Tying and Splicing Requirements for Welded Wire Reinforcement

Smooth Welded Wire Reinforcement
Because the surface of plain wire is smooth the building codes do not assign any bond strength value to lengths of plain wire. Therefore, lap splices of plain WWR depend solely on the welded intersections for bond and development. The plain WWR lap splice is made by overlapping one complete cross wire space plus 2 inches. The splice is then wire tied sufficiently to prevent the two adjoining sheets from displacing during concrete placement. Tying requirements vary widely with the wire size and spacing of the WWR, but it is common practice to tie at every 3 rd welded intersection, or about every 3 feet, whichever is less.

The ACI 318 lap splice length for plain WWR is one space plus 2 inches, minimum of 6 inches or 1.5 times the development length, whichever is greater. Large wires at close spacings necessitate longer development lengths and therefore the lap splice lengths are also longer (see ACI 318 Sections 12.8 and 12.19 for development length and lap splice design information).

The very nature of the splice creates inefficiencies within the splice zone. Larger wire spacings can create longer splice lengths, but there is a solution. Remember, the requirement of “one spacing of cross wires plus 2 inches” means two welded intersections from each sheet are required to be within the splice zone. The splice zone can be shortened by moving these two welded intersections closer together. To accomplish this, the spacing is reduced at the edges of the sheet. See the example shown in the figure.

The area of steel within the splice zone can be more than twice that of the rest of the WWR sheet. Again, there is a solution. Smaller wires replace the two edge wires at each side of the sheet. These wires must be at least 40% of the area of the wire welded in the opposite direction. In the example, W4.4 wires are used to replace the W11.0 wires at the edges of the sheets. These smaller edge wires combined with the reduced spacing resulted in a 58% reduction of material within the splice zone in this example.

Deformed Welded Wire Reinforcement
Deformed WWR sheets can be spliced by overlapping welded intersections (called a “welded deformed wire reinforcement lap splice” in ACI 318-05), or by simply lap splicing the straight sections of overhanging wires at the perimeter of the sheets (called a “wire lap splice” in ACI 318-05). ACI 318 Chapter 12 provides for both types of lap splices for use in structural reinforced concrete applications.

The “welded deformed wire reinforcement splice” takes advantage of the additional anchorage provided by the welded intersections in conjunction with the bond provided by the deformations along the surface of the wire. Unlike plain WWR splices, deformed WWR splices are required to have only one welded intersection from each sheet within the splice zone. The welded transverse wires must overlap one another by at least 2 inches and have sufficient wire length beyond the welded intersection to satisfy the minimum calculated splice length. Efficiencies can be gained by reducing the size of the transverse wires within the splice zone as long as the minimum cross sectional area of steel is provided in the transverse direction. This can be a very economical splice but can also create build-up at the splice zone, particularly where four sheets meet at one place. It is recommended that adjacent rows of sheets be offset slightly to avoid unnecessary build-up at the splice zone.

The “deformed wire splice” does not require the overlapping of welded intersections, nor does it require that a welded intersection be in the splice zone at all. The splice is designed identically to a rebar lap splice (for #6 bars and smaller). Deformed wire splices are longer than deformed WWR splices but the build-up can be reduced or totally eliminated when the WWR sheets are placed in the proper sequence.