Development Length, Lap Splices and curtailment of reinforcement

Transcription

1 Lecture 05 Development Length, Lap Splices and curtailment of reinforcement By: Prof Dr. Qaisar Ali Civil Engineering Department UET Peshawar 1 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 l dh must be satisfied Curtailment of reinforcement 2 1

2 Development Length If the force P applied on the concrete block is gradually increased then depending on the embedment length provided the following two conditions are possible: 1. The bar will be pulled out of the concrete block. 2. Or the steel will be yielded without pulling the bar out of concrete block l P 3 Development Length Development length (l d ) 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 (l d ), the bar will be pulled out of the concrete block which is termed as bond failure. l P 4 2

3 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. 5 Development Length of Compression Reinforcement 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. 6 3

4 Development Length of Compression Reinforcement 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. 7 ACI Provision for Development of Tension Reinforcement Basic Equation (ACI ) For deformed bars or deformed wire, l d shall be: l d = 3 40 λ Ψ Ψ Ψ + For practical construction the (c + K tr )/d b is taken

5 ACI Provision for Development of Tension Reinforcement 9 ACI Provision for Development of Tension Reinforcement Basic Equation (ACI ) When λ, Ψ t and Ψ s values are taken equal to 1 For for No. 7 and larger bars, (Ψ e = 1), the equation is reduced to l d = 20 For No. 6 and smaller bars, (Ψ e = 0.8), the equation is reduced to l d =

6 ACI Provision for Development of Tension Reinforcement l d for grades 40 and 60 l d = (No. 7 bars and larger) l d = (No. 6 bars and smaller) For more details refer to section 5.3 of Design of Concrete Structures 14th Ed. by Nilson, Darwin and Dolan. Bar No Grade 40 Grade 60 # # # # # # # ACI Provision for Development of 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 (l dh ) is given as follows l dh = 65 λ 12 6

7 ACI Provision for Development of Standard hook in Tension l dh (inches) for grades 40 and 60 ( f c = 3000 psi λ = 1) l dh = f y d b / (65 λ f c ) Bar No. Grade 40 Grade 60 # # # # # # # 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. For more details refer to section 5.4 of Design of Concrete Structures 14th Ed. by Nilson, Darwin and Dolan. 14 7

8 Various scenarios where l dh must be Beam Column Joint satisfied Development of beam reinforcement in column shall be > l dh Development of beam reinforcement in column shall be > l dh Beam Column 15 Various scenarios where l dh must be satisfied Development of column reinforcement in foundation 16 8

9 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. 17 Types Bar splicing can be done in three ways: Lap Splice Mechanical Splice Welded Splice 18 9

10 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 , 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. 19 Lap Splice According to ACI , 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 l d Class B splice l d Where l d is as per in ACI 25.4 (discussed earlier)

11 Lap Splice Lap splices in general must be class B splices according to ACI , 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 21 Lap Splice According to ACI , tension 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)

12 Mechanical Splice In this method of splicing, the bars in direct contact are mechanically connected through sleeves or other similar devices. According to ACI , a full mechanical splice shall develop in tension or compression, as required, at least 125 percent of specified yield strength f y 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. 23 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 , A full welded splice shall develop at least 125 percent of the specified yield strength f y of the bar. This is for the same reason as discussed for mechanical splices

13 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 , A full welded splice shall develop at least 125 percent of the specified yield strength f y 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. 25 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

14 Curtailment of reinforcement a b a b 27 Curtailment of reinforcement 28 14

15 Curtailment of reinforcement For more details refer to section 5.10 of Design of Concrete Structures 14th Ed. by Nilson, Darwin and Dolan. 29 References Design of Concrete Structures 14th Ed. by Nilson, Darwin and Dolan. Building Code Requirements for Structural Concrete (ACI ) 30 15