development length, lap splices and curtailment of ... 05-development leng… · for more details...

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Department of Civil Engineering, University of Engineering and Technology Peshawar, Pakistan

Prof. Dr. Qaisar Ali CE:320 Reinforced Concrete Design-I

Lecture 05

Development Length, Lap

Splices and Curtailment of

Reinforcement

By: Prof. Dr. Qaisar Ali

Civil Engineering Department

UET Peshawar

[email protected]



Topics Addressed

Development Length

Development Length of Compression Reinforcement

ACI provisions for Development of Tension Reinforcement

ACI provisions for Development of Standard Hook in Tension

Dimensions & Bends for Standard Hooks

Various Scenarios where ldh must be satisfied

Splices of Deformed Bars

Curtailment of reinforcement

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Objectives

At the end of this lecture, students will be able to;

Explain importance of Development Length in concrete

members to avoid bond failure

Outline location of curtailment of reinforcement

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Development Length

Consider a steel bar embedded in concrete block; If force P is

gradually increased, depending on the embedment length, either

The bar will come out of the concrete block, or

The steel will yield.

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P

ℓ

Pℓ

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Development Length

Development length (ℓd) is the minimum length of the bar to be

embedded in the concrete block so that the bar is yielded but not

pulled out of the concrete block due to bond failure.

If the provided embedment length (ℓ) is less than the development

length (ℓd), the bar will be pulled out of the concrete block which is

termed as bond failure.

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P

ℓ



Development Length

There are two types of Bond Failure

1. Direct pullout of reinforcement: Direct pullout of reinforcement

occurs in members subjected to direct tension.

2. Splitting of concrete: In members subjected to tensile flexural

stresses, the reinforcement causes splitting of concrete as

shown.

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Development Length of Compression

Reinforcement

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In the case of bars in compression, a part of the total force is transferred by

bond along the embedded length, and a part is transferred by end bearing

of the bars on the concrete.

As the surrounding concrete is relatively free of cracks and because of the

beneficial effect of end bearing, shorter basic development lengths are

permissible for compression bars than for tension bars.



Development Length of Compression

Reinforcement

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In the next portion of the lecture we will discuss the development length

of tension reinforcement only because it is the governing criteria in most

of the cases in reinforced concrete structures.

For more details refer to section 5.8 of Design of Concrete Structures

14th Ed. by Nilson, Darwin and Dolan.

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Development of Tension

Reinforcement

Tensile force (T), acting on bar (Ab) having yield strength fy , try to

pullout bar embedded in concrete member having bond strength (fb).

Resistance (R), to force (T) , offered by skin friction around bar.

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Tℓd

• R ≥ 𝑇

• Length * Circumference * fb ≥ 𝑓yAb

• ld* Лdb* fb ≥ 𝑓𝑦𝐴𝑏

• ld* Лdb* α 𝑓𝑐′≥ 𝑓y∗ Лdb

2

4

• ld = (𝑓𝑦

4α 𝑓𝑐′) 𝑑𝑏



ACI Provision for Development of

Tension Reinforcement

Basic Equation (ACI 318-19, 25.4.2.4)

For deformed bars or deformed wire, ld shall be:

For practical construction the (cb + Ktr)/db is taken 1.5

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ℓd =3

40

𝑓𝑦

λ 𝑓𝑐′

Ψ𝑡Ψ𝑒Ψ𝑠Ψ𝑔

𝑐𝑏 +𝐾𝑡𝑟𝑑𝑏

𝑑𝑏

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Ref: ACI 318-19





Basic Equation (ACI 318-19, 25.4.2.4)

For No. 7 and larger bars, (Ψs = 1), the equation is reduced to

ℓd (in)=𝑓𝑦

20 𝑓𝑐′

𝑑𝑏

For No. 6 and smaller bars, (Ψs = 0.8), the equation is reduced to

ℓd (in)=𝑓𝑦

25 𝑓𝑐′

𝑑𝑏

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When λ, Ψg, Ψt and Ψe values are taken equal to 1

(fy and fc′ are in Psi)

(db is in inches)

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ℓd (inches) for grades 40 and 60; fc′ = 3 ksi

ℓd =fy

20 fc′db (No. 7 bars and larger)

ℓd =fy

25 fc′db (No. 6 bars and smaller)

Bar No Grade 40 Grade 60

#3 11′′ 17′′

#4 15′′ 22′′

#5 19′′ 28′′

#6 22′′ 33′′

#7 32′′ 48′′

#8 37′′ 55′′

#9 41′′ 62′′




Standard hook in Tension

If a hook is provided at the end of the embedded bar, the requirement

on the straight length portion of embedded bar is reduced. The

development length with hook (ldh) is given as follows

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ℓdh =𝑓𝑦𝑑𝑏

65 λ 𝑓𝑐′ Pℓdh

( ACI 318-19, 18.8.5.1)

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ℓdh (inches) for grades 40 and 60; fc′ = 3 ksi λ = 1)

