Reinforced Masonry and Structural Systems
Chapter Introduction
This chapter covers the fundamental principles of reinforced masonry, masonry structural systems, and the requirements of the National Building Code of Canada (NBC) as well as CSA A371 (Concrete Masonry — Site Construction) and CSA S304 (Design of Masonry Structures). For the Red Seal exam, you must master not only installation techniques, but also basic calculations, reinforcement details, compliance criteria, and common pitfalls related to load-bearing structures and shear walls.
1. Definitions and Fundamental Principles
1.1 Reinforced vs. Unreinforced Masonry
Reinforced masonry is an assembly of blocks, bricks, or stones in which steel bars (reinforcement) are embedded in mortar or grout to resist tensile, shear, and flexural forces. Unreinforced masonry relies solely on the compressive strength of the units and mortar.
| Characteristic | Unreinforced Masonry | Reinforced Masonry |
|---|
| Tensile resistance | None or negligible | Yes, through steel |
| Typical maximum height | Limited | Higher |
| Seismic resistance | Low | High (with ductility) |
| Common use | Interior walls, partitions | Load-bearing walls, foundation walls, multi-storey buildings |
1.2 Components of the Reinforced System
Vertical reinforcement: bars placed in block cavities or in grout-filled cells.
Horizontal reinforcement: bars placed in mortar joints (bed joints) or in special blocks (U-blocks, bond beams).
Grout: fluid concrete with fine coarse aggregate (6 mm to 10 mm) pumped into cavities to encase the steel and tie the elements together.
Cell or conduit: the reserved space in the block to receive the reinforcement and grout.
1.3 Mechanical Behaviour Principle
Reinforced masonry acts as a composite material: the masonry resists compression, while the steel resists tension. The collaboration is ensured by bond (steel-grout adhesion) and by end anchorage (hooks, embedment).
> Key Point for the Exam: The nominal flexural resistance of a reinforced wall is calculated according to reinforced concrete principles, but with resistance factors specific to masonry (φm = 0.60 for masonry, φs = 0.85 for steel per CSA S304).
2. Regulatory Requirements and Applicable Codes
2.1 CSA S304 — Design of Masonry Structures
CSA S304 is the reference standard for the design of reinforced and unreinforced masonry in Canada. It defines:
Design strengths of materials (blocks, mortar, grout, steel).
Ductility requirements for seismic zones.
Minimum wall thicknesses.
Maximum reinforcement spacings.
Key rules to know:
Clause 4.2.2: Compressive strength of masonry (f'm) determined by testing or tabulated values.
Clause 7.3: Minimum reinforcement in load-bearing walls (0.13% of gross cross-sectional area for reinforced walls).
Clause 7.4: Maximum spacing of vertical reinforcement: 4 times the wall thickness, not exceeding 1200 mm.
Clause 7.5: Maximum spacing of horizontal reinforcement: 3 times the wall thickness, not exceeding 800 mm.
2.2 CSA A371 — Site Construction
CSA A371 governs field installation: block placement, mortar preparation, grout placement, and quality control. Essential points:
Section 5.4: Grout must be placed in the cavities before the mortar takes its initial set (within 30 minutes of block placement).
Section 6.2: Reinforcing bars must be clean, free of loose rust, and positioned with minimum cover of 20 mm (interior) and 40 mm (exterior or exposed to weather).
Section 6.5: Vertical reinforcement must be held in position by approved supports or ties, spaced no more than 1200 mm vertically.
2.3 National Building Code of Canada (NBC)
The National Building Code of Canada (NBC) references CSA standards for technical details. Fire resistance and compartment separation requirements are defined in the NBC, but structural calculations fall under CSA S304.
> Practical Rule: For the exam, remember that the NBC requires fire walls with a minimum thickness of 140 mm for concrete blocks, with a fire-resistance rating (FRR) of 45 minutes to 4 hours depending on thickness and aggregate type.
3. Types of Masonry Structural Systems
3.1 Load-Bearing Wall (Vertical Load)
The load-bearing wall transmits floor and roof loads to the foundations. It must resist compression, buckling, and shear.
Maximum Axial Load Calculation (Simplified Formula):
Allowable load = φm × f'm × Net area × Slenderness reduction factor (k)
Where:
φm = 0.60
f'm = specified strength (e.g., 15 MPa for standard blocks)
Net area = block section minus unfilled voids
k = factor depending on the height-to-thickness ratio (H/t)
Example: Wall 190 mm thick, 3.0 m high, f'm = 15 MPa, net area = 0.12 m² per linear metre.
H/t = 3000 / 190 = 15.8 → k = 0.65 (CSA S304 table)
Allowable load = 0.60 × 15 × 10⁶ Pa × 0.12 m² × 0.65 = 702 kN/m
3.2 Shear Wall (Bracing)
Shear walls resist horizontal forces (wind, seismic). They must be reinforced in both directions and anchored to foundations and floors.
