Rebar development length, lap length and anchorage

Governing equations

Development length in tension under IS 456 Clause 26.2.1 is given by the expression L_d equals phi times sigma_s divided by four times tau_bd, where phi is the bar diameter, sigma_s is the design stress in the bar under the design load case, and tau_bd is the design bond stress for the concrete grade and bar type. For deformed bars in tension, the design bond stress values tabulated in IS 456 Table are 1.2, 1.4, 1.5, 1.7 and 1.9 N per square millimetre for concrete grades M20, M25, M30, M35 and M40 respectively. The tabulated values are for plain bars; for deformed bars the values are increased by sixty percent.

Under ACI 318 Chapter 25 the tension development length is computed from the more detailed expression that includes modification factors for bar location, epoxy coating, bar size, lightweight concrete, cover and confining transverse reinforcement. A Class B tension lap splice, which is the default where more than half of the bars are lapped at the same location, is 1.3 times the development length, and in no case less than 300 millimetres.

Worked example: Fe 500 D twenty-millimetre bar in M30 concrete

Consider a twenty-millimetre Fe 500 D bar embedded in M30 concrete, with the bar acting in tension at its full design yield stress of 0.87 times 500 which equals 435 N per square millimetre. The design bond stress for M30 from IS 456 Table is 1.5 N per square millimetre; increased by sixty percent for deformed bars gives 2.4 N per square millimetre.

Development length: L_d equals 20 times 435 divided by four times 2.4, which equals 8700 divided by 9.6, which gives 906 millimetres. This rounds up to 910 millimetres in a detailing note. A Class B tension lap for the same bar is 1.3 times 910 which equals 1183 millimetres, rounded to 1200 millimetres. A standard 90-degree hook under IS 456 Clause 26.2.2.1 has a straight anchorage length beyond the bend of eight bar diameters, which is 160 millimetres, and the hook itself contributes a development length equivalent to sixteen bar diameters, so the total development beyond the critical section with a hook is 320 millimetres.

Modification factors under ACI 318

The product of these factors modifies the basic development length, and the composite expression is capped so that the combined factor for bar location and coating does not exceed 1.7.

Typical detailing outcomes

The values in the table assume deformed bars in tension at the design yield stress, with no top-bar increase and no epoxy coating. Designers should adjust for modification factors as applicable and should in every case round up to the nearest fifty millimetres when issuing detailing drawings.

Staggering and splice location

IS 456 Clause 26.2.5.1 requires that laps be staggered such that no more than fifty percent of the bars are spliced at any one section, and that laps be located in regions of low stress. In a beam this usually means locating laps in the compression zone at mid-span rather than at the support, and in a column it means locating laps above the floor slab in the lower third of the storey height rather than at the joint. Mechanical couplers conforming to IS 16172 are preferred for bars larger than 32 millimetres, and mandatory where the lap length would be impractical or where bar congestion would prevent proper concrete placement.

Related pages

See the companion technical articles on mortar mix ratios, concrete curing and MCB and RCCB sizing. The steel and rebar category page carries the grade selection table and brand roster referenced here.

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