Civil. Engineering INELASTIC BENDING CAPACITY IN COLD-FORMED STEEL MEMBERS Annual Stability Conference New Orleans, Louisiana

Transcription

1 Civil Engineering at JOHNS HOPKINS UNIVERSITY at JOHNS HOPKINS UNIVERSITY INELASTIC BENDING CAPACITY IN COLD-FORMED STEEL MEMBERS 2007 Annual Stability Conference New Orleans, Louisiana April 2007 Y.Shifferaw 1, B.W.Schafer 2 (1),(2) Department of Civil Engineering- Johns Hopkins University

2 Acknowledgments National Science Foundation Cheng Yu American Iron and Steel Institute American Institute of Steel Construction

3 Overview Background and motivation on inelastic bending capacity in cold-formed steel (CFS) members Mechanisms for inelastic reserve Observed inelastic reserve in CFS tests Recent testing on CFS beams & summary of test results FE modeling Modeling assumptions Material models Parametric FE study Design Methods Strength as a function of strain Strain as a function of slenderness DSM style inelastic strength prediction: Strength as a function of slenderness Future work Conclusions

4 Inelastic reserve M y <M<M p hot-rolled cold-formed Limit b f /2t f (FLB) Limit h/t w (WLB) M M p C and Z sections M M y Hats (stiffened element compression flange) M can be >M y

5 Motivation Stub-column interaction diagram P n P y hot-rolled steel Cold-formed steel cold-formed steel (DSM for hot -rolled steel?) M n including local, distortional, and LTB via Direct Strength Method M y M p M n DSM including Inelastic reserve?

6 Elementary mechanics: mechanism 1 for inelastic reserve ε max =ε y, C y =1 ε max =2ε y, C y =2 ε max =3ε y, C y =3 ε max =, C y = ε max =ε y, C y =1 ε max =2ε y, C y =2 ε max =3ε y, C y =3 ε max =, C y = fy (a) (b) (c) (d) M=M y M y <M<M p M=M p (a) (b) (c) (d) M=M y M y <M<M p M=M p symmetric section

7 Elementary mechanics: mechanism 2 for inelastic reserve C y =1/3 C y <1 C y =1.2 C y = ε max =ε y, C y =1 ε max =2ε y, C y =2 ε max =3ε y, C y =3 ε max =, C y = fy (a) (b) (c) (d) M=M y M y <M<M p M=M p (a) (b) (c) (d) C y-tension =1 C y-tension =1.5 C y-tension =4 C y-tension = M=M y M y <M<M p first yield in tension M=M p

8 Observed inelastic reserve in CFS tests count M test >0.95M y max M test /M y Section and Researcher Hats and Deck Sections Acharya (1997) Desmond (1977) Hoglund (1980) Papazian (1994) Winter (1946) C and Z Sections Cohen (1987) LaBoube and Yu (1978) Rogers (1995) Shan (1994) Yu and Schafer (2003) Yu and Schafer (2006)

9 Observed inelastic reserve in CFS tests count M test >0.95M y max M test /M y min M p /M y max M p /M y Section and Researcher Hats and Deck Sections Acharya (1997) Desmond (1977) Hoglund (1980) Papazian (1994) Winter (1946) C and Z Sections Cohen (1987) LaBoube and Yu (1978) Rogers (1995) Shan (1994) Yu and Schafer (2003) Yu and Schafer (2006) shape factor

10 Back-calculated strain at failure 2.50 first yield in tension comp. C y-tension (εmax-tension/εy) tension yielding inelastic buckling 0.50 no inelastic reserve C y (ε max-compression /ε y )

11 Recent testing on CFS beams Cold-formed steel beams may buckle instead of exhibiting inelastic reserve Recent testing has investigated local and distortional buckling as separate limit states on typically used CFS beams, even in the inelastic regime Yu and Schafer (2003,2006,2007)

12 buckling modes in a thin-walled beam M cr L cr local buckling distortional buckling lateral-torsional buckling

13 Local buckling tests 25 C and Z beam pairs local mode shape Standard panel fastener configuration, test and FE showed distortional buckling failures even though panel stiffness should be enough to restrict DB Yu and Schafer (2003)

