3. Bond , Anchorage and Shear • This chapter will …

1 3. bond , Anchorage and Shear This chapter will discuss the following topics: Outline the theory of calculating the Anchorage bond length. Determination of Anchorage bond length tension lap Determination of Anchorage bond length, tension lap length and compression lap length by using Table Table of the code. Outline the design formulae for the design of Shear reinforcement and the use of Table the Code. RC Design and Construction HKC 2004 (2ndEdition)3-13. bond , Anchorage and Shear Minimum Distance Between Bars The min. distance between bars are given in terms ofh(themaximumsizeofaggregate)hagg(thema ximumsizeofaggregate).

2 ClearHorizontalandVerticalDistancebetwee nbars ClearHorizontalandVerticalDistancebetwee nbars bardiameteror bardiameteror (hagg+ 5mm) or 20mmwhicheverisgreater Design and Construction HKC 2004 (2ndEdition) Minimum and Maximum Percentages of Reinforcement in Beams, Slabs and Columns Minimum % : Table the code Maximum %(a) Beam ( )Neither the area of tension reinforcement nor the area of compression preinforcement should exceed 4% of the gross cross-sectional area of concrete.(b)Column( )()()The longitudinal reinforcement should not exceed the following amounts, calculated as percentages of the gross cross-sectional area of the concrete :~ vertically cast columns: 6%~ horizontally cast columns : 8%~ laps in the vertically or horizontally cast column.

3 10%pyy(c) Wall ( )The area of vertical reinforcement should not exceed 4% of the concreteRC Design and Construction HKC 2004 (2ndEdition)3-3 The area of vertical reinforcement should not exceed 4% of the concrete cross-sectional area of the Anchorage bond The steel bar subjected to tension shown in length (embedded length) depends onthebondbetweenthebarandtheconcrete,and thethebondbetweenthebarandtheconcrete,an dthecontact F Fig. Anchorage BondRC Design and Construction HKC 2004 (2ndEdition) Anchorage BondL = Min. Anchorage length to prevent pull out.

4 =Barsizeornominaldiameter = Ultimate Anchorage bond Direct tensile or compressive stress in the forces on the bar,Tensile pull-out force = Area of bar * Stress of bar 2= 24 fsRC Design and Construction HKC 2004 (2ndEdition) Anchorage BondAnchorage force= Contact Area * bond Stress =(L ) f= (L ) fbu =(L ) fbu 2 fs( )bu4fs =bubusfffL4=4yThe ultimate Anchorage bond stress,.Valuesof Valuesof Anchorage length L can be written asLK (tbl84fthCd)RC Design and Construction HKC 2004 (2ndEdition)3-6L=KA ( ) Anchorage BondTable - Values of the bond coefficient Bt Bar type Bars in tension Bars in compression Plain bars Type 2 : deformed bars Fabric Normally deformed type 2 bars are used in construction works.

5 Furthermore Anchorage may be provided by hooksandbendsinthereinforcement(seeFig82 )andbendsinthereinforcement.( ).RC Design and Construction HKC 2004 (2ndEdition)3-7 Fig. Anchorage of LinksdfdReproduce from HK CodeRC Design and Construction HKC 2004 (2ndEdition) Lapping of Reinforcement Lapping of reinf. may be required if the reinf. is of insufficient length or there is curtailment (cuttinginsufficient length or there is curtailment (cutting off of reinf. when it is no longer required in order to save money) of save money) of Rulesforlapping are:- The laps should be staggered and be away from sectionswithhighstresses(ieavoidlappingo freinfatsectionswithhighstresses.)

