1. 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

2. 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)

3. 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

4. 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

5. 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

6. DSM basis: major axis yielding and elastic buckling

7. DSM basis: minor axis yielding and elastic buckling

8. DSM basis: biaxial yielding and elastic buckling

9. 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

10. Local FE

11. Major axis local for channel

12. Major axis local for eave strut

13. Distortional FE

14. Minor axis distortional for channel section

15. Minor axis distortional for eave strut section

16. 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

17. Preliminary DSM beam-column strength prediction
LOCAL

18. Local DSM vs major axis strength bounds for channel

19. Local DSM vs minor axis strength bounds for channel

20. Local DSM vs major axis strength bounds for eave

21. Local DSM vs minor axis strength bounds for eave

22. Preliminary DSM beam-column strength prediction
DISTORTIONAL

23. Distortional DSM vs major axis strength bounds
for channel

24. Distortional DSM vs minor axis strength bounds
for channel

25. Distortional DSM vs major axis strength bounds
for eave

26. Distortional DSM vs minor axis strength bounds
for eave

27. 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.

28. 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

29. ?