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Extending Direct Strength Design to Cold-Formed Steel Beams with Holes Cris Moen, Virginia Tech Ben Schafer, Johns H PowerPoint Presentation
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Extending Direct Strength Design to Cold-Formed Steel Beams with Holes Cris Moen, Virginia Tech Ben Schafer, Johns Hopkins University. 20 th International Specialty Conference on Cold-Formed Steel Structures Wednesday, November 3, 2010 St. Louis , MO. Outline.

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slide1

Extending Direct Strength Design to Cold-Formed Steel Beamswith Holes

Cris Moen, Virginia Tech Ben Schafer, Johns Hopkins University

20th International Specialty Conference on Cold-Formed Steel Structures

Wednesday, November 3, 2010

St. Louis, MO

outline
Outline
  • Direct Strength methodology
  • Elastic buckling (including holes!)
  • DSM design expressions ( considering holes!)
    • Global buckling
    • Local buckling
    • Distortional buckling
    • Validation
  • DSM (Holes) example
  • Conclusions and future work
direct strength methodology
Direct Strength methodology
  • The basic idea… use elastic buckling properties and the beam yield moment topredict capacity

Elastic buckling moments

  • Mcrl, Mcrd, Mcre

Beam yield moment

  • My

Distortional buckling failure

Picture - Yu, C., and Schafer, B.W. "Distortional buckling tests on cold-formed steel beams", ASCE Journal of Structural Engineering, 132(4), 2006, 515-528.

direct strength methodology4

for beamswith holes

Direct Strength methodology

including holes

  • The basic idea… use elastic buckling properties and the beam yield moment topredict capacity

including holes

Elastic buckling moments

  • Mcrl, Mcrd, Mcre

Unstiffened holes decrease distortional buckling capacity

Beam yield moment

  • My

Limit capacity to net section

  • Mynet
outline5
Outline
  • Direct Strength methodology
  • Elastic buckling (including holes!)
  • DSM design expressions ( considering holes!)
    • Global buckling
    • Local buckling
    • Distortional buckling
    • Validation
  • DSM (Holes) example
  • Conclusions and future work
elastic buckling with holes
Elastic Buckling (with holes)

Moen, C. D., and Schafer, B. W. (2009). "Elastic buckling of thin plates with holes in compression or bending." Thin-Walled Structures, 47(12), 1597-1607.

Moen, C. D., and Schafer, B. W. (2009). "Elastic buckling of cold-formed steel columns and beams with holes." Engineering Structures, 31(12), 2812-2824.

Local buckling – finite strip and classical plate approximations

Global buckling– “weighted average” approach

Holes decrease bending and torsional stiffness

Distortional buckling– finite strip

Assume local buckling dominates at a hole or between holes

Reduce web thickness to simulate reduced web bending stiffness

outline7
Outline
  • Direct Strength methodology
  • Elastic buckling (including holes!)
  • DSM design expressions ( considering holes!)
    • Global buckling
    • Local buckling
    • Distortional buckling
    • Validation
  • DSM (Holes) example
  • Conclusions and future work
global buckling
Global buckling

Use simplified methods or thin shell FE to calculate…

local global buckling interaction
Local-global buckling interaction

Limit capacity to the net section

Use simplified methods or thin shell FE to calculate…

distortional buckling
Distortional Buckling

Limit capacity to the net section

transition through inelastic buckling regime

Use simplified methods or thin shell FE to calculate…

distortional buckling11
Distortional Buckling

Holes decrease flexural capacity by 8%.

Mynet/My=0.90

No holes

dsm holes validation
DSM Holes Validation
  • Beam test database:
  • SSMA sections, 204 simulations (laterally braced)
  • Compare tests to DSM Holes predictions:

LRFD resistance factor f calculated with AISI Chapter F approach

dsm holes validation13
DSM Holes Validation
  • Existing test data (unstiffened holes):

Shan, M.Y., and LaBoube, R.A. "Behavior of Web Elements With Openings Subjected to Bending, Shear, and the Combination of Bending and Shear", Civil Engineering Study 94-2, Cold-formed Steel Series, University of Missouri-Rolla, Rolla, 1994.

Small holes, only local buckling considered…

  • Recent experiments (unstiffened holes):

Soroori Rad, B.H.T. (2010). "Experiments on Cold-Formed Steel Beams with Holes," M.S. Thesis, Virginia Tech, Blacksburg.

Schudlich, A., von der Heyden, A., Moen, C.D. (2011) “Distortional buckling of cold-formed steel joists with unstiffened holes,” Sixth International Conference on Thin-Walled Structures, Timisoara, Romania.

L, D, and G considered…

Focused on distortional buckling…

We are slowly building an experimental database. Tests on CFS beams with stiffened holes are coming soon…

outline14
Outline
  • Direct Strength methodology
  • Elastic buckling (including holes!)
  • DSM design expressions ( considering holes!)
    • Global buckling
    • Local buckling
    • Distortional buckling
    • Validation
  • DSM (Holes) example
  • Conclusions and future work
design example
Design Example

Laterally braced…

SSMA 550S162-33 joist with typical slotted holes

All example files (CUFSM, Mathcad) are available at www.moen.cee.vt.edu

local buckling
Local Buckling
  • Mcrl=min(Mcrlnh, Mcrlh)

Local buckling between holes

local buckling17
Local Buckling
  • Mcrl=min(Mcrlnh, Mcrlh)

Local buckling at a hole

local buckling18
Local Buckling

Local buckling at a hole controls.

  • Mcrl=min(17.5 kip-in., 10.5 kip-in.)

Local buckling dominates at the holes…

distortional buckling19
Distortional Buckling

Unstiffened holes decrease Mcrd

design summary
Design Summary
  • My=29.1 kip-in.
  • Mynet=29.0 kip-in.
  • Mne=29.0 kip-in. (29.1 kip-in.)
  • Mnl=17.5 kip-in. (20.1 kip-in.)
  • Mnd=19.4 kip-in. (21.9 kip-in.)

Holes have minimal effect on yield moment.

Holes

No holes

Local buckling at a hole controls flexural capacity…

The DSM prediction is contrary to AISI-S100-07 Section B2.4 which allows holes with dh/h<0.38

to be ignored when calculating flexural capacity.

conclusions and future work
Conclusions and future work
  • Elastic buckling infrastructure and DSM design expressions are ready for implementation
  • DSM for beams with holes coming soon
  • DSM for columns with holes – AISI ballot
  • DSM for members with stiffened holes – coming soon

Publications on theirway…

Moen, C. D., and Schafer, B. W. (In Press). "Direct Strength Method for the Design of Cold-Formed Steel Columns with Holes." ASCE Journal of Structural Engineering.

Moen, C. D., and Schafer, B. W. (In Preparation). "Direct Strength Method for the Design of Cold-Formed Steel Beams with Holes." ASCE Journal of Structural Engineering.

Grey, C., Moen, C. D., and Yu, C. (2011). ”Elastic buckling simplified methods for cold-formed steel columns and beams with stiffened holes.” SSRC Annual Conference, Pittsburg, PA.

slide22

Thanks for your attention!

DSM Holes design example:

SSMA 550S162-33 joist with slotted holesis available at:

www.moen.cee.vt.edu

Mechanics

Strength prediction of CFS members with holes