Chapter7 Bond Development Length Splices Lecture Goals Slab

Chapter-7 Bond Development Length & Splices

Lecture Goals • Slab design reinforcement • Bar Development • Hook development

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

The table is A-9 from Mac. Gregor’s book.

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. 10. 6. 4

Flexural Reinforcement in Slabs Maximum & Minimum reinforcement requirements • 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)

Flexural Reinforcement in Slabs Maximum & Minimum reinforcement requirements T&S Reinforcement (perpendicular to span) ACI Sec 7. 12

Flexural Reinforcement in Slabs T&S Reinforcement (perpendicular to span) ACI Sec 7. 12 Flexural Reinforcement (parallel to span) ACI Sec 10. 54 Smax from reinforced spacing

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

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

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.

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.

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: 1. Minimum edge distance and spacing of bars (smaller distance= smaller load) 2. Tensile strength of concrete. 3. Average bond stress along bar. (Increase in bond stress larger wedging forces)

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

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.

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

Development Length for Bars in Tension Development length, ld fc 12” ACI 12. 2. 1 10000 psi for Ch. 12 provisions for development length in ACI Codes. Development length, ld (simplified expression from ACI No. 6 and smaller No. 7 and larger 12. 2. 2) bars and deformed bars wires Clear spacing of bars being developed or spliced not less than db, clear cover not less than db, and stirrups or ties throughout ld not less than the code minimum or Clear spacing of bars being developed or spliced not less than 2 db and clear cover not less than db. Other cases

Development Length for Bars in Tension Development length, ld 2. 5 ACI 12. 2. 3 limit to safeguard against pullout type failure.

Factors used in expressions for Development Length (ACI 12. 2. 4) a = reinforcement location factor where ab < 1. 7 Horizontal reinforcement so placed that more than 12 in of fresh concrete is cast in the member below the development length or splice 1. 3 Other reinforcement 1. 0 b = coating factor (epoxy prevents adhesion & friction between bar and concrete. ) Epoxy-coated bars or wires with cover less than 3 db or clear spacing less than 6 db 1. 5 All other epoxy-coated bars or wires 1. 2 Uncoated reinforcement 1. 0

Factors used in expressions for Development Length (ACI 12. 2. 4) g = reinforcement size factor (Reflects more favorable performance of smaller f bars) No. 6 and smaller bars and deformed wire 0. 8 No. 7 and larger bars 1. 0 l = lightweight aggregate concrete factor (Reflects lower tensile strength of lightweight concrete, & resulting reduction in splitting resistance. When lightweight aggregate concrete is used. 1. 3 However, when fct is specified, shall be permitted to be taken as but not less than 1. 0 When normal weight concrete is used 1. 0

Factors used in expressions for Development Length (ACI 12. 2. 4) 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.

Factors used in expressions for Development Length (ACI 12. 2. 4) Kct = transverse reinforcement index (Represents the contribution of confining reinforcement across potential splitting planes. ) Atr = 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. fyt = Specified yield strength of transverse reinforcement, psi. s = maximum center-to-center spacing of transverse reinforcement within ld in. n = number of bars or wires being developed along the plane of splitting. Note: It is permitted to use Kct =0 as a design simplification even if transverse reinforcement is present.

Excess Flexural Reinforcement Reduction (ACI 12. 2. 5) Reduction = (As req’d ) / (As provided ) - Except as required for seismic design (see ACI 21. 2. 14) - Good practice to ignore this provision, since use of structure may change over time. - final ld 12 in.

Development Length for Bars in Compression (ACI 12. 3) Compression development length ldc = ldbc * applicable reduction factors 8 in. Basic Development Length for Compression, ldbc

Development Length for Bars in Compression (ACI 12. 3) Reduction Factors (ACI 12. 3. 3) - Excessive Reinforcement Factor = (As req’d)/(As 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 7. 10. 5 and spaced 4 in. on center. Factor = 0. 75 Note ldc < ld (typically) because - Beneficial of end bearing is considered - weakening effect of flexural tension cracks is not present for bars in compression.

Hooked Bar at Discontinuous Ends (ACI 12. 5. 4) If side cover and top (or bottom cover) 2. 5 in. Enclose hooked bar w/ ties or stirrup-ties: Spacing 3 db db =f of hooked bar Note: Multiplier for ties or stirrups (ACI 12. 5. 3. 3) is not applicable for this case.

Hooked Bar at Discontinuous Ends (ACI 12. 5. 4) Table A-11, A-12, A-13 (Back of textbook) - Basic Development lengths Others Mechanical Anchorage ACI (12. 6) Welded Wire Fabric ACI (12. 7) Bundled Bars ACI (12. 4)

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.

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.

Design of Standard Hooks for Tension Anchorage (ACI 12. 5) Development Length for Hooked Bar, ldb. Basic Development Length for Hooked Bar = lhb when fy = 60, 000 psi

Design of Standard Hooks for Tension Anchorage (ACI 12. 5) Conditions Bar Yield Strength Bars with fy 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 fy /60, 000 0. 7

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 ldh no farther apart than 3 db, where db is diameter of hooked bar. Multiplier As(req’d) / As(provided) 1. 3 0. 8

Design of Standard Hooks for Tension Anchorage (ACI 12. 5) Conditions Epoxy-coated Reinforcement Hooked bars with epoxy coating Multiplier 1. 2

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”