Analysis of the Full-Scale Unit Model

There are several possible failure scenarios during the cold-bending of a curtain-wall unit. Some of the major ones are: structural failure of the frame,

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014

Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.

FIG. 13—Comer of FE model of full-size test.

breakage of glass, structural silicone failure, or PIB seal separation in the IGU. Structural failure of the frame can be safely predicted through the typical struc­tural analysis involved in curtain-wall design. Glass can be designed to perform with the sustained stresses induced through cold-bending. Structural silicone is considered to be a strong component in the system and not a critical link. On the other hand, it is suspected that the PIB seal is the governing element during cold-bending of IGUs. Long-term performance of such a seal under sustained strains is unknown. However, the seal is known to fail under a regime of ASTM testing procedures even without applied strains.

Based on the results of the FE model, the locations of maximum edge-seal strains in the PIB were predicted. Preliminary FE analysis of the full-scale unit revealed that the maximum edge-seal deformations were located on the long side of the panel about 40 in. (1016 mm) away from the loaded corner. Intuition and engineering judgment, however, led to the conclusion that strains at the corner of the unit may also be significant. Therefore, these two locations were selected for the measurements of the edge-seal strains on assembly 2. The strain state at each of the locations was measured using three displacement dial gauges attached to the external ply of the glass to measure the relative in-plane edge displacements of the internal ply of the glass. Figure 7 shows locations of all six of these gauges. From these measurements the edge-seal strains in the PIB were calculated and presented in Table 3.

In addition to measuring the in-plane movements of the glass, several strain gauges were attached to the glass to understand its true bending behavior. These gauges were used to validate the results obtained between the three full-scale test assemblies and FE model. The locations of strain gauges in the

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014

Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.

full-size test specimen of tests one and three are shown in Figs. 4 and 8, respec­tively. Two rosette strain gauges (numbered 1 through 6) were placed at the cen­ter of the glass on the inner and outer pane. Rosette gauges 7 through 9 are placed at the location of maximum stress in the glass as predicted by FE model. These gauges helped in understanding the shape of deformation of the glass during its bending. Unidirectional strain gauges were placed on all three test assemblies on the outer glass layer at the midpoints of the free edges. Readings of gauges 10 and 11 are affected mainly by the stiffness of the frame members.

The amount of applied out-of-plane deformation of the IGU in the physical testing was measured using four LVDTs attached near the corners of the glass (Fig. 14). The out-of-plane displacement in the numerical model however, was applied using forced displacement values of the node at the loaded corner with the other corners of the frames restrained. Because these measurements were taken at different locations, a correlation between the out-of-plane deformation in the physical and numerical models needed to be developed. Therefore, the displacements obtained directly from LVDT readings in the physical test were

FIG. 14—One of four LVDTs measuring bending in the full-scale test specimen.

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

compared to the corresponding nodal displacement results of the glass in the FE model. This comparison between physical and FE model out-of-plane dis­placements (bending) is presented in Fig. 15. Data presented in this paper corre­sponds to displacements as measured in the physical test.