Muller, J. A.

Yen, B. T.

English

Lehigh University: Lehigh Preserve

Lehigh University
Lehigh Preserve
Fritz Laboratory Reports Civil and Environmental Engineering
1966
A summary report on the behavior of thin-web
plate girders under repeated loading august 1966
B. T. Yen
J. A. Muller
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Recommended Citation
Yen, B. T. and Muller, J. A., "A summary report on the behavior of thin-web plate girders under repeated loading august 1966" (1966).
Fritz Laboratory Reports. Paper 1860.
http://preserve.lehigh.edu/engr-civil-environmental-fritz-lab-reports/1860
."
Submitted to the
Welded Plate Girder Project Subcommittee
of the
Welding Research Council
for approval
for Publication in the
HIGHWAY RESEARCH RECORD
A SUMMARY REPORT ON
THE BEHAVIOR OF THIN-WEB PLATE GIRDERS
UNDER REPEATED LOADING
by
B. T. Yen
J. A. MuelJ,er
Lehigh University
Fritz Engineering Laboratory Report No. 303.13
August·, 1966
303.13
TABLE OF CONTENTS
SYNOPSIS
INTRODUCTION
MOVEMENT OF PLATE GIRDER WEBS
FATIGUE FAILURE OF THIN WEBS
STUDY OF WEB STRESSES
CONCLUSIONS~AND DISCUSSION
ACKNOWLEDGEMENTS
FIGURES
REFERENCES
i
ii
1
2
5
7
10
11
21
303.13
SYNOPSIS
ii
Thin-web plate girders behave in a different manner than beams
when subjected to repeated loading. Repeated lateral web movements
cause fatigue cracks along web boundaries. The pattern of web
deflections under load, the occurrence of cracks and their locations,
and the estimate of stresses in girders are all described on the
basis of experimental and analytical results. The possibility of
.~ .
using slender-web girders for highway bridges is discussed.
303.13
INTRODUCTION
-1
..
Plate girders are common' bridge members for carrying bending
moments and shearing forces beyond the capacities of beams. Plate
girders usually are much deeper than beams and their deep web
plates are susceptible to lateral buckling. The elimination of the
possibility of web buckling long has provided a limit in bridge
girder design. Specifically, the web depth to thickness ratio
(the slenderness ratio) and the spacing of transverse stiffeners
are kept low so that web panels will not buckle. The consequence
is that the functioning of a plate girder is identical to that of a
beam.
Results of studies have shown that a sudden web buckling
phenomenon does not exist in steel plate girders (!) and that the
static load carrying capacity of a plate girder is not its web
buckling load (~,2.,':D. Based on analytical methods of static
strength evaluation, girders for buildings are no longer designed
by the web buckling theory. Thin webs are permitted and stiffeners
are spaced relatively far apart in such girders C.~).
For plate girders in bridges, the utilization of the above-
mentioned post-buckling strength can be made only after the fatigue
behavior of girders under repeated loading has been investigated .
Since a girder is essentially a beam--except for the web-~and since
numerous fatigue studies have been made on beams (f), the emphasis
here is on the behavior of girder webs, or web panels.
303.13
MOVEMENT OF PLATE GIRDER WEBS
-2
Under gradually applied loads on a girder, the web of a panel
deflects laterally in a gradual manner. For a particular point on
the web, the load-lateral deflection relationship generally.takes
one of the two forms of Fig. 1 (]) .At points of compression and
high shear, deflections are relatively large at high loads and their
relationship is not linear (Fig. la). Points in the tension region
of a girder panel usually decrease their deflection when more load
is applied (Fig. lb). In both cases, there are initial lateral
deflections or out-of-straightness of the web; and there is no
sudden "buckling ll as would have been indicated by a discontinuity of
the P - ~ curves. The magnitudes of these deflections depend on the
geometry of the panel and the location of the point in question.
