Chapter 8: Detailing of Reinforced Concrete Structures

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8.1 DETAILS OF REINFORCEMENT

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8.1.1 Notation

• d = distance from extreme compression fibre to centroid of tension reinforcement, mm
• db = nominal diameter of bar, mm
• fy = specified yield strength of reinforcement, N/mm²
• h = overall thickness of member, mm
• ℓd = development length, mm

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8.1.2 Standard Hooks

The term “standard hook” as used in this Code shall mean one of the following: a) 180° bend plus an extension of at least 4 bar diameters, but not less than 60 mm at the free end of the bar. b) 90° bend plus an extension of at least 12 bar diameters at the free end of the bar. c) For stirrup and tie anchorage i) For 16 mm Ø bar and smaller, a 90° bend plus an extension of at least 6 bar diameters at the free end of the bar, ii) For 20 mm and 25 mm Ø bar, a 90° bend plus an extension of at least 12 bar diameters at the free end of the bar, iii) For 25 mm Ø bar and smaller, a 135° bend plus an extension of at least 6 bar diameters at the free end of the bar, iv) For closed ties and continuously wound ties, a 135° bend plus an extension of at least 6 bar diameters, but not less than 75 mm.

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8.1.3 Minimum Bend Diameters

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8.1.3.1

The minimum diameter of bend measured on the inside of the bar, for standard hooks other than for stirrups and ties in sizes 10 mm Ø through 16 mm Ø, shall not be less than the values shown in Table 6.8.1.

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8.1.3.2

For stirrups and tie hooks, inside diameter of bend shall not be less than 4 bar diameters for 16 mm Ø bar and smaller. For bars larger than 16 mm Ø, diameter of bend shall be in accordance with Table 6.8.1. Table 6.8.1: Minimum Diameters of Bend

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8.1.4 Bending

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8.1.4.1

Unless otherwise permitted by the engineer, all reinforcement shall be bent cold.

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8.1.4.2

Errors in alignment of reinforcement partially embedded in hardened concrete shall not be corrected by bending in place, except as permitted by the engineer.

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8.1.5 Surface Conditions of Reinforcement

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8.1.5.1

When concrete is placed, metal reinforcement shall be free from mud, oil, or other nonmetallic coatings that decrease bond.

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8.1.5.2

Metal reinforcement with rust, mill scale, or a combination of both, shall be considered satisfactory, provided the minimum dimensions (including height of deformations) and weight of a hand-wire-brushed test specimen are not less than applicable ASTM specification requirements.

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8.1.6 Placing of Reinforcement

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8.1.6.1

Reinforcement shall be accurately placed and adequately supported before concrete is placed, and shall be secured against displacement within tolerances permitted in Sec 8.1.6.2 below.

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8.1.6.2

Reinforcement shall be placed within the following tolerances unless otherwise specified by the engineer: a) Tolerances for depth d, and minimum concrete cover in flexural members, walls and compression members shall be as set forth in Table 6.8.2. Table 6.8.2: Tolerances for Placing Reinforcement b) Notwithstanding the provision of (a) above, tolerance for the clear distance to formed soffits shall be minus 6 mm and tolerance for cover shall not exceed minus ½ the minimum concrete cover required in the design drawings or specifications. c) Tolerance for longitudinal location of bends and ends of reinforcement shall be ± 50 mm, except at discontinuous ends of members where tolerance shall be ± 12 mm. d) Welding of crossing bars shall not be permitted for assembly of reinforcement unless authorized by the engineer.

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8.1.7 Spacing of Reinforcement

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8.1.7.1

The minimum clear spacing between parallel bars in a layer shall be equal to one bar diameter, but not less than 25 mm (also see Sec 5.2.2.2).

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8.1.7.2

Where parallel reinforcement is placed in two or more layers, bars in the upper layers shall be placed directly above those in the bottom layer with clear distance between layers not less than 25 mm.

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8.1.7.3

For compression members, the clear distance between longitudinal bars shall be not less than 1.5 bar diameters nor 35 mm (also see Sec 5.2.2.2).

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8.1.7.4

Clear distance limitation between bars shall apply also to the clear distance between a contact lap splice and adjacent splices or bars.

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8.1.7.5

In walls and one-way slabs the maximum bar spacing shall be three times the wall or slab thickness (h) but not more than 450 mm.

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8.1.7.6

For two-way slabs, maximum spacing of bars shall be 2h but not more than 450 mm.

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8.1.7.7

For temperature steel only, maximum spacing shall be 5h but not more than 450 mm.

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8.1.7.8 Bundled Bars

a) Groups of parallel reinforcing bars bundled in contact to act as a unit shall be limited to four in any one bundle. b) Bundled bars shall be enclosed within stirrups or ties. c) Bars larger than 35 mm Ø shall not be bundled in beams. d) Individual bars within a bundle terminated within the span of flexural members shall terminate at different points with at least 40db stagger. e) 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.

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8.1.8 Exposure Condition and Cover to Reinforcement

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8.1.8.1

The nominal concrete cover to all reinforcement (including links), maximum free water-cement ratio and minimum cement content required for various minimum concrete strengths used in different exposure conditions shall be as specified in Table 6.8.3. However, for mild environment, the minimum concrete cover specified in Sec 8.1.8.2 and 8.1.8.3 for various structural elements may be used. Table 6.8.3: Concrete Cover and other Requirements for Various Exposure Conditions

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8.1.8.2 Cast-in-place Concrete

The following minimum concrete cover may be provided for reinforcement for concrete surfaces not exposed to weather or in contact with ground: Slabs, Walls: Minimum cover, mm

• 45 mm and 55 mm Ø: 30
• 35 mm Ø bar and smaller: 20

Beams, Columns:

• Primary reinforcement: 40
• Ties, stirrups, spirals: 30

Shells, folded plate members:

• 20 mm Ø bar and larger: db
• 15 mm Ø bar and smaller: 15

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8.1.8.3 Precast Concrete (manufactured under plant control conditions)

The following minimum concrete cover may be provided for reinforcement for concrete surfaces not exposed to weather or in contact with ground: Slabs, Walls: Minimum cover, mm

• 45 mm and 55 mm Ø: 25
• 35 mm Ø bar and smaller: 15

Beams, Columns:

• Primary reinforcement: 20
• Ties, stirrups, spirals: 15

Shells, folded plate members:

• 20 mm Ø bar and larger: 15
• 15 mm Ø bar and smaller: 10

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8.1.9 Reinforcement Details for Columns

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8.1.9.1 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 compressive force in the offset bent 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 8.1.4). e) Where the face of the column above is offset 75 mm or more from the face of the column below, offset bars shall not be permitted. The splice shall be made by separate dowels. Lap splices shall conform to Sec 8.2.14.

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8.1.9.2 Steel Cores

Load transfer in structural steel cores of composite compression members shall be provided by the following: a) Ends of structural steel cores shall be accurately finished to bear at end bearing splices, with positive provision for alignment of one core above the other in concentric contact. b) At end bearing splices, bearing shall be considered effective to transfer not more than 50 per cent of the total compressive stress in the steel core. c) Transfer of stress between column base and footing shall be designed in accordance with Sec 6.10.6. d) Base of structural steel section shall be designed to transfer the total load from the entire composite member to the footing; or, the base shall be designed to transfer the load from the steel core only, provided ample concrete section is available for transfer of the portion of the total load carried by the reinforced concrete section to the footing by compression in the concrete and by reinforcement.

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8.1.10 Lateral Reinforcement for Columns

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8.1.10.1

Lateral reinforcement for compression members shall conform to the provisions of Sec 8.1.10.3 and 8.1.10.4 below and where shear or torsion reinforcement is required, shall also conform to provisions of Sec 6.2.7.

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8.1.10.2

Lateral reinforcement requirements for composite columns shall conform to Sec 6.3.10.8 and 6.3.10.9.

