A "mullion" refers to a vertical or horizontal structural element that divides adjacent window units or panels within a curtain wall or window system. Mullions are essential components in the design and construction of facades, providing both structural support and aesthetic division. They help to transfer loads from the glass or panels to the building structure, ensuring stability and integrity. Mullions can be made from various materials, including aluminum, steel, or wood, depending on the design requirements and architectural style of the building. In modern commercial construction, aluminum is a popular choice due to its strength, lightweight nature, and resistance to corrosion.

Structural Purpose

Mullions provide a structural framework to support the installation of multiple windows or doors in an opening . They also share the load with girts to support wall sheeting in gable end walls .

Design and Installation

In terms of design and installation, mullions can be vertical or horizontal, and their size and material may vary depending on the project requirements . For example, the minimum cavity width for mullions, except for steel mullions, is specified to be not less than 35 mm and not more than 75 mm .

Classification and Compliance

In Australian commercial construction projects, buildings are classified into different categories, such as class 5, 6d, 7b, 8, 9a, and 9b, according to the Building Code of Australia (BCA) . Mullions, as part of the structural framework, must comply with the relevant building codes and standards.