Key Requirements:
Minimum vertical reinforcement in boundary zones (edge elements).
Continuous horizontal reinforcement in bed joints or in special blocks.
Connection to floors using anchors (dowels) spaced no more than 1200 mm apart.
3.3 Retaining Wall (Lateral Loads)
Reinforced masonry retaining walls are common for foundations and below-grade walls. They must resist earth pressure (lateral pressure) and hydrostatic pressure.
Lateral Earth Pressure: p = K × γ × h
Where:
K = pressure coefficient (active: 0.33; at rest: 0.50)
γ = unit weight of soil (≈ 18 kN/m³)
h = height of retained soil
Example: Wall 2.5 m high, dry soil, K = 0.33.
Maximum pressure at base = 0.33 × 18 × 2.5 = 14.85 kN/m²
Total thrust = ½ × 14.85 × 2.5 = 18.56 kN/m
3.4 Curtain Wall and Infill Wall
Curtain walls (non-load-bearing) are attached to the main structure. They must be designed for wind loads and differential movements (expansion, settlement). Expansion joints must be provided every 6 to 12 m depending on block type and climatic conditions.
4. Reinforcement Details and Construction Arrangements
4.1 Vertical Reinforcement
Minimum diameter: 15M (15 mm) for reinforced load-bearing walls.
Maximum diameter: 25M (25 mm) to avoid grout placement problems.
Maximum spacing: 1200 mm (or 4 × wall thickness).
Base anchorage: bars must be embedded in the foundation with a standard hook of 90° or 180° (development length per CSA S304, Clause 8.3).
4.2 Horizontal Reinforcement
Minimum diameter: 10M (10 mm).
Maximum spacing: 800 mm (or 3 × wall thickness).
Placement: in bed joints (mortar) or in U-blocks filled with grout.
Lap splice: 40 × diameter for bars in tension (e.g., 40 × 10 mm = 400 mm).
4.3 Grout and Cover
| Element | Minimum Cover (mm) |
|---|
| Vertical reinforcement, interior | 20 |
| Vertical reinforcement, exterior (exposed) | 40 |
| Horizontal reinforcement in joint | 15 |
| Reinforcement in contact with soil | 50 |
4.4 Development Length and Lap Splices
The development length (ld) is the anchorage length required to transfer the full steel force to the masonry. It depends on bar diameter, grout strength, and position (horizontal or vertical).
Simplified formula (CSA S304):
ld = 0.45 × db × fy / (k × √f'm)
Where:
db = bar diameter (mm)
fy = steel yield strength (400 MPa for 400R bars)
k = 1.0 for vertical bars, 1.3 for horizontal bars (poorer bond conditions)
Example: 15M bar (db = 15 mm), fy = 400 MPa, f'm = 15 MPa, vertical.
ld = 0.45 × 15 × 400 / (1.0 × √15) = 2700 / 3.87 = 698 mm → round up to 700 mm
5. Practical Calculations for the Exam
5.1 Compressive Resistance Check
The axial resistance of a reinforced wall is given by:
Pr = φm × 0.80 × f'm × (Ae – As) + φs × fy × As
Where:
Ae = effective area of the section (mm²)
As = area of steel (mm²)
0.80 = reduction factor for eccentricity and imperfections
Example: Wall 190 mm × 1000 mm (1 m length), f'm = 15 MPa, As = 400 mm² (2–15M bars), fy = 400 MPa.
Ae = 190 × 1000 = 190,000 mm²
Pr = 0.60 × 0.80 × 15 × (190,000 – 400) + 0.85 × 400 × 400
Pr = 0.60 × 0.80 × 15 × 189,600 + 136,000
Pr = 1,365,120 + 136,000 = 1,501,120 N = 1501 kN
5.2 Flexural Check (Resisting Moment)
For a reinforced wall subjected to a bending moment (e.g., earth pressure), the resisting moment is:
Mr = φs × As × fy × (d – a/2)
Where:
d = distance from extreme compression fibre to the centre of reinforcement (mm)
a = depth of compression block = (φs × As × fy) / (0.85 × φm × f'm × b)
Example: Wall 190 mm, d = 150 mm, As = 400 mm², fy = 400 MPa, f'm = 15 MPa, b = 1000 mm.
a = (0.85 × 400 × 400) / (0.85 × 0.60 × 15 × 1000) = 136,000 / 7650 = 17.8 mm
Mr = 0.85 × 400 × 400 × (150 – 17.8/2) = 136,000 × 141.1 = 19,189,600 N·mm = 19.2 kN·m
5.3 Shear Check
The shear resistance of a reinforced wall is:
Vr = φm × (vm × b × d + 0.25 × N / d + φs × Av × fy × d / s)
Where:
vm = masonry shear strength (≈ 0.16 × √f'm MPa)
N = axial load (N)
Av = area of stirrups or horizontal reinforcement (mm²)
s = spacing of horizontal reinforcement (mm)
Example: Wall with N = 100 kN, b = 190 mm, d = 150 mm, f'm = 15 MPa, Av = 100 mm² (10M bar), s = 400 mm.