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8.1.10.3 Spirals

Spiral reinforcement for columns shall conform to Sec 6.3.6.4 and to the following: a) Spirals shall consist of evenly spaced continuous bar or wire of such size and so assembled as to permit handling and placing without distortion from designed dimensions. b) Size of spirals shall not be less than 10 mm diameter for cast-in-place construction. c) The minimum and maximum clear spacing between spirals shall be 25 mm and 75 mm respectively. d) Anchorage of spiral reinforcement shall be provided by 1.5 extra turns of spiral bar or wire at each end of a spiral unit. e) Splices in spiral reinforcement shall be lap splices of 48 spiral diameter, but not less than 300 mm. f) Spirals shall extend from the top of footing or slab in any storey to the level of the lowest horizontal reinforcement in members supported above. g) Spirals shall extend above termination of spiral to bottom of slab or drop panel, where beams or brackets do not frame into all sides of a column. h) Spirals shall extend to a level at which the diameter or width of capital is 2 times that of the column, in case of columns with capitals. i) Spirals shall be held firmly in place and true to line.

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8.1.10.4 Ties

Tie reinforcement for compression members shall conform to the following: a) All bars shall be enclosed by lateral ties, at least 10 mm Ø in size for longitudinal bars 30 mm Ø or smaller, and at least 12 mm Ø in size for 35 mm Ø to 55 mm Ø and bundled longitudinal bars. b) Vertical spacing of ties shall not exceed 16 longitudinal bar diameters or 48 tie diameters, or the least dimension of the compression members. c) 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 135°. No vertical bar shall be farther than 150 mm clear on each side along the tie from such a laterally supported bar. Where longitudinal bars are located around the perimeter of a circle, a complete circular tie is allowed. d) The lowest tie in any storey shall be placed within one-half the required tie spacing from the topmost horizontal reinforcement in the slab or footing below. 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. e) Where beams or brackets provide concrete confinement at the top of the column on all (four) sides, the top tie shall be within 75 mm of the lowest horizontal reinforcement in the shallowest of such beams or brackets.

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8.1.11 Lateral Reinforcement for Beams

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8.1.11.1

Compression reinforcement in beams shall be enclosed by ties or stirrups satisfying the size and spacing limitations in Sec 8.1.10.4 above. Such ties or stirrups shall be provided throughout the distance where compression reinforcement is required.

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8.1.11.2

Lateral reinforcement for flexural framing members subject to stress reversals or to torsion at supports shall consist of closed ties, closed stirrups, or spirals extending around the flexural reinforcement.

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8.1.11.3 Closed Ties or Stirrups

Closed ties or stirrups shall be formed in one piece by overlapping standard stirrup or tie end hooks around a longitudinal bar, or formed in one or two pieces lap spliced with a Class B splice (lap of 1.3ℓd) or anchored in accordance with Sec 8.2.10.

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8.1.12 Shrinkage and Temperature Reinforcement

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8.1.12.1

Where the flexural reinforcement extends in one direction only, reinforcement for shrinkage and temperature stresses shall be provided perpendicular to flexural reinforcement in structural slabs. Shrinkage and temperature reinforcement shall be provided in accordance with Sec 8.1.12.2 below.

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8.1.12.2

Deformed reinforcement conforming to Sec 5.3.2 shall be provided in accordance with the following: a) Area of shrinkage and temperature reinforcement shall provide at least the following ratios of reinforcement area to gross concrete area:

• 0.0020 for slabs where reinforcement with fy = 275 N/mm² or 350 N/mm² are used,
• 0.0018 for slabs where reinforcement with fy = 410 N/mm² are used,
• 0.0018 for slabs where reinforcement with fy exceeding 410 N/mm² are used.

In any case, the reinforcement ratio shall not be less than 0.0014. b) Area of shrinkage and temperature reinforcement for brick aggregate concrete shall be at least 1.5 times that provided in (a) above. c) Shrinkage and temperature reinforcement shall be spaced not farther apart than 5 times the slab thickness, nor 450 mm. d) At all sections where required, reinforcement for shrinkage and temperature stresses shall develop the specified yield strength fy in tension in accordance with Sec 8.2.

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8.1.13 Requirements for Structural Integrity

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8.1.13.1

In the detailing of reinforcement and connections, members of a structure shall be effectively tied together to improve integrity of the overall structure.

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8.1.13.2

The minimum requirements for cast-in-place construction shall be: a) In one-way slab construction, at least one bottom bar shall be continuous or shall be spliced over the support with a Class A tension splice. At noncontinuous supports, the bars may be terminated with a standard hook. b) Beams at the perimeter of the structure shall have at least one-sixth of the tension reinforcement required for negative moment at the support and one-quarter of the positive moment reinforcement required at midspan made continuous around the perimeter and tied with closed stirrups. Closed stirrups need not be extended through any joints. The required continuity may be provided with top reinforcement spliced at midspan and bottom reinforcement spliced at or near the support with Class A tension splices. c) When closed stirrups are not provided, in other than perimeter beams, at least one-quarter of the positive moment reinforcement required at midspan shall be continuous or shall be spliced over the support with a Class A tension splice. At noncontinuous supports the bars may be terminated with a standard hook.

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8.1.13.3

To effectively tie elements together, tension ties shall be provided in the transverse, longitudinal, and vertical directions and around the perimeter of the structure for precast concrete construction.

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8.2 DEVELOPMENT AND SPLICES OF REINFORCEMENT

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8.2.1 Notation

• a = depth of equivalent rectangular stress block as defined in Sec 6.2.3.7
• Ab = area of an individual bar, mm²
• As = area of tension reinforcement, mm²
• Atr = total cross-sectional area of transverse reinforcement (stirrup or tie) within a spacing s and perpendicular to plane of bars being spliced or developed, mm²
• Av = area of shear reinforcement within a distance s, mm²
• bw = web width, or diameter of circular section, mm
• d = distance from extreme compression fibre to centroid of tension reinforcement, mm
• db = nominal diameter of bar, mm
• fc’ = specified compressive strength of concrete, N/mm²
• fy = specified yield strength of reinforcement, N/mm²
• h = overall thickness of member, mm
• ℓa = additional embedment length at support or at point of inflection, mm
• ℓd = development length, mm
• ℓdb = basic development length, mm
• ℓdh = development length of standard hook in tension, measured from the critical section to the farthest point on the bar, parallel to the straight part of the bar, mm = ℓhb × applicable modification factors
• ℓhb = basic development length of standard hook in tension, mm
• Mn = nominal moment strength at section, N·mm
• N = number of bars, in a layer, being spliced or developed at a critical section
• s = spacing of stirrups or ties, mm
• Vu = factored shear force at section, N
• βb = ratio of area of reinforcement cut off to total area of tension reinforcement at section

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8.2.2 Development of Reinforcement - General

Calculated tension or compression stress in reinforcement at each section of reinforced concrete members shall be developed on each side of that section by embedment length, hook or mechanical device, or a combination thereof. Hooks may be used in developing bars in tension only.

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8.2.3 Development of Deformed Bars in Tension

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8.2.3.1

Development length ℓd for deformed bars in tension shall be computed as the product of the basic development length ℓdb of Sec 8.2.3.2 below and the applicable modification factors given in Sec 8.2.3.3 through 8.2.3.5 below, but ℓd shall not be less than 300 mm.

