Chapter-2 Parts of Steel Bridges

contents 2.1 Stringer 2.2 CROSS-GIRDERS 2.3 Main Girder 2.4 Bearings 2.5 Horizontal and transversal wind bracing 2.6 Classification of bridges

2.1 STRINGERS 2.1.1 Railway Bridges 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)(9.3.4)-(149), 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 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 P(145) back

2.1.2 Roadway bridges 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 and the calculation of bending moment are as given for railway bridge. The maximum deflection of stringers should be less than 1/600 of their span (p132). Back

2.2 CROSS-GIRDERS 2.2.1 Railway Bridges 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

2.2.2 Roadway bridges 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 and the arrangement of sidewalk are as given for railway bridge. the maximum deflection of the cross girder should be less than 1/600 of their span (p132). back

2.3 MAIN GIRDERS 2.3.1 Railway bridges 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.

2.3.2 Roadway bridges The main girder may be plate girders or trusses and the bridge may be through or deck bridge type. In the deck type, 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 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.

2.4 Bearings They are made from cast iron for small roadway bridges or cast steel; they distribute the reaction over a sufficient area of the abutment. We have:- a - Boll bearing. Free rotation and free expansion in all direction except vertical movement b- Movable or expansion bearing. One rotation and one horizontal movement M c- Hinged bearing. One rotation only

2.5 Horizontal and transversal wind bracing There will be:- a- Lateral bracing.(9.3.2.1(p146)) Whenever possible, two system of lateral bracing (upper wind bracing and lower wind bracing) may be used. b- transverse bracing (9.3.2.2( p146)) End and intermediate portal frames (for through bridge) . End and intermediate cross frames (for deck bridge). To transmit the horizontal reactions (of upper lateral bracing) to the bearing

c- Stringer bracing (for Railway Bridge). (9.3.2.4)-(p147) To carry the lateral shock of the train D-Braking force bracing (for Railway Bridge). (9.3.2.4)-(p147) To carry the braking force of the train They transmit the wind pressure, the centrifugal forces and other horizontal forces to the abutments. They reduce the buckling length of the compression chord

Sec(1-1) Upper bracing Lower bracing Truss Bridge

Lower bracing Plate Girder Bridge

Lower Bracing Back

Upper Bracing Back