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At both sides of any cross-section the force in each bar should be developed by an appropriate embedment length or other end anchorage. Provided this is done, local bond stress may be ignored.

3.12.8.2 Anchorage bond stress

Anchorage bond stress is assumed to be constant over the effective anchorage length. It may be taken as the force in the bar divided by its effective surface anchorage area (see 3.12.8.3). It should not exceed the appropriate value obtained from 3.12.8.4.

3.12.8.3 Design anchorage bond stress

The design anchorage bond stress fb is assumed to be constant over the anchorage length and is given by

the following equation:

fb = Fs/;Îel equation 48

where

fb is the bond stress;

Fs is the force in the bar or group of bars;

l is the anchorage length;

Îe is the effective bar size which, for a single bar is equal to the bar size and, for a group of bars

3.12.8.4 Values for design ultimate anchorage bond stress

Values for design ultimate anchorage bond stress fbu may be obtained from the equation:

For bars in tension in slabs or in beams where minimum links have been provided in accordance with Table 3.7, the values of ¶ may be taken from Table 3.26. These values include a partial safety factor (¾m)

of 1.4.

Table 3.26 — Values of bond coefficient ¶

In beams where minimum links in accordance with Table 3.7 have not been provided, the design anchorage bond stresses used should be those appropriate to plain bars irrespective of the type of bar used. This does not apply to slabs.

Values for anchorage lengths are given in Table 3.27 as multiples of bar size.

3.12.8.5 Design ultimate anchorage bond stresses for fabric

The value for design ultimate anchorage bond stress given in 3.12.8.4 for fabric is applicable to fabric manufactured from bars conforming to BS 4449 or wires conforming to BS 4482. This is provided that:

a) the fabric is welded in a shear resistance manner conforming to BS 4483; and

b) the number of welded intersections within the anchorage length is at least equal to 4As required/As

provided.

When condition b) is not satisfied, the anchorage bond stress should be taken as that appropriate to the individual bars or wires in the sheet.

3.12.8.6 Anchorage of links

A link may be considered to be fully anchored if it satisfies the following:

a) it passes round another bar of at least its own size, through an angle of 90°, and continues beyond for a minimum length of eight times its own size; or

b) it passes round another bar of at least its own size, through an angle of 180°, and continues beyond for a minimum length of four times its own size.

In no case should the radius of any bend in the link be less than twice the radius of a test bend guaranteed by the manufacturer of the bar.

NOTE When producing reinforcement schedules in accordance with BS 8666, the minimum lengths referred to in a) and b) above would be ten times and five times respectively.

3.12.8.7 Anchorage of welded fabric used as links

A link may be considered to be fully anchored when it has within the length of the anchorage either two welded transverse wires or a single welded wire of a size not less than 1.4 times the size of the wire being anchored.

fbu = ¶Æfcu equation 49

where

fbu is the design ultimate anchorage bond stress;

¶ is a coefficient dependent on the bar type.

Bar type ¶

Bars in tension Bars in compression

Plain bars 0.28 0.35

Type 1: deformed bars 0.40 0.50 Type 2: deformed bars 0.50 0.63 Fabric (see 3.12.8.5) 0.65 0.81

3.12.8.8 Anchorage of column starter bars in bases or pile caps

The compression bond stresses that develop on starter bars within bases or pile caps do not need to be checked provided:

a) the starters extend down to the level of the bottom reinforcement;

b) the base or pile cap has been designed for moments and shears in accordance with 3.11.

3.12.8.9 Laps and joints

Connections transferring stress may be lapped, welded or joined with mechanical devices7)_{. They should be}

placed, if possible, away from points of high stress and should preferably be staggered. Laps in fabric may be layered or nested to maintain the lapped bars in one plane.

3.12.8.10 Joints where imposed loading is predominantly cyclical

In such cases bars should not be joined by welding.

3.12.8.11 Minimum laps

The minimum lap length for bar reinforcement should be not less than 15 times the bar size or 300 mm, whichever is the greater, and for fabric reinforcement should be not less than 250 mm.

3.12.8.12 Laps in beams and columns with limited cover

Where both bars at a lap exceed size 20 and the cover is less than 1.5 times the size of the smaller bar, transverse links should be provided throughout the lap length. At the lap the links should be at least one-quarter the size of the smaller bar and the spacing should not exceed 200 mm.

3.12.8.13 Design of tension laps

The length should be at least equal to the design tension anchorage length (see 3.12.8.3 and 3.12.8.4) necessary to develop the required stress in the reinforcement. Lap lengths for unequal size bars (or wires in fabric) may be based upon the smaller bar. The following provisions also apply:

a) where a lap occurs at the top of a section as cast and the minimum cover is less than twice the size of the lapped reinforcement, the lap length should be increased by a factor of 1.4;

b) where a lap occurs at the corner of a section and the minimum cover to either face is less than twice the size of the lapped reinforcement or, where the clear distance between adjacent laps is less

than 75 mm or six times the size of the lapped reinforcement, whichever is the greater, the lap length should be increased by a factor of 1.4;

c) in cases where both conditions a) and b) apply, the lap length should be increased by a factor of 2.0. Values for lap lengths are given in Table 3.27 as multiples of bar size.

3.12.8.14 Maximum amount of reinforcement in a layer including tension laps

At laps, the sum of the reinforcement sizes in a particular layer should not exceed 40 % of the breadth of the section at that level.

3.12.8.15 Design of compression laps

The length should be at least 25 % greater than the compression anchorage length (see 3.12.8.3

and 3.12.8.4) necessary to develop the required stress in the reinforcement. Lap lengths for unequal size bars (or wires in fabric) may be based upon the smaller bar.

Table 3.27 — Ultimate anchorage bond lengths and lap lengths as multiples of bar size

3.12.8.16 Butt joints

3.12.8.16.1 Bars in compression

In such cases the load may be transferred by end bearing of square sawn-cut ends held in concentric contact by a suitable sleeve or other coupler. The concrete cover for the sleeve should be not less than that specified for normal reinforcement.

3.12.8.16.2 Bars in tension

The only acceptable form of full-strength butt joint for a bar in tension comprises a mechanical coupler satisfying the following criteria.

a) When a test is made of a representative gauge length assembly comprising reinforcement of the size, grade and profile to be used and a coupler of the precise type to be used, the permanent elongation after loading to 0.6fy should not exceed 0.1 mm.

b) The tensile strength of the coupled bar should exceed 287.5 N/mm2 _{for grade 250, 525 N/mm}2 _for

grade 500 A and 540 N/mm2 _{for grade 500 B.}

Reinforcement types Grade 250

plain Grade 500

Plain Deformed

type 1 Deformed type 2 Fabric