Steel reinforcement must be bonded to the concrete sufficiently so that the steel will yield before it is freed from the concrete. Despite assumptions made in the past to the contrary, bond stress between concrete and reinforcing bars is not uniform over a given length, not directly related to the perimeter of the bars, not equal in tension and compression, and may be affected by lateral confinement. The ACI 318 Building Code requirements therefore reflect the significance of average bond resistance over a length of bar or wire sufficient to develop its strength (development length).
The calculated tension or compression force in each reinforcing bar at any section [Eqs. (9.53) to (9.61) and (9.64)] must be developed on each side of that section by a development length Ld, or by end anchorage, or both. Hooks can be used to assist in the development of tension bars only.
The critical sections for development of reinforcement in flexural members are located at the points of maximum stress and where the reinforcement terminates or is bent.
The following requirements of the ACI 318 Building Code for the development of reinforcement were proposed to help provide for shifts in the location of maximum moment and for peak stresses that exist in regions of tension in the remaining bars wherever adjacent bars are cut off or bent. In addition, these requirements help minimize any loss of shear capacity or ductility resulting from flexural cracks that tend to open early whenever reinforcement is terminated in a tension zone.
Development for All Flexural Reinforcement
Reinforcement should extend a distance of d of 12db, whichever is larger, beyond the point where the steel is no longer required to resist tensile stress, where d is the effective depth of the member and db is the nominal diameter of the reinforcement.
This requirement, however, does not apply at supports of simple spans and at the free end of cantilevers.
Continuing reinforcement should extend at least the development length Ld beyond the point where terminated or bent reinforcement is no longer required to resist tension.
Reinforcement should not be terminated in a tension zone unless one of the following conditions is satisfied:
Development for Positive-Moment Reinforcement
A minimum of one-third the required positive-moment reinforcement for simple beams should extend along the same face of the member into the support, and in beams, for a distance of not less than 6 in.
A minimum of one-fourth the required positive-moment reinforcement for continuous members should extend along the same face of the member into the support, and in beams, for a distance of at least 6 in.
For lateral-load-resisting members, the positive-moment reinforcement to be extended into the support in accordance with the preceding two requirements should be able to develop between the face of the support and the end of the bars the yield strength ƒy of the bars.
Positive-moment tension reinforcement at simple supports and at points of inflection should be limited to a diameter such that the development length, in computed for ƒy with Eqs. (9.54) to (9.58) and (9.61) does not exceed
The value of Mn /Vu can be increased by 30% when the ends of the reinforcement are confined by a compressive reaction. It is not necessary to satisfy Eq. (9.53) for reinforcing bars that terminate beyond the center of simple supports with a standard hook, or terminate with a mechanical anchorage equivalent to a standard hook.
Development for Negative-Moment Reinforcement
Negative-moment reinforcement in continuous, restrained, or cantilever members should be developed in or through the supporting member.
Negative-moment reinforcement should have sufficient distance between the face of the support and the end of each bar to develop its full yield strength.
A minimum of one-third of the required negative-moment reinforcement at the face of the support should extend beyond the point of inflection the greatest of d, 12db, or one-sixteenth of the clear span.
Computation of Development Length
Tension development length, Ld, is the length of deformed bar or deformed wire required to develop, or to transfer to the concrete, the full tensile capacity of the bar or wire. The tension development length of an uncoated bar or wire in normal weight concrete is expressed as a function of yield strength of the bar; ƒy; the square
Increased Ld is required for bundled bars: in 3-bar bundles, 20%; in 4-bar bundles, 33%. For determining the appropriate modifying factors for use with bundled bars, a unit of bundled bars should be treated as a single bar with a diameter derived from the equivalent total area.
