REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
Bond, Development Lengths, and Splices
The design ob beams is so far based on the maximum moments to provide the calculation of area of steel reinforcement needed. The area computed are based at midspan for positive reinforcement and near the support for negative reinforcement. However, the moment varies with distance at some points along the length allowing a reduction of area of steel theoretically to reduce cost. The figure shown below explains how bars can be reduced or cutoff with moments as basis for the reductions. Assuming a beam with 6 bars as reinforcement, then the possible reduction can be theoretically be cut as shown. kN/m
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
BOND STRESS The basic assumption in the design of concrete is, concrete and steel reinforcement should act as one even if they are made of different materials. It means that there is no slippage of steel bars with the surrounding materials. The acting together is what we call “Bond”. If the materials will not bond as one, and if steel bars are not anchored at its end, then it will be pulled out of the concrete. As a result, deformed bars are used in almost all concrete works. Plain bars are also allowed to be used for lateral reinforcement in compression members. Reinforcing bars are made of rib type deformations and thus, bond is supplied by the bearing on the ribs as shown.
To illustrate the type of bond failures on concrete, the figure below provides a picture of how concrete splits depending on several factors like, concrete that covers around the bars, spacing of bars, coating on the bars, type of aggregates, transverse confining effect of stirrups etc.
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
DEVELOPMENT LENGTHS FOR TENSION REINFORCEMENT. To provide a monolithic behavior of the materials, the steel and concrete, the bond stresses must be transferred to the concrete by bond between the steel and concrete before the bars are cut off. In this case, the bars must be extended some distance back into the support and out into the beam to anchor or develop their strength, this extended distance is what we call as “Development Length, ld” as shown in the figure below.
ACI Equation 12-1
ld
c ktr db
k tr
9 10
f y c ktr f c' db
db
shall not be taken greater than 2.5.
Atr f yt 10sn
, transverse reinforcement index.
This equation provides a result in terms of diameter of bars being used in the design. The values of the different coefficients are given in ACI 12.2.4 or NSCP 412.3.4 DIOSDADO B. DIMALANTA, MSCE, CE DEPARTMENT, CSU-ENGINEERING
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
NOTE: 1. ACI 12.1.2, the value of f c' used in the equations shall not exceed 25/3 Mpa. 2. ACI 12.2.5, Excess reinforcement.-reduction in development length may be permitted. The ld/db may be multiplied by (As ,required / As, provided ).
DEVELOPMENT LENGTHS FOR BUNDLED BARS The use of bundled bars requires greater development lengths because there is not a “core” concrete between the bars to provide resistance to slipping. ACI 12.4.1 Development of individual bars within a bundle, in tension and compression, shall be that for the individual bar, increased 20 percent for three-bar bundle, and 33 percent for four-bar bundle.
10 mm Ø stirrups
DIOSDADO B. DIMALANTA, MSCE, CE DEPARTMENT, CSU-ENGINEERING
3-25 mm Ø
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
HOOKS In actual design implementation due to limited dimensions of members or sufficient space is not available to anchor tension bars by running them straight for their required development lengths, hooks may be used to provide the necessary development length. (Hooks are considered useless for compression bars for development length purposes).
D=6db for 10 to 25 mm Ø D=8db for 28, 32, & 36 mm Ø D=10 db for higher than 36 mm Ø
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ACI 12.5.1 Development length ldh for deformed bars in tension terminating in a standard hook.
ld
0.24f y f
' c
db
ld >8db nor less than 150 mm The development length ld is measured from the critical section of the bar to the outer side end or edge of the hook as shown in the figure.
4db for 10 to 25 mm Ø 5db for 28, 32, & 36 mm Ø 6db for higher than 36 mm Ø
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
DEVELOPMENT LENGTH FOR COMPRESSION BARS For compression bars, there are no cracks that encourage slipping, and the bearing on the ends of the reinforcement on concrete helps develop the load, hence it is obvious that development length for compression bars is smaller than those required for tension bars. ACI 12.3.2 For deformed bars and deformed wire, the ld shall be taken as the larger of
ld
0.002 f y d b f c'
l d (0.0003 f y )d b
But ld ≥ 200 mm
Note that 0.0003 has a unit of mm2/N EFFECT OF COMBINED SHEAR AND DIAGRAM ON DEVELOPMENT LENGTHS
MOMENT
ACI 12.10.3 Reinforcement shall extent beyond the point at which it is no longer required to resist flexure to a distance equal to the effective depth of member or 12db whichever is greater, except at supports of simple spans and at free end of cantilevers. Commentary 12.10.3 The extension of effective depth of member or 12db is to account for the shifting location of maximum moment or inflection point due to changes in loading, support settlement, application of lateral loads and other factors.
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
BAR SPLICES IN FLEXURAL MEMBERS Construction is the implementation of design phase of a project. In actual situation, reinforcing bars are limited to standard lengths of 6, 7.5, 9, 10.5, 12 and 13.5 m. The splicing of bars is necessary to provide continuity of reinforcement, hence, locations and types of splicing is very important to be specified by the designer. The most common method of spicing is Lapping. The code requires that lapping is only allowed for diameters 36 mm or smaller. However, lap splicing may not be satisfactory for several situations where it may cause congestions, lap maybe very long, and big diameters like 36 mm and above are not permitted by the code. Splices should be located away from location of maximum tensile stresses. Furthermore, the bars should not be spliced at the same locations- that is splices should be staggered as required by the code. TENSION SPLICES
ACI 12.15.1 Stipulates the two type of splices into two; Class A and Class B and it is dependent on the level of stress and percentage of steel that is spliced at a particular location. Class A Splices are those where reinforcement is lapped for a minimum distance of 1.0ld (but not less than 300mm) and where one-half or less of the reinforcing is spliced at any one location. Class B Splicing are those where reinforcement is lapped for a minimum distance of 1.3ld (but not less than 300mm) and where all the reinforcing is spliced at same location.
As provided As required Equal or greater than 2 Less than 2 DIOSDADO B. DIMALANTA, MSCE, CE DEPARTMENT, CSU-ENGINEERING
Maximum percent of As spliced within required lap length 50 100 Class A Class B
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COMPRESSION SPLICES ACI 12.16.1 Compression lap splice length shall be 0.0005fydb for fy of Grade 60(415 Mpa) or less, or (0.0009 fy – 24)db for fy greater than Grade 60, but not less than 300 mm. For fc’ less than 20 Mpa, length of lap shall be increased by one-third.
Example of Mechanical Connectors (Coupler)
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
EXAMPLE 1. Determine the development length for the 6-25 mm Ø bars for the following specification and requirements: fc’ = 20.7 Mpa, fy = 275 Mpa Using: 1. Straight bar development length 2. If 180o hook is used 3. If 90o hook is used
63mm
350 mm 63mm
50 mm
6-25 mm Ø
6-25 mm Ø
500 mm 387 mm
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REINFORCED CONCRETE DESIGN NOTES CIVIL-STRUCTURAL ENGINEERING
DIOSDADO B. DIMALANTA, MSCE, CE DEPARTMENT, CSU-ENGINEERING
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