Transcript The Direct Strength Method of Cold

Local buckling design without effective width
new developments in the building industry
TRB - A2C06 Committee Meeting
January 2004
Ben Schafer, Ph.D.
The Johns Hopkins University
Profiles evolve – how does design keep up?
Direct Strength
In the building industry we
have recently added a
new alternative method
for local buckling design
that:
(1) requires no effective
width calculation, and
(2) incorporates a numerical
analysis that accurately
predicts local buckling
even for complicated
cross-sections.
Fictitious profile
Examine bending
Local buckling occurs at 0.75 of the reference bending load
Idea for incremental profile improvement
Local buckling at 4 times reference
Local buckling at 2.7 times reference
Big boost from 0.75! I want to take advantage of that….
(1) hard to calculate by hand
(2) effective width becomes overly complicated
Conventional approach
Via a specification we get
• all local buckling modes,
• effective widths,
• effective properties, and
finally the strength.
Direct Strength Approach
M n 
M cr  M cr
 1  0.22
My 
My  My


M n  Nominal bending capacity
of the profile
M y  Max bending capacity
- no local buckling
M cr  Local buckling bending
moment (CUFSM)
Via alternative we get
• all local buckling modes
calculated exactly, and
the strength.
Direct Strength for Pipe…
M n 
M cr
 1  0.22
My 
My

or
 M
cr

 My

Tn 
Tcr  Tcr
 1  0.22
Ty 
Ty  Ty


and
M
T

1
M n Tn
(could be strain based)
M,T
Thrust - Columns
•Lipped channels
•Lipped zeds
•Lipped channels with int. web stiffener
•Hat sections
•Rack post sections
267 columns , b = 2.5, f = 0.84
Bending - Beams
•
•
•
•
569 beams, b=2.5, f=0.9
Lipped and plain channels
Lipped zeds
Hats with and without intermediate
stiffener(s) in the flange
Decks with and without intermediate
stiffener(s) in the web and the flange
DSM for flexible pipe?
• More research would be needed to bring the
Direct Strength Method to flexible pipe, but the
existing research plus the success of the
AASHTO method demonstrate it is possible.
• Design for thin-walled flexible pipe has benefited
from existing research, new design methods
provide new opportunities for simplification and
optimization.
www.ce.jhu.edu/bschafer