Plate girder

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Chapter 6

• Plate girder

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5.1 Introduction 5.3 Fiber stress of plate
girder 5.4 Width of compression flange
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5.1 Introduction

• A plate girders consist of a vertical plate
  called web, and two flanges each consisting of
  horizontal plate. Sufficient weld must be used to
  insure that the bottom flange, top flange and the
  web plate acts as one unit. For spans less than
  15 m the rolled beams or plated beams are used.
  But above that (15 m) and till spans to 30 -35 m,
  the plate girders are economic. The weight of the
  plate girder is greater than that of truss of the
  same span but the fabrication costs and
  maintenance are small.

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1. Economic web depth of girders For simply
supported main girders of railway bridges the
height of web could not be less than 1/10 of the
bridge span. And for roadway bridges the height
of web could not be less than 1/12 of the bridge
span. While for continuous and cantilever girders
the web height could not be less than (1/10
1/14) of the bridge span. For stringers and
cross-girders the height could not be less than
(1/12 1/10) respectively.
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1.                  Thickness of the web Girder
without long stiffener (with or without
transverse stiffener)
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Girder with long stiffener at (d/5 d/4) tw
? 60 of values in clause 7.3.2 (clause
7.3.3) Nothing given for stiffeners at d/2 In the
web of a plate girder there are - a.a.    In a
vertical plan there are normal stresses
due to B.M. and shear stress due to
S.F.   b. b.   On a horizontal plan we have only
shear stresses. Unit shear stresses at section
b-b
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where, Ix cross section moment of inertia of
the whole section about axis x-x. Sx gross
statical moment of shaded area about axis
x-x. Aweb gross area of the web, qb shear
buckling stress c. The principal stresses in the
web occur in inclined plans. d. The buckling of
the web in a diagonal direction due to principal
compression stresses should be considered
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a.   e.    We provide for this buckling by using
a lower shear stress in the web (increase t) and
by adding vertical and horizontal stiffeners For
panel under Q and M If qact gt 0.6 qb
The all bending stress shall be limited to
or assume the flanges alone resist total bending
without reducing Fb
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Check shear stress ? d1/ d
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If
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If
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5.3 Fiber stress of plate girder Each flange of a
plate consists of one or more flange plates.
Theses parts are connected together and the web
by a sufficient weld, to guarantee that the
girders act as a solid beam. The fiber stresses
at any point may be computed from the beam
formula
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Approximate method (flange area method) This
method is used to obtain a reasonable cross
section before we check with the exact method.
The flange material is grouped quite closed
together and nearly the moment of inertia of the
flanges is equal to 85 of the total moment of
inertia of the whole section. If we assumed that
the two flanges acts as the upper and lower chord
of truss, i.e the flange stress is nearly
constant. Hence, the required flange area
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h? effective depth distance between the
assumed centroids of the two flanges height of
web (5-10) cm 97 hweb Fb 0.58 Fy Fb
1.4 for St 37. Fb 1.6 for St 44. Fb 2.1
for St 52.
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 a.  A part of the web is considered to belong
the flange can be calculated as Gross moment of
inertia of the web
     b.  If we assumed that an area of web
Aweb/6, is placed in each flange, the moment of
inertia is
Gross moment of inertia of the web
   C.   Hence, 1/6 gross area of the web acts
with the flange (welded section only) Gross
moment of inertia of the web
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d.  Finally, the required flange area can be
obtained by
The number of flange plates should be minimized
as many as you can but not less than one flange
plate can be used.
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  e. Flange plates thickness may be chosen as
following Plate thickness t 10 12 14
mm, for smaller spans bridges. .Plate thickness
t 14 26 mm, for bigger spans bridges.  f.
Check the value of ? to ensure that the assumed
value h? is correct, otherwise repeat the
calculations.
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5.4 Width of compression flange a. a.   The
compression flange is liable to buckle
perpendicular to the plan of web. In the plan of
web it will prevented from buckling by the
stiffness of the web.    b.   The width of
compression flange (b) must be chosen in such
away that buckling may be prevented.
  c.  C.   In a deck bridge the buckling length
of the compression flanges is limited by on upper
wind bracing and is equal to ?
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d. In a through bridge where an upper wind
bracing is not possible, the upper flanges must
be laterally supported by bracket plates bolted
to the cross girders. As, these brackets are
elastic supports, the buckling length of the
compression flange is not the distance between
the brackets but it could be calculated as in
clause 4.3.4, code 2001.
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 e.  There are cross girders and stiffeners
forming U-frames provide lateral restrained.
Hence, the effective buckling length is according
to clause 4.3.2.3 (Table 4.9)-
E 2100 t/cm2 Iy moment of inertia of plate
girder compression flange IY flange only a
spacing between U-frames (a ? Lu ? 2 a)
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d1 dw Hx.G. d2 dw Hx.G./2 I1 moment of
inertia of bracket. I2 IX moment of inertia
of X-G. about the axis of
bending B the distance between centers of
Main Girders.
(Table 2.1c)
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