Tying and Supporting Reinforcing Steel

Attaching and Supporting Reinforcement

Chapter Introduction

Attaching and supporting reinforcement is a crucial step in the placement of rebar. Poorly tied or inadequately supported reinforcement can shift during concrete placement, compromising the structural position of the bars and the durability of the structure. This chapter covers all the theoretical and practical knowledge required for the Red Seal exam, including tie types, supports, dimensional tolerances, lap splice calculations, and the requirements of the National Building Code of Canada (NBC) and CSA standards.

2. Role and Fundamental Principles of Tying

2.1 Why Tie Reinforcement?

Tying reinforcing bars serves three essential functions:

8.Position maintenance: preventing bar displacement during concrete placement and vibration.

9.Structural assembly: ensuring mechanical continuity at intersections and lap splices.

10.Corrosion protection: ensuring compliance with prescribed minimum concrete covers.

Poorly tied reinforcement can shift several centimetres, reducing the effective depth (d) of the section and compromising the load-bearing capacity of the beam or slab.

2.2 Basic Principles of Good Tying

Strength: the tie must resist handling forces, worker foot traffic, and the pressure of fresh concrete.

Speed of execution: tying time is a major productivity factor on site.

Material economy: use the appropriate gauge of tie wire without excess.

Compatibility: the wire must be compatible with the reinforcing steel (no galvanic corrosion).

2.3 Types of Tie Wire

Wire Type	Gauge (AWG)	Diameter (mm)	Typical Use
Black annealed wire	16	1.6	General use, single ties
Black annealed wire	15	1.8	Double ties, heavy reinforcement
Black annealed wire	14	2.0	Very heavy reinforcement, columns
Galvanized wire	16	1.6	Corrosive environments, stainless steel
Stainless steel wire	16	1.6	Stainless steel, marine environments

Black annealed wire is the most common. Annealing makes the wire malleable, allowing easy bending and effective tightening. Galvanized wire is used when reinforcement is exposed to moisture before concreting or in corrosive environments.

2.4 Tying Tools

Manual tying pliers: basic tool, allows precise control of tension.

Automatic tying tool (rebar tying gun): increases productivity by 3 to 4 times, ideal for large slab areas.

Rebar hook: used for twist ties.

Cutting pliers: for cutting excess wire.

3. Tying Techniques

3.1 Snap Tie

The snap tie consists of crossing the wire diagonally over the intersection of two bars and twisting it half to three-quarters of a turn. This technique is fast but offers limited strength. It is suitable for light reinforcement (bars ≤ 15M) in slabs and walls.

Procedure:

31.Cut a piece of wire 300 to 450 mm in length.

32.Bend the wire in half around the intersection.

33.Cross the ends diagonally.

34.Twist with the pliers until firmly tight (do not over-tighten to avoid weakening the wire).

35.Fold down the ends to prevent them from puncturing the formwork or injuring workers.

3.2 Wrap and Snap Tie

The wrap and snap tie makes a full turn around the intersection before twisting. It offers superior strength and is used for medium reinforcement (20M to 25M) and areas with high reinforcement density.

3.3 Figure-Eight Tie

The figure-eight tie wraps both bars perpendicularly, forming a figure-eight pattern. This technique is used for columns and deep beams where stability is critical. It prevents lateral sliding of the bars.

3.4 Lift Tie

Used for pre-assembled reinforcement cages that must be lifted by crane. The lift tie is reinforced with additional turns of wire and sometimes special metal ties.

3.5 Continuous Tie

For long runs of parallel bars (walls, slabs), the continuous tie uses a single continuous wire that zigzags between intersections. This method is economical but less rigid than individual ties.

4. Reinforcement Supports

4.1 Function of Supports

Supports (or chairs) hold the reinforcement at the required height to ensure the minimum concrete cover prescribed by the standards. Insufficient cover exposes the steel to corrosion and reduces fire resistance. Excessive cover reduces the effective depth of the section.

