5.3 Bar Supports, Wire Chairs, Bolsters & Precast Concrete Dobies (CRSI Standards)

5.3 Bar Supports, Wire Chairs, Bolsters & Precast Concrete Dobies

Bar supports establish reinforcement elevation or lateral position. Ties hold bars in their intended arrangement. A support can be the correct nominal height while the cage still moves if the support tips, settles into the base or spans too far between bearing points. Inspect the installed assembly before placement and check the approved procedure for keeping it in position during placement.

1. Match the support to its function

Under specified ACI 301-20 Section 3.2.1.8, support types must meet the contract requirements and conform to CRSI RB4.1. Section 3.3.2.4 covers their installation. Use the approved product’s capacity, height and application. Consider the support material and whether its contact points will affect the finished surface or the membrane underneath.

CRSI describes concrete block supports with or without tie wires. Match the furnished product to the project’s requirements for attachment and material. If the support schedule requires a concrete block of a particular strength, check its documentation. A brick, loose stone or other improvised block is not evidence of conformity to the specified support. Verify the disposition of proposed substitutions.

Worked example: Support height and the nearest steel

A footing bottom is cast against earth and the approved detail requires 3 inches of cover. A rectangular support block is 2 by 3 by 4 inches and is approved for that position. If it rests flat with the 3-inch dimension vertical and bears directly against the closest steel, that geometry gives 3 inches of cover before any settlement. If the closest steel hangs 1/2 inch below the bar resting on the block, the actual cover is $3-0.5=2.5$ inches. Inspect the nearest steel and the actual bearing condition. Compare the observation to the specified dimension and the applicable placement criteria. Follow the approved support detail rather than choosing a tolerance limit as a target.

2. Match spacers and ties to the reinforcement

ACI 301-20 Section 3.3.2.4 specifies epoxy- or polymer-coated tie wires for epoxy-coated and dual-coated reinforcement. In walls with those coatings, use the specified epoxy-coated, polymer-coated or composite spacers between reinforcement layers. For stainless steel bars the provision permits stainless steel, plastic or precast concrete bar supports, with stainless or plastic-coated tie wire. Inspect the supplied supports and fasteners and check for coating damage before concrete is placed.

CRSI illustrates several tying methods. A snap tie is commonly used in flat horizontal work. A wrap-and-snap tie adds a wrap to help secure a crossing on a vertical assembly. A saddle or wrap-and-saddle tie can secure bars to stirrups or ties. The figure-eight tie is another option for heavier assemblies. Use the approved tie arrangement and check that the bar position is stable. A count of wire ties alone cannot establish that the reinforcement will remain in its specified location.

3. Check spacing, access and placement stability

ACI 301-20 Section 3.3.2.1 requires reinforcement to be placed, supported and fastened to maintain its location during concrete placement. Verify the approved support layout and the measured bar position between supports, especially at openings, lap groups and mat edges. Section 3.3.2.5 separately addresses welded wire reinforcement. For welded wire reinforcement lighter than W4.0 or D4.0 in elevated formed slabs under Section 3.3.2.5(b), where that subsection applies, continuous support spacing cannot exceed 12 inches perpendicular to the span.

Inspect support bearing before and after other trades pass through the area. Check for displaced spacers, damaged retarder, loosened ties and conflicts with sleeves. Record the specified and observed bar position. If a support fails or steel shifts before concealment, report the condition to the contractor and verify the corrected geometry and support arrangement.