vm = 0.16 × √15 = 0.62 MPa
Vr = 0.60 × (0.62 × 190 × 150 + 0.25 × 100,000 / 150 + 0.85 × 100 × 400 × 150 / 400)
Vr = 0.60 × (17,670 + 1667 + 12,750) = 0.60 × 32,087 = 19,252 N = 19.3 kN
6. Installation and Quality Control
6.1 Grout Preparation and Placement
Grout must have a fluid consistency (slump of 200 to 250 mm on the Abrams cone).
Grouting is done in lifts of 1.2 m maximum, with rodding or vibration to ensure complete filling.
Grout must be placed within 30 minutes of block placement (CSA A371, Section 5.4).
In hot weather (> 30 °C), grout must be placed within 20 minutes.
6.2 Reinforcement Inspection
Verify the positioning of bars before grouting (cover, spacing).
Verify lap splices: minimum length of 40 × db for bars in tension.
Verify hooks: 90° or 180° angle, tail length = 12 × db.
Bars must be free of rust or any substance that would reduce bond.
6.3 Testing and Sampling
| Test | Standard | Frequency |
|---|
| Mortar strength (cubes) | CSA A179 | 1 set per day or per 50 m³ |
| Grout strength (cubes) | CSA A371 | 1 set per day or per 10 m³ |
| Grout slump | CSA A23.2 | Each delivery |
| Block strength | CSA A165 | 1 set per lot of 10,000 blocks |
7. Pitfalls to Avoid
Here are the most frequent errors on the Red Seal exam on this topic:
146.Confusing resistance factors: φm = 0.60 for masonry, φs = 0.85 for steel. Do not mix them up.
147.Forgetting the 0.80 factor in the axial resistance calculation (eccentricity and imperfections).
148.Using gross area instead of net area for unfilled hollow blocks.
149.Neglecting minimum cover: 20 mm interior, 40 mm exterior, 50 mm in contact with soil.
150.Confusing maximum vertical and horizontal spacing: 1200 mm vertical, 800 mm horizontal.
151.Forgetting that grout must be placed within 30 minutes of block placement.
152.Using the shear formula without including the axial load contribution (the term 0.25 × N / d).
153.Not checking the development length for horizontal bars (factor k = 1.3).
154.Confusing units: convert MPa to N/mm² (1 MPa = 1 N/mm²).
155.Ignoring seismic requirements: in seismic zones, reinforcement spacing is reduced (600 mm max vertical, 400 mm max horizontal).
8. Summary
Reinforced masonry combines the compressive strength of masonry with the tensile strength of steel.
The reference standards are CSA S304 (design) and CSA A371 (construction).
Vertical reinforcement is spaced no more than 1200 mm apart; horizontal reinforcement no more than 800 mm.
Grout must be fluid, placed in lifts of 1.2 m, and within 30 minutes of placement.
Resistance calculations use the factors φm = 0.60 and φs = 0.85.
Axial resistance includes a 0.80 factor for imperfections.
Shear resistance includes the contribution of axial load and horizontal reinforcement.
Minimum covers are 20 mm (interior), 40 mm (exterior), and 50 mm (soil).
Lap splices are 40 × db minimum for bars in tension.
Quality control tests cover mortar, grout, and blocks, with frequencies defined by CSA standards.
9. Self-Assessment Questions
170.What is the fundamental difference between reinforced and unreinforced masonry?
171.What are the resistance factors for masonry and steel per CSA S304?
172.What is the maximum spacing of vertical reinforcement in a reinforced load-bearing wall?
173.What is the development length of a vertical 15M bar (fy = 400 MPa, f'm = 15 MPa)?
174.A wall 190 mm thick, 3.0 m high, f'm = 15 MPa, is subjected to a load of 500 kN/m. Verify whether the resistance is sufficient (Ae = 0.12 m²/m, As = 400 mm²/m).
175.What is the minimum cover for reinforcement in contact with soil?
176.Within what time must grout be placed after block placement?
177.What are the three terms that make up the shear resistance of a reinforced wall?
10. Normative References
CSA S304: Design of Masonry Structures
CSA A371: Concrete Masonry — Site Construction
CSA A165: Concrete Masonry Blocks
CSA A179: Mortar and Grout for Masonry
National Building Code of Canada (NBC): General requirements and fire resistance
> Final Advice: For the exam, memorize the key numerical values (φm, φs, spacings, covers, time limits) and practice solving axial and flexural resistance calculations using the simplified formulas. Precision in units and adherence to the coefficients are the most heavily evaluated criteria.