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8.2.3.2

Basic development length, ℓdb shall be $$\ell_{db} = \frac{0.02 A_b f_y}{\sqrt{f'_c}} \quad \text{for 35 mm Ø bar and smaller}$$ $$\ell_{db} = \frac{25 f_y}{\sqrt{f'_c}} \quad \text{for 45 mm Ø bar}$$ $$\ell_{db} = \frac{35 f_y}{\sqrt{f'_c}} \quad \text{for 55 mm Ø bar}$$

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8.2.3.3

To account for bar spacing, concrete cover, splitting strength, actual design stress in the bar and enclosing transverse reinforcement, the basic development length shall be multiplied by a factor from (a), (b) or (c) below which may be modified by (d) or (e) but shall not be less than that specified in (f). a) For all bars satisfying any one of the following conditions: 1.0   i) Bars in beams or columns with minimum cover not less than that specified in Sec 8.1.8.1, transverse reinforcement satisfying tie requirements of Sec 8.1.10.4, minimum stirrup requirements of Sec 6.2.7.4(d) and 6.2.7.4e(ii) along the development length, and with clear spacing of not less than 3db.   ii) Bars in beams or columns with minimum cover not less than that specified in Sec 8.1.8.1 and enclosed within transverse reinforcement Atr along the development length satisfying the following: $$A_{tr} \geq \frac{d_b s N}{40} \tag{8.2.1}$$   iii) Bars in the inner layer of slab or wall reinforcement and with clear spacing of not less than 3db.   iv) Any bars with cover of not less than 2db and with clear spacing of not less than 3db. b) For bars with a cover of db or less or with a clear spacing of 2db or less: 2.0 c) For other bars not included in (a) or (b) above: 1.4 d) The factors in (a) through (c) above shall be multiplied by 0.8 for 35 mm Ø bars and smaller, with clear spacing not less than 5db, and with at least 2.5db clear from face of member to edge of bar. e) The factors in (a) through (c) above shall be multiplied by 0.75 for reinforcement enclosed within spiral reinforcement not less than 6 mm diameter and not more than 100 mm pitch. f) The basic development length multiplied by the applicable factor of (a) through (c) above with modifiers of (d) and/or (e) above shall not be taken less than $$\frac{0.375 d_b f_y}{\sqrt{f'_c}}$$

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8.2.3.4

Basic development length ℓdb as modified by Sec 8.2.3.3 above shall also be multiplied by the applicable factor or factors for: a) Top Reinforcement — Horizontal reinforcement so placed that more than 300 mm of concrete is cast in the member below the bar: 1.3 b) Epoxy Coated Reinforcement:   i) Bars with cover less than 3db or clear spacing less than 6db: 1.5   ii) For all other conditions: 1.2 The product of factor for top reinforcement of (a) and the factor for epoxy-coated reinforcement of (b) need not be taken greater than 1.7.

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8.2.3.5 Excess Reinforcement

Development length may be reduced by the factor (As required)/(As provided) where reinforcement in a flexural member is in excess of that required by analysis except where anchorage or development for fy is specifically required or the reinforcement is designed under the provisions of Sec 8.3.3.1(d).

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8.2.4 Development of Deformed Bars in Compression

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8.2.4.1

Development lengths ℓd for deformed bars in compression shall be computed as the product of the basic development length ℓdb of Sec 8.2.4.2 below and applicable modification factors of Sec 8.2.4.3 but ℓd shall be not less than 200 mm.

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8.2.4.2

Basic development length ℓdb shall be $$\ell_{db} = \frac{0.24 d_b f_y}{\sqrt{f'_c}} \quad \text{but not less than } 0.04 d_b f_y$$

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8.2.4.3

Basic development length ℓdb shall be multiplied by applicable factors from the following: a) Where reinforcement is provided in excess of that required by analysis: (As required)/(As provided) b) Where reinforcement is enclosed within spiral reinforcement not less than 6 mm Ø and not more than 100 mm pitch or ties not less than 12 mm Ø in conformity with Sec 8.1.10.4 spaced at not more than 100 mm on centres: 0.75

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8.2.5 Development of Bundled Bars

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8.2.5.1

Development length of individual bars within a bundle, in tension or compression, shall be that for the individual bar, increased 20 per cent for 3 bar bundles and 33 per cent for 4 bar bundles.

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8.2.5.2

A unit of bundled bars shall be treated as a single bar of a diameter derived from the equivalent total area for determining the appropriate factors in Sec 8.2.3.3 and 8.2.3.4(b).

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8.2.6 Development of Standard Hooks in Tension

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8.2.6.1

Development length ℓdh for deformed bars in tension terminating in a standard hook shall be computed as the product of the basic development length, ℓhb of Sec 8.2.6.2 below and the applicable modification factor or factors of Sec 8.2.6.3, but ℓdh shall be not less than 8db nor less than 150 mm.

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8.2.6.2

Basic development length ℓhb for a hooked bar with fy = 410 N/mm² shall be $$\ell_{hb} = \frac{100 d_b}{\sqrt{f'_c}}$$

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8.2.6.3

Basic development length ℓhb shall be multiplied by applicable factor or factors for: a) Bars with fy other than 410 N/mm²: fy/410 b) For 35 mm Ø bars and smaller, side cover not less than 60 mm, and for 90° hook, cover on bar extension beyond hook not less than 50 mm: 0.7 c) For 35 mm Ø bar and smaller, hook enclosed vertically or horizontally within ties or stirrups with spacing along the full development length ℓdh not greater than 3db, where db is diameter of hooked bar: 0.8 d) Where anchorage or development for fy is not specifically required, reinforcement is provided in excess of that required by analysis: (As required)/(As provided)

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8.2.6.4

For bars being developed by a standard hook at discontinuous ends of members with both side cover and top or bottom cover less than 60 mm, hooked bar shall be enclosed within closed stirrups or ties spaced along the full development length ℓdh not greater than 3 times the diameter of hooked bar. For this case, factor 0.8 of Sec 8.2.6.3(c) shall not apply.

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8.2.6.5

Hooks shall not be considered effective in developing bars in compression.

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8.2.7 Development of Flexural Reinforcement - General

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8.2.7.1

Tension reinforcement may be developed by bending across the web to be anchored or made continuous with reinforcement on the opposite face of member.

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8.2.7.2

Critical sections for development of reinforcement in flexural members are at points of maximum moment and at points within the span where adjacent reinforcement terminates, or is bent. In addition, the provisions of Sec 8.2.8.3 shall also be satisfied.

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8.2.7.3

Reinforcement shall extend beyond the point at which it is no longer required to resist flexure for a distance not less than d nor less than 12db, except at supports of simple spans and at free end of cantilevers.

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8.2.7.4

Continuing reinforcement shall have an embedment length not less than the development length ℓd beyond the point where the bent or terminated tension reinforcement is no longer needed to resist bending.

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8.2.7.5

No flexural bar shall be terminated in a tension zone unless one of the following conditions is satisfied: a) The shear at the location of termination is not over two-thirds that normally permitted, including the shear strength of shear reinforcement provided. b) Stirrups in excess of those normally required for shear and torsion are provided over a distance along each terminated bar equal to 0.75d from the point of cutoff. Excess stirrup area Av shall be not less than 0.4bws/fy. Spacing s shall not exceed d/(8βb), where βb is the ratio of area of reinforcement cut off to total area of tension reinforcement at the section. c) For 35 mm Ø bar and smaller, the continuing bars provide twice the area required for flexure at the cutoff point and the shear does not exceed three-quarters of that permitted.

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8.2.7.6

Where the reinforcement stress is not directly proportional to moment, such as in sloped, stepped, or tapered footings, brackets, deep flexural members, or members in which tension reinforcement is not parallel to the compression face, adequate anchorage shall be provided for the tension reinforcement.

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8.2.8 Development of Positive Moment Reinforcement

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8.2.8.1

At least one-third of the positive moment reinforcement in simple members and one-fourth of the positive moment reinforcement in continuous members shall extend along the same face of member into the support. In beams, such reinforcement shall extend into the support at least 150 mm.

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8.2.8.2

When the flexural member is a part of the primary lateral load resisting system, positive moment reinforcement extended into the support by Sec 8.2.8.1 above shall be anchored to develop the specified yield strength fy in tension at the face of support.

