Background

Four-sided SSG systems have been designed, since their inception, using the prin­ciples found in ASTM C-1401 [8]. The accepted maximum allowable stress in a sealant subjected to wind loading has been established as 20 psi (138 kPa). This was based on loading probabilities and the strength of sealants at the time ASTM C 1401 [8] was initially published. The probability of occurrence of the building code level design wind event in any one year is 0.02 (50-yr recurrence interval) [9], which corresponds to an effective load duration of 1 min, as discussed by Zargha – mee et al. in 1996 [2]. The probability of occurrence of the building code level design seismic event in any one year is 0.002 (500-yr recurrence interval) [10], which corresponds to an effective loading duration of 1 s, per the Uniform Build­ing Code as it existed in 1996 [11]. For these reasons, and the increased strength of sealants since the late 1970 s, in 1996, Zarghamee proposed that 50 psi would be an acceptable maximum allowable level of sealant stress when designing for seismic loading [2]. The authors believe that based on the results generated by the present study, this proposed sealant maximum allowable design stress is reasona­ble and defendable for a four-sided SSG system in a high seismic zone.

As noted, two-sided SSG systems, which employ silicone for attaching two sides of the glass lite and mechanical gaskets to attach the opposite two sides, have been tested under racking conditions [4,5]. In these studies, two-sided SSG systems were compared to “dry glazed" systems, in which all four sides are mechanically captured with gaskets. Key conclusions from these works show that serviceability and drift capacities of two-sided SSG systems are significantly higher than their dry-glazed counterparts. Additionally, Memari et al. [5] noted by air leakage testing that both two-sided SSG and dry-glazed systems do leak air and that air leakage is increased after racking, even if the sealant sustains no visi­ble damage. Based on the results of these studies, it can be assumed that a dis­continuously sealed system with dry gaskets or sealant/gasket transitions will not maintain as high a serviceability performance level after seismic racking as com­pared to a continuously sealed system such as four-sided SSG. Data from actual buildings experiencing earthquakes has not yet been collected and published to conclusively prove this assumption and quantify the increased serviceability of a four-sided SSG system. However, a recent study on a four-sided SSG stick-built system [6] does confirm that a four-sided SSG system will have better overall seis­mic performance than a two-sided SSG and dry-glazed system. The authors of the paper presented here hope that the results of the research/testing discussed in the following sections will further address this issue in a meaningful way.