4.2 Types of Supports

Support Type	Material	Use
Concrete block	Precast concrete	Slabs on grade, foundations
Plastic chair	Polypropylene	Slabs, walls, covers 20–75 mm
Wire bar chair	Galvanized or stainless steel	Slabs, beams, columns
Continuous bar chair	Steel	Long spans, thick slabs
High chair	Steel	Thick slabs, top reinforcement
U-chair	Steel	Beams, top reinforcement
Spring chair	Steel	Walls, vertical reinforcement

4.3 Support Spacing

The maximum support spacing depends on the bar diameter and slab thickness. The following table gives typical values for slabs cast on grade:

Bar Diameter	Slab Thickness (mm)	Maximum Spacing (mm)
10M	≤ 150	750
10M	150–250	600
15M	≤ 150	900
15M	150–250	750
20M	≤ 200	1200
20M	200–300	900

For suspended slabs, spacing is generally reduced by 25 to 30%.

4.4 Supports for Top Reinforcement

The top reinforcement of a slab (shrinkage or negative reinforcement) must be supported by high chairs or continuous bar supports. The density of these supports must be sufficient to prevent sagging under the weight of workers and concrete.

Rule of thumb: for a 200 mm thick slab with 15M top reinforcement, use high chairs spaced at 900 mm in both directions.

4.5 Supports for Column Reinforcement

Vertical column bars must be held in position using plastic chairs or wire supports attached to the stirrups. The vertical spacing of supports must not exceed 1.2 m.

5. Tolerances and Code Requirements

5.1 Position Tolerances

The National Building Code of Canada (NBC) and CSA A23.1 (Concrete: Constituent Materials and Execution of Work) prescribe the following tolerances:

Parameter	Tolerance
Nominal concrete cover	± 10 mm (if cover ≤ 75 mm)
Nominal concrete cover	± 15 mm (if cover > 75 mm)
Bar spacing	± 15 mm
Vertical position in slabs	± 10 mm
Horizontal position in walls	± 15 mm
Effective depth (d)	± 10 mm

5.2 Minimum Concrete Covers (CSA A23.1, Table 16)

Element	Minimum Cover (mm)
Concrete cast on ground (slab on grade)	75
Concrete exposed to weather (walls, beams)	50
Concrete not exposed to weather (interior)	40
Concrete exposed to water or soil	75
Precast concrete (interior)	25
Precast concrete (exterior)	40
Columns (interior)	40
Columns (exterior)	50

5.3 Lap Splices (CSA A23.1, Clause 12.5)

The lap splice length (overlap) of bars depends on the diameter, steel grade, exposure class, and distance between bars.

Simplified formula for tension lap splices:

L_splice = 0.5 × L_d × factor

Where:

L_d = development length (mm)

factor = 1.3 for Class A splices (50% of bars spliced at the same location)

factor = 1.7 for Class B splices (more than 50% of bars spliced)

Typical L_d values for 400 MPa steel (15M to 25M bars):

Diameter	30 MPa Concrete	35 MPa Concrete
10M	300 mm	270 mm
15M	450 mm	400 mm
20M	600 mm	540 mm
25M	750 mm	675 mm
30M	900 mm	810 mm

5.4 Standard Hooks (CSA A23.1, Clause 12.4)

Hook Type	Angle	Development Length
90° hook	90°	12 × d_b (minimum)
135° hook	135°	12 × d_b (minimum)
180° hook	180°	4 × d_b (minimum) + extension

Where d_b = bar diameter.

Example: for a 20M bar (d_b = 19.5 mm), a 90° hook requires a development length of 12 × 19.5 = 234 mm, rounded up to 240 mm.

6. Practical Calculations for Tying and Support

6.1 Tie Wire Quantity

Tie wire consumption depends on the element type and reinforcement density. The following values are averages for estimation:

Element	Consumption (kg of wire / tonne of steel)
Slab on grade	8 to 12
Suspended slab	12 to 16
Wall	10 to 14
Beam	14 to 18
Column	16 to 20
Foundation	8 to 10

6.2 Number of Ties per Intersection

The number of ties required depends on the configuration:

Configuration	Ties per Intersection
Slab (single mat)	1 (snap tie)
Slab (double mat)	2 (wrap and snap ties)
Wall (single mat)	1
Wall (double mat)	2
Beam (stirrups on longitudinal bars)	1 per stirrup
Column (stirrups on vertical bars)	1 per stirrup

6.3 Calculating Wire Length per Tie

The wire length required for a snap tie is approximately:

L_wire = 4 × (d_b1 + d_b2) + 150 mm

Where d_b1 and d_b2 are the diameters of the bars at the intersection.

