Prof'Dr'Nabil Mahmoud

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Steel Bridge

• By
• Prof.Dr.\Nabil Mahmoud

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Content

• 1-Introduction
  
• 2-Parts of steel Bridge
• 3-Loads on bridge
• 4-Allowable stresses
• 5-Fatigue
• 6-Plate girder
• 7-Length of flange plate girder
• 8-Connection of flange plate with web
• 9-Web stiffnner
• 10-Web splice
• 11-Splice of flange plate
• 12-Maximum defleciton in Bridge
• 13-Bearing
• 14-Road way bridge

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1-Introdution

• 1.1 Introduction
• 1.2 Railway Bridge
• 1.3 Highway Bridge

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1.1 Introduction

• Most bridges are built for the transportation of
  highway or railway traffic across natural or
  artificial obstacles. A deck bridge supports the
  roadway or railway on its top chords (truss
  bridge) or flanges (plate girder bridge), while a
  through bridge supports the floor system at or
  near the lower chords or flanges, so that traffic
  passes through the supporting structure (main
  girder).

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• The rolled-beam bridge supports its roadway
  directly on the top flanges of a series of rolled
  beams placed parallel to the direction of traffic
  and extending from abutment to abutment. It is
  simple and economical. It may also be used for
  multiple spans where piers or intermediate bents
  can be built economically. Beam bridges may be
  economical for spans up to 15 m. a typical beam
  bridge for highway traffic is illustrated in Fig.
  (1.1).

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For crossings greater than those which can be
spanned economically by a rolled-beam bridge,
(deck or through) plate-girder bridges may be
used. In this simplest form, a plate girder
consists of three plates (two flanges and web)
welded together in the form of an I. Ties and
rails for railway bridges may rest directly on
the top flanges of the deck plate-girder bridge.
When clearance below the structure is limited, a
through plate girder bridge is used. The floor
system may consist of a single line of stringers
under each rail, supported by floor beams (cross
girders) forming into the main girders just above
their lower flanges.
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If an open floor is objectionable, ballast may be
laid on concrete or steel-plate decking supported
by closely spaced floor beams (cross girders)
without stringers. Knee braces (U frames) are
used to support the top flanges of through
bridges, as illustrated in Fig. (1.2) . Highway
plate-girder bridges are usually of the deck
type. The floor slab is usually supported
directly on the main girder, as in the beam
bridge Fig. (1.1). In orthotropic steel-deck
plate construction the floor consists of a steel
deck plate stiffened in two mutually
perpendicular directions by a system of
longitudinal and transverse ribs welded to it
(Fig. (1.3)). The deck structure functions as the
top flange of the main girder and floor beams.
This system makes efficient and economical use of
materials, particularly for long-span
construction.
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When the crossing is too long to be spanned
economically by plate girders, a through or deck
truss bridge may be used. Deck bridges are more
economical than through bridges because the
trusses can be placed closer together, so that
the span of the floor beam is shortened. For
multiple spans there is also a saving in the
height of the piers.
back
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1.2 Railway Bridge

• The stringers, cross girders and the main girders
  are the main load carrying members. The design of
  various elements is done in the sequence in which
  the load is transmitted. In railway bridge, there
  will be either an open timber floor or a
  ballasted floor.
• STRINGERS
• CROSS-GIRDERS
• MAIN GIRDERS

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STRINGERS

• The stringers are placed parallel to the main
  girders, and the stringers span between adjacent
  cross-girders (floor beams). The stringers have
  spans from 3 m to 5 m. the stringers are assumed
  to be simply supported or continuous according to
  their connection to the cross girder. The
  stringers are placed about 10 to 15 cm outside
  the main stock rails (standard-gauge 1.435m),
  so that the effect of impact is less.

