Have you ever looked up at a high rise skyscraper or big building of any shape or size and seen the walls completely made out of what seems to be just glass or large windows, magically attached to the building somehow and thought that looks nice but it is definitely not obvious how something so heavy as a glass wall can just sit on the building like that? This is curtain wall, and it’s one of a few types of structural glazing. To be honest, when my education was finished in architecture school, I only had a vague idea of what curtain wall really is. Through working in the industry my knowledge on this subject has skyrocketed. This is why I want to explain some of the most commonly used structural glazing systems found today: to help others, in the industry or not, understand more about large scale structural fenestration in a building envelope.
Different Types of Structural Glazing and How They Work
-Unitized curtain wall is a vertically load bearing wall consisting of prefabricated units which are assembled individually in a plant, then shipped to site where they are fitted together on the building, unit by unit. The wall is structurally attached to steel edge beams or edge of slab via a steel anchor system, providing support for lateral loads. Main components of the wall include: aluminum mullions (vertical and horizontal), anchors, fasteners, gaskets, caulk/sealant, vision glass (see through) or spandrel glass (not see through), coated and uncoated aluminum panels, stiffeners, and insulation. The wall system is always tested before erection to provide a watertight seal from the outside, but can be custom made to also provide superior solar protection and temperature insulation with different types of glass or a combination of glass, insulation, and panels.
–Stick built systems are similar to unitized as far as most parts go. The main differences are in the production, assembly, vertical mullions, and the anchoring of the product. The parts are prepared and sent to site. Parts are assembled together on site to create the wall system. Due to on-site construction rather than prefab interlocking units, vertical mullions used are one-piece tubes, not two-piece male and female generally found in unitized. Anchor types vary widely depending on the condition. Stick built systems can be used on a whole building (considered curtain wall) or only a small part. One excellent example of stick built is a store front system, as seen above. Due to a smaller amount of material being held up, wall depth is usually less than a unitized system, and are used at the street level.
–Point supported glazing differs greatly from unitized and stick built systems. It is completely site assembled. There are four main components: glass, fittings, silicone, and additional structure, usually minimal to the eye. The glass is thicker than in the other two systems to take on wind loads differently, and is supported only at points held together with fittings. Fittings are small and come in different shapes with varying amounts of points. The picture above shows spider fittings. Silicone is placed all around the glass between each piece butted against each other piece. The wall only works with some additional structure, either steel structure (long pieces or cable) or glass panel (used above). This system is also the most commonly used one for both vertical and horizontal applications.
The Design Process
Architects will send the glazing or curtain wall design contractor (or design-build contractor) architectural and structural shop drawings of their building design. Once received, the contractor designs a realistic solution based on the systems they offer. This means the architectural drawings are used as a guideline to show intention, but does not necessarily reflect real system dimensions and parts. A design team would consist of drafters and/or engineers with knowledge and experience in structural glazing and architectural building systems. The contractor will draw their own shop drawings of the structural glazing and send to the architect for design approval in accordance with deadline dates. When the architect approves the contractor’s shop drawings, purchase orders are filled out and approved, material can be ordered, fabrication drawings for parts can be started and manufacturing the product commences, whether that be complete units or only some parts needing prep work before being shipped to site. Usually, when a part of the building has similar glazing, or the project is extremely large, production drawings are broken down into different packages, or phases, referring to the part of the building being addressed in that package. Breaking down the building envelope into different parts eases the process of ordering, creating fabrication drawing packages, manufacturing, shipping, and assembly.