Background

The majority of SSG construction is of the two-sided type, which commonly consists of attaching the two vertical glass pane edges to mullions using struc­tural sealant, whereas the two horizontal edges are captured within glazing frame pockets using rubber gaskets based on typical dry-glazed construction practices. In four-sided SSG systems, all four sides of a glass pane are attached to the glazing frame using structural sealant [6,7]. Major guidelines for design, testing, and construction of SSG in the U. S. can be found in ASTM C1401-09 [1] and in Europe in ETAG 002 [8]. Because of the lack of a mechanical capture for the glass panes in four-sided SSG, one must rely heavily on the adhesion prop­erty of the sealant material to the glass and aluminum substrates. Although seal­ant manufacturers, curtain-wall designers, and glazing installers generally follow well-established standards, guidelines, and procedures for specifications, design, detailing, fabrication, and installation of four-sided SSG systems; none­theless, some concerns about their seismic performance still exists. For this rea­son, full-scale mockup testing is necessary to establish satisfaction of the code’s seismic provisions. Of course, such concerns in the past have been more about the shear deformation capacity of the structural sealant in stick-built curtain – wall systems. Recent experimental studies on racking test evaluation of two­sided and four-sided SSG curtain-wall systems [9,10] provide some insight to seismic performance of stick-built SSG systems.

Most of such concerns have been resolved recently through the use of unit­ized construction of four-sided SSG systems. Whereas in stick-built construction the glazing frame is usually continuous over multiple stories, and, therefore, the glazing panel will be forced to rack under story drift and subsequently transfer large strains to structural sealants, the unitized system is structurally discontinu­ous from story to story. This is accomplished through shop glazing and prefabri­cating the complete panels and simply attaching adjacent panels to one another through stack joints that easily allow sliding between panels, resulting in lower stresses in the structural sealant. As in stick-built designs, setting blocks are used to carry the dead load of the glass. For this project, typical stack joint details, as shown in the renderings of Fig. 1, are to be used.

For preliminary testing in this project, however, it was decided to investi­gate a worst-case scenario, which would mean a unitized system failing to

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FIG. 1—Rendering of stack joints and anchor system for the unitized four-sided SSG system.

behave as intended and, in turn, to perform as a stick-built system by racking under in-plane story drift. The objective was to understand how the sealants perform under a racked frame condition. By showing that the performance of glass and sealants satisfied the intent of the code provision in a stick-built con­struction, there will be assurance that the system will perform satisfactorily under an actual unitized construction condition.