Lecture 21 Splices and Shear

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Lecture 21 Splices and Shear

• February 5, 2003
• CVEN 444

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Lecture Goals

• Spice
• Shear
• Shear Design

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Bar Splices
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Tension Lap Splices
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Types of Splices
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Types of Splices
Class B Spice
(ACI 12.15.2)
All tension lay splices not meeting requirements
of Class A Splices
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Tension Lap Splice (ACI 12.15)
where As (reqd) determined for bending ld
development length for bars (not allowed
to use excess reinforcement modification
factor) ld must be greater than or
equal to 12 in.
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Tension Lap Splice (ACI 12.15)
Lap Spices shall not be used for bars larger than
No. 11. (ACI 12.14.2) Lap Spices should be placed
in away from regions of high tensile stresses
-locate near points of inflection (ACI 12.15.1)

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Compression Lap Splice (ACI 12.16.1)
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Compression Lap Splice (ACI 12.17)

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Example Splice Tension
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Example Splice Tension
For 8 bars, db 1.0 in. and a ?b ? g ?l ?
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Example Splice Tension
The As(provided) /As(required) gt 2, class ?
splice applies
The As(provided) /As(required) lt 2, class ?
splice applies
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Example Splice Compression
Calculate the lap splice length for a 10
compression bar in tied column when fc 5 ksi and
when a) fy 60 ksi and b) fy 80 ksi
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Example Splice Compression
For 10 bars, db ? in.
Check ls gt 0.005 db fy
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Example Splice Compression
For 10 bars, db ? in. The ld 2? in. Check
ls gt (0.0009 fy 24) db So use
ls ? in.
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Shear Design
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Uncracked Elastic Beam Behavior
Look at the shear and bending moment diagrams.
The acting shear stress distribution on the beam.
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Uncracked Elastic Beam Behavior
The acting stresses distributed across the
cross-section.
The shear stress acting on the rectangular beam.
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Uncracked Elastic Beam Behavior
The equation of the shear stress for a
rectangular beam is given as
Note The maximum 1st moment occurs at the
neutral axis (NA).
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Uncracked Elastic Beam Behavior
The ideal shear stress distribution can be
described as
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Uncracked Elastic Beam Behavior
A realistic description of the shear distribution
is shown as
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Uncracked Elastic Beam Behavior
The shear stress acting along the beam can be
described with a stress block
Using Mohrs circle, the stress block can be
manipulated to find the maximum shear and the
crack formation.
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Inclined Cracking in Reinforced Concrete Beams
Typical Crack Patterns for a deep beam
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Inclined Cracking in Reinforced Concrete Beams
Flexural-shear crack - Starts out as a flexural
crack and propagates due to shear
stress. Flexural cracks in beams are vertical
(perpendicular to the tension face).
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Inclined Cracking in Reinforced Concrete Beams
For deep beam the cracks are given as The shear
cracks Inclined (diagonal) intercept crack
with longitudinal bars plus vertical or inclined
reinforcement.
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Inclined Cracking in Reinforced Concrete Beams
For deep beam the cracks are given as The shear
cracks fail due two modes - shear-tension
failure - shear-compression failure

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Shear Strength of RC Beams without Web
Reinforcement
Total Resistance vcz vay vd (when no
stirrups are used)
vcz - shear in compression zone va - Aggregate
Interlock forces vd Dowel action from
longitudinal bars Note vcz increases from (V/bd)
to (V/by) as crack forms.
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Strength of Concrete in Shear (No Shear
Reinforcement)
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Strength of Concrete in Shear (No Shear
Reinforcement)
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Strength of Concrete in Shear (No Shear
Reinforcement)
(3) Shear span to depth ratio, a/d (M/(Vd))

Deep shear spans more detail design required
Ratio has little effect
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Strength of Concrete in Shear (No Shear
Reinforcement)
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Strength of Concrete in Shear (No Shear
Reinforcement)
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Function and Strength of Web Reinforcement
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Function and Strength of Web Reinforcement

• Uncracked Beam Shear is resisted uncracked
  concrete.
• Flexural Cracking Shear is resisted by vcz,
  vay, vd

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Function and Strength of Web Reinforcement

• Flexural Cracking Shear is resisted by
  vcz, vay, vd and vs

Vs increases as cracks widen until yielding of
stirrups then stirrups provide constant
resistance.
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Designing to Resist Shear
Shear Strength (ACI 318 Sec 11.1)
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Designing to Resist Shear
Shear Strength (ACI 318 Sec 11.1)
Nominal shear resistance provided by concrete
Nominal shear provided by the shear reinforcement
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Shear Strength Provided by Concrete
Bending only
Simple formula More detailed Note
Eqn 11.3
Eqn 11.5
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Shear Strength Provided by Concrete
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Typical Shear Reinforcement
Stirrup - perpendicular to axis of members
(minimum labor - more material)
ACI Eqn 11-15
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Typical Shear Reinforcement
Bent Bars (more labor - minimum material) see
reqd in 11.5.6
ACI 11-5.6
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Stirrup Anchorage Requirements
Vs based on assumption stirrups yield
Stirrups must be well anchored.
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Stirrup Anchorage Requirements
Refer to Sec. 12.13 of ACI 318 for development of
web reinforcement. Requirements

• each bend must enclose a long bar
• 5 and smaller can use standard hooks 90o,135o,
  180o
• 6, 7,8( fy 40 ksi )
• 6, 7,8 ( fy gt 40 ksi ) standard hook plus a
  minimum embedment

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Stirrup Anchorage Requirements
Also sec. 7.10 requirement for minimum stirrups
in beams with compression reinforcement, beams
subject to stress reversals, or beams subject to
torsion
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Design Procedure for Shear
(1) Calculate Vu (2) Calculate fVc Eqn 11-3 or
11-5 (no axial force) (3) Check
If yes, add web reinforcement (go to 4)
If no, done.
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Design Procedure for Shear
Provide minimum shear reinforcement
(4)
Also (Done)
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Design Procedure for Shear
(5)
Check
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Design Procedure for Shear
(6)
Solve for required stirrup spacing(strength)
Assume 3, 4, or 5 stirrups
from 11-15
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Design Procedure for Shear
(7) Check minimum steel requirement (eqn 11-13)
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Design Procedure for Shear
(8) Check maximum spacing requirement (ACI
11.5.4)
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Design Procedure for Shear
(9) Use smallest spacing from steps 6,7,8
Note A practical limit to minimum stirrup
spacing is 4 inches.
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Location of Maximum Shear for Beam Design
Non-pre-stressed members
Sections located less than a distance d from face
of support may be designed for same shear, Vu, as
the computed at a distance d.
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Location of Maximum Shear for Beam Design
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Location of Maximum Shear for Beam Design
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Homework
Determine the development length required for the
bars shown . fc 4-ksi and fy 60-ksi. Check the
anchorage in the column. If it is not
satisfactory, design an anchorage using a 180o
hook and check adequacy.
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Homework
Considering the anchorage of the beam bars into a
column, determine the largest bar that can be
used with out a hook. fc 3-ksi and fy 40ksi
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Homework
A simple supported uniformly loaded beam carries
a total factored design load of 4.8 k/ft
(including self-weight) on a clear span of 34 ft.
fc 3 ksi and fy40 ksi. Assume that the
supports are 12 in wide and assume that the bars
are available in 30 ft lengths. Design a
rectangular beam Determine bar cutoffs. Locate
splices and determine the lap length.