

Comment

Consideration for bond and detail designfor anchorage, development length andstructural integrity requirements are

important to have proper structuralperformance of the building.

114458688 Bond Anchorage and Development Length

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