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8.2.8.3

At simple supports and at points of inflection, positive moment tension reinforcement shall be limited to a diameter such that ℓd computed for fy by Sec 8.2.3 satisfies Eq (8.2.2), except that Eq (8.2.2) need not be satisfied for reinforcement terminating beyond the centreline of simple supports by a standard hook or a mechanical anchorage at least equivalent to a standard hook. $$\ell_d \leq \frac{M_n}{V_u} + \ell_a \tag{8.2.2}$$ where

• Mn = nominal moment strength assuming all reinforcement at section to be stressed to fy
• Vu = factored shear force at section
• ℓa = embedded length of bar beyond centre of support or past point of zero moment. In the latter case, it shall not be taken more than the greater of d or 12db.

The value of Mn/Vu may be increased 30 per cent when the ends of reinforcement are confined by a compressive reaction.

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8.2.9 Development of Negative Moment Reinforcement

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8.2.9.1

Negative moment reinforcement in a continuous, restrained, or cantilever member, or in any member of a rigid frame, shall be anchored in or through the supporting member by embedment length, hooks or mechanical anchorage.

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8.2.9.2

Negative moment reinforcement shall have an embedment length into the span as required by Sec 8.2.2 and 8.2.7.3.

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8.2.9.3

At least one-third of the total tension reinforcement provided for negative moment at the support shall be extended beyond the extreme position of the point of inflection a distance not less than one-sixteenth the clear span, or 12db, whichever is greater.

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8.2.10 Development of Shear Reinforcement

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8.2.10.1

Shear reinforcement shall be carried as close to compression and tension surfaces of member as cover requirements and proximity of other reinforcement will permit.

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8.2.10.2

The ends of single leg, simple U-, or multiple U-stirrups shall be anchored by one of the following means: a) By a standard hook around longitudinal reinforcement for 16 mm Ø stirrups and smaller and for 20 mm Ø and 25 mm Ø stirrups with fy ≤ 275 N/mm². b) For 20 mm Ø and 25 mm Ø stirrups with fy greater than 275 N/mm², a standard stirrup hook around a longitudinal bar plus an embedment between mid-height of the member and the outside end of the hook equal to or greater than 0.175 db fy / √f’c.

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8.2.10.3

Each bend in the continuous portion of a simple U-stirrup or multiple U-stirrup shall enclose a longitudinal bar between anchored ends.

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8.2.10.4

If extended into the region of tension, longitudinal bars bent to act as shear reinforcement shall be continuous with longitudinal reinforcement and, if extended into a region of compression, shall be anchored beyond mid-depth d/2 as specified for development length in Sec 8.2.3 for the stress required to develop in the bent bar to satisfy Eq (6.2.26).

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8.2.10.5

Pairs of U-stirrups or ties so placed as to form a closed unit shall be considered properly spliced when length of laps are 1.3ℓd. In members at least 450 mm deep, such splices with Abfy not more than 40 kN per leg shall be considered adequate if stirrup legs extend the full available depth of member.

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8.2.11 Development of Plain Bars

For plain bars, the minimum development length shall be twice that of deformed bars specified in Sec 8.2.2 through Sec 8.2.10 above.

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8.2.12 Splices of Reinforcement - General

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8.2.12.1

Splices of reinforcement shall be made only as required or permitted on design drawings, or in specifications, or as authorized by the engineer.

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8.2.12.2 Lap Splices

a) Lap splices shall not be used for 35 mm Ø bars and larger, except as provided in Sec 8.2.14.2. b) Lap splices of bundled bars shall be based on the lap splice length required for individual bars within the bundle, increased in accordance with Sec 8.2.5. Individual bar splices within a bundle shall not overlap. Entire bundles shall not be lap spliced. c) 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 150 mm.

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8.2.12.3 Welded Splices and Mechanical Connections

a) Welded splices and other mechanical connections are allowed. b) Except as provided in the Code, all welding shall conform to “Structural Welding Code - Reinforcing Steel” (AWS D1.4). c) Welded splices shall be butted and welded to develop in tension at least 125 per cent of specified yield strength fy of the bar. d) A full mechanical connection shall develop in tension or compression, as required, at least 125 per cent of specified yield strength fy of the bar. e) Welded splices and mechanical connections not meeting the requirements of (c) or (d) above are allowed in accordance with Sec 8.2.13.4.

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8.2.13 Splices of Deformed Bars in Tension

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8.2.13.1

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: $1.0\ell_d$ Class B splice: $1.3\ell_d$

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8.2.13.2

Lap splices of deformed bars 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 the total reinforcement is spliced within the required lap length.

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8.2.13.3

Where area of reinforcement provided is less than twice that required by analysis, welded splices or mechanical connections used shall meet the requirements of Sec 8.2.12.3(c) or 8.2.12.3(d) above.

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8.2.13.4

Welded splices or mechanical connections used where area of reinforcement provided is at least twice that required by analysis shall meet the following: a) Splices shall be staggered at least 600 mm and in such manner as to develop at every section at least twice the calculated tensile force at the section but not less than 140 N/mm² for total area of reinforcement provided. b) Spliced reinforcement may be rated at the specified splice strength, in computing tensile force developed at each section. Unspliced reinforcement shall be rated at that fraction of fy defined by the ratio of the shorter actual development length to ℓd required to develop the specified yield strength fy.

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8.2.13.5

Splices in tension tie members shall be made with a full welded splice or full mechanical connection in accordance with Sec 8.2.12.3(c) or 8.2.12.3(d). Splices in adjacent bars shall be staggered at least 750 mm.

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8.2.14 Splices of Deformed Bars in Compression

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8.2.14.1

The minimum length of lap for compression splice shall be $0.07f_yd_b$ for fy equal to 410 N/mm² or less or $(0.13f_y - 24)d_b$ for fy greater than 410 N/mm², but not less than 300 mm. For $f'_c$ less than 20 N/mm², length of lap shall be increased by one-third.

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8.2.14.2

When bars of different diameters are lap spliced in compression, the splice length shall be the larger of the development length of the larger bar, or the splice length of the smaller bar.

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8.2.14.3

Welded splices or mechanical connections used in compression shall satisfy the requirements of Sec 8.2.12.3(c) or 8.2.12.3(d).

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8.2.14.4 End Bearing Splices

a) Compression splices for bars required to transmit compressive stress only may consist of end bearing of square cut ends held in concentric contact by a suitable device. b) Bar ends shall terminate in flat surfaces within 1.5 deg of a right angle to the axis of the bars, and shall be fitted within 3 deg of full bearing after assembly. c) End bearing splices shall be used only in members containing closed ties, closed stirrups or spirals.

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8.2.15 Special Splice Requirements for Columns

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8.2.15.1

Lap splices, butt welded splices, mechanical connections, or end-bearing splices shall be used with the limitations of Sec 8.2.15.2 through 8.2.15.4 below. A splice shall satisfy the requirements for all load combinations for the column.

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8.2.15.2 Lap Splices in Columns

a) Lap splices shall conform to Sec 8.2.14.1, 8.2.14.2, and where applicable to 8.2.15.2(d) or 8.2.15.2(e) below, where the bar stress due to factored loads is compressive. b) Where the bar stress due to factored loads is tensile and does not exceed 0.5fy in tension, lap splices shall be Class B tension lap splices if more than one-half of the bars are spliced at any section, or Class A tension lap splices if half or fewer of the bars are spliced at any section and alternate lap splices are staggered by ℓd. c) Where the bar stress due to factored loads is greater than 0.5fy in tension, lap splices shall be Class B tension lap splices. d) In compression members, if lateral ties are used having an area of at least 0.0015hs, lap splice length may be multiplied by 0.83, but lap length shall not be less than 300 mm. Tie legs perpendicular to dimension h shall be used in determining effective area. e) If spiral reinforcement confines the splice, the lengths required may be multiplied by 0.75, but lap length shall not be less than 300 mm.