Example: intersection of 15M (d_b = 16 mm) and 20M (d_b = 19.5 mm) bars:

L_wire = 4 × (16 + 19.5) + 150 = 4 × 35.5 + 150 = 142 + 150 = 292 mm

Round up to 300 mm.

6.4 Calculating Support Spacing

For a slab of thickness h with top reinforcement of diameter d_b, the maximum support spacing is:

S_max = (h - 2 × cover - d_b) / 2

Example: 250 mm slab, 40 mm cover, 15M bars (16 mm):

S_max = (250 - 2 × 40 - 16) / 2 = (250 - 80 - 16) / 2 = 154 / 2 = 77 mm

This value is theoretical; in practice, spacing is governed by manufacturer recommendations and site loading conditions.

7. Specific Requirements for Different Elements

7.1 Slabs on Grade

Reinforcement must be supported by concrete blocks or plastic chairs at a height ensuring the minimum 75 mm cover.

Supports must be placed at every bar intersection in high-load areas.

Top reinforcement (mesh or bars) must be supported by high chairs spaced at 900 mm maximum.

7.2 Suspended Slabs

Bottom reinforcement is supported by bar chairs spaced at 750 to 900 mm.

Top (negative) reinforcement requires high chairs with distribution rails (continuous high chair) to spread the load.

Supports must be re-checked after worker foot traffic.

7.3 Walls

Vertical reinforcement is held by spring chairs or plastic chairs attached to the horizontal bars.

Vertical support spacing is 600 to 900 mm.

Horizontal bars are tied to vertical bars at every intersection.

7.4 Beams

Bottom reinforcement is supported by bar chairs or concrete blocks.

Top reinforcement (if present) is supported by U-chairs or high chairs.

Stirrups are tied to longitudinal bars at every intersection.

7.5 Columns

Vertical bars are held by plastic chairs attached to the stirrups.

Stirrup spacing follows the drawings; maximum spacing is generally 300 mm in seismic zones.

Ties must be double ties to resist vibration forces.

8. Quality Control and Inspection

8.1 Pre-Pour Checks

Before concrete placement, the inspector must verify:

127.Bar position: diameter, spacing, alignment.

128.Concrete cover: measure with a gauge or rebar detector.

129.Lap splices: length and position of overlaps.

130.Ties: correct number and type, adequate tightness.

131.Supports: spacing, type, stability.

132.Stirrups: spacing, hook angle (135° in seismic zones).

133.Cleanliness: no excessive rust, oil, or dirt.

8.2 Inspection Tolerances

Parameter	Inspection Tolerance
Bar spacing	± 15 mm
Concrete cover	± 10 mm (cover ≤ 75 mm)
Lap splice position	± 50 mm
Lap splice length	+ 50 mm / - 0 mm
Column bar verticality	± 10 mm per metre

8.3 Critical Control Points

Seismic zones: stirrups must have 135° hooks with a 6 × d_b extension (minimum 75 mm).

High corrosion zones: increase cover by 10 to 15 mm.

Construction joints: reinforcement must be continuous through the joint or spliced according to the drawings.

9. Site Safety

9.1 Hazards Related to Tying and Support

Cuts: un-folded wire ends can cause lacerations.

Impalement: unprotected vertical bars pose a fall hazard.

Falls: working on elevated reinforcement without protection.

Collapse: an improperly stabilized reinforcement cage can collapse.

9.2 Mandatory Safety Measures

Wear protective gloves and safety glasses.

Fold all wire ends toward the inside of the reinforcement.

Install bar protectors (caps) on exposed vertical bars.

Use work platforms for elevated areas.

Verify cage stability before releasing from lifting supports.

