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Modular Prefab: The Manufacture of Unitized Curtainwall Systems

Thursday, September 8, 2011

Fabrication processes have become increasingly important to the construction industry, and ever more interesting to building designers. With no other building system is this truer than with the building skin. Let’s take a quick run through the manufacturing process for unitized curtainwall systems, which are specifically designed to benefit from the favorable conditions provided by a factory-controlled environment.

Early curtainwall designs were known as “stick systems.” Here the framing members, most often linear aluminum extrusions called mullions, were fabricated and attached individually to the building. Long vertical framing members would be attached first, and incremental horizontals would then be attached to the verticals. Finally, the cladding materials were installed into the framed grid, fixed in place, and the weather seal applied.

The concept of prefabrication has been gaining traction in the AEC industry, and the term has become one of the latest industry buzzwords in recent years. Architects, engineers, and contractors are finding novel ways to fabricate large assemblies offsite as a means to maximize quality and minimize field labor, thereby speeding the construction process. This is old news in the facade industry, where prefabrication has been commonplace for decades.

Prefabricated curtainwall systems are typically referred to as “unitized” systems by the industry. Modular panelized “units” are assembled from fabricated framing members and infill cladding materials, such as glass, stone, or metal panels. Each unit generally contains multiple cladding panels.

The unitized strategy requires that the vertical mullions at each side of the unit, and the horizontal mullions at the top and bottom of the unit, be split into two pieces, such that the second piece becomes part of the adjacent unit relative to each mullion. The horizontal mullion interface is called the stack joint, and is a critical consideration in curtainwall system design. Obviously, the design of unitized systems is considerably more complex than for simple stick systems. In addition to the significant benefits mentioned above, the unitized systems are much superior in accommodating the movements characteristic of tall buildings. Consequently, unitized systems now predominate in multistory building facades.

The manufacturing process of unitized systems can be categorized into two primary functions: fabrication and assembly. Fabrication involves all processing needed to prepare the materials required for unit assembly, with the aluminum extrusions chief among them. A supplier specializing in the extrusion process typically provides extrusions. They are often painted prior to fabrication. Processing of the extrusions involves various combinations of notching, grooving, drilling, and cutting to length. This is most effectively accomplished with the use of CNC machines specially configured to process long lengths of uniform section to very high tolerances. Gaskets are often installed on the extruded framing components as part of the fabrication process. Glass, as the predominant infill cladding material, is ordered to fit requiring no further processing, and the same is true of most other cladding components required for unit assembly.

The assembly process is comprised of building up the unit frames by screwing together the fabricated framing components. Anchors and lifting lugs are attached to the frames as required to facilitate lifting the units in the field and attaching them to the building. Remaining gaskets, glazing tape, backer rod, and other materials are applied to the frames as appropriate. Silicone sealant and structural silicone is then applied to adhere and/or seal the panel materials within the frame. The silicone is a two-part material with a cure time as short as only a couple of hours. The units are then cleaned and carefully bunked (packed) for shipping to the building site.

A typical building project will require many variations of unit configuration. The identification and numbering of the individual units becomes of critical importance, as is the sequence of their delivery to the building site. There are many nuances to the efficient manufacture of a quality unitized curtainwall system, and the foregoing only outlines the essential process.

Enclos operates manufacturing plants at strategic locations throughout the United States. In addition to our own fabrication facilities, we maintain a global supply chain that assures top quality, unlimited capacity, and optimal economy to our clients. We have developed a lean, flexible, and scalable assembly technology that we attempt to mobilize in relatively close proximity to a building site, thus minimizing site storage requirements, and with just-in-time delivery from the assembly operation to build site.

You are cordially invited to visit one of our manufacturing facilities at your convenience. We welcome the opportunity to share our knowledge of and enthusiasm for the curtainwall manufacturing process. Contact curtainwall@enclos.com to enquire about an Enclos facility near you.