Discussion of Glazing System Design for Wind and Seismic Loads

The horizontal and vertical movement joints between each typical panel allow for movement and flexibility within the system. Individual unitized panels can sway independent of each other. Whereas some binding in the stack joint may occur, forcing the panels to rack slightly, they will not rack to the degree that a conventional stick system, firmly attached to each floor level, would. At the cor­ners, the panels will be allowed to rock to accommodate the building drift. The

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continuous horizontal fin, incorporated in the sill extrusion, carries the setting blocks and the dead load forces of the unit above. The horizontal joint is sized appropriately to accommodate slab edge deflection and thermal expansion within each window unit.

For wind loading (perpendicular to a given building elevation), the glass lites span in two-way action to their perimeter edges. Each of the four sides is adhered to the mullions with SSG sealant, which transfers all of this loading in either tension or compression. In tension under out – of-plane wind loading, the SSG sealant has an allowable capacity of 20 psi (138 kPa) [1]. Out-of-plane seis­mic loading is frequently much lower than wind loading and consequently not a governing load case. For the unitized system under study, the only in-plane restraint occurs at the head of the unit by means of the mechanical connections to the slab. The bottom (sill) of the unit is not restrained horizontally or verti­cally. Because in-plane restraint is not provided at the bottom of each unit, the primary mode of behavior for a unit caused by in-plane seismic loading is to sway. However, as shown in Fig. 4, adjacent vertical panels attached through the horizontal stack joint still will experience some in-plane resistance to move­ment because of the gaskets attaching the upper and lower sill mullions. Fur­thermore, for a hypothetical situation where for any reason the sway behavior fails and stack joints do not function as expected, the system would behave more like a stick-built system, i. e., it would rack. Racking here means that under in-plane lateral load at top of the panel, the panel will deform into a shape of a parallelogram. However, sway is the condition when the panel will slide in­plane at the horizontal stack joint keeping its rectangular shape. The racking condition will then require that each unit resist this loading either by frame action with each horizontal and vertical mullion connection resisting shear, ten­sion, and moment, or the glass lites must act as shear-resisting elements with all stresses being transferred through the SSG sealant connecting the glass to the mullions. Whereas some moment can be transferred between the horizontal to vertical glazing frame member connections, this connection is primarily designed for direct shear and tension. The walls of aluminum mullions are typi­cally thin (less than 0.125 in. (3.2 mm)) and, therefore, fairly flexible when sub­jected to moments resisted solely by screw fasteners between the members. The glass lites, however, present a relatively stiff element into the unit construction. This stiffness is tempered by the flexibility of the SSG sealant, the clearance between the edge of the glass, and the protruding fins of the mullions (Fig. 4).

For seismic design of four-sided SSG systems, it is essential to evaluate the maximum stresses experienced by the SSG sealant and make sure the sealant bead size is sufficient to keep stresses below the sealant allowable value. For this project, the sealant bead was originally sized to accommodate the maximum wind load on the largest lite of glass at just below the 20 psi (138 kPa) allowable. The typical sealant bead is 3/8 in. (9.5 mm) thick by 7/16 in. (11.1 mm) wide on the back face of the heat-strengthened glass lite (#4 surface). The SSG bead then wraps around the edge of the inside lite of glass to the #3 surface. The overall effective width for the L-shaped sealant bead is 5 in. (12.7 mm). The bead is Dow Corning 983 SGS two-part structural silicone sealant, which will be shop – installed. The structural sealant bead attaches the inboard lite of glass to the

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aluminum mullion. All primary structural loads are transferred through the structural bead. For seismic loading, the allowable stress, based on a 5:1 safety factor requested by OSHPD, is 28 psi (193 kPa). Based on finite-element analysis of elastic models of the wall system, the sealant bead under in-plane seismic load­ing typically experiences stresses less than 20 psi (138 kPa) with few exceptions where the stresses reach approximately 24 psi (165 kPa).

The finite-element modeling performed indicates that the sealant stresses will remain below the 5:1 safety factor when the panel sways because of seismic loading. The results from the AAMA 501.6 testing discussed next have further shown that even when subjected to full racking, the SSG structural sealant beads perform adequately.