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Chapter-7 Bond Development Length & Splices. Lecture Goals Slab design reinforcement Bar Development Hook development.

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Presentation on theme: "Chapter-7 Bond Development Length & Splices. Lecture Goals Slab design reinforcement Bar Development Hook development."— Presentation transcript:

1 Chapter-7 Bond Development Length & Splices

2 Lecture Goals Slab design reinforcement Bar Development Hook development

3 Flexural Reinforcement in Slabs Flexural Reinforcement in Slabs For a 1 ft strip of slab is designed like a beam A s(req’d) is in units of (in 2 /ft)

4 The table is A-9 from MacGregor’s book.

5 Flexural Reinforcement in Slabs The minimum spacing of the bars is given as: Also, check crack control - important for exterior exposure (large cover dimensions) - ACI Sec

6 Flexural Reinforcement in Slabs Thin slabs shrink more rapidly than deeper beams. Temperature & shrinkage (T&S) steel is provided perpendicular to restrain cracks parallel to span. (Flexural steel restrains cracks perpendicular to span) Maximum & Minimum reinforcement requirements

7 Flexural Reinforcement in Slabs Maximum & Minimum reinforcement requirements T&S Reinforcement (perpendicular to span) ACI Sec 7.12

8 Flexural Reinforcement in Slabs T&S Reinforcement (perpendicular to span) ACI Sec 7.12 Flexural Reinforcement (parallel to span) ACI Sec S max from reinforced spacing

9 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements A. Concept of Bond Stress and Rebar Anchorage Internal Forces in a beam Forces in Rebar Bond stresses provide mechanism of force transfer between concrete and reinforcement. Forces developed in the beam by loading.

10 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements Equilibrium Condition for Rebar  = bond stress (coefficient of friction) Note: Bond stress is zero at cracks

11 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements Sources of Bond Transfer (1) Adhesion between concrete & reinforcement. (2) Friction Note: These properties are quickly lost for tension.

12 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements Sources of Bond Transfer (3)Mechanical Interlock. The edge stress concentration causes cracking to occur. Force interaction between the steel and concrete.

13 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements Splitting cracks result in loss of bond transfer. Reinforcement can be used to restrain these cracks. Splitting Load is Affected by: Minimum edge distance and spacing of bars (smaller distance= smaller load) Tensile strength of concrete. Average bond stress along bar.(Increase in bond stress larger wedging forces)

14 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements Typical Splitting Failure Surfaces.

15 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements General splitting of concrete along the bars,either in vertical planes as in figure (a) or in horizontal plane as in figure (b). Such splitting comes largely from wedging action when the ribs of the deformed bar bear against the concrete. The horizontal type of splitting frequently begins at a diagonal crack. The dowel action increases the tendency toward splitting. This indicates that shear and bond failure are often intricately interrelated.

16 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements ACI Code expression for development length for bars in tension/in compression. B. Development Length, l d Shortest length of bar in which the bar stress can increase from zero to the yield strength, f y. ( l d used since bond stresses,  vary along a bar in a tension zone)

17 Development Length for Bars in Tension Development length, l d 12” ACI f c psi for Ch. 12 provisions for development length in ACI Codes. Development length, l d (simplified expression from ACI ) Clear spacing of bars being developed or spliced not less than d b, clear cover not less than d b, and stirrups or ties throughout ld not less than the code minimum or Clear spacing of bars being developed or spliced not less than 2d b and clear cover not less than d b. Other cases No. 6 and smaller No. 7 and larger bars and deformed bars wires

18 Development Length for Bars in Tension Development length, l d ACI limit to safeguard against pullout type failure.

19 Factors used in expressions for Development Length (ACI )  reinforcement location factor Horizontal reinforcement so placed that more than 12 in of fresh concrete is cast in the member below the development length or splice Other reinforcement  coating factor (epoxy prevents adhesion & friction between bar and concrete.) Epoxy-coated bars or wires with cover less than 3d b or clear spacing less than 6d b All other epoxy-coated bars or wires Uncoated reinforcement where  < 1.7

20 Factors used in expressions for Development Length (ACI )  reinforcement size factor (Reflects more favorable performance of smaller  bars) No.6 and smaller bars and deformed wire No. 7 and larger bars  lightweight aggregate concrete factor (Reflects lower tensile strength of lightweight concrete, & resulting reduction in splitting resistance. When lightweight aggregate concrete is used. However, when f ct is specified, shall be permitted to be taken as but not less than When normal weight concrete is used

21 Factors used in expressions for Development Length (ACI ) c = spacing or cover dimension, in. Use the smaller of either (a) the distance from the center of the bar or wire to the nearest concrete surface. or (b) one-half the center-to-center spacing of the bar or wires being developed.

