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Evaluating the LRFD Resistance Factor for Cold-Formed Steel Compression Members Karthik Ganesan Ϯ & Dr. Cris Moe

20 th International Specialty Conference on Cold-Formed Steel Structures St. Louis, Missouri – Nov 3 rd - 4 th , 2010. Evaluating the LRFD Resistance Factor for Cold-Formed Steel Compression Members Karthik Ganesan Ϯ & Dr. Cris Moen. www.moen.cee.vt.edu.

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Evaluating the LRFD Resistance Factor for Cold-Formed Steel Compression Members Karthik Ganesan Ϯ & Dr. Cris Moe

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  1. 20th International Specialty Conference on Cold-Formed Steel Structures St. Louis, Missouri – Nov 3rd - 4th, 2010. Evaluating the LRFD Resistance Factor for Cold-Formed Steel Compression MembersKarthik GanesanϮ & Dr. Cris Moen • www.moen.cee.vt.edu Charles E. Via Jr. Dept. of Civil and Environmental Engineering

  2. Outline Historical background Objectives Scope & Methodology Results & Conclusions

  3. Historical background • 1991 AISI Spec –LRFD is implemented for CFS M, F, P fcbased on AISC 1986 Spec! 264 col. tests! Hsiao, L.E., W.W. Yu, and T.V. Galambos (1988). “Load and Resistance Factor Design of Cold-Formed Steel: Calibration of the AISI Design Provisions,” Ninth Progress Report, Civil Engineering Study 88-2, University of Missouri-Rolla, February 1988.

  4. Historical background • 1996 AISI Spec –column curve is revised (299 tests). From AISC – LRFD! Peköz, T.B., Sümer, Ö. (1992). “Final Report – Design Provisions for Cold-Formed Steel Columns and Beam Columns,” AISI Report, September 1992.

  5. Historical background • 2001 AISI Spec (2004 Supp.) is published: • DSM added – elastic buckling modes for whole member, not “element-by-element”. Schafer, B.W. (2002). "Local, distortional, and Euler buckling of thin-walled columns." Journal of Structural Engineering, 128(3), 289-299.

  6. Historical background - DSM • 2 additional columncurves! • Local buckling eqns. are different!

  7. Historical Background • 2007 AISI Spec – Distortional buckling added to Main Spec Compare DSM Main Spec

  8. Objectives Unchanged since 1991, increase? Only 264 col. tests, more tests? Main Spec  Eqns. changed DSM introduced Distortional buckling Slenderness Limit State

  9. LRFD Method Failure,≤ 0 β = 2.5  6 in 1000 cols fail! What is β?? Higher β better design! Measures safety of design! Reliability Index! Ravindra, M. K., and Galambos, T. V. (1978). “Load and Resistance Factor Design for Steel." J. Strct. Div., ASCE, 104(9), 1337-1353

  10. Objectives (First order Approx.) Need to find just Pm and VP. • AISI-S100-07 Chapter F: • βo = Target reliability index = 2.5 for LRFD • CΦ = Calibration coefficient = 1.52 for LRFD • Mm=1.1 , Fm =1and Pm =test-to-predicted mean (Ptest/Pn) • VF = 0.05, VQ = 0.21, VM=0.10 • VP = C.O.V. of test-to-predicted ratio Hsiao, L.-E., Yu, W.-W., and Galambos, T. V. (1990). "AISI LRFD method for cold-formed steel structural members." Journal of structural engineering, New York, N.Y., 116(2), 500-517

  11. Types of Columns H Angle Section

  12. Column Test Database • 1991 AISI LRFD calibration– based on 264 tests • Current database: 455 lipped C 49 plain C 72 lipped Z 13 plain Z 11 Hats 161 with holes294 without holes Total of 600 tests(concentrically loaded)

  13. Scope & Methodology • Scope • Main Spec (with and without holes) • DSM (without holes) • Procedure • Created custom Matlabcode • Validatedwith example from AISI Design Manual, DSM Design Guide • Predicted strength of columns from database (Pn) • Calculated (Ptest/Pn)  Mean (Pm); C.O.V  (VP) • Chapter F  fc

  14. Results H/t <472 • Same Global Buckling equations  Same фc! • Different Local Buckling equations  Significantly differentфc! • Same Distortional Buckling equations  Almost sameфc! • Expand Limits!

  15. Conclusions • Resistance factor was calculated using first order second moment reliability approach. • Expansion of Main Spec limits & DSM prequalified limits. • Significant differences in local buckling prediction, DSM φc is higher! • Distortional buckling is most accurately predicted! • To increase φc,consider separate resistance factors for different limit states with φc=0.95 for distortional buckling, φc=0.90 for local buckling; φc=0.80 for global buckling

  16. Acknowledgements AISI Moldovan, A. Pekoz, T Dat, D. T Shanmugam Mulligan, G. P. Young, B. Winter, T AISC Galambos, T. V. DeWolf, J Sivakumaran, K. S Yu, W.W Polyzois, D Abdel-Rahman, N Loh, T. S. Schafer, B. W Shanmugam, N. E Loughlan, J. Ortiz-Colberg, R. A Chodraui, G Rasmussen, K. J. R Hancock, G. J Thomasson, P. O.

  17. Q e t o s ? u s i n

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