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The stingers carry the weight of stock
rails, guard rails ( 250 kg/m of track) (p149),
fastenings, weight of sleepers ( 350 kg/m of
track) and the self-weight. The self weight of
stringers may be assumed. The stringers are
subjected to maximum vertical live load, lateral
shock and impact load, when the one complete span
of stringer, (i.e. the distance between adjacent
cross-girder) is fully loaded.
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The dead load, live load, lateral shock and
impact load is computed per track. Then, the
total load is found per stringer. The simply
supported stringers are designed for the maximum
Mx and the corresponding My and checked for
maximum shear force. In case the rolled steel
beam sections may furnish the required modulus of
section for the stringers, then the rolled steel
beams are provided, otherwise a plate girder
sections are adopted for the stringers. The
stringers are connected at their ends to the
cross-girders with suitable connections.
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The stringers transmit the load to the
cross-girders. The stringers are also braced
similar to the main plate girders in the deck
type bridges. In the case of rolled steel
sections the depth of stringers shall preferably
be not less than 1/12 of their span (p145).
However, the maximum deflection of stringers
should be less than 1/800 of their span
(p132). In the calculation of continuous
stringers, unless otherwise obtained by a
structural analysis, the following bending
moments may be assumed (p145)
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Positive moment in end span.......................
......................0.9 M? Positive moment in
intermediate span................................0
.8 M? Negative moment at support..................
...........................0.75 M? Where M? is
the maximum bending moment for a simply supported
stringer. The same value of bending moment shall
be assumed for stringer fitted between cross
girders and provided with top and bottom plates
resisting the full negative moment at the
support. In all other cases, stringers shall be
calculated as simply supported beams
back
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CROSS-GIRDERS

• The cross-girders (floor-beams) span right angles
  to the main girders and shall be rigidly
  connected thereto. The span of cross-girder is
  equal to the distance between center to center of
  the main girders.
• The cross-girders carry the weight of stock
  rails, guard rails, fastening, and weight of
  sleepers, weight of stingers and the self weight.
  The self weight of cross-girder acts as uniformly
  distributed load.

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• The cross-girders are subjected to maximum live
  load and impact load when both the adjacent
  stringers are loaded. These live load and impact
  load, along with dead load (weight of stock
  rails, guard rails, fastenings and sleepers)
  excluding self-weight act as two concentrated
  loads at the points at which the stringers are
  connected to the cross-girder. The maximum
  bending moment and shear force are found for
  corresponding loading. The rolled steel beam
  sections or plate girder sections are provided.

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The cross-girders are connected as near the
bottom or top flange of the main girders as
possible. These points of connections are known
as panel points. The cross-girders transmit the
load to the main girders at the panel points. The
depth of cross girders shall preferably be not
less than 1/10 of their span (p145). However, the
maximum deflection of the cross girder should be
less than 1/800 of their span (p132). Sidewalk
brackets shall be connected in such a way that
the bending stresses will be transferred directly
to the cross girder (p145).
Back
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MAIN GIRDERS

• The main girder may be plate girders or trusses
  and the bridge may be through or deck bridge
  type. These girders in through type railway
  bridges are spaced with sufficient width to suit
  the clearance requirement. The spacing of main
  girders necessary for the clearance requirements
  is sufficient to develop lateral strength and
  rigidity and to resist the overturning with the
  specified wind pressure and load conditions.

Back
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• The plate girder carry the weight of stock rails,
  guard rails, fastenings, sleepers, weight of
  stringers, weight of cross-girders and
  self-weight. In addition to this dead load, the
  plate girders also carry the live load and impact
  load. When the spacing of cross-girders is up to
  4 m, then, the load transmitted by the
  cross-girder is treated as uniformly distributed
  load.

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1.3 Highway Bridge

• The floor is a part of bridge which carries the
  load directly. The floor system in case of
  Highway Bridges generally consists of reinforced
  concrete slab or steel deck plate and wearing
  surface. In case of deck type plate girder
  Highway Bridges, the slab is supported directly
  by the plate girders. In case of through type
  Highway Bridges, the reinforced concrete slab is
  supported on stringers, and cross-girders, or by
  the cross-girders alone. Many times, the
  reinforced concrete slab provides its own traffic
  surface. In addition to this, the bituminous,
  asphalt or carpet surface is also furnished. This
  acts as a wearing surface. The design of
  reinforced concrete slab has not been discussed
  in the text.
• STRINGERS
• CROSS-GIRDERS
• MAIN GIRDERS