22 Factors used in expressions for Development Length (ACI ) K ct = transverse reinforcement index (Represents the contribution of confining reinforcement across potential splitting planes.) Total cross-section area of all transverse reinforcement within the spacing s, which crosses the potential plane of splitting along the reinforcement being developed with in the development length, in 2. Specified yield strength of transverse reinforcement, psi. maximum center-to-center spacing of transverse reinforcement within l d in. number of bars or wires being developed along the plane of splitting. A tr = f yt = s = n = Note: It is permitted to use K ct =0 as a design simplification even if transverse reinforcement is present.

23 Excess Flexural Reinforcement Reduction (ACI ) Reduction = (A s req’d ) / (A s provided ) - Except as required for seismic design (see ACI ) - Good practice to ignore this provision, since use of structure may change over time. - final l d 12 in.

24 Development Length for Bars in Compression (ACI 12.3) Compression development length l dc = l dbc * applicable reduction factors 8 in. Basic Development Length for Compression, l dbc

25 Development Length for Bars in Compression (ACI 12.3) Reduction Factors (ACI ) - Excessive Reinforcement Factor = (A s req’d)/(A s provided) - Spiral and Ties If reinforcement is enclosed with spiral reinforcement 0.25 in. diameter and 4 in. pitch or within No. 4 ties according to and spaced 4 in. on center. Factor = 0.75 Note l dc < l d (typically) because - Beneficial of end bearing is considered - weakening effect of flexural tension cracks is not present for bars in compression.

26 Hooked Bar at Discontinuous Ends (ACI ) If side cover and top (or bottom cover) 2.5 in. Enclose hooked bar w/ ties or stirrup-ties: Spacing 3d b d b =  of hooked bar Note: Multiplier for ties or stirrups (ACI ) is not applicable for this case.

27 Hooked Bar at Discontinuous Ends (ACI ) Table A-11, A-12, A-13 (Back of textbook) - Basic Development lengths OthersMechanical Anchorage ACI (12.6) Welded Wire Fabric ACI (12.7) Bundled Bars ACI (12.4)

28 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements C. Use of Standard Hooks for Tension Anchorage Hooks provide additional anchorage when there is insufficient length available to develop a bar. Note: Hooks are not allowed to developed compression reinforcement.

29 Reinforcement Development Lengths, Bar Cutoffs, and Continuity Requirements C. Use of Standard Hooks for Tension Anchorage Standard Hooks are defined in ACI 7.1. Hooks resists tension by bond stresses on bar surface and bearing on on concrete inside the hook.

30 Design of Standard Hooks for Tension Anchorage (ACI 12.5) Development Length for Hooked Bar, l db. Basic Development Length for Hooked Bar = l hb when f y = 60,000 psi

31 Design of Standard Hooks for Tension Anchorage (ACI 12.5) Conditions Bar Yield Strength Bars with f y other than 60,000 psi Concrete Cover for 180 Degree Hooks For No. 11 bars and smaller. Side cover (normal to plane of hook) 2.5 in. Concrete Cover for 90 Degree Hooks For No. 11 bars and smaller. Side cover (normal to plane of hook) 2.5 in. Cover on bar extension beyond hook tail 2 in. Multiplier f y /60,

32 Design of Standard Hooks for Tension Anchorage (ACI 12.5) Conditions Excessive Reinforcement Where anchorage or development for fy is not specified required. Lightweight Aggregate Concrete Ties or Stirrups For No. 11 bar and smaller. Hook enclosed vertically or horizontally within ties or stirrup-ties spaced along full l dh no farther apart than 3d b, where d b is diameter of hooked bar. Multiplier A s (req’d) / A s (provided)

33 Design of Standard Hooks for Tension Anchorage (ACI 12.5) Conditions Epoxy-coated Reinforcement Hooked bars with epoxy coating Multiplier 1.2

34 Example Example 4 GIVEN: A #5 Grade 40 bar is in tension as shown below. Use LIGHTWEIGHT concrete with f’c = 4000 PSI. REQUIRED: Determine the min. required hook dimensions “X”, “Y” and “Z”

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