Points far away from the paDel boundaries usually have much larger
deflection magnitudes than those of points near the flanges and
stiffeners. Thin web plates usually deflect much more than thick
plates.
When repeated loads are applied ona girder, each load cycle
constitutes a gradual increase and decrease of load within the
period of the loading. The lateral deflection of a point increases
and decreases with the load as described by the P - ~ diagrams above.
The result is that, under repeated loads, web points move laterally
back and forth (Fig. 2). This movement is synchronized with the
repeated loading and is characteristic of thin-web plate girders (~).
303.13 -3
.,
Cross-sectional shapes of thin-web girders under loads have
been measured (Z) and the results, when sketched out, give a
pictorial indication of the web movement (Fig. 3). If the applied
load fluctuates repeatedly between two magnitudes, the cross
sections of the web move repeatedly laterally between two deflected
positions. It is quite clear then that thin-web plate girders
under fatigue loading are subjected tq repeated bending (secondary
bending) of the web plate.
Because the lateral movement of a web panel involves all points
within the panel, the deflected shape of the whole web, rather than
that of cross sections or the P - ~ diagrams of individual web
points, is more descriptive in revealing the behavior of the web.
Lateral deflection contours of web panels therefore are constructed.
Depending upon the loading condition on a panel, there are three
patterns of deflection contours for thin-web panels (2). Under pure
bending moment, the compression zone of a web panel deflects much
more than the tension region, as is indicated by the close distance
between contour lines in the upper part of the panel in Fig. 4. At
high shear, a pattern of inclined contour lines is usually present
(Fig. 5). The inclination follows generally the direction of the
panelTs tension diagonal. When both bending moment and shearing
forces are appreciable, the pattern of the lateral deflection
contours is the combination of the previous two. The typical
inclined contour lines for shear are dominant and the magnitudes of
deflections are higher in the compression zone (Fig. 6). In any of
,303.13
the three cases, a thin web of a girder panel moves between two
deflected positions of similar pattern when the fatigue load range
is small, or between a relatively flat position and that of Figs.
4, 5 and 6 when the load range is large.
-4
•
The significant 'phenomenon is that plate girder webs repeatedly
move laterally in synchronization with the loads on the girder (~).
The movement, although small for relatively thick webs, is never-
the~ess measurable. For thin webs, this movement can be seen by
I
eye if the web slenderness ratio is high, if stiffeners are spaced
far apart, or when high loads (as compared to the static strength
of the girder) are applied .
,303.13
FATIGUE FAILURE OF THIN WEBS
-5
•
The consequence of lateral web movement is the possibility of
fatigue crack formation along' web boundaries. Results of experi-
mental investigations show that, indeed, such cracks occur in thin
webs at high loads and large load ranges (~,~,10). These cracks are
usually observed at the web toes of the boundary fillet welds in
welded plate girders, very often initiate on one surface of the web
and then propagate through the thickness to the opposite ~urface (~).
The uniqueness of these fatigue cracks in the web is that they
were not detected in beams where webs do not move laterally. Also,
these cracks differ from other beam and girder fatig'ue cracks--such
as those at the termination of transverse stiffeners or partial
l~ngthcoverplates--inthat the web'cracks have much less apparent
effect on the fatigue performance of the member. Often with web
cracks of considerable length, a plate girder can continue to sustain
fatigue test loads without increasing deflections.
