Steel Bridge By : Prof.Dr.\Nabil Mahmoud
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
1-Introdution • 1.1 Introduction • 1.2 Railway Bridge • 1.3 Highway Bridge
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).
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. back
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
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.
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.
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.
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):
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
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.
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.
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
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
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.
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
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,
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
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
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
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.
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.
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.
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. Back
Figure 1-1 Figure 1-2 Back Figure 1-3
Back Figure 1-4