ColdFormed Steel Design by the Direct Strength Method: ByeBye Effective Width

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Cold-Formed Steel Design by the Direct Strength
MethodBye-Bye Effective Width

• NASCC SSRC sessions
• April 2003
• Ben Schafer, Ph.D.

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Specification complication

• Anyone who has ever attempted to design a
  light-gage member following the Specification
  provisions probably realized how tedious and
  complex the process was.
• When such cold-formed framing is needed one of
  two things tend to happen to the engineers they
  either uncritically rely on the suppliers
  literature, or simply avoid any cold-formed
  design at all
• Alexander Newman 1997, in Metal Building Systems

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Rinchen (1998) - Australia
Kesti (2000) - Finland
Landolfo and Mazzolani (1990) - Italy
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Specification complication explained

• Sections are not doubly-symmetric
• Element elastic buckling calculation (ks)
• Effective width
• effective width f(stress,geometry)
• stress f(effective properties e.g., Aeff,
  Ieff)
• iteration results
• Web crippling calculations
• Inclusion of system effects

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Specification complication explained

• Sections are not doubly-symmetric
• Element elastic buckling calculation (ks)
• Effective width
• effective width f(stress,geometry)
• stress f(effective properties e.g., Aeff,
  Ieff)
• iteration results
• Web crippling calculations
• Inclusion of system effects

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www.ce.jhu.edu/bschafer/cufsm
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finite strip resource
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LGSI Z12-25-14g
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My192 kip-in.
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Local buckling
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Distortional
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Lateral-torsional
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Direct strength prediction

• Pn f (Py, Pcre, Pcrd, Pcrl)?
• Input
• Squash load, Py
• Euler buckling load, Pcre
• Distortional buckling load, Pcrd
• Local buckling load, Pcrl
• Output
• Strength, Pn

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Elastic buckling
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Elastic buckling
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Direct Strength Curve(university of sydney
testing)
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Columns

• Lipped channels
• Lipped zeds
• Lipped channels with intermediate web stiffener
• Hat sections
• Rack post sections

Kwon and Hancock (1992), Lau and Hancock (1987),
Loughlan (1979), Miller and Peköz (1994),
Mulligan (1983), Polyzois et al. (1993),
Thomasson (1978)
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267 columns , b 2.5, f 0.84
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Pnmin(Pne,Pnl,Pnd)
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Beams

• Lipped and plain channels
• Lipped zeds
• Hats with and without intermediatestiffener(s)
  in the flange
• Trapezoidal decks with and without intermediate
  stiffener(s) in the web and the flange

• Cees and Zeds Cohen 1987, Ellifritt et al. 1997,
  LaBoube and Yu 1978, Moreyara 1993, Phung and Yu
  1978, Rogers 1995, Schardt and Schrade 1982,
  Schuster 1992, Shan 1994, Willis and Wallace 1990
• Hats and Decks Acharya 1997, Bernard 1993,
  Desmond 1977, Höglund 1980, König 1978, Papazian
  et al. 1994

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569 beams, b2.5, f0.9
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Direct strength advocacy

• No effective width, no elements, no iteration
• Gross properties
• Element interaction
• Distortional buckling
• Wider applicability and scope
• Encourage cross-section optimization

Your computer performs analysis that employs
fundamental mechanics instead of just mimicking
old hand calculations. DSM integrates known
behavior into a straightforward design procedure.
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provided examples
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Plenty of future research needed

• beam-columns and eccentric loads,
• isolated and patterned perforations,
• laterally un-braced flexural members,
• significant neutral axis shift in the
  post-buckling regime,
• geometric limitations and definition of
  applicability,
• fine-tuning and further calibration of strength
  expressions,
• interaction of distortional buckling with other
  modes,
• shear and shear interaction issues,
• calibration of new cross-sections, and
• elastic distortional buckling of all
  cross-sections.

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Concluding thoughts

• DSM represents an opportunity for a new direction
  in cold-formed steel design.
• By taking advantage of simple, yet fundamental,
  mechanics solutions (member elastic buckling via
  finite strip) we have the means to vastly
  simplify and at the same time improve design.
• DSM can be used now for unusual sections via the
  rational analayis clause in AISI, and will be
  adopted as an alternative design procedure in the
  next Specification.

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Resources

• Research
• www.ce.jhu.edu/bschafer
• Finite strip
• www.ce.jhu.edu/bschafer/cufsm
• Direct Strength
• www.ce.jhu.edu/bschafer/direct_strength

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How does Direct Strength work?

• ELASTIC BUCKLING
• You must determine all relevant elastic buckling
  values for your section, e.g., for a column the
  local, distortional, and flexural-torsional
  buckling loads.
• DIRECT STRENGTH CURVES
• Given the elastic buckling loads and the yield
  load empirical expressions (e.g., SSRC column
  curves) are used to predict the capacity.

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How does Direct Strength work?

• ELASTIC BUCKLING
• You must determine all relevant elastic buckling
  values for your section, e.g., for a column the
  local, distortional, and flexural-torsional
  buckling loads.
• DIRECT STRENGTH CURVES
• Given the elastic buckling loads and the yield
  load empirical expressions (e.g., SSRC column
  curves) are used to predict the capacity.

Finite Strip (CUFSM)
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