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8.2.15.3 Welded Splices or Mechanical Connectors in Columns

Welded splices or mechanical connectors in columns shall meet the requirements of Sec 8.2.12.3(c) or 8.2.12.3(d).

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8.2.15.4 End Bearing Splices in Columns

End bearing splices complying with Sec 8.2.14.4 may be used for column bars stressed in compression provided the splices are staggered or additional bars are provided at splice locations. The continuing bars in each face of the column shall have a tensile strength at least 0.25fy times the area of the vertical reinforcement in that face.

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8.2.16 Splices of Plain Bars

For plain bars, the minimum length of lap shall be twice that of deformed bars specified in Sec 8.2.12 through Sec 8.2.15 above.

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8.2.17 Mechanical Anchorage

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8.2.17.1

Any mechanical device capable of developing the strength of reinforcement without damage to concrete is allowed as anchorage.

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8.2.17.2

Mechanical device may be used only when its adequacy can be proven by test results to the satisfaction of the engineer.

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8.2.17.3

Development of reinforcement may consist of a combination of mechanical anchorage plus additional embedment length of reinforcement between the point of maximum bar stress and the mechanical anchorage.

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8.3 SPECIAL PROVISION FOR SEISMIC DESIGN

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8.3.1 Notation

• Ach = cross-sectional area of a structural member measured out to out of transverse reinforcement, mm²
• Acp = area of concrete section resisting shear of an individual pier or horizontal wall segment, mm²
• Acv = net area of concrete section bounded by web thickness and length of section in the direction of shear force considered, mm²
• Ag = gross area of section, mm²
• Aj = effective cross-sectional area within a joint, see Sec 8.3.7.3, in a plane parallel to plane of reinforcement generating shear in the joint. The joint depth shall be the overall depth of the column. Where a beam frames into a support of larger width, the effective width of the joint shall not exceed the smaller of: (a) beam width plus the joint depth; (b) twice the smaller perpendicular distance from the longitudinal axis of the beam to the column side (Sec 8.3.7.3)
• Ash = total cross-sectional area of transverse reinforcement (including cross ties) within spacing s and perpendicular to dimension hc
• b = effective compressive flange width of a structural member, mm
• bw = web width or diameter of circular section, mm
• db = bar diameter, mm
• E = load effects of earthquake or related internal moments and forces
• f’c = specified compressive strength of concrete
• fy = specified yield strength of reinforcement
• fyh = specified yield strength of transverse reinforcement
• hc = cross-sectional dimension of column core measured centre to centre of confining reinforcement
• hw = height of entire wall (diaphragm) or of the segment of wall (diaphragm) considered
• ℓd = development length for a straight bar
• ℓdh = development length for a bar with a standard hook
• ℓo = minimum length, measured from joint face along axis of structural member, over which transverse reinforcement must be provided, mm
• ℓw = length of entire wall (diaphragm) or of segment of wall (diaphragm) considered in the direction of shear force
• Mpr = probable flexural moment strength of members, with or without axial load, determined using the properties of the member at the joint faces assuming a tensile strength in the longitudinal bars of at least 1.25fy and a strength reduction factor φ of 1.0
• Ms = portion of slab moment balanced by support moment
• s = spacing of transverse reinforcement measured along the longitudinal axis of the structural member, mm
• so = maximum spacing of transverse reinforcement, mm
• Vc = nominal shear strength provided by concrete
• Ve = design shear force
• Vn = nominal shear strength
• Vu = factored shear force at section
• αc = coefficient defining the relative contribution of concrete strength to wall strength
• ρ = ratio of tension reinforcement = As/bd
• ρg = ratio of total reinforcement area to cross-sectional area of column
• ρn = ratio of distributed shear reinforcement on a plane perpendicular to plane of Acv
• ρs = ratio of volume of spiral reinforcement to the core volume confined by the spiral reinforcement (measured out to out)
• ρv = Asv/Acv; where Asv is the projection on Acv of area of distributed shear reinforcement crossing the plane of Acv
• φ = strength reduction factor

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8.3.2 Definitions

For the purposes of this section: BASE OF STRUCTURE: The level at which earthquake motions are assumed to be imparted to a structure. This level does not necessarily coincide with the ground level. BOUNDARY MEMBERS: Members along wall and diaphragm edges strengthened by longitudinal and transverse reinforcement. These members do not necessarily require an increase in the thickness of the wall or diaphragm. If required, edges of openings within walls and diaphragms shall be provided with boundary members. COLLECTOR ELEMENTS: Elements that are used to transmit the inertial forces within the diaphragms to members of the lateral force resisting systems. CROSS TIE: A continuous bar having a hook not less than 135 deg with at least a six diameter extension at one end but not less than 75 mm, and a hook not less than 90 deg with at least a six diameter extension at the other end. The hooks shall engage peripheral longitudinal bars. The 90 deg hooks of two successive cross ties engaging the same longitudinal bars shall be alternated end for end. DEVELOPMENT LENGTH FOR A BAR WITH A STANDARD HOOK: The shortest distance between the critical section and a tangent to the outer edge of the 90 deg hook. HOOP: A hoop is a closed tie or continuously round tie. A closed tie can be made up of several reinforcing elements with 135 hooks having a six diameter extension at each end (but not less than 75 mm). A continuously round tie shall have at each end a 135 hook with a six diameter extension that engages the longitudinal reinforcement but not less than 75 mm. LATERAL FORCE RESISTING SYSTEM: That portion of the structure composed of members designed to resist forces related to earthquake effects. SHELL CONCRETE: Concrete outside the transverse reinforcement confining the concrete. STRUCTURAL DIAPHRAGMS: Structural members, such as floor and roof slabs, which transmit inertial forces to lateral force resisting members. STRUCTURAL WALLS: Walls designed to resist combinations of shears, moments, and axial forces induced by earthquake motions. A shear wall is a structural wall. STRUT: An element of a structural diaphragm used to provide continuity around an opening in the diaphragm. TIE ELEMENTS: Elements used to transmit inertial forces and prevent separation of building components.

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8.3.3 General Requirements

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8.3.3.1 Scope

a) This section contains special requirements for design and construction of reinforced concrete members of a structure for which the design forces, related to earthquake motions, have been determined on the basis of energy dissipation in the nonlinear range of response. b) The provisions of Chapter 6, shall apply except as modified by the provisions of this section. c) In regions of moderate seismic risk, Zone 2 (see Chapter 2), reinforced concrete frames resisting forces induced by earthquake motions shall be built to satisfy the requirements of Sec 8.3.10 in addition to the requirements of Chapter 6. d) In regions of high seismic risk, Zone 3 (see Chapter 2), all reinforced concrete structures shall satisfy the requirements of Sec 8.3.3 through 8.3.9 in addition to the requirements of Chapter 6.

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8.3.3.2 Analysis and Proportioning of Structural Members

a) The interaction of all structural and nonstructural members shall be considered in the analysis. b) Rigid members which are not a part of the lateral force resisting system are allowed provided their effect on the response of the system is considered and accommodated in the structural design. Consequences of failure of structural and nonstructural members which are not a part of the lateral force resisting system shall also be considered. c) Structural members below base of structure required to transmit forces resulting from earthquake effects to the foundation shall also comply with the requirements of this section. d) All structural members which are not a part of the lateral force resisting system shall conform to Sec 8.3.9.

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8.3.3.3 Strength Reduction Factors

Strength reduction factors shall be in accordance with Sec 6.1.4.

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8.3.3.4 Concrete in Members Resisting Earthquake Induced Forces

Compressive strength f’c of the concrete shall be not less than 20 N/mm².

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8.3.3.5 Reinforcement in Members Resisting Earthquake Induced Forces

Reinforcement resisting earthquake induced flexural and axial forces in frames and wall boundary members shall comply with ASTM A706, ASTM A615 and BDS 1313. Reinforcement with fy = 275 N/mm² and fy = 410 N/mm² are allowed in these members if (a) the actual yield strength based on mill tests does not exceed the specified yield strength by more than 125 N/mm² (retests shall not exceed this value by more than an additional 20 N/mm²), and (b) the ratio of the actual ultimate tensile strength to the actual tensile yield strength is not less than 1.25.

