Structural Glazing and Curtain Walls

Chapter Introduction

Structural glazing and curtain walls are among the most technical areas of the glazier trade. These systems are not simply windows: they form the complete building envelope and must withstand wind, thermal, seismic, and gravitational loads while ensuring water and air tightness. For the Red Seal exam, you must master the engineering principles, installation procedures, tolerances, and applicable Canadian standards. This chapter covers all required knowledge, with an emphasis on common pitfalls and checkpoints.

Definitions and Essential Terminology

Curtain Wall

A curtain wall is a non-load-bearing exterior building envelope. It supports only its own weight and the loads directly applied to it (wind, accumulated snow, seismic loads), then transfers them to the main structure through anchors. It does not contribute to the building's structural stability. The fundamental distinction: the curtain wall is suspended from the structure, whereas a load-bearing wall supports the floors above.

Structural Glazing

Structural glazing is an installation method where glass is attached to the frame (typically aluminum) using a structural silicone sealant. This sealant transfers wind and glass weight loads to the structure. There is no visible mechanical pressure (screws, bolts) from the exterior; the glass appears to float on the facade. This technique is often integrated into curtain walls but can also be used on individual windows or skylights.

Key Terms to Know

Term	Definition
Mullion	Main vertical member of a curtain wall
Transom	Horizontal member connecting mullions
Pressure plate	Plate securing the glass to the frame
Gasket	Rubber or neoprene sealing joint
Setting block	Neoprene support block placed under the glass
Shim	Adjustment shim for alignment
Glazing pocket	Frame groove receiving the glass
Butt joint	Joint between two glass panels without a visible mullion
Structural silicone	Silicone-based sealant designed to transmit loads
Weep hole	Water drainage opening in the frame

Design Principles and Load Transfer

Load Path

In a curtain wall system, loads follow a precise path:

15.Wind load: applied to the glass surface → transferred to the frame (via structural silicone or pressure plates) → transferred to mullions and transoms → transferred to anchors → transferred to the building structure.

16.Gravitational load (glass weight): supported by setting blocks made of neoprene placed at the quarter points of the panel width (at 25% and 75% of the span) → transferred to the horizontal frame → transferred to mullions.

17.Thermal load: caused by differential expansion between glass, aluminum, and the structure. Expansion joints and sliding clearances within frames must absorb these movements.

Allowable Deflection Calculation

Deflection (deformation) of mullions under wind load is a critical parameter. The Canadian standard requires that the maximum mullion deflection not exceed L/175 (where L is the clear span of the mullion) for curtain walls, and L/360 for operable windows. For structural glazing, deflection is limited to L/175 with an absolute maximum of 19 mm (3/4 in).

Calculation example: a 3.6 m (12 ft) span mullion under a 1.5 kPa wind load. The allowable deflection is:

L/175 = 3600 mm / 175 = 20.6 mm

But limited to 19 mm → the allowable deflection is therefore 19 mm

Wind Pressure Calculation

The design wind pressure (q) is determined according to the National Building Code of Canada (NBCC) , Part 4 (loads and effects). The basic formula:

q = 0.5 × ρ × V² × Cg × Cp

Where:

ρ = air density (1.2929 kg/m³ at 15 °C)

V = reference wind speed (m/s) based on location

Cg = gust coefficient

Cp = external pressure coefficient

In practice, the glazier does not perform these calculations (they are done by the engineer), but you must understand the impact of wind pressure on glass selection and installation method.

Types of Curtain Wall Systems

Stick System

The stick system is assembled piece by piece on site. Mullions are first attached to the anchors, then transoms are inserted, and finally glass panels are installed. Advantages: flexibility, lower cost for smaller areas, easy transport. Disadvantages: more joints to seal on site, increased risk of leaks if labor is not qualified.

Unitized System

Prefabricated panels are assembled in the factory (glass, frame, seals) then transported to site and hoisted into place. Advantages: factory-controlled quality, rapid installation, less work at height. Disadvantages: higher initial cost, requires a crane, stricter installation tolerances.

System Comparison

Criterion	Stick System	Unitized System
Assembly	On site	In factory
Initial cost	Lower	Higher
Installation speed	Slow	Fast
Quality control	Variable	High
Water tightness	Depends on labor	Factory-tested
Transport	Easy (components)	Requires special trailers
Tolerances	More forgiving	Very strict

Structural Glazing System

In this system, glass is bonded to the aluminum frame with structural silicone sealant. The silicone must have certified tensile and shear strength. The silicone joint width is calculated by the engineer based on wind load and sealant strength. Typically, the joint width ranges from 6 mm to 25 mm.

Structural glazing can be:

Four-sided: glass is bonded on all four sides (most common)

Two-sided: glass is bonded on two opposite sides, with the others mechanically retained

Materials and Components

Glass

Glass used in curtain walls is almost always tempered or laminated for safety and thermal resistance reasons.

