Test of 8.5" deep Zees, t=0.059in. (nominal)

professor ben schafer's thin-walled structures research group - johns hopkins university

Testing on the first pair of 8.5" deep Zees with a nominal thickness of 0.059 in. has been completed. These are the thinnest 8.5" deep Zees that will be tested. The 0.059 specimens were selected because of the family of 8.5" deep Zees they have the greatest propensity to fail in the local mode. (FEA results verify that the paired panel-to-purlin screw pattern employed in specimen 8.5Z073-4E3W will result in elastic local buckling before distortional buckling in the t=0.059 in. specimens)

 

8.5" Zees, t = 0.073 in. (nominal)	8.5" Zees t = 0.059 in. (nominal)

The experimental results, shown above, indicate that the failure mechanism in the t = 0.059 in. specimen is similar to that observed in the t = 0.073 in. specimen, with the same panel-to-purlin screw configuration.

 

Local Buckling - 1/2 wavelength ~ 5 in.	Distortional Buckling - 1/2 wavelength ~ 27 in.

Final failure is by a "mechanism", as shown by the strong yield lines in the initial pictures, not buckling. That said, one generally tries to determine which instability "triggers" or is most strongly "related" to the failure mechanism. To this end, we generally discuss whether a not a failure is in the "local" mode, or in the "distortional mode" (as shown in the pictures above). The goal of this testing has been to study failures in the "local" mode, are we succeeding?

The failure mechanism of 8.5Z073-4E3W and 8.5Z059-4E3W 

- involves vertical translation at the flange/lip juncture consistent with distortional buckling

- occurs at a short half-wavelength, consistent with local buckling

- involves rotation at the flange/lip junction, consistent with local buckling

- involves some deformation in flange, consistent with local buckling

Definitive conclusions are difficult, but it is clear that some aspects of both modes exist in the failure mechanism. Realizing that for all of the 8.5" deep Zees, distortional buckling occurs at a lower buckling stress than local buckling in the unrestrained case - the panel and panel-to-purlin connectors are strongly influencing the results. The distortional mode has been highly restricted and many characteristics consistent with a local mode have been successfully triggered. It appears that the restraint of the panel is being signficantly realized in this configuration and the distortional mode has been significantly (though perhaps not completely) restricted. 

It is suggested that the testing on 8.5" deep Zees be completed with this detail (paired panel-to-purlin) screws pending an examination of the test-to-predicted ratios of the already tested specimens. 

last updated 04/15/02