beam in bending
TRANSCRIPT
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Beam in bending:
• Ideal beam – without imperfections:
characteristic is critical moment Mcr
• Real beam – with imperfections
characteristic is resistance of beam Mb,Rd
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A. Ideal beam – without imperfections:
Critical moment Mcr
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Elastic critical moment Mcr
„fork“ boundary conditions, uniform bending moment M,
uniform nonwarping cross-section: warping stiffness EIw= 0 kNm4
L
GIEIM
tz
cr
EIz bending stiffness
GIt torsional stiffness
L span
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Lateral Torsional Buckling
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Positive influence of warping stiffness
on Mcr
• Rectangular cross-section
Iw ≈ 0 m6
Iw = b3h3/36
• Double symmetric cross-section
L
GIEIM
tz
cr
L
LEIGIEIM
w )/( 22
tz
cr
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Positive influence of vertical
deflection on Mcr
,)/( 22
tzcr
L
LEIGIEIM
w
1
t
w
z
t
y
z
y
z
GIL
EI
EI
GI
I
I
I
I2
2
111
1
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Broude 1953 in Russia,
Trahair cca 20 years later in Australia
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Mcr for any boundary conditions and
any type of loading• 1958 Mrázik, obtained coefficients for 11 cases:
• 1960 Clark, Hill, colected coefficients for 21 cases:
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Mcr for any boundary conditions and
any type of loading
• 1974 Djalaly, obtained coefficients for 37 cases:
• ENV 1993-1-1:1992 a ENV 1999-1-1:1998
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Relations between cofficients of
different formulae
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Formulae from ČSN and STN 73 1401
• cl. H.2: loading prependicular to axis of symmetry
• It is simplification of Mráziks formula
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Formulae from ČSN and STN 73 1401
cl. H.6: loading in the direction of symmetry axis
e, ac are used with sign + or -
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Formulae from ČSN and STN 73 1401
cl. H.6: loading in direction of symmetry axis
It is again simplification of Mrazics formula
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1ck
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Relations between cofficients of
different formulae
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Mcr according to DIN 18 800
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2
,2
ely
cr
EWM
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Not correct things in European
prestandards ENV 1993, ENV 1999
• Some coefficients Ci are incorrect (e.g. instead of C1=0.411 it is used C1=1.73,etc.)
• No information is given about used torsion boundary conditions (kw=1) in the cases of end moments
• No information is given about used lateral bending boundary conditions (fixed is left beam end)
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Not correct things in European
prestandards ENV 1993, ENV 1999
• No definition of Mcr location (location is in middle of span)
• Influence of torsion stiffness and warping stiffness are not taken into account in calculation of C1 coefficients
• For beams with nonsymmetric cross-sections under nonsymmetric bending end moments are Ci coefficients incorrect
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ENV 1993-1-1, ENV 1999-1-1
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ENV 1993-1-1, ENV 1999-1-1
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prEN 1993-3: October 2001
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prENV 1993-3: October 2001
EN 1993-1-1 – no formulae for calculation of Mcr
EN 1999-1-1 and NA to STN EN 1993-1-1 contain
our proposals how to calculate Mcr
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EN 1999-1-1 and NA to STN EN 1993-1-1
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C1, C2, C3 are cofficients depending on
type of loading and boundary conditions
For symmetric cross-section to axis y-y is zj = 0
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In STN 73 1401 positive axis z goes
down, in STN EN goes up !!!
a) Loading in the direction of symmetry axis
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Loading perpendicular to axis of symmetry:
b) single symmetric,
c) double- or point-symmetric
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For any ratio of end moments when kz=1
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Calculation of Mcr with the help of
computer programs (no limitation)
• DRILL (emeritus Prof. Friemann, TU Darmstadt)
• BT II (Prof. Ostrerrieder)
• IBDSQ (Dickel, Prof. Rothert, …)
• RSTAB (Dlubal)
• CalcMcr (Baláž)
• LTBeam (CTICM Paris)
• ALPHAcr (Lennert, TU Graz)
• many others
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B. Real beam – with imperfections:
Resistance of beam Mb,Rd
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When it is not necessary to verify
LTB of beam ?
• beam flange in compression is lateraly
supported
• beam has closed profile (big torsional
stiffness)
• beam is loaded in direction of minor
stiffness
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Die Espe Die Birke
BRZOZAOSIKA
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Structure magazine, February 2008
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1ck
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Interval in which increasing of reduction
coefficient χLT by factor f is effective
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Obr.3
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Theory of the 2. order of beams with imperfection
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Obr.2
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Obr. 3
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Reduction factors χLT
Eurocode: Perry – Robertson formula
DIN 18 800: Merchant – Rankine formula
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Resistance of cross-section
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.11.1
0
a0 ( )
0.1
2 a0 ( )
0.1
1 2 a0 ( )
20
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20
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0.2
0.3
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0.5
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0.7
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1
1.11.1
0
d ( )
0.1
2 d ( )
0.1
1 2 d ( )
20