Lecture 15 Bar Development

1
Lecture 15 - Bar Development

• October 23, 2001
• CVEN 444

2
Lecture Goals

• Bar Development
• Hook development
• Bar Cut-off Points
• Spice

3
Example
Design anchorage of 4 8 top bars in column.
4
Bar Cut of Points
Why do you want to put in cut off points?
Economy!
5
Bar Cut of Points
Factors Affecting Bar Cut-off Points (Sec. 8-6)
Bars no longer needed to resist tensile forces or
remaining bars are adequate (Use moment and shear
envelopes) Bars must be extended on each side of
section to develop bar force at that section.
1.) 2.)
6
Bar Cut of Points
Factors Affecting Bar Cut-off Points (Sec. 8-6)
Major stress concentrations occur when tension
bars are cutoff in regions of moderate to high
shear forces. Leads to cracking. Code
specified construction requirements (good
practice)
3.) 4.)
7
Bar Cut of Points
Factors Affecting Bar Cut-off Points (Sec. 8-6)
Load Uncertainties (Seismic Considerations) Keep
Cutoffs to a minimum Simplify design
Construction
5.)
8
Determining Locations of Flexural Cutoffs
Given a simply supported beam with a distributed
load.
9
Determining Locations of Flexural Cutoffs
Note Total bar length Fully effective length
Development length
10
Determining Locations of Flexural Cutoffs
ACI 12.10.3 All longitudinal tension bars must
extend a min. distance d or 12 db (usually
larger) past the theoretical cutoff for flexure
(Handles uncertainties in loads, design
approximations,etc..)

11
Determining Locations of Flexural Cutoffs

Development of flexural reinforcement in a
typical continuous beam. ACI 318R-95
12
Bar Cutoffs - General Procedure
Determine theoretical flexural cutoff points for
envelope of bending moment diagram. Extract the
bars to satisfy detailing rules (from ACI Section
7.13, 12.1, 12.10, 12.11 and 12.12) Design extra
stirrups for points where bars are cutoff in zone
of flexural tension (ACI 12.10.5)
1. 2. 3.
13
Bar Cutoffs - General Rules
All Bars Rule 1. Rule 2.
Bars must extend the longer of d or 12db past the
flexural cutoff points except at supports or the
ends of cantilevers (ACI 12.11.1)
Bars must extend at least ld from the point of
maximum bar stress or from the flexural cutoff
points of adjacent bars (ACI 12.10.2 12.10.4 and
12.12.2)
14
Bar Cutoffs - General Rules
Positive Moment Bars Rule 3.

• Structural Integrity
• Simple Supports At least one-third of the
  positive moment reinforcement must be extend 6
  in. into the supports (ACI 12.11.1).
• Continuous interior beams with closed stirrups.
  At least one-fourth of the positive moment
  reinforcement must extend 6 in. into the support
  (ACI 12.11.1 and 7.13.2.3)

15
Bar Cutoffs - General Rules
Positive Moment Bars Rule 3.

• Continuous interior beams without closed
  stirrups. At least one-fourth of the positive
  moment reinforcement must be continuous or shall
  be spliced near the support with a class A
  tension splice and at non-continuous supports be
  terminated with a standard hook. (ACI 7.13.2.3).

16
Bar Cutoffs - General Rules
Positive Moment Bars Rule 3.

• Continuous perimeter beams. At least one-fourth
  of the positive moment reinforcement required at
  midspan shall be made continuous around the
  perimeter of the building and must be enclosed
  within closed stirrups or stirrups with 135
  degree hooks around top bars. The required
  continuity of reinforcement may be provided by
  splicing the bottom reinforcement at or near the
  support with class A tension splices (ACI
  7.13.2.2).

17
Bar Cutoffs - General Rules
Positive Moment Bars Rule 3.

• Beams forming part of a frame that is the primary
  lateral load resisting system for the building.
  This reinforcement must be anchored to develop
  the specified yield strength, fy, at the face of
  the support (ACI 12.11.2)

18
Bar Cutoffs - General Rules
Positive Moment Bars Rule 4.

• At the positive moment point of inflection and at
  simple supports, the positive moment
  reinforcement must be satisfy the following
  equation for ACI 12.11.3. An increase of 30 in
  value of Mn/Vu shall be permitted when the ends
  of reinforcement are confined by compressive
  reaction (generally true for simply supports).

19
Bar Cutoffs - General Rules
Negative Moment Bars Rule 5.

• Negative moment reinforcement must be anchored
  into or through supporting columns or members
  (ACI Sec. 12.12.1).

20
Bar Cutoffs - General Rules
Negative Moment Bars Rule 6.

• Structural Integrity
• Interior beams. At least one-third of the
  negative moment reinforcement must be extended by
  the greatest of d, 12 db or ln/16 past the
  negative moment point of inflection (ACI Sec.
  12.12.3).
• Perimeter beams. In addition to satisfying rule
  6a, one-sixth of the negative reinforcement
  required at the support must be made continuous
  at midspan. This can be achieved by means of a
  class A tension splice at midspan (ACI 7.13.2.2).

21
Example
A preliminary design of reinforcement at sections
C and D is shown. Cross section are shown.
Select cutoff points for locations C and D based
on following requirements . For both locations
assume that the flexural design is adequate and
that no excess reinforcement is provided. Girder
16 in. fc 5000 psi fy 60000 psi
22
Example
All the negative moment reinforcement at C will
be extended past the negative moment point of
inflection. Determine the cutoff point for the
No 7 negative moment bars at each end.
(A)
23
Example
Cutoff 1 No.7 positive moment bar when it is no
longer needed at each end. Provide the distance
for location D (at midspan) to the cutoff point
at each end of the bar.
(B)
24
(No Transcript)
25
Bar Splices
Why do we need bar splices? -- for long
spans Types of Splices 1. Butted
Welded 2. Mechanical Connectors 3. Lay Splices
Must develop 125 of yield strength
26
Tension Lap Splices
Why do we need bar splices? -- for long
spans Types of Splices 1. Contact Splice 2. Non
Contact Spice (distance 6 and 1/5 splice
length) Splice length is the distance the two
bars are overlapped.
27
Types of Splices
Class A Spice
(ACI 12.15.2)
When over entire
splice length. and 1/2 or less of
total reinforcement is spliced win the reqd lay
length. All tension lay splices not meeting
requirements of Class A Splices
Class B Spice
28
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. 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)
29
Compression Lap Splice (ACI 12.16)
Lap, reqd 0.0005fy db for fy lt 60000 psi
Lap, reqd (0.0009fy -24)
db for fy gt 60000 psi
Lap, reqd gt 12 in
For fc lt 3000 psi, required lap splice shall be
multiply by (4/3) (ACI 12.16.1)
In tied column splices with effective tie area
throughout splice length 0.0015 hs factor
0.83 In spiral column
splices, factor 0.75 But final splice length
12 in.