Field-emission Microscopy Modeling Overview

Frequently on large projects, full-scale performance mock-ups testing protocols are required. Included in these testing protocols is the option to perform a static in-plane racking test AAMA 501.4 [21]. These performance mock-ups may not occur until a year or more after the curtain wall system has been designed. For dry-glazed mechanically captured or two-sided SSG systems whose perform­ance is well documented in industry and research studies, this design and test­ing sequence is appropriate. For an essential service building (i. e., hospitals, police stations, etc.) with a four-sided SSG system, an AAMA 501.6 [7] test may be employed to prove the suitability of this design because the building will have more stringent serviceability requirements than non-essential construc­tion. The results of the AAMA 501.6 racking test can confirm the design efficacy for actual seismic events. Because this is being used to prove the design, it must be conducted early in the design process. This adds cost to the project not only because of the addition of a second full scale mock-up test but also because a

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physical mock-up must be constructed when not all of the design criteria has been established. It would be favorable in the future to be able to predict system performance without a full-scale curtain wall system being tested. To this end, a FE model of the full-scale mock-up tested as part of this research was created to compare and calibrate analytical results with the test results.

The FE model was constructed in a program called Visual Analysis by Inte­grated Engineering Software [22]. This FE modeling program focuses primarily on linear-elastic modeling of structures. Figure 15 shows an overall view of the completed model. The aluminum mullions (vertical and horizontal) are repre­sented by two-node beam elements. Because the influence of the silicone sealant is expected to cause the glass lites to act as “shear" resisting elements, the moment restraints were released where the horizontal mullion beam elements attach to the vertical mullion beam elements. The stiffening influence of a nomi­nal moment connection between the aluminum mullions is expected to be insig­nificant compared to the stiffening influence of the glass lites, which are attached to the aluminum mullions by the silicone sealant. The structural sili­cone sealant and the glass lites are represented by three – or four-node plate ele­ments. Figure 16 shows close-up views of glass and sealant plate elements at glass corners. The appropriate linear-elastic material properties for all three materials were entered into the model. For the silicone sealant, the anisotropic and non-linear material behaviors cannot be accounted for in this specific FE modeling software. The Young’s modulus, E, of the silicone sealant was varied

Releases

FIG. 15—FE model overall view.

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FIG. 16—FE model close-up view.

in separate runs of the model in an attempt to capture both the tension and shear behaviors of the sealant. This was only a rudimentary attempt to capture their effects. A more sophisticated FE software program would be needed to more fully integrate these non-linear sealant properties into the analysis. Addi­tionally, as a result of these modeling limitations, the FE results will be com­pared only with the test results in the expected linear-elastic range of the sealant.

The FE modeling results presented here only correspond to boundary con­dition 3, “rack." Once a good correlation/evaluation of the FE results has been established for this boundary condition as it relates to the corresponding physi­cally tested boundary condition, the FE analysis of the remaining two boundary conditions can be pursued. For boundary condition 3, “rack," both the top and bottom of the vertical mullions are attached to the testing apparatus with steel angles. In the FE software, these are represented by restraints in the X, Y, and Z directions. No rotational restraints are added at these support locations.

For the actual testing, movements in the mock-up specimens were meas­ured at discrete displacement values. When trying to obtain analytical results from computer software that can be appropriately compared to testing results, it is important to simulate as closely as possible the actual test conditions. Therefore, the FE model was displaced to the same values that were measured in testing. Figure 17 shows a graphical representation of the displaced shape of the FE model at a given displacement. A close up of one of the glass lite intersec­tions can be seen in Fig. 18. In both of these figures, the glass is represented by a “blue" color and the silicone sealant by a “grey" color.

The first step in evaluating the FE model is to review the overall results and determine if they generally correlate with the testing results and what might be considered to be appropriate behavior. From an overall view of the displaced model (Fig. 17), it is evident that the plate elements representing the glass lites are displacing and rotating rigidly. A close-up view of the glass lite intersection

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FIG. 17—FE model displaced shape.

(Fig. 18) also reveals that the silicone sealant is being stretched in an elastic manner. Figure 19 shows actual movement of the corners of the glass lite when the mock-up is in a similar displaced condition to the FE model view in Fig. 18. The FE model clearly demonstrates similar movement behavior to that of the actual test. Because the Young’s modulus, E, of the glass is 10100 ksi (69,637 MPa) and the E of the silicone sealant varies between 45 psi (310 kPa) in shear and 400 psi (2758 kPa) in tension (Tables 2 and 3) this type of result would be expected. The silicone sealant is substantially more flexible than the glass lites. The beam elements representing the aluminum mullions are not visible in either of the FE model views represented by Figs. 17 and 18. The aluminum mullion can be seen through the glass on the actual mock-up shown in Fig. 19. Because the strong and weak axis rotational df at each end of these elements are released (no moment) there is no resistance to lateral displacement from these elements in the FE model.

The next step in evaluating the FE model results is to calculate the elonga­tion in the plates that represent the silicone sealant so that they can be com­pared with the test results. The elongation in the sealant is calculated based on the distance between two nodes that represent the front and back edges of the sealant, which attach a specific location on the glass lite to a corresponding location on the aluminum mullion. The difference in this distance before and af­ter the model is displaced represents the relative elongation of the sealant for a given displacement.

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FIG. 18—FE close-up displaced shape at glass life corners.