Application of all the various interdependent tension development length requirements to each structural element in design would be extremely difficult and a waste of design time. The authors recommend that the designer check the actual dimensions available for tension development in the connection (or from a cutoff point established as a fraction of the span on typical design drawing details), compare to a table of development lengths required for each bar size, and select the bar size allowable. Table 9.8, which is based on the direct short-cut method, presents values of tension Ld for each size bar for normal-weight concrete with compressive strengths of 3000, 4000 and 5000 psi. Note that separate values are tabulated for ‘‘top bars’’ and ‘‘other bars.’’
Anchorage with Hooks
For rebars in tension, standard 90 and 180 end hooks can be used as part of the length required for development or anchorage of the bars. Table 9.9 gives the minimum tension embedment length Ldh required with standard end hooks (Fig. 9.17 and Table 9.9) and Grade 60 bars to develop the specified yield strength of the bars.
Development for Welded-Wire Fabric in Tension
For deformed welded-wire fabric (WWF) with at least one cross wire within the development length not less than 2 in. from the point of critical section (Fig. 9.18), the tension development length is the length calculated from Eqs. (9.54) and (9.56) using the direct short-cut method or from Eq. (9.58) using the more rigorous method and then multiplied by a wire fabric factor. The wire fabric factor is the larger of
The resulting development length should be at least 8 in except for determining lap splice lengths. When using Eqs. (9.54), (9.56) or (9.58), an epoxy-coated welded wire fabric factor of 1.0 can be taken for B . For deformed WWF with no cross wires within the development length or with a single cross wire less than 2 in from the point of the critical section, the wire fabric factor should also be taken as 1.0.
Tension Lap Splices
Bar sizes No. 11 or less and deformed wire may be spliced by lapping. Tension lap splices are classified in two classes, A and B, depending on the stress in the bars to be spliced. The minimum lap length Ls is expressed as a multiple of the tension development length Ld of the bar or deformed wire (Art. 9.49.4).
Class A tension lap splices include splices at sections where the tensile stress due to factored loads does not exceed 0.5ƒy and not more than one-half the bars at these sections are spliced within one Class A splice length of the section. For Class A splices,
Compression development length Ld is calculated by multiplying Ldb by optional modification factors. When bars are enclosed by a spiral at least 1⁄4 in in diameter and with not more than a 4-in pitch, or by ties at least size No. 4 with a spacing not more than 4 in., a modification factor of 0.75 may be used but the lap should be at least 8 in. It excess reinforcement is provided, Ldb may be reduced by the ratio of the area of steel required to area of steel provided. For general practice, with concrete compressive strength psi, use 22db ƒ’c => 3000 for compression em- c bedment of dowels (Table 9.11).
For bundled bars in compression, the development length of each bar within the bundle should be increased by 20% for a three-bar bundle and 33% for a four-bar bundle.
Compression Lap Splices
Minimum lap-splice lengths of rebars in compression Ls vary with nominal bar diameter db and yield strength ƒy of the bars. For bar sizes No. 11 or less, the compression lap-splice length is the largest of 12 in or the values computed from Eqs. (9.65a) and (9.65b):
When is less than 3000 psi, the length of lap should be one-third greater than ƒc the values computed from the preceding equations.
When the bars are enclosed by a spiral, the lap length may be reduced by 25%.
For general practice, use 30 bar diameters for compression lap splices (Table 9.11).
Spiral should conform to requirements of the ACI 318 Building Code: Spirals should extend from top of footing or slab in any story to the level of the lowest horizontal reinforcement in members supported above. The ratio of volume of spiral reinforcement to the total volume of the concrete core (out-to-out of spirals) should be at least that given in Art. 9.83. Minimum spiral diameter in cast-in-place construction is 3⁄8 in. Clear spacing between spirals should be limited to 1 to 3 in.
Spirals should be anchored by 11⁄2 extra turns of spiral bar or wire at each end of a spiral unit. Lap splices, or full mechanical or welded splices can be used to splice spiral reinforcement. Lap splice lengths should comply with Table 9.12, but not be less than 12 in.