If the initiation of fatigue crdcks in the web is considered the
failure of the web panel, then failure modes of thin-web panels of
plate girders may be classified according to their loading conditions
(~). In a TTshear mode TT of failure, where high shearing forces are
applied to a panel and an inclined pattern of web deflection contours
is present, web cracks may appear along the flanges or the transverse
stiffeners at short distances away from the corners of the panel
where the tension diagonal is present (Fig. 7). For panels in pure
,303.13 -6
bending the more frequent cracks are those near the tension ends of
transverse stiffeners which are common to beams as well as to
girders. In addition, a possible location of web cracks is along
the transverse stiffeners just below the neutral axis (Fig. 8).
In a failure mode of combined bending and shear, the effects of
large lateral web deflections in the compression region cause web
cracks to occur more likely along the upper boundaries of the web
panel (Fig. 9).
Definitions of fatigue failure notwithstanding, it seems certairi
that the initiation of web cracks is as much related to the lateral
movement of the web as to other factors such as local stress
concentrations and primary stresses. When subjected to high
magnitudes of lateral web deflections, cracks along web boundaries
generally (but not always) develop earlier than when the deflections
are small. The more important fact is that only when the lateral
web deflections are of high magnitudes do the webs crack along the
boundaries.
303.13
STUDY OF WEB STRESSES
-7
The correlation between lateral web deflections and web
boundary cracks is through the evaluation of stresses at the
boundaries. High magnitudes of deflections may create high
stresses which in turn cause fatigue cracks. Hence, the obvious
procedure to investigate fatigue cracks in thin-web plate girders
is to evaluate stresses along the web boundaries from web deflec-
tions and then consider the stress-cycle relationship (S-N curves).
Unfortunately, up to the present, there are no applicable accurate
mathematical solutions to relate deflections and stresses because
of the complexity of initial deflections and the uncertainty of
boundary 'restraints to the web. Neither is there sufficient
information to construct S-N curves for web boundaries. Experi-
mental or semi-empirical methods have to be employed.
From measurements made during p13te girder tests, it has been
found that secondary web bending stresses due to lateral deflections
of the web can-be as high as the yield point of the girder mate~l
(~). At certain locations of cracks where no direct measurements
were made, simple assumptions give approximate values of these
stresses from measured web deflections. For the few cracks where
approximation is possible, the resulting stress-cycle relationship
shows a definite trend (Fig. 10) similar to that of common S-N
curves. Further study in this direction is in progress.
303.13
CONCLUSIONS AND DISCUSSION
-8
"
From the very brief description above on the behavior of thin
webs of plate girders," it can be concluded that:
1. Plate girders differ from beams in that some finite
amount of lateral web deflection takes place in
girders subjected to load.
2. If repeated loads are applied to a girder, its web
moves repeatedly between two lateral positions and
in synchronization with the lO3.d.
3. The consequence of severe lateral web movement is the
possibility of fatigue cracks along the boundaries of
thin webs.
4. "Modes of fatigue cracks in girder panels depend on the
patterns of lateral web deflections which, in turn, are
governed by the loading conditions on the web panels.
5. Stresses due to lateral deflections of thin webs can be
as high as the yield point of the girder material. An
empirical stress-cycle relationship for web boundary
cracks has the familiar sh~pe of common S-N curves.
