DETAILING OF REINFORCEMENT IN CONCRETE STRUCTURES
( According to BNBC 2017 )
Contents of this presentation
· General requirements of detailing
• Standard and seismic hook
• Minimum bend diameter
• Tolerance for placing reinforcement
• Spacing of reinforcement
• Bundled bar
• Exposure condition and cover
• Column detailing
• Temperature and shrinkage bars
• Structural integrity
• Development length
For free end of bar,
- 180o bend plus an extension≥4db 65 mm
- 90o bend plus an extension ≥ 12db
§ For stirrup and tie anchorage:
- For db≤16mm, 90o bend plus an extension ≥ 6db
- For 19≤db≤25mm, 90o bend plus an extension≥12db
- For db≤25mm, 135o bend plus ≥ 6db
- For closed ties and continuously wound ties, 135o bend plus an extension ≥ 4db 65 mm
For free For stirrup and tie anchorage of bar
- Seismic hook bend should be ≥ 1350
- Circular hoops bend should be ≥ 900
· Seismic hook extension ≥ 6db, 75mm
Minimum bend diameters
The minimum diameter of bend for standard hook and for tie and stirrup hooks if db ≥16mm
Bar Size------------------------------Minimum Diameter of Bend
10 mm ≤ d_b ≤ 25 mm---------6d_b
25 mm < d_b ≤ 40 mm----------8d_b
40 mm < d_b ≤ 57 mm----------10d_b
· For stirrups and tie hooks, if db≤16mm, inside diameter of bend ≥4 db
Spacing of Reinforcement
· The minimum clear spacing between parallel bars of slab ≥1db , 25 mm
· 1.33* maximum nominal size of coarse aggregate
· Where parallel reinforcement is placed in two or more layers, clear distance between layers ≥25 mm
· For compression members, the clear distance between longitudinal bars ≥1.5 db , 40 mm
· 1.33* maximum nominal size of coarse aggregate
· In walls and one-way slabs the maximum bar spacing ≤3*wall or slab thickness h, 450 mm
· For two-way slabs, maximum spacing of bars ≤ 2* slab thickness h, 450 mm
· For temperature steel, maximum spacing ≤ 5* slab thickness h , 450 mm
· Groups of parallel reinforcing bars bundled in contact to act as a single unit shall be limited to four.
· Bundled bars shall be enclosed within stirrups or ties.
· Bars larger than 32 mm diameter shall not be bundled in beams.
· Individual bars within a bundle terminated within the span of flexural members shall terminate at different points with at least 40db stagger.
· Where spacing limitations and minimum concrete cover are based on bar diameter db, a unit of bundled bars shall be treated as a single bar of a diameter derived from the equivalent total area
· Exposure Condition and Minimum Cover
Exposure Condition and Recommended Cover
· Cast-in-place concrete :
- Minimum concrete cover (permanently exposed to earth)= 75 mm.
- Concrete exposed to earth or weather, the minimum clear cover shall be as under
19 mm to 57 mm bar diameter: 50 mm
16 mm diameter bar and smaller: 40 mm
Concrete in Corrosive Environments
- A specified concrete cover for reinforcement not less than 50 mm for walls and slabs and not less than 65 mm for other members may be used. For precast concrete members a specified concrete cover not less than 40 mm for walls and slabs and not less than 50 mm for other members may be used.
- Minimum compressive strength of concrete for the corrosive environment or other severe exposure conditions shall be 25 MPa with minimum cement of 400 kg per cubic meter. Coarse aggregate shall be 20 mm down well-graded stone chips and fine aggregate shall be coarse sand of minimum FM 2.20.
- For any non-structural member like drop wall, railing, fins etc., 12 mm down well graded stone chips may be used as coarse aggregate.
- Use of brick chips (khoa) as coarse aggregate is strictly prohibited for the corrosive environment or other severe exposure conditions.
- Water cement ratio shall be between 0.4-0.45. Potable water shall be used for all concreting
8.1.9 Lateral Reinforcement for Columns
· Size of spirals for cast in situ construction ≥ 10 mm diameter
· Clear spacing between spirals 25 mm ≤ s ≤ 75 mm
· Anchorage of spiral reinforcement shall be provided by 1.5 extra turns of spiral bar or wire at each end of a spiral unit.
· Lap Splices ≥ 48 spiral diameter for deformed uncoated bar or wire, 72 spiral diameter for other cases, 300 mm
· Vertical spacing of ties ≤ 16 db , 48 dtie, Least dimension of column
8.1.8 Reinforcement Details for Columns
Offset Bars: Offset bent longitudinal bars shall conform to the following:
(a) The maximum slope of inclined portion of an offset bar with axis of column shall not exceed 1 in 6.
(b) Portions of bar above and below an offset shall be parallel to the axis of column.
(c) Horizontal support at offset bends shall be provided by lateral ties, spirals, or parts of the floor construction. Horizontal support provided shall be designed to resist 1.5 times the horizontal component of the computed force in the inclined portion of the offset bars. Lateral ties or spirals, if used, shall be placed not more than 150 mm away from points of bend.