Glass Type	Strength	Typical Use
Annealed (float)	Low	Interior, small windows
Tempered (thermally)	4 to 5 times annealed	Curtain walls, safety glazing
Laminated	Impact-resistant, stays in place	Guardrails, roofs, high-risk areas
Tempered-laminated	Combination of both	Critical applications
Insulating (double/triple glazing)	Thermal	All modern curtain walls

Insulating glass units (IGUs) consist of two or three glass panes separated by an air or gas space (argon, krypton) and hermetically sealed. The glazier must verify the perimeter seal integrity and the desiccant condition.

Aluminum

Aluminum frames are extruded in alloys 6063-T5 or 6063-T6. Aluminum has a thermal expansion coefficient of approximately 23 × 10⁻⁶ /°C (compared to 9 × 10⁻⁶ for glass). This differential expansion must be absorbed by silicone joints and sliding clearances.

Sealants and Gaskets

Structural silicone sealant: used for structural glazing, must be certified to ASTM C1184 (Standard Specification for Structural Silicone Sealants)

Silicone caulking sealant: for sealing joints between panels, conforming to ASTM C920

Neoprene gaskets: preformed EPDM or neoprene seals, used for mechanical water tightness

Setting blocks: neoprene blocks with a Shore A hardness of 90, placed under the glass to distribute weight

Installation Procedures

Preparation and Pre-Installation Verification

Before any installation, the glazier must:

59.Verify plans and specifications: dimensions, glass types, frame types, specified tolerances

60.Inspect the structure: anchors must be in place, aligned, and conform to the plans

61.Verify structural tolerances: the building structure must be within acceptable tolerances (generally ± 6 mm on vertical and horizontal alignment)

62.Plan the installation sequence: bottom to top for curtain walls, or according to the plans

Anchor Installation

Anchors connect the curtain wall to the building structure. They must allow three-dimensional adjustment (x, y, z) to compensate for structural tolerances. Anchors are typically galvanized steel or aluminum, secured by bolts or welding.

Critical points:

Anchors must be adjustable before final tightening

Galvanic corrosion between aluminum and steel must be prevented with isolation (neoprene pad or coating)

Bolts must be torqued to the specified value (typically 20-40 N·m depending on diameter)

Mullion and Transom Installation

Mullions are installed vertically, using a laser level or plumb bob. The vertical tolerance is ± 1.5 mm over 3 m (1/16 in over 10 ft). Transoms are inserted between mullions and secured with brackets or screws.

Expansion joints between mullions (every 6 to 9 m) must be provided to absorb thermal expansion. A gap of 12 to 20 mm is typical, filled with a silicone joint.

Glass Installation

The glass installation procedure in a stick system:

74.Clean the glazing pocket: remove all dust, oil, or debris

75.Install setting blocks: at 25% and 75% of the panel width, to distribute weight

76.Install interior gaskets: preformed seals in the groove

77.Place the glass panel: using suction cups, centering it in the groove

78.Install exterior gaskets: pushed into place with a gasket tool

79.Install pressure plates: secured to the frame to hold the glass

80.Apply finishing sealant: on joints between panels and at intersections

For structural glazing, the procedure differs:

82.Glass is bonded to the frame in the factory or on site in a controlled environment

83.The bonding surface must be perfectly clean (solvent cleaning, then primer)

84.Structural silicone is applied in a continuous bead, free of air bubbles

85.Cure time is 7 to 14 days before full handling

86.The glass is then installed as a prefabricated panel

Water Tightness and Water Testing

Water tightness is tested according to AAMA 501.1 (Standard Test Method for Water Penetration of Exterior Windows, Curtain Walls and Doors). The test involves applying a water spray at a specified pressure (typically 300 to 700 Pa) for 15 minutes and verifying that no infiltration occurs.

Weep holes (drainage openings) must be clear to allow drainage of condensed or infiltrated water. They are placed at the base of mullions and transoms, typically 6 mm in diameter, every 600 mm.

Applicable Canadian Standards and Codes

National Building Code of Canada (NBCC)

The NBCC is the primary reference for structural and safety requirements. Relevant sections for the glazier:

Part 4: Loads and effects (calculation of wind, snow, and seismic loads)

Part 5: Element separation (air and water tightness, vapor control)

Part 9: Housing (for small windows, but not for curtain walls)

CSA Standards

CSA A440: Windows (performance standard for windows, including air and water tightness tests)

CSA A440.2: Energy-efficient windows and doors (energy classification)

CSA A500: Curtain walls (specific standard for curtain walls, including performance and testing requirements)

AAMA Standards (American Architectural Manufacturers Association)

Although American, these standards are widely used in Canada:

AAMA 501.1: Water infiltration test

AAMA 501.4: Dynamic performance test (wind pressure)

AAMA 503: Specification for structural silicone glazing

Key NBCC Requirements for Glazing

The NBCC requires that glass in curtain walls be:

Tempered or laminated if located within 1.8 m of finished floor (impact risk)

Laminated for guardrails and skylights above circulation areas

Capable of resisting the calculated wind load with a safety factor of 2.5

Practical Calculations for the Glazier

Allowable Deflection Calculation

For a mullion with span L (in mm):

Curtain wall: max deflection = L/175 (max 19 mm)

Operable window: max deflection = L/360

Example: 4.2 m (4200 mm) mullion in a curtain wall.