14 Local buckling tests Total 25 C and Z beams local mode shape Panel fastener configuration to insure panel is engaged and distortional buckling restricted Yu and Schafer (2003)

15 Distortional buckling tests Total 24 C and Z beams Distortional mode shape Yu and Schafer (2006)

16 Typical test results P Test 8.5Z092 P P crd P y local distortional Local buckling test 99% of NAS* Δ Distortional buckling test 83% of NAS* *North American Specification for the Design of Cold-Formed Steel Structural Shapes 2007 version adopted new rules to account for distortional buckling in beams

17 Summary test results inelastic reserve DSM-Local DSM-Distortional Local buckling tests Distortional buckling tests M test /M y (M y /M cr ) 0.5

18 Overview Background and motivation on inelastic bending capacity in cold-formed steel (CFS) members Mechanisms for inelastic reserve Observed inelastic reserve in CFS tests Recent testing on CFS beams & summary of test results FE modeling Modeling assumptions Material models Parametric FE study Design Methods Strength as a function of strain Strain as a function of slenderness DSM style inelastic strength prediction: Strength as a function of slenderness Future work Conclusions

19 Objective Finite element modeling To study inelastic reserve in typical (symmetric) CFS beams for local and distortional limit states. Method Parametric study motivated from testing of Yu and Schafer (2003,2006) Material and geometric nonlinear analysis in ABAQUS using S9R5 shell element models Models generated from purpose-built Matlab code (performs elementary mechanics, C y, FSM and FEM models)

20 Boundary conditions (distortional buckling) Warping restraint (a la FSM) ends are constrained to act rigidly about tie point rotation (moment) enforced at ends compression flange is unrestrained.

21 Boundary conditions (local buckling) Warping restraint (a la FSM) ends are constrained to act rigidly about tie point rotation (moment) enforced at ends compression flange rot. Restrained at mid width of flange

22 Sections considered 3.62 in. to 6 in. SSMA sections thickness ( in) vary t MBMA member company section 8.5 in.

23 Geometric imperfections Shape = local or distortional buckling mode shape Magnitude = 50% probability of exceedance based on statistics gathered by Schafer and Peköz (1998) as summarized below d d Type 1 Type 2 Type 1 Type 2 (local) (distortional) Probability (X < x) Type 1 Type 2 P(X < x) d/t d/t μ σ CDF of Maximum Imperfection

24 Material models stress (ksi) fy=73.4ksi fy=62ksi fy=56ksi fy=44ksi fy=33ksi strain σ ε curves measured, Yu and Schafer (2007)

25 Strength as function of slenderness? LOCAL DISTORTIONAL Z C C C054 D85Z120-1 D362C Z C C C054 D85Z120-1 D362C054-4 M u l/m y 1.10 M ud /M y λ l = (M y /M crl ) λ d = (M y /M crd ) 0.5 Direct DSM-style relationship is difficult to justify with initial results Sensitivities to imperfection magnitude (direction) observed (Camotim et al. 2006)

26 Strain as function of slenderness? LOCAL DISTORTIONAL Z C C C054 D85Z120-1 D362C Z C C C054 D85Z120-1 D362C054-4 C y 6 C y λ l = (M y /M crl ) λ d = (M y /M crd ) 0.5 Strain demands greater in distortional buckling Slenderness where inelastic C y >1 occurs much lower for local

27 Strain as function of strength? LOCAL DISTORTIONAL M u l/m y 1.1 M ud /M y Z Z120 6C C C C C C054 D85Z120-1 D85Z120-1 D362C054-4 D362C C y C y Strain to strength similar for local and distortional buckling Elementary mechanics is driving inelastic reserve

28 Overview Background and motivation on inelastic bending capacity in cold-formed steel (CFS) members Mechanisms for inelastic reserve Observed inelastic reserve in CFS tests Recent testing on CFS beams & summary of test results FE modeling Modeling assumptions Material models Parametric FE study Design Methods Strength as a function of strain Strain as a function of slenderness DSM style inelastic strength prediction: Strength as a function of slenderness Future work Conclusions