ℓdh = fy db / (65 λ √fc′)


#3 5′′ 7′′

#4 6′′ 9′′

#5 7′′ 11′′

#6 9′′ 13′′

#7 10′′ 15′′

#8 12′′ 17′′

#9 13′′ 19′′





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Comparison between ℓd and ℓdh (fc′ = 3 ksi)


ℓd ℓdh ℓd ℓdh

#3 11′′ 5′′ 17′′ 7′′

#4 15′′ 6′′ 22′′ 9′′

#5 19′′ 7′′ 28′′ 11′′

#6 22′′ 9′′ 33′′ 13′′

#7 32′′ 10′′ 48′′ 15′′

#8 37′′ 12′′ 55′′ 17′′

#9 41′′ 13′′ 62′′ 19′′

Class Activity:

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Dimensions and bends for standard

hooks

Standard bends in reinforcing bars are described in terms of the inside

diameter of bend since this is easier to measure than the radius of bend.

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Ref: ACI 318-19



Various scenarios where ldh

must be

satisfied

Beam Column Joint

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Development of beam

reinforcement in column

shall be > ldh

Development of column

reinforcement in beam

shall be > ldh

Column

Beam

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Various scenarios where ldh

must be

satisfied

Development of column reinforcement in foundation

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Introduction

Splice means “to join”.

In general, reinforcing bars are stocked by supplier in lengths upto

60′. For this reason, and because it is often more convenient to

work with shorter bar lengths, it is frequently necessary to splice

bars.

Splices in the reinforcement at points of maximum stress should be

avoided.

Splices should be staggered.

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Types

Bar splicing can be done in three ways:

Lap Splice

Mechanical Splice

Welded Splice

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Lap Splice

Splices for #11 bars and smaller are usually made simply lapping

the bars by a sufficient distance to transfer stress by bond from

one bar to the other.

The lapped bars are usually placed in contact and lightly wired so

that they stay in position as the concrete is placed.

According to ACI 25.5.1.3, bars spliced by noncontact lap splices

in flexural members shall not be spaced transversely farther apart

than one-fifth the required lap splice length, nor 6 inches.

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Lap Splice

According to ACI 25.5.2.1, minimum length of lap for tension lap

splices shall be as required for Class A or B splice, but not less

than 12 inches, where:

Class A splice ................................................... 1.0ld

Class B splice ................................................... 1.3ld

Where ld as per ACI 25.4 (discussed earlier).

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Lap Splice

Lap splices in general must be class B splices according to ACI

25.5.2.1, except that class A splice is allowed when the area of the

reinforcement provided is at least twice that required by analysis

over the entire length of the splice and when ½ or less of the total

reinforcement is spliced within the required lap length.

The effect of these requirements is to encourage designers to

locate splices away from regions of maximum stress to a location

where the actual steel area is at least twice that required by

analysis and to stagger splices.

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Lap Splice

According to ACI 25.5.5.2, compression lap splices shall not be

used for bars larger than #11 (Because of lack of adequate

experimental data on lap splices of No. 14 and No. 18 bars).

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Mechanical Splice

In this method of splicing, the bars in direct

contact are mechanically connected through

sleeves or other similar devices.

According to ACI 25.5.7.1, a full mechanical

splice shall develop in tension or compression,

as required, at least 125 percent of specified

yield strength fy of the bar.

This ensures that the overloaded spliced bar

would fail by ductile yielding in the region away

from the splice, rather than at the splice where

brittle failure is likely.

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Welded Splice

Splicing may be accomplished by welding in which bars in direct

contact are welded so that the stresses are transferred by weld

rather than bond.

According to ACI 25.5.7.1, a full welded splice shall develop at

least 125 percent of the specified yield strength fy of the bar.

This is for the same reason as discussed for mechanical splices.

For more details refer to section 5.13 of Design of Concrete

Structures 14th Ed. by Nilson, Darwin and Dolan.

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Lap splice location

The splicing should be avoided in the critical locations, such as at

the maximum bending moment locations and at the shear critical

locations.

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It is common practice to cut off bars where they are no longer needed to resist

stress. In the case of simply supported beams, figure shows cut off locations for

various percentages of reinforcement curtailment.

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a

a

b

b



Prof. Dr. Qaisar Ali CE:320 Reinforced Concrete Design-I 30


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For nearly equal spans, uniformly loaded, in which not more than about one-half the tensile steel

is to be cut off, the locations shown in fig. are satisfactory.

Note, in Fig. that the beam at the exterior support at the left is shown to be simply supported. If

the beam is monolithic with exterior columns or with a concrete wall at that end, details for a

typical interior span could be used for the end span as well.





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For more details refer to section 5.10 of Design of Concrete Structures

14th Ed. by Nilson, Darwin and Dolan.



References

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Design of Concrete Structures 14th Ed. by Nilson, Darwin and

Dolan.

Building Code Requirements for Structural Concrete (ACI 318-

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