Discussion of the Racking Test Results

To put the overall tested displacements in context, it is important to discuss the maximum expected displacements/drifts the building code allows a structure to undergo/experience during a design level seismic event. Per ASCE 7-05 [9], the maximum allowed story displacement for an essential service building is 1.25 % of the story height. For the tested mock-up, which is 11 ft (3353 mm) tall this corresponds to displacement of 1.65 in. (42 mm). The displacement before which glass fallout must not occur (ASCE 7, Ch 13 [9]) is: Afaiiout > 1.25*I*DP, where I is the importance factor, in this case 1.5, and Dp is the design displace­ment or the allowable displacement, in this case 1.65 in. (42 mm) and Af^lout is the drift associated with a piece of glass with an area of at least 1 in. (645 mm2) breaking away from the panel [2]. For this mock-up then, Afailout > 3.10 in. (79 mm), which means that if no glass fallout occurs prior to a displacement of 3.10 in. (79 mm), the mock-up has passed the ASCE 7-05 Af^out criterion.

The focus of the results reported here will be from the 0-3 in. (0-76 mm) range, which captures performance of the system up to the displacement at which the code indicates that glass fallout, Afallout, cannot occur. This also pro­vides a range of results that can be correlated with the FE Model. Past 3 in. (76 mm) of movement, a non-linear model is probably more appropriate.

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FIG. 19—Video still showing actual displaced shape at glass lite corners.

Boundary condition 1, “sway," was tested all the way to 4 1/2 in. (114 mm) of displacement with no visible evidence of sealant damage (Fig. 20). The testing was stopped at this displacement because one of the index clips (Fig. 5) dis­lodged in the cavity between the male-female vertical mullions and wedged the vertical mullions apart. Testing beyond this point without removing and rein­stalling the index clips would have resulted in additional damage to this mock – up unit making it unusable for a second boundary condition test. Boundary condition 2, “rack with vertical slip," was tested all the way to 6 in. (152 mm) of displacement, which is the limit of the testing apparatus. The first signs of visi­ble sealant damage/tearing occurred at 5 in. (127 mm) of displacement (Table 4). At 6 in. (152 mm) of displacement, the sealant around the perimeter of the center lite of glass showed approximately 15 % damage/tearing. All of this seal­ant damage was cohesive with no evidence of adhesive failure. The center lite of glass (Fig. 3) was still firmly secured to the aluminum mullions and not in immi­nent danger of falling out. This system was tested well beyond the Afallout criteria of 3.1 in, (79 mm), and did not result in any glass fallout even at 6 in. (152 mm) of displacement (Table 4). Therefore, by definition in the code, the “delta fall­out" condition was exceeded. As delta fallout indicates the displacement level at which glass actually falls out during testing, the defined delta fallout then for

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Lower Left Center Lite – Sealant Bead Elongation
(L4 Camera)

boundary condition 2 would be beyond 6 in, (152 mm). For boundary condition 3, minor visible sealant damage (one tear less than 1 in, (25 mm) in length) became evident at 4 in, (102 mm) of displacement. At 6 in, (152 mm) of displace­ment, there was complete sealant failure around the perimeter of the center lite of glass and it fell from the mock-up framing. The Afal]out criteria was again exceeded in this test, but as glass did fall out in this test, the delta fallout level would be referenced as 6 in, (152 mm) for boundary condition 3 (rack). With the center lite of glass completely disengaged from the mullions a full inspection of the sealant bead was possible both on the aluminum mullions and the broken lite of glass. This inspection revealed that the sealant damage was predomi­nantly cohesive as we would want to see. There were some isolated locations that indicated localized adhesive failure but they amounted to less than 2 % to 3 % of the entire sealant bead, which secured the lite of glass to the aluminum mullions.

From analysis of the video footage, it can be determined that the sealant is elongating about 4 % in boundary condition 1, “sway," at a displacement of 3 in, (76 mm), well within its movement capability. As expected, in boundary

Boundary Condition

BC1

BC2 BC3

TABLE 4—Sealant damage at varying displacements and boundary conditions.

Displacements at first sealant tear

N/A

5 in. (127 mm)

4 in. (102 mm)

Displacements at glass fallout

N/A

N/A

6 in. (152 mm)

condition 3, “rack," the sealant undergoes greater movement, and from video analysis, the sealant in this condition strains approximately 15% at 3 in, (76 mm) displacement. In the hybrid condition, boundary condition 2, in which the vertical mullions are allowed to slip, the sealant elongates approximately 24 % at 3 in, (76 mm) of displacement.

It was expected when the testing began that the sealant elongation for the hybrid condition (boundary condition 2), and subsequently any sealant damage noted would fall between the sway and rack conditions. The sealant perform­ance results from visual inspection of sealant damage support this assumption, as sealant damage was not noted at all for the sway condition, and was noted at 4 in, (102 mm) for the rack condition. It follows then that sealant damage at any displacement between 4 in, (102 mm) and 6 in, (152 mm) would make sense for the hybrid condition. In fact, sealant damage was observed at 5 in, (127 mm) for the hybrid condition, exceeding the performance of the rack condition as expected, yet sustaining some damage not seen in the sway condition. Addition­ally at 6 in, (152 mm) of displacement there was approximately 15 % of sealant tearing around the center lite for boundary condition 2, while 100% sealant tearing and glass fallout occurred on the same lite of glass for boundary condi­tion 3, thus the racking results for boundary condition 2 again fell between the sway and rack condition. This is also supported by the hysteresis curves (Fig. 23). The load cell on the test apparatus requires less force to displace the mock – up an equivalent displacement with the sway (boundary condition 1) than for the racking (boundary condition 3).

In analyzing the sealant movement through video images, it was expected then that the elongation in the sealant for boundary condition 2 would be some­where between 4 % and 15 %. However, in the analysis of the sealant movement through video images, the hybrid condition actually shows the sealant moving more—24 %—than in the rack condition, which showed 15 %. This does not cor­relate with the visual inspection of the sealant for damage; however, it may be within the bounds of error for the study given the cameras, images, and ruler methods required to arrive at these measurements.

The corner condition did not turn out to be a limiting factor on the overall system performance. For boundary condition 1, “sway," the corner unit was the only significant source of restraint. Without the corner unit, it is likely that the strains in the sealant would have been even lower than those shown in Figs. 20 and 21. There was also no evidence of glass-to-glass contact where the primary and corner units joined (Fig. 6). The detailing of the glass lites at the corner allowed the glass lite on the primary unit to move past the edge of the corner unit glass lite. Glass-to-glass contact between the center lite and its surrounding lites of glass did not occur during the testing of boundary conditions 1 or 2. Glass-to-glass contact did occur while testing boundary condition 3 at around 5 in, (127 mm) of displacement.