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8.3.3.6

Reinforcement required by factored load combinations which include earthquake effect shall not be welded except as specified in Sec 8.3.4.2(d) and 8.3.5.3(b). In addition, welding shall not be permitted on stirrups, ties, inserts, or other similar elements to longitudinal reinforcement required by design.

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8.3.4 Flexural Members of Frames

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8.3.4.1 Scope

Requirements of this section shall apply to frame members, (i) resisting earthquake induced forces, and (ii) proportioned primarily to resist flexure. These frame members shall also satisfy the following conditions: a) Factored axial compressive force on frame member shall not exceed $0.1A_g f'_c$. b) Clear span for the member shall not be less than four times its effective depth. c) The width to depth ratio shall be at least 0.3. d) The width shall not be (i) less than 250 mm and (ii) more than the width of the supporting member (measured on a plane perpendicular to the longitudinal axis of the flexural member) plus distances on each side of the supporting member not exceeding three-fourths of the depth of the flexural member.

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8.3.4.2 Longitudinal Reinforcement

a) At any section of a flexural member and for the top as well as for the bottom reinforcement, the amount of reinforcement shall be not less than $1.38b_wd/f_y$ and the reinforcement ratio, ρ shall not exceed 0.025. At least two bars shall be provided continuously both top and bottom. b) The positive moment strength at the face of the joint shall be not less than one-half of the negative moment strength provided at that face. Neither the negative nor the positive moment strength at any section along the member length shall be less than one-fourth the maximum moment strength provided at the face of either joint. c) Lap splices of flexural reinforcement shall be permitted only if hoop or spiral reinforcement is provided over the lap length. Maximum spacing of the transverse reinforcement enclosing the lapped bars shall not exceed d/4 nor 100 mm. Lap splices shall not be used (i) within the joints, (ii) within a distance of twice the member depth from the face of the joint, and (iii) at locations where analysis indicates flexural yielding caused by inelastic lateral displacements of the frame. d) Welded splices and mechanical connections conforming to Sec 8.2.12.3(a) through 8.2.12.3(d) are allowed for splicing provided not more than alternate bars in each layer of longitudinal reinforcement are spliced at a section and the centre to centre distance between splices of adjacent bars is 600 mm or more measured along the longitudinal axis of the frame member.

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8.3.4.3 Transverse Reinforcement

a) Hoops shall be provided in the following regions of frame members:   i) At both ends of the flexural member, over a length equal to twice the member depth measured from the face of the supporting member toward midspan.   ii) Over lengths equal to twice the member depth, on both sides of a section where flexural yielding is likely to occur in connection with inelastic lateral displacements of the frame. b) The first hoop shall be located not more than 50 mm from the face of the supporting member. Maximum spacing of the hoops shall not exceed (i) d/4, (ii) eight times the diameter of the smallest longitudinal bars, (iii) 24 times the diameter of the hoop bars, and (iv) 300 mm. c) Where hoops are required, longitudinal bars on the perimeter shall have lateral support conforming to Sec 8.1.10.4(c), and where hoops are not required, stirrups shall be spaced not more than d/2 throughout the length of the member. d) Hoops in flexural members are allowed to be made up of two pieces of reinforcement consisting of a U-stirrup having hooks not less than 135 deg with 6 diameter but not less than 75 mm extension anchored in the confined core and a cross tie to make a closed hoop. Consecutive cross ties engaging the same longitudinal bar shall have their 90 deg hooks at opposite sides of the flexural member. If the longitudinal reinforcing bars secured by the cross ties are confined by a slab only on one side of the flexural frame member, the 90 deg hooks of the cross ties shall all be placed on that side.

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8.3.5 Frame Members Subjected to Bending and Axial Load

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8.3.5.1 Scope

The requirements of this section shall apply to columns and other frame members serving to resist earthquake forces and having a factored axial force exceeding $0.1A_g f'_c$. These frame members shall also satisfy the following conditions: a) The shortest cross-sectional dimension shall not be less than 300 mm. b) The ratio of the shortest cross-sectional dimension to the perpendicular dimension shall not be less than 0.4.

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8.3.5.2 Minimum Flexural Strength of Columns

a) Flexural strength of any column designed to resist a factored axial compressive force exceeding $0.1A_g f'_c$ shall satisfy (b) or (c) below. Lateral strength and stiffness of columns not satisfying (b) below shall be ignored in calculating the strength and stiffness of the structure but shall conform to Sec 8.3.9. b) The flexural strength of the columns shall satisfy the following relation: $$\sum M_e \geq 1.2 \sum M_g \tag{8.3.1}$$ where

• $\sum M_e$ = sum of moments, at the centre of the joint, corresponding to the design flexural strength of the columns framing into that joint. The lowest flexural strength of the columns, calculated for the factored axial force, consistent with the direction of the lateral forces considered, shall be used.
• $\sum M_g$ = sum of moments, at the centre of the joint, corresponding to the design flexural strengths of the girders framing into that joint.

Flexural strengths shall be summed such that the column moments oppose the beam moments. Eq (8.3.1) shall be satisfied for beam moments acting in both directions in the vertical plane of the frame considered. c) If the requirements of (b) above is not satisfied at a joint, columns supporting reactions from that joint shall be provided with transverse reinforcement as specified in Sec 8.3.5.4 over their entire height.

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8.3.5.3 Longitudinal Reinforcement

a) The reinforcement ratio, ρg, shall not be less than 0.01 and shall not exceed 0.06. b) Lap splices are permitted only within the centre half of the member length and shall be designed as tension splices. Welded splices and mechanical connections conforming to Sec 8.2.12.3(a) through 8.2.12.3(d) are allowed for splicing the reinforcement at any section provided not more than alternate longitudinal bars are spliced at a section and the distance between splices is 600 mm or more along the longitudinal axis of the reinforcement.

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8.3.5.4 Transverse Reinforcement

a) Transverse reinforcement shall be provided as specified below unless a larger amount is required by Sec 8.3.8.   i) The volumetric ratio of spiral or circular hoop reinforcement, ρs, shall not be less than that indicated by the following equation: $$\rho_s = \frac{0.12 f'_c}{f_{yh}} \tag{8.3.2}$$   and shall not be less than that required by Eq (6.3.3).   ii) The total cross-sectional area of rectangular hoop reinforcement shall not be less than that given by the following equations: $$A_{sh} = 0.3\left(sh_cf'_c/f_{yh}\right)\left[\left(A_g/A_{ch}\right)-1\right] \tag{8.3.3}$$ $$A_{sh} = \frac{0.09sh_cf'_c}{f_{yh}} \tag{8.3.4}$$   iii) Transverse reinforcement shall be provided by either single or overlapping hoops or cross ties of the same bar size and spacing. Each end of the cross ties shall engage a peripheral longitudinal reinforcing bar. Consecutive cross ties shall be alternated end for end along the longitudinal reinforcement.   iv) If the design strength of member core satisfies the requirements of the specified loading combinations including earthquake effect, Eq (8.3.3) and (6.3.3) need not be satisfied. b) Transverse reinforcement shall not be spaced more than one-quarter of the minimum member dimension nor 100 mm. c) Spacing of cross ties or legs of overlapping hoops shall not be more than 350 mm on centre in the direction perpendicular to the longitudinal axis of the member. d) The volume of transverse reinforcement in amount specified in (a) through (c) above shall be provided over a length ℓo from each joint face and on both sides of any section where flexural yielding is likely to occur in connection with inelastic lateral displacements of the frame. The length ℓo shall not be less than (i) the depth of the member at the joint face or at the section where flexural yielding is likely to occur, (ii) one-sixth of the clear span of the member, and (iii) 450 mm. e) If the factored axial force in columns supporting reactions from discontinued stiff members, such as walls, exceeds $0.1A_g f'_c$ they shall be provided with transverse reinforcement as specified in (a) through (c) above over their full height beneath the level at which the discontinuity occurs. Transverse reinforcement shall extend into the discontinued member for at least the development length of the largest longitudinal reinforcement in the column in accordance with Sec 8.3.7.4. If the lower end of the column terminates on a wall, transverse reinforcement as specified above shall extend into the wall for at least the development length of the largest longitudinal reinforcement in the column at the point of termination. If the column terminates on a footing or mat, transverse reinforcement as specified in above shall extend at least 300 mm into the footing or mat. f) Where transverse reinforcement as specified in (a) through (c) above, is not provided throughout the full length of the column, the remainder of the column length shall contain spiral or hoop reinforcement with centre to centre spacing not exceeding the smaller of 6 times the diameter of the longitudinal column bars or 150 mm.