Based on the research work conducted thus far, it is anticipated
that the web buckling limitation will be relaxed in bridge girder
design. Although the use of more slender webs has the potential of
causing fatigue cracks, it is no more severe than that created by
J303.13
current bridge details such 3S transverse stiffeners and partial~
length coverplates. Whereas these details are permitted with
precaution, it seems logical also to permit the use of more
slender webs, with proper limitations. These limitations probably
will be expressed through the web slenderness ratio or stiffener
spacing which will prevent excessive lateral web deflections.
-9.
1303.13
ACKNOWLEDGEMENTS
The results summarized in this report are from investigations
-ill
303.13· -11
p p
Point b
Point a
Fig. 1 Load Versus L~teral Deflection of. Web
1303.13
p
-12
Fig. 2 Web Fluctuation Under Repeated Load
.. '
303.13
p=
-13
Fig. 3 Laterally Deflected Web Cross Section
(deflections exaggerated)
,..
303.13
-- ~ - .,..- .--. -- ---
""",. ...--- .- -- -- --- ..- ----..... ~ "-
/ ./_----------,/ ',... - --./ / /" - - - - -::::-- -- '- ...... "I / I 1//,'- ::: -= =~ ~~~~, '\", ,
I I / - - - "" " '" ....1I11 / /'~-;::-::::::- :"\""'~\\'
I I I I I, I / <- ,... --::: - "'" \'\ ,'\ \ , \\l \ \ \\\ \ \. l '" (. <. .36_ ) I / ) ) ) I JJI ),
\ \ \\\'~~~ ..... -::. -~__-~.::://///J,I
\ \ \\' ' .........- -"24--/////1 /" , - . - -" / /.,/ / -I\ \ '" '- " - is.:::= -:::..~ // / / /'" , - . - - ~"..,..,/\" '--1-2--:::'-" / ., ..... ----..-, ....... --- -;::;.--
" - -'-=-~;;: - c:::---J
~.06'--..-J
.03
Fig. 4 Lateral Web Deflection Contour,
Panel Under Bending
-14
j'-,
303.13
Fig. 5 Lateral Web Deflection Contour,
Panel In Shear
,
-15
)'
303.13 '-16
•
Fig. 6 Lateral Web Deflection Contour, Panel
Under Combined Bending and Shear
1303.13
I ..~"'"~
'\
"'~~.'0~
'"
" I
<tt.
'7
<II:.~
?"
Fig. 7 Locations of Web Cracks,
Panel in Shear
-17
4"
303.13
""~ ....!-
- f-- - - -
"'
I--
-
I
...~
...
~
I ,
Fig. 8 Locations of Web Cracks,
Panel in Bending
-18
J.~'
303.13
• • •
"
( ,
'"
'7
""~~"'0.,
"",. ~ <lit.
Fig. 9 Locations of Web Cracks, Panel
Under Combined Bending and Shear
-19
•,
'..
303.13
s
-.•
• • •
-20
(;
Log N
Fig. 10 Approximate Web Bending Stress
Versus Cycles at Cracks
).
.,
..
303.1~
1.
2 .
3.
4.
-21
REFERENCES
Basler, K., Yen, B. T., Mueller, J. A. and
ThUrlimann, B.
WEB BUCKLING TESTS ON WELDED PLATE GIRDERS,
WRC Bulletin No. 64,' (Sept,.. 1960)
Basler, K. and Thurlimann, B.
STRENGTH OF PLATE GIRDERS IN BENDING, Proc.
ASCE, Vol. 87, ST7 (Aug. 1961)
Basler, K.
STRENGTH OF PLATE GIRDERS IN SHEAR, Proc. ASCE,
Vol. 87, ST7, (Oct. 1961)
Basler, K.
STRENGTH OF PLATE GIRDERS UNDER COMBINED
BENDING AND SHEAR, Proc. ASCE, Vol. 87, ST7,
(Oct. 1961)
;~ 5.
,
6.
7.
8.
9.
:
American Institute of Steel Construction
SPECIFICATIONS FOR THE DESIGN, FABRICATION AND
ERECTION OF STRUCTURAL STEEL FOR BUILDINGS, 1963
Munse, W. H. and Grover, L. M.
FATIGUE OF WELDED STEEL STRUCTURES, WRC, 1964
Dudley, K. E., Mueller, J. A. and Yen, B. T.
LATERAL WEB DEFLECTIONS OF WELDED TEST GIRDERS,
Lehigh University Fritz Engineering Laboratory'
Report No. 303.4, June, 1966
Yen, B. T. and Mueller, J. A.
FATIGUE TESTS OF LARGE-SIZE WELDED PLATE GIRDERS,
Lehigh University, Fritz Engineering Laboratory
Report No. 303.10, June, 1966
Hall, L. R. and Stallmeyer, J. E.
THIN WEB GIRDER FATIGUE BEHAVIOR AS INFLUENCED BY
BOUNDARY RIGIDITY, Univ. of Illinois, Civil
Engineering Studies, Structural Research Series,
No. 278, Jan. 1964
••
lO. Toprac, A. A.
FATIGUE STRENGTH OF HYBRID PLATE GIRDERS, Univ.
of Texas, Structural Fatigue Research Laboratory
Report No. 04-64, 'July, 1965


A summary report on the behavior of thin-web plate girders under repeated loading august 1966

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A summary report on the behavior of thin-web plate girders under repeated loading august 1966

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