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Concrete In Australia : September 2009
Concrete in Australia Vol 35 No 3 27 A two tiered approach is proposed for the development length of a deformed bar in tension. In any situation, a designer may adopt the simpler lower tier approach of Clause 220.127.116.11 and specify the development length (Lsy.t) as the basic development length (Lsy.tb) given in Eq. 18.104.22.168. Alternatively, in situations where the beneficial effects of transverse reinforcement and/or transverse confining pressure exist along the development length, the designer may opt for the refined upper tier approach of Clause 22.214.171.124. e expression for the basic development length given in Eq. 126.96.36.199 is significantly different to the expression in AS3600-2001 and has been calibrated to provide an appropriate factor of safety against bond failure at a developing bar. A wide range of test data was used in the calibration. When specifying the development length using Eq. 188.8.131.52 there is no need to consider or include a strength reduction factor (φ) as an appropriate strength reduction factor has been incorporated into the expression. Unlike the previous expression in AS3600-2001, the new expression for the basic development length provides development lengths that have an adequate and consistent factor of safety against brittle bond failure and that are compatible with the development lengths specified in the other major international Standards including ACI 318-08 (American Concrete Institute, 2008) and Eurocode 2 (European Committee for Standardisation [CEN], 2004). e factor k1 in Eq. 184.108.40.206 (and Eq. 2 above) accounts for the position of the bar in the structure and increases the development length for bars with more than 300 mm of concrete cast below the bar (such as the top bars in a beam or thick slab). Such bars may be subjected to a reduction in bond strength due to settlement of fresh concrete below the bar and an accumulation of bleed water. Both effects occur along the underside of the bar. e factor applies only to horizontal bars in slabs, walls, beams and footings; it does not apply to sloping or vertical bars, to fabric, or to fitments. ere is a step increase in the value of k1 when the depth of concrete cast below the bar reaches 300 mm (ie. the value jumps from 1.0 to 1.3). ere is evidence that bond loss can occur with even shallower concrete depths and it may be prudent to linearly vary k1 from 1.0, when the depth of concrete cast below the bar is less than or equal 200 mm, to 1.3 when the depth is 300 mm (or more). e factor k2 accounts for the reduction in the average ultimate bond stress as the diameter of the reinforcing bar increases and varies linearly fromk2=1.2whendb=12mmtok2=0.92whendb=40mm. e factor k3 accounts for the confining effect of the concrete surrounding the bar and depends on the concrete cover to the anchored bar (c1 or c in FIGURE 13.1.2(A)) or the clear distance to the next parallel bar (a in FIGURE 13.1.2(A)). e dimension cd is used in the expression for k3, where cd is the thickness of the appropriate concrete ring surrounding the development length shown in Figure 3 (cd is the smaller of the side cover, c1, the cover to the soffit (or top) surface, c, or half the clear distance to the next parallel bar, a/2). When cd is less than or equal to the bar diameter, k3 = 1.0. When cd is greater than or equal to twice the bar diameter, k3 = 0.7. When cd is between db and 2db , k3 varies linearly between 1.0 and 0.7. e average ultimate bond stress is directly related to the tensile strength of concrete, which is taken in the Standard to be proportional to ¢fc and, hence, the term ¢fc is included in Eq. 220.127.116.11. Due to the very limited experimental data available for development lengths of deformed bars in high strength concrete, an upper limit of 65 MPa has been placed on the concrete strength. e minimum value of Lsy.tb (29 k1db) is applicable to a steel yield stress of 500 MPa and is based on the formula 0.058db fsy from AS1480, (1982) and Ferguson (1988). Due to the reduced average ultimate bond stress, the development length for an epoxy-coated bar is significantly longer than for an uncoated bar and, accordingly, Lsy.tb shall be multiplied by 1.5 for epoxy-coated bars. e tensile strength of lightweight concrete is significantly less than for normal weight concrete and so the average ultimate bond stress is also lower. e standard specifies that the basic development length shall be multiplied by 1.3 when lightweight concrete is used and when the structural element containing the deformed bar is built with slip forms. Refined Development Length In situations where there is significant transverse reinforcement along the development length or where there is transverse pressure, the average ultimate bond stress increases and a reduced development length may be possible by multiplying the basic development length Lsy.tb (obtained from Eq. 18.104.22.168) by two factors, k4 and k5. e factor k4 (= 1.0 -- Kλ) accounts for the presence of transverse reinforcement and is equal to 1.0 when there is no transverse reinforcement and may reduce to a minimum value of 0.7 depending on the amount and location of the transverse reinforcement. e term λ depends on the total cross-sectional area of transverse reinforcement along the basic development length (ΣAtr), as well as the cross-sectional area of the single anchored bar being developed (As) and is given by λ = (ΣAtr ΣAtr.min)/As, where ΣAtr.min is the Figure 3. Concrete confinement dimension cd .