Allowable deflection = 4200 / 175 = 24 mm → but limited to 19 mm → 19 mm

Structural Silicone Joint Width Calculation

The minimum joint width (w) is calculated by:

w = (P × A) / (S × 2)

Where:

P = design wind pressure (kPa)

A = glass panel area (m²)

S = silicone shear strength (typically 0.14 MPa for a certified sealant)

The factor 2 represents distribution over two sides (for four-sided glazing, use the shorter side)

Example: 1.5 m × 2.0 m panel, 2.0 kPa wind pressure, silicone with S = 0.14 MPa.

A = 1.5 × 2.0 = 3.0 m²

Total load = 2.0 kPa × 3.0 m² = 6.0 kN

Joint width = (6.0 kN) / (0.14 MPa × 2 × 1.5 m) = 6000 N / (0.14 × 10⁶ N/m² × 3.0 m) = 0.0143 m = 14.3 mm

The minimum width is therefore 15 mm (rounded up).

Glass Weight Calculation

Glass weighs approximately 2.5 kg/m² per mm of thickness.

Example: 12 mm tempered glass panel, dimensions 1.5 m × 2.0 m.

Area = 3.0 m²

Weight = 3.0 m² × 12 mm × 2.5 kg/m²/mm = 90 kg

This calculation is essential for determining the number of suction cups required and lifting capacity.

Pitfalls to Avoid

Measurement and Tolerance Errors

Not verifying actual dimensions of the opening before ordering glass. Plans may differ from reality. Always measure on site.

Confusing tolerances: glass manufacturing tolerance (± 1.5 mm) is not the same as frame installation tolerance (± 6 mm).

Forgetting expansion clearance: glass must have a minimum clearance of 3 mm on each side within the groove to allow for thermal expansion.

Installation Errors

Placing setting blocks in the wrong position: they must be at the quarter points of the width (25% and 75%), not at the center or ends.

Over-tightening pressure plates: this can create stress concentration points and crack the glass. The tightening torque must be respected.

Using non-structural sealant for structural glazing: ordinary silicone does not have certified strength. Verify ASTM C1184 certification.

Cleaning bonding surfaces with a product that leaves residue: use only recommended solvents (isopropanol, methyl ethyl ketone) and allow to dry completely.

Design and Code Errors

Ignoring differential expansion between aluminum (23 × 10⁻⁶ /°C) and glass (9 × 10⁻⁶ /°C). Over a 6 m length, the difference is 14 mm for a ΔT of 50 °C.

Not providing expansion joints in mullions every 6 to 9 m.

Using annealed glass in an application where the code requires tempered or laminated glass (near floor level, guardrails, skylights).

Forgetting safety requirements for glass located within 1.8 m of the floor: it must be tempered or laminated per the NBCC.

Exam-Specific Pitfalls

Confusing standards: CSA A440 (windows) vs CSA A500 (curtain walls). The exam may ask you which standard applies.

Forgetting the safety factor of 2.5 for glass strength calculations.

Not knowing expansion values: aluminum 23 × 10⁻⁶ /°C, glass 9 × 10⁻⁶ /°C, steel 12 × 10⁻⁶ /°C.

Confusing allowable deflection: L/175 for curtain wall, L/360 for operable window.

Not knowing that setting blocks are neoprene with a Shore A hardness of 90.

Exam Tips

158.Memorize key values: expansion of aluminum and glass, glass weight (2.5 kg/m²/mm), allowable deflections (L/175, L/360), setting block hardness (Shore A 90).

159.Understand the load path: glass → frame → anchors → structure. Load transfer questions are common.

160.Know how to identify system types: stick system vs unitized system, and their respective advantages/disadvantages.

161.Know standards by their full names: "National Building Code of Canada", "CSA A440", "CSA A500", "ASTM C1184".

162.Practice calculations: deflection, glass weight, silicone joint width. Calculation questions are often on the exam.

163.Visualize procedures: the installation sequence (anchors → mullions → transoms → glass → sealing) is a classic question.

Summary

Structural glazing and curtain walls are complex systems that require a thorough understanding of structural principles, materials, and standards. The essential points to remember:

The curtain wall is non-load-bearing and transfers loads to the structure via anchors

Structural glazing uses certified silicone (ASTM C1184) to transmit loads

Allowable deflections are L/175 (curtain wall) and L/360 (operable window)

Setting blocks are placed at the quarter points of the panel width

Glass weight is 2.5 kg/m² per mm of thickness

Installation tolerances are ± 1.5 mm over 3 m for verticality

Expansion joints are required every 6 to 9 m

Key standards are the NBCC (Parts 4 and 5), CSA A440, CSA A500, and AAMA 501.1

Glass near the floor (< 1.8 m) must be tempered or laminated

Weep holes ensure water drainage and must be kept clear

Mastering these concepts, combined with practicing calculations and knowing the standards, will help you succeed on this topic in the Red Seal exam.

Structural Glazing and Curtain Walls