Seismic Performance of Two-Side Structural Silicone Glazing Systems

ABSTRACT: This paper presents the results of the first phase of an experimental research program on simulated seismic performance of structural silicone glazed (SSG) curtain wall systems. Full-scale, two – side SSG mockups made up of three side-by-side glass panels were tested under cyclic racking displace­ments to determine serviceability and ultimate behavior responses. Variables were glass type (annealed and fully tempered) and panel configuration (monolithic, laminated, and insulating glass). In the experi­ments carried out, damage states such as gasket deformation/pullout, sealant failure (e. g., adhesion/ cohesion, etc ), glass cracking glass and fallout were identified and their corresponding drift levels were determined. The extent of damage to silicone sealant was determined through air leakage tests.

KEYWORDS: silicone sealant, structural glazing, architectural glass, seismic behavior, curtain wall, racking test, air leakage test

Introduction

Since its introduction nearly four decades ago, structural silicone glazing (SSG) has become a popular glazing method for curtain wall construction. Perhaps the most important reason for SSG popularity is the aesthetics associated with a mullionless curtain wall system [1,2]. The major difference between SSG systems and the more widely used “dry-glazed” systems is that glass lites or panels in SSG systems are adhered to the supporting glazing frame with structural silicone sealant along either two edges of the glass panel or all four edges. Common practice consists of attaching the two vertical glass panel edges to mullions located behind the glass with structural silicone sealant, while the two horizontal glass panel edges are held in aluminum framing pockets by gaskets (dry glazed). This glazing procedure, which is used to create a “strip window” system, is known as “two-side” SSG construction [3]. A similar procedure referred to as “four-side” SSG construction, uses structural silicone sealant on all four edges of the glass panel. The vertical edges of adjacent glass panels in two-side SSG systems and all edges of adjacent glass panels in four-side SSG systems are separated by silicone weatherseal joints.

Glass panel edges in dry-glazed curtain walls are mechanically captured; and thus, out-of-plane loads are transferred from the glass to glazing frame through confining gaskets. In contrast, SSG systems rely upon the silicone sealant to transfer such loads to the supporting frame. For this reason, the strength and modulus properties of the silicone sealant and the quality of the bond between the sealant and the sub­strates (glass edge and glazing frame) are of great importance in SSG systems design.

Structural sealant material properties and guidelines for its use are usually provided by sealant manu­facturers using a number of standard test methods (e. g., [4]). However, most tests carried out by manu­facturers are limited to coupon tests to determine tensile and shear strain capacities, modulus properties, adhesion, and material compatibilities. Data from tests on components such as glass panels attached with

silicone to glazing frames are scarce. Although design guidelines for SSG systems under wind loading conditions are well developed, there is a lack of knowledge related to the behavior of SSG systems under in-plane displacements experienced by buildings during earthquakes. It is generally believed that SSG systems perform well in seismic regions due to the “resilient attachment” of glass to the glazing frame. In fact, it is noted in ASTM Standard C 1401 [5] that, “Since the lite or panel is not captured in a metal glazing pocket, the opportunity for it to impact the metal glazing pocket surfaces is minimized, eliminating a primary cause of breakage.” This notion has merits in four-side SSG systems, but may need moderation for two-side systems, wherein the top and bottom edges are typically captured in metal glazing pockets. It is further noted in ASTM Standard C 1401 [5] that, “Depending on system design, however, adjacent glass lite or panel edges could contact each other and cause breakage or other effects.” Despite these claims, there is only limited full-scale experimental test data from in-plane racking tests on SSG systems [6,7] to substantiate them.

Seismic design requirements based on building codes (e. g., [8]) require that nonstructural components including curtain walls accommodate building interstory drifts. For glazing design purposes, the code normally requires the drift capacity of the glazing system corresponding to glass fallout to be known. Currently, the only full-scale glass fallout data available in the literature was collected during the limited tests conducted by Behr [6].

In order to develop a better understanding of seismic behavior of SSG systems, an experimental program is underway at Penn State University. The objective of this paper is to present the results of cyclic racking tests on two-side SSG curtain walls. The serviceability and ultimate performance of conventional two-side SSG systems are evaluated for two different glass types and three different configurations.