6 ( large bending moment as it induces large tensile stressin reinforcement) Min. lap length shouldbe not less than thegreaterof 15 or Design and Construction HKC 2004 (2ndEdition) Lapping of ReinforcementThe tension lap length should be at least equal to the design tension Anchorage length. Lap lengths for unequal size bars may be based hdifhll bhfll iiilupon the diameter of the smaller bar. The following provisions also apply: where a lap occurs at the top of a section as cast and the where a lap occurs at the top of a section as cast and the minimum cover is less than twice the diameter of the lapped reinforcement, the lap length should be increased by a factor of where a lap occurs at a corner of a section and the minimum cover to either face is less than twice the diameter of thecover to either face is less than twice the diameter of the lapped reinforcement, the lap length should be increased by a factor of where both conditions above apply.

7 The lap length should be increased by a factor of Design and Construction HKC 2004 (2ndEdition)3-10 Figure and table are Lapping of Reinforcement Rules for lapping are:- Compression laps should be at least 25% greater than theCompression laps should be at least 25% greater than the compression Anchorage length. Lap lengths for unequal size bars may be based on the smaller Cover < 2 , multiple lap length by (Lap length = Anchorage length x )Fig. Lapping of tension reinforcementRC Design and Construction HKC 2004 (2ndEdition)3-11 Fig. Lapping of tension Shear reinforcement can be in the form of shearlinksandinclinedbars(bent-upbars).

8 However,linksandinclinedbars(bentupbars) .However, k(Sti) ShearLinks(Stirrups) Links can be broadly classified into Shear link andnominal link (minimum link) andthe designformulae for these types of links are as follows:RC Design and Construction HKC 2004 (2ndEdition)3-12 Design Formulae for Shear Reinforcementff 40 MPfor fcu 40 MPa Shear Links: Shear Links:-()csvvvbA Minimum (nominal) RC Design and Construction HKC 2004 (2ndEdition)3-13 Design Formulae for Shear ReinforcementWhere:-A= Total crosssectional area of the vertical legs of theAsv = Total cross-sectional area of the vertical legs of of beamb = Breadth of beamfyv= Characteristic strength of the Shear reinf.

9 = fyL itdi lif h liksv= Longitudinal spacing of Shear linksv= Design Shear stressD ihf/(bd)V/(bd)= Design Shear force/(bd) = V/(bd)vc= The ultimate Shear stress that can be resisted by the concrete. RC Design and Construction HKC 2004 (2ndEdition)3-14 Design Formulae for Shear ReinforcementThe values of vcis given in table the Code. It shall be noted that the values given in table is for grade 25 conc. For other grades of conc., adjustment shall be made by multiplying fcu13 The values of vcincreases for shallow members and those25 The values of vcincreases for shallow members and those with larger percentages of tensile Design and Construction HKC 2004 (2ndEdition) Enhanced Shear Resistance Within a distance of 2d from a support or aconcentratedload,thedesignconcreteshear stressconcentratedload,thedesignconcrete shearstressvcmay be increased tov (2d/a)vc (2d/av) The distance avis measuredfrom the support orconcentrated load to the section being designed.

10 AverageshearstressshouldneverexceedtheAv erageshearstressshouldneverexceedtheless er ofor 7 Design and Construction HKC 2004 (2ndEdition)3-16 Figure Shear failure near supportsRdf HKCdRC Design and Construction HKC 2004 (2ndEdition)3-17 Reproduce from HK CodeTable - Values of vcDesign Concrete Shear StressRdf HKCdRC Design and Construction HKC 2004 (2ndEdition)3-18 Reproduce from HK CodeTable - Minimum Percentage of ReinforcementRdf HKCdRC Design and Construction HKC 2004 (2ndEdition)3-19 Reproduce from HK CodeTable Ultimate Anchorage bond Lengths (lb)Reproduce from HK CodeRC Design and Construction HKC 2004 (2ndEdition)3-20 Table Ultimate Lap LengthsReproduce from HK CodeRC Design and Construction HKC 2004 (2ndEdition)3-21 Figure - Factors for lapping barsReproduce from HK CodeRC Design and Construction HKC 2004 (2ndEdition)3-22 Self-Assessment the Anchorage bond length from the firstprinciple for a T32 bar in tension (deformed type 2)and fcu= 35 N/mm2.