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Direct Strength Design for Cold-Formed Steel Members with Perforations

Direct Strength Design for Cold-Formed Steel Members with Perforations. Progress Report 1 C. Moen and B.W. Schafer AISI-COS Meeting February 21, 2006. Outline. Objective and challenges Project overview FE stability studies fundamentals, plates and members with holes

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Direct Strength Design for Cold-Formed Steel Members with Perforations

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  1. Direct Strength Design for Cold-Formed Steel Members with Perforations Progress Report 1 C. Moen and B.W. Schafer AISI-COS Meeting February 21, 2006

  2. Outline • Objective and challenges • Project overview • FE stability studies • fundamentals, plates and members with holes • Modal identification and cFSM • Existing experimental column data • elastic buckling studies: hole effect, boundary conditions • strength prediction by preliminary DSMstub columns, long columns • Conclusions

  3. Objective Development of a general design method for cold-formed steel members with perforations.

  4. Direct strength for members with holes Pn = f (Py, Pcre, Pcrd, Pcrl)? Does fstay the same? Explicitly model hole(s)? Accuracy? Efficiency? Identification? Just these modes? Gross or net, or some combination?

  5. Project Update • Originally proposed as a three year project. Year 1 funding was provided, we are currently ½ way through year 1. • Project years 1: Benefiting from existing data 2: Identifying modes and extending data 3: Experimental validation & software

  6. Outline • Objective and challenges • Project overview • FE stability studies • fundamentals, plates and members with holes • Modal identification and cFSM • Existing experimental column data • elastic buckling studies: hole effect, boundary conditions • strength prediction by preliminary DSMstub columns, long columns • Conclusions

  7. 4w Local plate stability with a hole

  8. SSMAS162-33 w/ hole Member Study L = 1220mm = 48 in.

  9. Local (L) buckling • Pcrl no hole = 0.28Py, with hole = 0.28Py

  10. Distortional (D) buckling • Pcrd no hole = 0.64Py, with hole = 0.65Py

  11. Global flexural torsional (GFT) buckling • Pcrd no hole = 0.61Py, with hole = 0.61Py

  12. Distortional (DH) buckling around the hole • Pcrd no hole = 0.64Py, with hole = 0.307Py

  13. Hole size* and member buckling modes *this graph depicts effect of a circular hole, not the ‘SSMA’ oval hole

  14. Modal identification • Mixing of modes (a) complicates the engineer/analyst work (b) may point to post-buckling complications • We need an unambiguous way to identify the buckling modes • A significant future goal of this research is the extension of newly developed modal identification tools to members with holes

  15. cFSM and modal decomposition

  16. cFSM and modal identification

  17. Outline • Objective and challenges • Project overview • FE stability studies • fundamentals, plates and members with holes • Modal identification and cFSM • Existing experimental column data • elastic buckling studies: hole effect, boundary conditions • strength prediction by preliminary DSMstub columns, long columns • Conclusions

  18. Study of experimentally tested members • Collection of experimental column data • Estimation of elastic buckling Pcrl, Pcrd, Pcre using FE to capture influence of hole and reflect test boundary conditions • Examination of initial DSM strength predictions for tested sections

  19. Elastic local buckling in stub columns Pcrl,no hole = pin free- to-warp boundary conditions increasing hole size

  20. Boundary condition effect: local buckling

  21. Distortional buckling (effect of holes) (stub column data) identification of D modes can be challenging, minimum D mode = “DH” mode

  22. Stub column testing restrains distortional buckling

  23. Global buckling in long columns(effect of holes) Effect of holes on global buckling modes greater than anticipated, still under study...

  24. lowest local mode in an FE model with hole and test boundary conditions Pne set to Py Pne set to Py ? ? ? ? Py,gross Py,gross Py,gross Py,gross Py,net Py,net Py,net Py,net lowest distortional mode (includes DH) in an FE model w/ hole and test bc’s Preliminary DSM for stub columns • Local strength • Distortional strength

  25. DSM prediction for stub columns NET quite a few specimens have strength greater than Py,net mean test-to-predicted = 1.18 standard deviation = 0.16 *Pcr by FE reflects test boundary conditions, minimum D mode selected, Py=Py,net

  26. DSM prediction* for stub columns GROSS mean test-to-predicted = 1.04 standard deviation = 0.16 *Pcr by FE reflects test boundary conditions, minimum D mode selected, Py=Py,g

  27. Preliminary DSM for long columns Global buckling Local buckling Distortional buckling

  28. Global buckling in long columns

  29. Local-global in long columns

  30. Preliminary DSM for long columns member length/web depth (L/H)

  31. Conclusions • We are off and running on columns with holes • Local buckling (a) doesn’t really follow unstiffened element approximation (at least for elastic buckling) (b) should be modeled consistent with application, i.e., stub column boundary conditions, no square plates • Distortional buckling is even more of a mess than usual as it appears to get mixed with local buckling, particularly around hole locations. What does PcrdPcrl imply? We need better modal identification tools! • Global buckling needs further study, Pcre sensitivity to isolated holes here is a bit surprising • DSM (preliminary) based on gross section yield instead of net section yield has the best accuracy, what does this imply? The boundary conditions of the test and the hole should be explicitly modeled for finding Pcr. • Existing data does not cover distortional buckling well. We need additional experimental work and nonlinear FE modeling!

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