The ACI 318 Building Code contains provisions for lap splicing bars of different sizes in compression. Length of lap should be the larger of the compression development length required for the larger size bar or the compression lap-splice length required for the smaller bar. It is permissible to lap-splice the large bar sizes, Nos. 14 and 18, to No. 11 and smaller bars.
Mechanical and Welded Splices
As an alternative to lap splicing, mechanical splices or welded splices may be used.
When traditional lap splices satisfy all requirements, they are generally the most economical. There are conditions, however, where they are not suitable: The ACI 318 Building Code does not permit lap splices of the large-size bars (Nos. 14 and 18) except in compression to No. 11 and smaller bars. Lap splices cause congestion at the splice locations and their use then may be impracticable. Under certain conditions, the required length of tension lap splices for No. 11 and similar-size bars can be excessive and make the splices uneconomical. For these reasons, mechanical splices or welded splices may be suitable alternatives.
Mechanical splices are made with proprietary devices. The ACI 318 Building Code requires a full mechanical splice to have a capacity, in tension or compression, equal to at least 125% of the specified ƒy of the bar. End-bearing mechanical splices may be used where the bar stress due to all conditions of factored loads is compressive.
For these types of compression-only splices, the ACI 318 Building Code prescribes requirements for the squareness of the bars ends. Descriptions of the commercially-available proprietary mechanical splice devices are given in ‘‘Mechanical Connections of Reinforcing Bars,’’ ACI 439.3R, and ‘‘Reinforcement Anchorages, and Splices,’’ Concrete Reinforcing Steel Institute.
For a full-welded splice, the ACI 318 Building Code requires the butt-welded bars to have a tensile capacity of at least 125% of the specified ƒy of the bar.
Welding should conform to ‘‘Structural Welding Code—Reinforcing Steel’’ (ANSI/ AWS D1.4), American Welding Society.
Anchorage of Web Reinforcement
Stirrups are reinforcement used to resist shear and torsion. They are generally bars, wire or welded-wire fabric, either single leg or bent into L, U, or rectangular shapes.
Stirrups should be designed and detailed to be installed as close as possible to the compression and tension surfaces of a flexural member as concrete cover requirements and the proximity of other reinforcing steel will permit. They should be installed perpendicular or inclined with respect to flexural reinforcement and spaced closely enough to cross the line of every potential crack. Ends of singleleg, simple U stirrups, or transverse multiple U stirrups should be anchored by one of the following means:
1. A standard stirrup hook around a longitudinal bar for stirrups fabricated from No. 5 bars or D31 wire or smaller sizes. Stirrups fabricated from bar sizes Nos. 6, 7, and 8 in Grade 40 can be anchored similarly.
Each leg of simple U stirrups made of plain welded-wire fabric should be anchored by one of the following means:
1. Two longitudinal wires located at the top of the U and spaced at 2 in.
2. One longitudinal wire located at a distance of d/4 or less from the compression face and a second wire closer to the compression face and spaced at least 2 in from the first wire. (d distance, in from compression surface to centroid of tension reinforcement.) The second wire can be located on the stirrup leg beyond a bend, or on a bend with an inside diameter of at least 8db.
Each end of a single-leg stirrup, fabricated from plain or deformed welded-wire fabric, should be anchored by two longitudinal wires spaced at 2 in minimum. The inner wire of the two longitudinal wires should be located at least the larger of d/4 or 2 in from the middepth of the member d/2. The outer longitudinal wire at the tension face of the member should be located not farther from the face than the portion of primary flexural reinforcement closest to the face.
Between anchored ends, each bend in the continuous portion of a simple U or multiple U stirrup should enclose a longitudinal bar.
Pairs of U stirrups or ties placed to form a closed unit may be considered properly spliced when the legs are lapped over a minimum distance of 1.3Ld. In members at least 18 in deep, such splices may be considered adequate for No. 3 bars of Grade 60 and Nos. 3 and 4 bars of Grade 40 if the legs extend the full available depth of the member.