SPLICES, DEVELOPMENT AND STANDARD HOOKS FOR CONCRETE MASONRY, 2003 & 2006 IBC

INTRODUCTION

Building codes include requirements for minimum reinforcement development lengths and splice lengths, as well as requirements for standard hooks, to ensure the adequate transfer of stresses between the reinforcement and the masonry. This TEK presents these requirements, based on the provisions of both the 2003 and 2006 editions of the International Building Code (IBC) (refs. 1, 2). TEK 12-4D (ref. 3) includes basic material requirements, corrosion protection and placement tolerances for reinforcement used in concrete masonry construction. In addition, prestressing steel is discussed in Post-Tensioned Concrete Masonry Wall Construction, TEK 3-14 (ref. 4).

SPLICES AND DEVELOPMENT

Minimum development lengths are necessary to adequately transfer stresses between reinforcement and the grout or mortar in which it is embedded. Splicing of reinforcement serves a similar purpose; to adequately transfer stresses from one reinforcing bar to another.

Reinforcement can be developed by embedment length, hook, or mechanical anchoring device. The development of the reinforcing bars relies on mechanical interlock of the bar deformations, hook, and/or anchor along with sufficient masonry cover to prevent splitting of the masonry. Reinforcing bars may be spliced by lapping the reinforcement, by proprietary mechanical splices or by welding.

The required length of lap or development is determined according to the design procedure used and type of detail employed. In addition, these detailing requirements have been frequently revised in recent years. As a result, the minimum lap and development lengths can vary considerably from one code to the next as well as from one design method to another. For this reason, the following sections present the requirements for both the 2003 IBC and 2006 IBC for both allowable stress and strength design. Because the detailing requirements for these two codes can be significantly different, designers using the 2003 IBC are encouraged to compare these requirements to the corresponding 2006 IBC requirements.

ALLOWABLE STRESS DESIGN

2003 IBC Allowable Stress Design

By reference to the 2002 edition of Building Code Requirements for Masonry Structures (MSJC) (ref. 5), the 2003 IBC requires the minimum development length for reinforcing bars and wires designed by the allowable stress method to be calculated using Equation 1 (see Tables 1 and 2).

but not less than 12 in. (305 mm) for bars or 6 in. (152 mm) for wires.

When epoxy coated bars or wires are used, the development length determined by Equation 1 is required to be increased by 50 percent.

Although development lengths and lap splice lengths have historically been assumed to be related and calculated in a similar manner, the 2003 IBC provides a unique design equation for determining the minimum length of lap for reinforcing bars, in accordance with Equation 2. Table 5 contains tabulated values for common design variables shown in Equation 2.

but not less than 15 in. (380 mm)

For use in Equation 2, the reinforcement size factor, γ, is taken equal to 1.0 for No. 3 through No. 5 (M#10–M#16) reinforcing bars; 1.4 for No. 6 and No. 7 (M#19 and M#22) reinforcing bars; and 1.5 for No. 8 and No. 9 (M#25 and M#29) reinforcing bars. Reinforcing bars larger than No. 9 (M#29) are required to be spliced using mechanical connectors. When noncontact lap splices are used, the bars must be spaced no farther apart than one-fifth the required length of lap nor more than 8 in. (203 mm).

As an alternative to lap splicing, reinforcing bars can be spliced by welding. Welded splices require the bars to be butted or shortly lapped and welded to develop in tension at least 125% of the specified yield strength of the bar. All welding is required to conform to AWS D 1.4 (ref. 6). In practice, however, welding tends to be an expensive splicing option.

Finally, mechanical splicing of reinforcement typically employs proprietary couplers specifically designed for this application. Mechanical splices are required to have the bars connected to develop in tension or compression, as required, at least 125% of the specified yield strength of the bar.

Reinforcing bars can also be spliced using end-bearing splices, but only in members containing closed ties, closed stirrups or spirals for bars subject to compression only. End-bearing splices rely on the transmission of compressive stress by bearing of square cut ends held in concentric contact by a suitable device. The bar ends are required to terminate in flat surfaces within 1 ½ degrees of a right angle to the axis of the bars and be fitted within 3 degrees of full bearing after assembly.

2006 IBC Allowable Stress Design

With the publication of the 2006 IBC, which in turn references the 2005 MSJC (ref. 7), the majority of the splicing and development detailing requirements have remained the same, with the notable exception that Equation 2 has been superseded for determining the minimum length of lap splices.

While the 2005 MSJC includes an equation to determine development length, which is also used to determine lap splice length, the 2006 IBC modifies the MSJC lap splice length. In accordance with the 2006 IBC, the minimum required lap length for spliced reinforcing bars is determined using Equation 3. Table 4 lists tabulated values.

but not less than 12 in. (305 mm) or 40d_b, whichever is greater

Further, in regions of flexure where the design tensile stresses in the reinforcement, f_s, exceed 80% of the allowable steel tensile stress, F_s, the required length of lap determined by Equation 3 must be increased by 50%. Alternatively, equivalent means of stress transfer to accomplish the same 50% increase is permitted. Where epoxy coated bars are used, lap length is also required to be increased by 50%.

