ACI 318-11: Development length for reinforcement

The design is based on the reinforcement attaining the yield stress, so the reinforcement must be properly bonded to the concrete for a finite length in order not to slip. This sufficient length to anchor bars near the end of connections is referred to as the development length, ld. The development length equation according to ACI 318 is presented here:

where:

d_b, The bar diameter

c_b, The bar radius

ψ_t, Modifier for reinforcement location , 1.3 for top bars, 1.0 for other bars

ψ_e, modifier for epoxy coated bars

• 1.5 when cover < 3db or clear spacing < 6db

• 1.2 for other epoxy coated reinforcing

• 1.0 for non-epoxy coated reinforcing

The product, ψ_tψ_e, should not exceed 1.7

ψ_s, Modifier for bar size

• 0.8 for #6 and smaller

• 1.0 for #7 and larger

l, Modifier for lightweight concrete

• 1.3 for lightweight concrete

• 1.0 for other concrete

K_tr, transverse reinforcement index. According to ACI 318:

where:

A_tr = total cross sectional area of all transverse reinforcement which is within the spacing, s, and which crosses the potential plane of splitting through the reinforcement being developed.

f_yt = the yield strength of the transverse reinforcing

s = maximum C-C spacing of transverse reinforcement within the development length

n = number of longitudinal bars being developed along the plane of splitting.

Development Length in Tension

With the proper bar to bar spacing and cover, the common development length equations are:

#6 bars and smaller: or 12 in. minimum

#7 bars and larger: or 12 in. minimum

Development Length in Compression

Hook Bends and Extensions

The minimum hook length is:

Figure: Minimum requirements for 90° and 180° respectively.