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8.3.6 Structural Walls and Diaphragms

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8.3.6.1 Scope

The requirements of this section apply to structural walls serving as parts of the earthquake force resisting systems as well as to diaphragms, struts, ties, chords and collector members which transmit forces induced by earthquake.

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8.3.6.2 Reinforcement

a) The reinforcement ratio, ρv, for structural walls shall not be less than 0.0025 along the longitudinal and transverse directions. Reinforcement spacing each way shall not exceed 450 mm. Reinforcement provided for shear strength shall be continuous and shall be distributed across the shear plane. If the design shear force does not exceed $0.083 A_{cv}\sqrt{f'_c}$, the shear reinforcement may conform to Sec 6.9.7. b) At least two layers of reinforcement shall be used in a wall if the in-plane factored shear force assigned to the wall exceeds $0.17 A_{cv}\sqrt{f'_c}$. c) Structural truss members, struts, ties, and collector members with compressive stresses exceeding $0.2f'_c$ shall have special transverse reinforcement, as specified in Sec 8.3.5.4 over the total length of the member. The special transverse reinforcement is allowed to be discontinued at a section where the calculated compressive stress is less than $0.15f'_c$. Stresses shall be calculated for the factored forces using a linear elastic model and gross section properties of the members considered. d) All continuous reinforcement in structural walls, diaphragms, trusses, struts, ties, chords, and collector members shall be anchored or spliced in accordance with the provisions for reinforcement in tension as specified in Sec 8.3.7.4.

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8.3.6.3 Boundary Members for Structural Walls and Diaphragms

a) Boundary members shall be provided at boundaries and edges around openings of structural walls and diaphragms for which the maximum extreme fibre stress exceeds $0.2f'_c$ unless the entire wall or diaphragm member is reinforced to satisfy Sec 8.3.5.4(a) through 8.3.5.4(c). The boundary members may be discontinued where the calculated compressive stress is less than $0.15f'_c$. Stresses shall be calculated for the factored forces using a linearly elastic model and gross section properties. b) Where required, boundary members shall have transverse reinforcement as specified in Sec 8.3.5.4(a) through 8.3.5.4(c). c) Boundary members of structural walls shall be designed to carry all factored gravity loads on the wall, including tributary loads and self weight, as well as the vertical force required to resist overturning moment calculated from factored forces related to earthquake effect. d) Boundary members of structural diaphragms shall be proportioned to resist the sum of the factored axial force acting in the plane of the diaphragm and the force obtained from dividing the factored moment at the section by the distance between the edges of the diaphragm at that section. e) Transverse reinforcement in walls with boundary members shall be anchored within the confined core of the boundary member to develop the tensile yield stress. f) Transverse reinforcement terminating at the edges of structural walls without boundary elements shall have a standard hook engaging the edge reinforcement or the edge reinforcement shall be enclosed in the U-stirrups having the same size and spacing as, and spliced to, the transverse reinforcement, except when Vu in the plane of the wall is less than $0.083 A_{cv}\sqrt{f'_c}$.

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8.3.6.4 Construction Joints

All construction joints in walls and diaphragms shall conform to Sec 5.16.4 and contact surfaces shall be roughened as specified in Sec 6.13.3.15(j).

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8.3.6.5 Discontinuous Walls

Columns supporting discontinuous walls shall be reinforced in accordance with Sec 8.3.5.4(e).

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8.3.7 Joints of Frames

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8.3.7.1 General Requirements

a) Forces in longitudinal beam reinforcement at the faces of joints of reinforced concrete frames shall be determined for a stress of 1.25 fy in the reinforcement. b) Joint strength shall be calculated by the appropriate strength reduction factors specified in Sec 6.1.4. c) Beam longitudinal reinforcement terminated in a column shall be extended to the far face of the confined column core and anchored in tension according to Sec 8.3.7.4 below and in compression according to Sec 8.2.

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8.3.7.2 Transverse Reinforcement

a) As specified in Sec 8.3.5.4, transverse hoop reinforcement shall be provided within the joint, unless the joint is confined by structural members as specified in (b) below. b) Within the depth of the shallowest framing member, transverse reinforcement equal to at least one-half the amount required by Sec 8.3.5.4(a) shall be provided where members frame into all four sides of the joint and where each member width is at least three-fourths the column width. At these locations, the spacing specified in Sec 8.3.5.4(b) may be increased to 150 mm. c) As required by Sec 8.3.5.4, transverse reinforcement shall be provided through the joint to provide confinement for longitudinal beam reinforcement outside the column core if such confinement is not provided by a beam framing into the joint.

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8.3.7.3 Shear Strength

The nominal shear strength for the joint shall be taken not greater than the forces specified below:

• $1.66\sqrt{f'_c}A_j$ for joints confined on all four faces
• $1.24\sqrt{f'_c}A_j$ for joints confined on three faces or on two opposite faces
• $1.0\sqrt{f'_c}A_j$ for others

A member that frames into a face is considered to provide confinement to the joint if at least three-quarters of the face of the joint is covered by the framing member. A joint is considered to be confined if such confining members frame into all faces of the joint.

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8.3.7.4 Development Length of Bars in Tension

a) The development length, ℓdh, for a bar with a standard 90° hook shall be not less than (i) $8d_b$, (ii) 150 mm, and (iii) the length required by Eq (8.3.5). $$\ell_{dh} = \frac{0.185f_yd_b}{\sqrt{f'_c}} \tag{8.3.5}$$ for bar sizes 10 mm Ø through 35 mm Ø. b) For bar sizes 10 mm Ø through 35 mm Ø, the development length, ℓd, for a straight bar shall be not less than (i) 2.5 times the length required by (a) above, if the depth of the concrete cast in one lift beneath the bar does not exceed 300 mm, and (ii) 3.5 times the length required by (a) above, if the depth of the concrete cast in one lift beneath the bar exceeds 300 mm. c) Straight bars terminated at a joint shall pass through the confined core of a column or of a boundary member. Any portion of the straight embedment length not within the confined core shall be increased by a factor of 1.6.

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8.3.8 Shear Strength Requirements

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8.3.8.1 Design Forces

a) Frame Members Subjected Primarily to Bending: The design shear force Ve shall be determined from consideration of the statical forces on the portion of the member between faces of the joints. It shall be assumed that moments of opposite sign corresponding to probable strength Mpr act at the joint faces, and that the member is loaded with the factored tributary gravity load along its span. b) Frame Members Subjected to Combined Bending and Axial Load: The design shear force Ve shall be determined from consideration of the maximum forces that can be generated at the faces of the joints at each end of the member. These joint forces shall be determined using the maximum probable moment strengths Mpr of the member associated with the range of factored axial loads on the member. The member shears need not exceed those determined from joint strengths based on the probable moment strength Mpr of the transverse members framing into the joint. In no case, Ve shall be less than the factored shear determined by the analysis of the structure. c) Structural Walls and Diaphragms: The design shear force Ve shall be obtained from the lateral load analysis in accordance with the factored loads and combinations specified in Chapter 2, Loads.