For wires, minimum development length is determined in accordance with Equation 1 (see Table 2). For reinforcing bars, however, the 2005 MSJC minimum development length was not modified by the IBC, and so is determined by Equation 4 (see Table 3).

but not less than 12 in. (305 mm)

For use in Equation 4, the reinforcement size factor, γ, is taken equal to 1.0 for No. 3 through No. 5 (M#10–M#16) reinforcing bars; 1.3 for No. 6 and No. 7 (M#19 and M#22) bars; and 1.5 for No. 8 through No. 11 (M#25–M#36) bars. When epoxy coated bars are used, the development length determined by Equation 4 is required to be increased by 50%.

STRENGTH DESIGN

2003 IBC Strength Design

By reference to the 2002 MSJC (ref. 5), the 2003 IBC requires the minimum development length and lap splice length for reinforcing bars designed by the strength method to be calculated by Equation 5 (see Table 7).

but not less than 12 in. (305 mm)

For use in Equation 5, the reinforcement size factor, γ, is taken equal to 1.0 for No. 3 through No. 5 (M#10–M#16) reinforcing bars; 1.4 for No. 6 and No. 7 (M#19 and M#22) reinforcing bars; and 1.5 for No. 8 and No. 9 (M#25 and M#29) reinforcing bars.

The 2003 IBC modifies the MSJC requirements for welded and mechanical splices as follows. The IBC stipulates that mechanical splices be classified as Type 1 or 2 according to Section 21.2.6.1 of ACI 318 (ref. 8). Type 1 splices may not be used within the plastic hinge zone nor within a beam-column joint of intermediate or special reinforced masonry shear walls or special moment frames.

Welded splices must be able to develop at least 125% of the bar’s specified yield strength in tension or compression, as required. Welded splices must use ASTM A706 (ref. 9) steel reinforcement. Welded splices are not permitted to be used in plastic hinge zones of intermediate or special reinforced walls nor in special moment frames of masonry.

2006 IBC Strength Design

For development and lap splice length requirements, the 2006 IBC references the 2005 MSJC (see Equation 6 and Table 6), but limits the maximum length to 72d_b.

but not less than 12 in. (305 mm) and not more than 72d_b,

For Equation 6, the reinforcement size factor, γ, is taken equal to 1.0 for No. 3 through No. 5 (M#10–M#16) reinforcing bars; 1.3 for No. 6 and No. 7 (M#19, M#22) bars; and 1.5 for No. 8 and No. 9 (M#25, M#29) bars. When epoxy coated bars are used, the development length determined by Equation 6 is required to be increased by 50%. Bars spliced by noncontact lap splices must be spaced no farther apart than one-fifth the required length of lap and no more than 8 in. (203 mm).

The 2006 IBC includes the same modifications for mechanical and weld splices listed under 2003 IBC Strength Design.

STANDARD HOOKS

Figure 1 illustrates the requirements for standard hooks, when reinforcing bars are anchored by hooks or by a combination of hooks and development length. Table 8 lists minimum dimensions and equivalent embedment lengths for standard hooks of various sizes. The equivalent embedment length for allowable stress design (l_e = 11.25d_b) is based on an allowable stress of 7,500 psi (51.7 MPa), the accepted permissible value in masonry design (ref. 10). To achieve this capacity, the hook itself must be embedded a sufficient amount to prevent breakout/pullout. It is recommended that a minimum of 2½ in. (630 mm) embedment over the hook be provided (ref. 8), unless a more detailed analysis is performed.

A combination of hook and development length must be used when the equivalent embedment length of the hook, l_e, is less than the required minimum development length, l_d. In this case, development length equal to (l_d – l_e) must be provided in addition to the hook. This additional development length is measured from the start of the hook (point of tangency with the main portion of the bar).

JOINT REINFORCEMENT SPLICES

While not currently addressed by the MSJC or IBC, wire and joint reinforcement is typically spliced 6 in. (152 mm) minimum to transfer shrinkage stresses. Slippage of the deformed side wires is resisted by surface bond as well as by mechanical anchorage of the embedded portions of the cross wires.

NOTATIONS:

D_i = min. inside diameter of bend for standard hooks, in. (mm)
d_b = nominal diameter of reinforcement, in. (mm)
K = the least of the masonry cover, 5d_b and the clear spacing between adjacent reinforcement, in. (mm)
F_s = allowable tensile stress in reinforcement, psi (MPa)
f’_m = specified compressive strength of masonry, psi (MPa)
f_s = calculated tensile or compressive stress in steel, psi (MPa)
f_y = specified yield strength of steel, psi (MPa)
l_d = embedment length or lap splice length of straight reinforcement, in. (mm)
l_e = equivalent embedment length provided by standard hooks measured from the start of the hook (point of tangency), in. (mm)
γ = reinforcement size factor
Φ = strength reduction factor

NCMA and the companies disseminating this technical information disclaim any and all responsibility and liability for the accuracy and the application of the information contained in this publication.