10. Pitfalls to Avoid

Here are the most frequent errors made by Red Seal exam candidates:

156.Confusing cover and lap splice: cover is the distance between the concrete surface and the bar; a lap splice is the overlap of two bars.

157.Forgetting the class factor for lap splices: a Class B splice (1.7 × L_d) is required when more than 50% of bars are spliced at the same location.

158.Using the wrong support type: plastic chairs are not suitable for thick slabs (> 300 mm); use steel supports.

159.Neglecting support spacing for top reinforcement: top reinforcement sags if supports are too far apart.

160.Over-tightening the wire: excessive tightening weakens the wire and can damage high-strength reinforcing steel.

161.Ignoring cover tolerances: a 40 mm cover instead of 50 mm can lead to premature corrosion.

162.Confusing nominal and actual diameters: a 15M bar has an actual diameter of 16 mm, not 15 mm.

163.Forgetting 135° hooks in seismic zones: 90° hooks are not acceptable in seismic zones.

164.Not checking cage stability before pouring: the weight of concrete can displace an improperly stabilized cage.

165.Using galvanized wire with black steel: galvanic corrosion can occur in certain environments.

11. Summary

Key Point	Detail
Tie types	Snap, wrap and snap, figure-eight, lift, continuous
Tie wire	Black annealed (general use), galvanized (corrosion), stainless steel (marine environments)
Supports	Concrete, plastic, wire bar chair, high chair, U-chair, spring chair
Minimum cover	75 mm (grade), 50 mm (exterior), 40 mm (interior)
Tolerances	Cover ± 10 mm, spacing ± 15 mm, position ± 10 mm
Lap splices	Class A: 1.3 × L_d, Class B: 1.7 × L_d
Hooks	90°: 12 × d_b, 135°: 12 × d_b, 180°: 4 × d_b
Support spacing	750–900 mm for slabs, 600–900 mm for walls
Reference standards	CSA A23.1, NBC (National Building Code of Canada)

Key Points to Remember for the Exam

170.Tying maintains bar position and ensures structural continuity.

171.Black annealed 16-gauge wire is the most common.

172.Minimum cover is 75 mm for concrete cast on ground.

173.Cover tolerances are ± 10 mm for covers ≤ 75 mm.

174.Class B lap splices require 1.7 × L_d.

175.135° hooks are mandatory in seismic zones.

176.Support spacing depends on bar diameter and slab thickness.

177.High chairs are required for top reinforcement in slabs.

178.Wire consumption is 8 to 20 kg per tonne of steel depending on the element.

179.Safety requires folding down wire ends and protecting vertical bars.

12. Self-Assessment Questions

182.What is the cover tolerance for a nominal cover of 50 mm?

Answer: ± 10 mm (because ≤ 75 mm)

184.What type of support is used for the top reinforcement of a 250 mm slab?

Answer: Steel high chair

186.What is the development length of a 90° hook for a 25M bar (d_b = 25.2 mm)?

Answer: 12 × 25.2 = 302.4 mm, rounded to 305 mm

188.How many ties per intersection for a single-mat slab?

Answer: 1 snap tie

190.What is the minimum cover for an exterior wall exposed to weather?

Answer: 50 mm

192.What is the typical wire consumption for a beam?

Answer: 14 to 18 kg per tonne of steel

194.What type of hook is mandatory in seismic zones?

Answer: 135° hook with a 6 × d_b extension (minimum 75 mm)

196.What is the vertical position tolerance in slabs?

Answer: ± 10 mm

13. Normative References

CSA A23.1: Concrete: Constituent Materials and Execution of Work (Clauses 12.4, 12.5, Table 16)

CSA A23.2: Test Methods and Standard Practices for Concrete

National Building Code of Canada (NBC): Part 4 (Structural Design)

CSA G30.18: Steel Reinforcing Bars (Rebar)

CSA S16: Design of Steel Structures (for composite connections)

These standards are cited in the Red Seal exam; it is essential to know their main requirements, particularly the cover tables and development lengths.

This chapter covers all the knowledge required for the "Attaching and Supporting Reinforcement" section of the Red Seal exam. The numerical values conform to current CSA standards. For project-specific values, always refer to the drawings and specifications.