By :By :Prof.Dr.\Nabil MahmoudProf.Dr.\Nabil Mahmoud

ContentContent1-Introduction 2-Parts of steel Bridge3-Loads on bridge4-Allowable stresses5-Fatigue6-Plate girder 7-Length of flange plate girder8-Connection of flange plate with web9-Web stiffnner10-Web splice11-Splice of flange plate12-Maximum defleciton in Bridge13-Bearing14-Road way bridge

1-Introdution1-Introdution1.1 Introduction 1.2 Railway Bridge 1.3 Highway Bridge1.4 Types of Bridges

1.1 Introduction1.1 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).

•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).

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.

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.

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.

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1.2 Railway Bridge1.2 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

1.3 Highway Bridge1.3 Highway BridgeThe 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

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

, 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.

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.

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Figure 1-2

Figure 1-3

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Figure 1-1

Figure 1-4

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1.4 Types of Bridges

Introduction

In designing the different parts of a bridge we must investigate carefully how the loads are transmitted from one member to the next. We must follow the loads from the point of application up to the abutment. All members and all connections should have the same factor of safety. The strength of the whole structure depends on the weak part. The design of the details is just as important as the design of the main members; failure is generally caused by a weak or wrong detail. For the computation and design of the different parts of a steel bridge, the same method is used as for the corresponding members of steel buildings

But on account of the bigger spans and greater loads, much bigger cross section is required. Bridges can be classified according to many factors like purpose of the bridge, system of main girders, considering the position of the bridge floor, square or skew bridge, and fixed or movable bridges. In the following section we can see these different classifications.

1.5 Classification of bridges 1.5.1 Classification according to purpose of the bridge

 

a.   Railway Bridge.

b.    Roadway Bridge.

c.     Foot bridge.

d.     Combined bridge as Embaba Bridge.

1.5.2 Classification according to system of Main Girder

a-Simple Bridge.

The main girders are resting on two supports only. They may be: - beams, plate

girders or trusses. One of the supports is hinged while the other is movable and thus these bridges are externally statically determinate. But internally they may be either determinate or indeterminate.

b-Continuous Bridge.

The main girders are continuous trusses or plate girders on three or more supports. One bearing only of each girder is hinged, while all the other must be movable to avoid

temperature stresses. Vertical loads acting on a continuous girder give also vertical reactions, but the bridge is statically indeterminate. A settlement of one of the piers produces additional stresses; therefore continuous bridges should be built in places where we have firm soils.

c- Cantilever Bridge.

The main girders extended over several spans but they have many intermediate hinges that the reactions are statically determine. For n supports we have to odd n-2 hinges. In a cantilever bridge the settlement of support does not affect the stresses. When foundation is not firm enough, either simple bridges or cantilever bridges should be used.

    

D- Arch Bridges.

An arch is a structure which under vertical loads produces inclined reactions at both supports.

We have 3-hinged, 2-hinged and fixed arches.

1-Three-hinged arches are statically determinate; hence, horizontal displacement of the abutments does not produce any additional stresses on the structural system.

2- Two-hinged arches and the fixed arches are statically indeterminate; hence, displacement of the abutments produces additional stresses in the structural system. Furthermore, foundations of such arches should be on rock or on very solid gravel.

e- Suspension Bridges.

Cables of suspension bridges are made from very high tensile steel. The allowable deflection is about 10 cm.

The floor is hung by vertical suspenders from cables. These cables are carried by vertical steel towers A-Q, B-V over which it posses and are anchored at P and V.

A saddle top of each tower is provided to relieve the tower from B.M. The reaction at top of tower is nearly vertical. Stiffening, girders must be used to reduce the deflection and vibration of the bridge due to the moving loads. Suspension bridges are of good appearance but they are economical only for long spans (> 300 m). F- Cable stayed bridges

G- Three Chord System Bridges.

The arch trusses with a tie bow-string are simply supported. They are externally statically determinate, and once internally statically indeterminate. They are good appearance but rather expansive than trusses with two chords.

1.5.3 Classification according to position of bridge floor Fig(2-7)

 

a- Deck Bridge.

In which the floor is or near the top chord or flange of the main girders.

b- Through Bridge.

In which the floor is or near the bottom chord or flange of the main girders. The distance (h) is called the height of construction, it is the height between the top of rails or road way and the lowest line of the bracing.

If there is a sufficient height of construction a deck bridge should always be arranged as it is more economical stiffer, and of better appearance than through bridge.

In a railway deck bridge the distance between the two main girders can be made less than in a through bridge therefore the weight of the cross-girder and wind bracing would be less.In Roadway bridge, the reinforced concrete floor may rest on several main girders.

1.5.4 Classification according to the layout of the bridge (square or skew bridges)

The centerline of the square bridge is perpendicular to centerline of the canal, while in skew bridge they are at oblique angle. Fig(2-8)

1.5.5 Classification according to Fixed bridges and Movable bridges.

 

Movable spans are required in bridges crossing navigable streams if the height below the bridge is not sufficient for the passage of ships.

Three major types of movable bridges are in common use:-

a- The vertical lift bridge.

b- The bascule bridge.

c-The swing bridge.

Fig 2-1

L.W.B

X.G

A

SEC (A-A)

X.G

End bracket

Lower wind bracing

StringerM.G M.GX.G bracing

AEnd bracket

Scale=(1:50)

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Fig 2-2

L1 C1 L1C1

L1C1 C1

End Stiff.

Structural System

Cantilever Plate Girder Bridge

Web Plate

Lower Flange

Upper Flange

L1

L2

L2

C1C1 L1L1 L2

Structural System

Upper Flange

L1

Cantilever Plate Girder Bridge

L1 = (0.6 - 0.75) L2 L2

Systems of Main Girder

Web Plate

End Stiff.

Lower Flange

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Structural System

Upper Flange

L1

ContinuousTruss Bridge

L1 = (0.6 - 0.75) L2 L2

L1L2L1 = 0.8 L2

Systems of Main Girder

ContinuousTruss Bridge

Web Plate

End Stiff.

Lower Flange

Web systemUpper Chord

Lower Chord

BracingTransverse

Fig 2-3 Back

L

Fixed Arch Bridge

L

2-Hinged Arch Bridge

3-Hinged Arch Bridge

L

Systems of Main Girder

Figure 2.4 Back

Systems of Main Girder

L

Suspension Bridge

Cable SteelTower

SaddleSaddle

Stiffening Girder

Figure 2.5 Back

Three Chord Truss Girder Bridge

Bow-String Truss Girder Bridge

Bow-String Plate Girder BridgeL

L

L

Systems of Main Girder

Fig 2-6 Back

B = Bridge Width

h =

L/ 1

0

1.5 mSleeper

Lower Chord

Upper Chord

Back

B = Bridge Width

h =

L/ 1

0

Railway Deck Bridge

Railway Through Bridge

B = Bridge Widthh

= L/

10

Figure 2.7 Back

B =

Brid

ge W

idth

M.G.Square Bridge

M.G.

B =

Brid

ge W

idth

SkewBridgeM.G.

M.G.

Fig 2-8 Back