29 Strain as function of reserve strength (M test or FE -M y )/(M p -M y ) Test FE - Dist. FE - local Test FE - Dist. FE - local M M n p M M y y = 1 1 C y M M M n p M y y = 1 1 C y C y

30 Strain as function of slenderness if if λ < λ, C l ly λ < λ, C dy yl yd λl = λly λd = λdy whereλ whereλ dy ly = = C y FE - Dist. FE - Local Test - Dist. controlled Test - Local controlled λ l = (M y /M crl ) 0.5 or λ d = (M y /M crd ) 0.5

31 Direct Strength Method for Beams

32 M = M + ( M M ) nd y p 1 2 λ d Inelastic y λ reserve strength prediction dy 1 (M test -M y )/(M p -M y ) C y =4 C y =3 C y =2 cutoff Test - Local controlled Test - Dist. controlled FE - Local FE - Distortional D M p /M y = slenderness: λ l =(M y /M crl ) 0.5 or λ d =(M y /M crd ) 0.5 L

33 M = M + ( M M ) nd y p 1 2 λ d Inelastic y λ reserve strength prediction dy if LOCAL DISTORTIONAL 0.9 λ ( ) λl l < λ y, M n = M y + M p M y 1 ( ) λd l l if λ <, = + λ 1 ly d λdy M nd M y M p M y λdy 2 Table 1 Prediction statistics for inelastic bending capacity Section and M test /M n * M test /M n for M n >M y Researcher ave. st.dev. n ave. st.dev. n Hats and Decks Acharya (1997) Desmond (1977) Hoglund (1980) Papazian (1994) C and Z s Cohen (1987) LaBoube and Yu (1978) Rogers (1995) Shan (1994) Yu and Schafer (2003) Yu and Schafer (2006) All test data FE Studies Local models Distortional models * these statistics are provided only when Eq. 14 and 15 are employed for prediction, i.e., when the predicted M n is greater than M y (or equivalently λ l <λ ly or λ d <λ dy )

34 Why cutoff strain? Local demands can be far in excess of average (elementary mechanics) strain demands Table 1 Strain demands summary table C ymax-membrane /C y C ymax-surface /C y ave. st.dev. max ave. st.dev. max Local models Distortional models * C y without any modifier is taken as the average of the flange membrane strain

35 Future work Impact of strain hardening Further study on sensitivity to inward vs. outward inelastic buckling collapse mechanisms Extent to which LTB, and unbraced length influence solution particularly in distortional buckling Comprehensive verification of FE model Further study on using elastic slenderness to correlate to inelastic strength

36 Conclusions Inelastic bending reserve exists in CFS beams developing from one of two distinct mechanisms. From FE models, average strain demands in inelastic distortional buckling were found to be much greater than in inelastic local buckling at equivalent slenderness. Peak strain demands exceed average member strains predicted via elementary mechanics. Simple expressions for strain demand in inelastic local and distortional buckling as function of elastic cross-section slenderness as well as inelastic bending reserve are established. Preliminary Direct Strength Method design expressions for inelastic bending reserve in local and distortional buckling as a function of elastic section slenderness are established.

37 ? Any Questions?

38 Observed inelastic reserve in CFS beams 1.5 transition in inelastic regime, where M n >M y has not been explored for cold-formed sections. M M test y DSM local buckling DSM distortional buckling Experiments failing in local mode Experiments failing in distortional mode Elastic buckling cross-section slenderness λ max = My Mcr

39 Finite Element Modeling by ABAQUS Shell element S4R for sections, panels and tubes, solid element C3D8 for load beam. Geometric imperfection and material nonlinearity are considered. Residual stress is not considered. The auto Stabilization method in ABAQUS were used for postbuckling analysis.

40 Finite Element Modeling - geometric imperfections Geometric imperfection is obtained by the superposing two eigenmodes which are calculated by finite strip method (software CUFSM). + = Local buckling shape Distortional buckling shape Input geometric imperfection

41 Basic test setup

42 Formulas for strain at failure y' h σ dxdy + 0 y' σdxdy = 0 σ = f(ε) and ε = (y/y lim )ε max M = A f ( ε)yda = A f ( ( y y ) lim ε max )yda ε max-compression = C y ε y and ε max-tesion = C y-tension ε y