Lecture on CE 4014 Design of Concrete Structures

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Lecture onCE 4014 Design of Concrete Structures

• Yangon Technological University
• Department of Civil Engineering

(Bond, Anchorage and Development Length) Part (I)
Dr. Khin Than YuProfessor and Head
20-3-2008
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Design of Concrete Structures

• Text and Reference

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FUNDAMENTALS OF FLEXURAL BOND

• In reinforced concrete beams it is assumed that
  strain in the embedded reinforcing bar is the
  same as that in the surrounding concrete.
• Therefore, it is essential that bond force is
  developed on the interface between concrete and
  steel to prevent significant slip from occurring
  at the interface.

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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 action
  even for bond broken beam.
• Force in the steel,
• T Mmax / z
• Deformed bar providing bond force via the
  shoulders of the projecting ribs bear on the
  surrounding concrete.

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Bond Stress Based on Simple Cracked Section
Analysis
u local average unit bond stress?o
sum of the perimeter of all barsJd internal
lever arm between tensile and
compressive force resultantsdx short piece of
length of beam
dT dM / jdFor local equilibrium, change
in bar force bond force
at the contact surfaceu ?o
dx dT, u dT / ?o dx dM / ?o
jd dx dV / ?o jd
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b. Actual Distribution of Flexural Bond Stress

• Pure bending case
• Concrete fails to resist tensile stresses only
  where the actual crack is located. Steel T is
  maximum and
• T max M / jd .
• Between cracks , concrete does resist moderate
  amount of tension introduced by bond.
• u is proportional to the rate of change of bar
  force, and highest where the slope of the steel
  force curve is greatest.
• Very high local bond stress adjacent to the
  crack.

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• Beam under transverse loads,
• According to simple crack sectional theory, T is
  proportional to the moment diagram and u is
  proportional to shear force diagram.
• In actual, T is less than the simple analysis
  prediction everywhere except at the actual
  cracks.
• Similarly, u is equal with simple analysis
  prediction only at the location where slopes of
  the steel force diagrams are equals .If the slope
  is greater than assumed, bond stress is greater
  if the slope is less bond stress is less.

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ULTIMATE BOND STRENGTH AND DEVELOPMENT LENGTH

• Types of bond failure
• Direct pullout of bars
• (small diameter bars are used with
  sufficiently large concrete cover distances and
  bar spacing)
• Splitting of the concrete along the bar (cover or
  bar spacing is insufficient to resist the lateral
  concrete tension resulting from the wedging
  effect of bar deformations)

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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 of the 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 the beam.
• 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 length is the length of embedment
  necessary to develop the full tensile strength of
  bar, controlled by either pullout or splitting.
• In Fig., let
• maximum M at a and zero at support
• fs at a? T Ab fs _
• Development length concept ?total tension force
  must be transferred from the bar to the concrete
  in the distance l by bond stress on the
  surface.
• To fully develop the strength ? T Ab fy
• ? ld
  , development length
• Safety against bond failure the length of the
  bar from any point of given steel stress to its
  nearby end must be at least equal to its
  development length. If the length is inadequate,
  special anchorage can be 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 ktr) / db 2.5 for pullout case
• vfc are not to be greater 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 be required to develop its
  compressive strength by embedment under various
  circumstances.
• ACI basic development length in compression
• ldb 0.02db fy/vfc

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BAR CUTOFF AND BEND POINTS IN BEAMS

• Theoretical points of cutoff or bend
• T As fs M/z
• T function of (M)
• ACI Code uniformly loaded, continuous beam of
  fairly regular span may be designed using moment
  coefficients.

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b. Practical Considerations and ACI Code
Requirements
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• If cutoff points are in tension zone (to prevent
  formation of premature flexural and diagonal
  tension cracks) no flexural bar shall be
  terminated unless the following conditions are
  specified.

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• Standard Cutoff and Bend Points
• For not more than 50 of tensile steel is to be
  cutoff or bent

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c. Special Requirements near the Point of
Zero Moment

• It is necessary to consider whenever the moments
  over the development length are greater than
  those corresponding to a 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 flexural
• strength provided by those
• bars extend to the
• point of inflection.

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• For assumed (conservatively) uniformed slope of
  moment diagram Vu towards the positive moment
  region, length a at M 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,
  total collapse can be prevented through
  relatively minor changes in bar detailing owing
  to accidental or abnormal loading.
• If some reinforcement properly confined is
  carried continuously through a support catenary
  action of beam can prevent from total collapse
  even if the support is damaged.
• ACI Code

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Comment

• Consideration for bond and detail design for
  anchorage, development length and structural
  integrity requirements are important to have
  proper structural performance of the building.