WWR Supports

Project specifications should indicate the requirements for properly supporting the reinforcement. Various types of supports for WWR are commercially available. The types of supports include wire and welded wire supports, individual high chairs with plates, bolsters with plates, all-plastic supports, concrete blocks, and others. Individual high chairs or bolsters without plates may be used with a firm sub-base or mud mat.

Position of Reinforcement and Cover

The proper position for the steel reinforcement is a decision based on the design itself and is controlled by the intended function of the steel. When one layer of reinforcement is used, then it should be located at or above the mid-depth of the slab. Some architects/ engineers require that the single layer be placed 2 inches below the top surface of the slab. In thicker slabs, the reinforcement must be low enough so that it will not interfere with saw cutting. Others recommend that the layer be placed at one-third the depth below the top surface. Any of these locations can be the appropriate choice, depending on the design concept — for example, whether the slab is reinforced for crack control, or is reinforced for structural reasons, or designed for shrinkage-compensating concrete. When one layer of reinforcement is used, it should not be allowed to be below mid-depth. In general, positioning at one-third the depth below the surface is sufficient.

When two layers of reinforcement are used, the question of cover applies to both layers. The upper layer should be placed at least 1 inch below the top surface of the slab. The specified depth of any saw cut must also be considered and the upper layer placed below that saw cut. In the case of the lower layer, when the concrete slab is placed on a well-constructed base course (normally graded, compacted and porous), many who design floors consider 1-1/2˝ of clear cover below the steel to be adequate. Additional cover should not be necessary unless the governing building code requires a bottom cover of up to 3 inches.

 Common Support Types

Continuous support (used on firm subbase or mud mat)	Slab bolster with base plate
All plastic high chair	Continuous support (two layers)
Wired block (generally used on sloping grade)	High chair with base plate
Plain concrete block	All plastic support (two layers)
High chair with base plate

Slab on Grade Base Conditions on Selection of Supports

Most manufacturer’s brochures will indicate the base surfaces required for their products. The ACI Committee 360 report recommends a graded granular fill, appropriate for compaction and trimming, as the base material for slabs-on-grade. Gravel bases, when compacted, fit this description. Compacted granular fill allows a greater variety of supports for consideration due to the inherent strength and stability of gravel. The so-called sand cushion (a few inches of uniformly graded sand) is usually not stable or stiff, and thus demands the use of supports with base plates or concrete blocks. The supports must not penetrate the base (subgrade) during the construction process because the specified position of the reinforcing steel could be changed and its beneficial effect diminished. When polyethylene sheeting is used under a slab, the selected supports must not puncture the sheeting. Other materials may be required under a slab due to special circumstances or conditions. For example, the floors in cold storage or freezer warehouses are usually placed upon insulation boards. The selected supports must not penetrate the insulation board.

Spacing and Strength of Supports

Generally supports are spaced 2 to 6 (or more) feet apart, depending on the stiffness and weight of the WWR being supported. Between the supports, the reinforcement must not deflect or sag excessively. While there are no criteria for limiting this deflection, the reinforcement must not deflect beyond any required clearances. There is limited information available on requirements for support spacings for welded wire reinforcement. There are several factors to consider before determining support spacings. These factors include the diameter and spacing of the reinforcement (larger wire diameters with wider support spacings will allow workers to step through rather than on the reinforcement); and general recognition of any construction loads that will be applied before and during concrete placement. The welded intersections of WWR provide a very rigid sheet of reinforcement. The suggested spacings of supports in Table 1 may be used for estimating and construction. However, the preceding factors should be considered.

TABLE 1. SUGGESTED SPACINGS OF SUPPORTS

WWR Range	WWR Spacing	Suggested Support Spacing
W OR D9 OR LARGER*	12" AND GREATER	4-6 FT.
W OR D5 TO W OR D8	12" AND GREATER	3-4 FT.
W OR D9 AND LARGER	LESS THAN 12"	3-4 FT.
W OR D4 TO W OR D8	LESS THAN 12"	2-3 FT.
LESS THAN W OR D4**	LESS THAN 12"	2-3 FT. OR LESS

*Spacing of supports for WWR with wires larger than W or D9 could possibly be increased over the spacings shown depending on the construction loads applied.
** Consider using additional rows of supports when large deflections or deformations occur — also spacing of supports may be increased provided supports are placed and properly positioned as concrete is screeded.  

For welded wire reinforcing sheets, spacings of the individual wires should be a consideration to avoid permanent displacement due to workers walking on the reinforcement. This spacing should be 12 inches or more (up to 18 inches may be specified). If the design requirements do not allow larger spacings, then the wire stiffness and the support strength and spacings must be adequate to carry all anticipated construction loads. The strength of the supports and their spacings required to carry construction loads, other equipment and workers must also be considered. There are no exact guidelines, but the requirement for strength and stability cannot be ignored.

Source for this website page: - The WRI Institute
Publication Tech Facts - TF 702-R-03