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STRINGERS

• The stringers support the reinforced concrete
  slab in case of through type Highway Bridges. The
  stringers are supported by the cross-girder. The
  stringers may be supported on the top of
  cross-girders or may be framed into the
  cross-girders by the use of suitable connections.
  When the reinforced concrete slabs are used, then
  either the stringers should be supported on the
  top of the cross-girders or in case the stringers
  are framed into the cross-girders,

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• then the top of stringers should be on the same
  level as the cross-girders. The stringers carry
  the dead load, which consists of the weight of
  wearing coat, the weight of reinforced concrete
  slab and the self-weight. In addition to this,
  the stringers also support the live load and the
  impact load due to highway standard vehicles or
  trains. The preferable depth of the stringers,
  the maximum deflection and the calculation of
  bending moment are as given for railway bridge

Back
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CROSS-GIRDERS

• The load from floor system is carried to the
  cross girders by means of the stringers or the
  loads may be carried to the cross-girders
  directly by the reinforced concrete slab. The
  cross-girders carry dead load, which consists of
  the weight of wearing coat, the weight of
  reinforced concrete slab, the reaction from the
  stringers and the self-weight. In addition to
  this, the cross-girders carry live load and
  impact load due to highway standard vehicles or
  trains. The preferable depth of the cross girder,
  the maximum deflection and the arrangement of
  sidewalk are as given for railway bridge.

back
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MAIN GIRDERS

• The main girder may be plate girders or trusses
  and the bridge may be through or deck bridge
  type. In the deck type Highway Bridges, the
  spacing between the main girders is kept
  sufficient to develop lateral strength and
  rigidity, and to resist the overturning with the
  specified

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• wind pressure and the load conditions. In the
  through type Highway Bridges the spacing between
  plate girders is kept sufficient to suit the
  clearance requirement. The spacing of plate
  girders required for the clearance requirement is
  sufficient to resist the overturning with the
  specified wind pressure and load conditions, and
  to develop lateral strength and rigidity.

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In the deck type Highway Bridges, the two plate
girders are used for single lane carriageway
width and three or four plate girders depending
upon the design, as used for two lane carriageway
Highway Bridge. The reinforced concrete slabs
inclusive of wearing coat is supported directly
by the plate girders. The plate girders carry
dead load. The dead load consists of the wearing
coat, the weight of reinforced concrete slab and
self-weight of plate girders. In addition to the
dead load, the plate girders carry the live and
impact load due to the highway standard vehicles.
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In case of through type highway bridges, the
plate girders carry the dead load. The dead load
consists of the weight of wearing coat, the
weight of reinforced concrete slab, the weight of
stringers, the weight of the cross-girders, and
self-weight. In addition to this, the plate
girders carry the live load and the impact load
due to the highway standard vehicles.
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Figure (1.4) shows the common types of
simple-span bridge trusses. By varying the depth
of a truss throughout its length (Fig. 1.4c)
forces in the chord members can be more nearly
equalized and the forces in the web reduced.
Trusses of economical proportions usually result
if the angle between diagonals and verticals
ranges from 45 to 60?. However, if long-span
trusses are made deep enough for adequate
rigidity as well as for economy
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, a suitable slope of the diagonals may produce
panels too long for an economical floor system.
Using the subdivided panels (Figure 1.4(f and g))
solve this problem. Certain objections to
subdivided panels overcome with the invention of
the K truss (Fig. 1.4h). Cantilever bridges(Fig.
2.2) , continuous bridges (Fig. 2.3), arch
bridges (Fig. 2.4), suspension bridges (Fig. 2.5)
and three Chord Bridge (Fig. 2.6) are common
types of structures suitable for long spans.
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A cantilever bridge consists of two shore, or
anchor, spans flanked by cantilever arms
supporting a suspended simple span. Positive
bending moments are decreased because of the
shorter simple beam, while the cantilever and
anchor arms subjected to negative bending
moments. Positive bending moments in continuous
bridges are reduced because of the negative
moments at the piers. Arch bridges may be fixed,
single-hinged, two-hinged, or three-hinged. The
principal supporting elements of the
suspension-bridge superstructure are the cables
which pass over the towers to be anchored in
foundations at each end.
Back
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Figure 1-1
Figure 1-2
Figure 1-3
Back
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Back
Figure 1-4