(d) Offset bars shall be bent before placement in the forms (see Sec 8.1.3).
(e) Where the face of the column above is offset 75 mm or more from the face of the column below, longitudinal bars shall not be permitted to be offset bent. The longitudinal bars adjacent to the offset column faces shall be lap spliced using separate dowels. Lap splices shall conform to Sec 8.2.14.
(f) The lowest tie in any storey shall be placed within one-half the required tie spacing from the top most horizontal reinforcement in the slab or footing below.
(g) The uppermost tie in any storey shall be within one-half the required tie spacing from the lowest horizontal reinforcement in the slab or drop panel above.
(h) Where beams or brackets provide concrete confinement at the top of the column on all (four) sides, top tie shall be within 75 mm of the lowest horizontal reinforcement
(i) Ties shall be arranged such that every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie with an included angle not more than 135o.
(j) No vertical bar shall be farther than 150 mm clear on each side along the tie from such a laterally supported bar
Shrinkage and Temperature Reinforcement
· Area of shrinkage and temperature reinforcement shall provide at least the following ratios of reinforcement area to gross concrete area:
· In any case, the reinforcement ratio shall not be less than 0.0014.
· Area of shrinkage and temperature reinforcement for brick aggregate concrete shall be at least 1.5 times that provided in above.
· Spacing of shrinkage and temperature reinforcement ≤ 5*slab thickness, 450 mm
Brick aggregate concrete
Area of shrinkage and temperature reinforcement for brick aggregate concrete shall be at least 1.5 times that provided in above.
Development of Deformed Bars and Deformed Wires in Tension
· Development length for deformed bars and deformed wire in tension, ld ≥300 mm
· For deformed bars or deformed wire, ld shall be as follows:
Clear spacing of bars or wires being developed or spliced not less thand_b, clear cover not less than d_b, and stirrups or ties throughout l_d not less than the Code minimum
Or, Clear spacing of bars or wires being developed or spliced not less than 2d_b and clear cover not less than d_b *****
((f_y ψ_t ψ_e)/(2.1λ√(f'_c ))) d_b
· ld=development length
· fy=yield strength of the tension rebars (Mpa)
· fc’=compressive strength of concrete (Mpa)
· db=bar diameter (mm)
· ψt=rebar location factor that accounts for the position of rebars in freshly placed concrete
· ψe=rebar coating factor reflecting the effects of epoxy coating
· ψs= rebar size factor
· λ= lightweight aggregate concrete factor
The factors used in the expressions for development of deformed bars and deformed wires in tension are as follows:
· Where horizontal reinforcement is placed such that more than 300 mm of fresh concrete is cast below the development length or splice, ψt=1.3. For other cases, ψt=1.0.
· For epoxy-coated bars or wires with cover less than 3db, or clear spacing less than 6db, ψe=1.5. For all other epoxy-coated bars or wires, ψe=1.2. For uncoated and zinc-coated (galvanized) reinforcement, ψe=1.0. However, the product ψt ψe need not be greater than 1.7.
· For 19 mm diameter and smaller bars, and deformed wires, ψs=0.8. For 20 mm diameter and larger bars, ψs=1.0.
· Where lightweight concrete is used, λ shall not exceed 0.75 unless fct is specified. Where normal weight concrete is used, λ=1.0.
o Development of Deformed Bars and Deformed Wires in Compression
· Development length for deformed bars and deformed wire in compression, ldc ≥ 200 mm
· Development length for deformed bars and deformed wire in
compression, ldc≥ (0.24f_y d_b)/(λ√(f_c^' )) 0.043f_y d_b
Where the constant 0.043 carries the unit of mm2/N
Lap splices in bundled bar
· Lap splices of bundled bars shall be based on the lap splice length required for individual bars within the bundle.
· Individual bar splices within a bundle shall not overlap. Entire bundles shall not be lap spliced.
Splices of Deformed Bars and Deformed Wire in Tension
· The minimum length of lap for tension splices shall be as required for Class A or B splice, but not less than 300 mm, where the classification shall be as follows:
Class - A splice: 1.0 L
Class - B splice: 1.3 L
· Lap splices of deformed bars and deformed wire in tension shall be class B splices except that Class A splices are allowed when the area of reinforcement provided is at least twice that required by analysis over the entire length of the splice, and one-half or less of total reinforcement is spliced within the required lap length.
Minimum Splices in Compression
· The minimum length of lap for compression splice shall be 0.071f_y d_b for f_y =420 N/mm2 or less or (0.13f_y-24)d_b for f_y greater than 420 N/mm2, but not less than 300 mm. For f_c^' less than 21 N/mm2, length of lap shall be increased by one-third.
· Development length and lap length will increase 33% for brick aggregate concrete
· Development length and lap length will increase 30 - 50% for epoxy coated rebar