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8.3.8.2 Transverse Reinforcement in Frame Members

a) For determining the required transverse reinforcement in frame members, the quantity Vc shall be assumed to be zero if the factored axial compressive force including earthquake effects is less than $0.05A_gf'_c$ when the earthquake-induced shear forces, calculated in accordance with Sec 8.3.8.1(a), represents one-half or more of total design shear. b) Stirrups or ties required to resist shear shall be closed hoops over lengths of members as specified in Sec 8.3.4.3, 8.3.5.4 and 8.3.7.2.

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8.3.8.3 Shear Strength of Structural Walls and Diaphragms

a) Nominal shear strength of structural walls and diaphragms shall be determined using either (b) or (c) below. b) Nominal shear strength, Vn of structural walls and diaphragms shall be assumed not to exceed the shear force calculated from $$V_n = A_{cv}\left(0.17\sqrt{f'_c} + \rho_nf_y\right) \tag{8.3.6}$$ c) For walls and wall segments having a ratio of $(h_w/\ell_w)$ less than 2.0, nominal shear strength of wall and diaphragm shall be determined from $$V_n = A_{cv}\left(\alpha_c\sqrt{f'_c} + \rho_nf_y\right) \tag{8.3.7}$$ where the coefficient $\alpha_c$ varies linearly from 0.25 for $(h_w/\ell_w) = 1.5$ to 0.17 for $(h_w/\ell_w) = 2.0$. d) Value of ratio $(h_w/\ell_w)$ used in (c) above for determining Vn for segments of a wall or diaphragm shall be the larger of the ratios for the entire wall (diaphragm) and the segment of wall (diaphragm) considered. e) Walls and diaphragms shall have distributed shear reinforcement providing resistance in two orthogonal directions in the plane of the wall. If the ratio $(h_w/\ell_w)$ does not exceed 2.0, reinforcement ratio, ρv shall not be less than reinforcement ratio ρn. f) Nominal shear strength of all wall piers sharing a common lateral force shall not be assumed to exceed $0.67A_{cv}\sqrt{f'_c}$, where Acv is the total cross-sectional area, and the nominal shear strength of any one of the individual wall piers shall not be assumed to exceed $0.83A_{cp}\sqrt{f'_c}$ where Acp represents the cross-sectional area of the pier considered. g) Nominal shear strength of horizontal wall segments shall be assumed not to exceed $0.83A_{cp}\sqrt{f'_c}$, where Acp represents the cross-sectional area of a horizontal wall segment.

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8.3.9 Frame Members not Proportioned to Resist Forces Induced by Earthquake Motion

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8.3.9.1

Frame members assumed not to contribute to lateral resistance shall be detailed according to (a) or (b) below depending on the magnitude of moments induced in those members when subjected to twice the lateral displacement under the factored lateral forces. a) Members with factored gravity axial forces not exceeding $0.1A_gf'_c$ shall satisfy Sec 8.3.4.2(a) and 8.3.8.1(a) and members with factored gravity axial forces exceeding $0.1A_gf'_c$ shall satisfy Sec 8.3.5.4, 8.3.7.2(a) and 8.3.8.1(b) when the induced moment exceeds the design moment strength of the frame member. b) The member shall satisfy Sec 8.3.4.2(a) when the induced moment does not exceed the design moment strength of the frame members.

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8.3.9.2

All frame members with factored axial compressive forces exceeding $0.1A_gf'_c$ shall satisfy the following special requirements unless they comply with Sec 8.3.5.4. a) Ties shall have hooks not less than 135° with extensions not less than 6 tie bar diameter or 60 mm. Cross ties as defined in Sec 8.3.2 are allowed. b) The maximum tie spacing shall be so over a length ℓo measured from the joint face. The spacing so shall be not more than (i) eight diameters of the smallest longitudinal bar enclosed, (ii) 24 tie bar diameters, and (iii) one-half the least cross-sectional dimension of the column. The length ℓo shall not be less than (i) one-sixth of the clear height of the column, (ii) the maximum cross-sectional dimension of the column, and (iii) 450 mm. c) The first tie shall be within a distance equal to 0.5so from the face of the joint. d) The tie spacing shall not exceed 2so in any part of the column.

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8.3.10 Requirements for Frames in Regions of Moderate Seismic Risk, Zone 2

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8.3.10.1

In regions of moderate seismic risk, structural frames proportioned to resist forces induced by earthquake motions shall satisfy the requirements of Sec 8.3.10 in addition to those of Chapter 6.

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8.3.10.2

Reinforcement details in a frame member shall satisfy Sec 8.3.10.4 below if the factored compressive axial load for the member does not exceed $0.1A_gf'_c$. If the factored compressive axial load is larger, frame reinforcement details shall satisfy Sec 8.3.10.5 below unless the member has spiral reinforcement according to Eq (6.3.3). If a two-way slab system without beams is treated as part of a frame resisting earthquake effect, reinforcement details in any span resisting moments caused by lateral force shall satisfy Sec 8.3.10.6 below.

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8.3.10.3

Design shear strength of beams, columns, and two-way slabs resisting earthquake effect shall not be less than either (a) the sum of the shear associated with development of nominal moment strengths of the member at each restrained end of the clear span and the shear calculated for factored gravity loads, or (b) the maximum shear obtained from design load combinations which include earthquake effect.

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8.3.10.4 Beams

a) The positive moment strength at the face of the joint shall not be less than one-third the negative moment strength provided at that face. Neither the negative nor positive moment strength at any section along the length of the member shall be less than one-fifth of the maximum moment strength provided at the face of either joint. b) At both ends of the member, stirrups shall be provided over lengths equal to twice the member depth measured from the face of the supporting member toward midspan. The first stirrup shall be located not more than 50 mm from the face of the supporting member. Maximum stirrup spacing shall not exceed (a) d/4, (b) 8 times the diameter of the smallest longitudinal bar enclosed, (c) 24 times the diameter of the stirrup bar, and (d) 300 mm. c) Stirrups shall be placed at not more than d/2 throughout the length of the member.

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8.3.10.5 Columns

a) Maximum tie spacing shall not exceed so over a length ℓo measured from the joint face. The spacing so shall not exceed (i) 8 times the diameter of the smallest longitudinal bar enclosed, (ii) 24 times the diameter of the tie bar, (iii) one-half of the smallest cross-sectional dimension of the frame member, and (iv) 300 mm. The length ℓo shall not be less than (i) one-sixth of the clear span of the member, (ii) maximum cross-sectional dimension of the member, and (iii) 450 mm. b) The first tie shall be located not more than so/2 from the joint face. c) Joint reinforcement shall conform to Sec 6.3.8. d) Tie spacing shall not exceed 2so throughout the length of the member.

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8.3.10.6 Two-way Slabs without Beams

a) The factored slab moment at the supports relating to earthquake effect shall be determined for load combinations specified in Chapter 2, Loads. All reinforcement provided to resist the portion of slab moment balanced by support moment shall be placed within the column strip defined in Sec 6.4.2.2. b) The fractional part of the column strip moment shall be resisted by reinforcement placed within the effective width specified in Sec 6.4.4.5(b). c) Not less than one-half of the total reinforcement in the column strip at the support shall be placed within the effective slab width specified in Sec 6.4.4.5(b). d) Not less than one-quarter of the top steel at the support in the column strip shall be continuous throughout the span. e) Continuous bottom reinforcement in the column strip shall be not less than one-third of the top reinforcement at the support in the column strip. f) Not less than one-half of all bottom reinforcement at midspan shall be continuous and shall develop its yield strength at the face of support. g) At discontinuous edges of the slab all top and bottom reinforcement at the support shall be developed at the face of the support.