Chapter -9
WEB STIFFENERS
contents
Introduction
 Bearing stiffeners
 Intermediate stiffeners
 Horizontal stiffeners (Longitudinal stiffeners)
 Design of transverse stiffeners
 Design Longitudinal stiffeners

1-Introduction
The web of plate girder is so thin that there is always
tendency for diagonal buckling and vertical buckling.
Therefore, stiffeners are provided. In welded plate girders,
plates, angles, tees and closed sections are used as
stiffeners (Fig. 7.1 p126). The outstand of the stiffener
should not be less than that given in clause 7.4.1 (p124).
(d/30 + 5 cm) or (d/30 + 10 cm)
There are two kinds of transverse stiffeners:-
1-Bearing stiffeners.
2-Intermediate stiffeners.
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2-Bearing stiffeners
The function of these stiffeners is to distribute the load to
the web. These are provided at the supports and at the
points of concentrated loads.
These stiffeners shall be symmetrical about the web and
bear tightly against the flange plates.
The bearing stiffeners shall be designed as a column
assuming the section to consist of the pair of stiffeners
together with a length of web equal to 12, 25 times the
web thickness for end and intermediate stiffeners
respectively. The radius of gyration shall be taken about
the axis parallel to the web, and the working stress shall be
in accordance with the appropriate allowable value for a
compression member assuming an effective length equal to
0.80 of the length of stiffeners. From this, value of l/r is
found. Allowable compression stress is found from clause
2.6.4 (p15).
Where possible, specially at supports, the stiffeners
should extend to the outer edges of the flange plates.
Ends of all bearing stiffeners shall be machined or ground
to fit tightly
both at top and bottom and the stiffeners shall be
provided with sufficient welds to transmit to the web the
whole of the concentrated load (reaction).
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3-Intermediate stiffeners
Transverse intermediate stiffeners shall be provided
throughout the length of the girder when the average
calculate shear stress (qact= Q/Aw) is larger than the
allowable buckling shear stress (qb) Clause 2.6.3.2, p. 14.
The stiffeners may be pairs one on either side of the web
or single. When the single stiffeners are used, these should
be placed alternately on the opposite sides of the web. The
stiffener shall extend from flange to flange.
The intermediate stiffeners shall be designed as a
compression member (with effective length = 0.80 the
length of stiffener) to resist a force;
 0.35 *F y 
Cs  0.65 * 
 1 * Qact.
 qb

If
q b  0.35  Fy
(Eq. 7.8)
 C s  0.0
The connection between the transverse stiffeners and the
web should be designed on the transverse stiffeners
force. For intermediate stiffeners, this connection is
designed in such a way that the fastener in either the
upper or the lower thirds of the stiffeners should transfer
the design force.
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4- Horizontal stiffeners (Longitudinal stiffeners)
When the thickness of the web is less than d Fy / 190 or d /120
(eq. 7.4& 7.5). horizontal stiffeners (with or without
d d
 ) from
transverse stiffeners) shall be provided at (
5 4
the compression flange. The stiffeners shall be designed so
that moment of inertia I is not less than ( 4d  t 3w ).
d
Additional horizontal stiffeners at
may be provided
2
and its moment of inertia is not less than ( d  t 3w ).
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5-Design of transverse stiffeners
1. Lb = 0.8 d
2. Intermediate 25 tw
3. End stiffener
4. Design as a compression member
5. Force on intermediate stiffener
 0.35 *F y 
Cs  0.65 * 
 1 * Qact.
 qb

If
q b  0.35  Fy
 C s  0.0
6. Design end stiffener on R, intermediate on Cs
7. Weld for end stiffener (Force R, length d)
8. Weld for intermediate (Force Cs, length d/3)
d/4 - d/5
d/4 - d/5
d/4 - d/5
d/4 - d/5
d/2
tw
d
d1
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6-Design Longitudinal stiffeners
Iy for longitudinal stiffeners
I y  4d  t 3w
d d
at ( - )
4 5
Iy  d  t
d
at ( )
2
3
w
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