Towards a Direct Strength Method for Cold-Formed Steel Beam-Columns Structural Stability Research Council Orlando, Florida May 2010 Y.Shifferaw1 , B.W.Schafer2 (1),(2) Department of Civil Engineering- Johns Hopkins University

Acknowledgments National Science Foundation NSF – this project Cheng and AISI is work leading to this project AISC is future work and some motiviation (AISC Faculty Fellowship, Mina Seif)

Overview Introduction Basis of DSM: yield and elastic critical buckling Finite element collapse analysis in the P-M space Direct Strength Method preliminaries for local and distortional buckling in the P-M space Conclusion Future research

Introduction Current and postulated beam-column design approaches Py Postulated b n curve for all P and M ratios Postulated b for a given P and M ratio My Py

Sections considered 3.625 in. Channel Eave Strut 1.625 in. 1.75 in fy=55.9 ksi 250 Look up the flange width, maybe add to slide Subset of tested sections investigated by nonlinear FE analysis Thickness systematically vareid so that inelastic reserve can be examined… Impact of varying thickness is different on local and distortional buckling so this leads to some complication, but the boundary conditiosn used attempt to restrict local bucklign models to local buckling only – distortional bucklgin models, may still fail in local bucklig – but (a) only distortional imperfections are used and (b) no local buckling observed in the models completed to date with DB boundary conditiosn

DSM basis: major axis yielding and elastic buckling

DSM basis: minor axis yielding and elastic buckling

DSM basis: biaxial yielding and elastic buckling

Finite element modeling Objective To study combined P-M collapse loads in CFS beam-columns for local and distortional limit states. Method Material and geometric nonlinear analysis in ABAQUS using S9R5 shell element models; geometric local and distortional imperfections considered Models generated from purpose-built Matlab code

Local FE

Major axis local for channel

Major axis local for eave strut

Distortional FE

Minor axis distortional for channel section

Minor axis distortional for eave strut section

Overview Introduction Basis of DSM: yield and elastic critical buckling Finite element collapse analysis in the P-M space Direct Strength Method preliminaries for local and distortional buckling in the P-M space Conclusion Future research

Preliminary DSM beam-column strength prediction LOCAL

Local DSM vs major axis strength bounds for channel

Local DSM vs minor axis strength bounds for channel

Local DSM vs major axis strength bounds for eave

Local DSM vs minor axis strength bounds for eave

Preliminary DSM beam-column strength prediction DISTORTIONAL

Distortional DSM vs major axis strength bounds for channel

Distortional DSM vs minor axis strength bounds for channel

Distortional DSM vs major axis strength bounds for eave

Distortional DSM vs minor axis strength bounds for eave

Conclusion Under combined loading the assumptions in linear interaction equations are invalidated in CFS members due to Un-symmetric shapes of common CFS sections Consideration of cross-section stability Finite element models for local and distortional models are developed to examine load-bending collapse envelopes. Preliminary Direct Strength Method design expressions for beam-columns in local and distortional buckling as a function of elastic section slenderness are established and compared with the FE models developed. Significant efficiency in the proposed DSM approach in comparison with traditional design.

Future work Incorporation of recently proposed inelastic bending provisions Further preliminary studies including global buckling Beam-column tests Comprehensive FE parametric study Formal DSM proposals for beam-columns

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