Classification of steel bridges
Steel bridges are classified according to
- The Type Of Traffic Carried
- The Type Of Main Structural System
- The Position Of The Carriage Way Relative To The Main Structural System
These are briefly discussed in this section.
Classification based on type of traffic carried
- Bridges are classified as
- Highway or road bridges
- Railway or rail bridges
- Road – cum – rail bridges
Classification based on the main structural system
Many different types of structural systems are used in bridges depending upon the span, carriageway width and types of traffic. Classification, according to make up of main load carrying system, is as follows:
(i) Girder bridges - Flexure or bending between vertical supports is the main Structural action in this type. Girder bridges may be either solid web girders or truss girders or box girders. Plate girder bridges are adopted for simply supported spans less than 50 m and box girders for continuous spans upto 250 m. Cross sections of a typical plate girder and box girder bridges are shown in Fig.1 (a) and Fig. 1(b) respectively. Truss bridges [See Fig.1(c)] are suitable for the span range of 30 m to 375 m. Cantilever bridges have been built with success with main spans of 300 m to 550 m. In the next chapter girder as
bridges are discussed in detail. They may be further, sub-divided into simple spans, continuous spans and suspended-and-cantilevered spans, as illustrated in Fig.7. 3.
Fig.1 (a) Plate girder bridge section
Fig.1 (b) Box girder bridge section
Fig.1 (c) Some of the trusses used in steel bridges
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Fig.2Typical girder bridges
(ii) Rigid frame bridges - In this type, the longitudinal girders are made structurally continuous with the vertical or inclined supporting member by means of moment carrying joints [Fig.3]. Flexure with some axial force is the main forces in the members in this type. Rigid frame bridges are suitable in the span range of 25 m to 200 m.
Fig.3 Typical rigid frame bridge
(iii) Arch bridges
Fig.4 Typical arch bridges
The loads are transferred to the foundations by arches acting as the main structural element. Axial compression in arch rib is the main force, combined with some bending. Arch bridges are competitive in span range of 200 m to 500 m. Examples of arch bridges are shown in Fig. 4.
(iv) Cable stayed bridges - Cables in the vertical or near vertical planes support the main longitudinal girders. These cables are hung from one or more tall towers, and are usually anchored at the bottom to the girders. Cable stayed bridges are economical when the span is about 150 m to 700 m. Layout of cable stayed bridges are shown in Fig. 5.
Fig.5 Layout of cable stayed bridges
(v) Suspension bridges - The bridge deck is suspended from cables stretched over the gap to be bridged, anchored to the ground at two ends and passing over tall towers erected at or near the two edges of the gap. Currently, the suspension bridge is best solution for long span bridges. Fig. 6 shows a typical suspension bridge. Fig. 7 shows normal span range of different bridge types.
Fig.6 Suspension bridge
7.3.3 Classification based on the position of carriageway
The bridges may be of the "deck type", "through type" or "semi-through type". These are described below with respect to truss bridges.
(i) Deck type bridge - The carriageway rests on the top of the main load carrying members. In the deck type plate girder bridge, the roadway or railway is placed on the top flanges. In the deck type truss girder bridge, the roadway or railway is placed at the top chord level as shown in Fig. 8(a).
Fig.7 Normal span ranges of bridge system
Fig.8 Typical deck, through and semi-through type truss bridges
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(ii) Through type bridge - The carriageway rests at the bottom level of the main load carrying members [Fig. 8(b)]. In the through type plate girder bridge, the roadway or railway is placed at the level of bottom flanges. In the through type truss girder bridge, the roadway or railway is placed at the bottom chord level. The bracing of the top flange or lateral support of the top chord under compression is also required.
(iii) Semi through type bridge - The deck lies in between the top and the bottom of the main load carrying members. The bracing of the top flange or top chord under compression is not done and part of the load carrying system project above the floor level as shown in Fig. 8(c). The lateral restraint in the system is obtained usually by the U-frame action of the verticals and cross beam acting together.
Loads and Load combinations
Loads on bridges
The following are the various loads to be considered for the purpose of computing stresses, wherever they are applicable.
- Dead load
- Live load
- Impact load
- Longitudinal force
- Thermal force
- Wind load
- Seismic load
- Racking force
- Forces due to curvature.
- Forces on parapets
- Frictional resistance of expansion bearings
- Erection forces
Dead load – The dead load is the weight of the structure and any permanent load fixed thereon. The dead load is initially assumed and checked after design is completed.
Live load – Bridge design standards specify the design loads, which are meant to reflect the worst loading that can be caused on the bridge by traffic, permitted and expected to pass over it. In India, the Railway Board specifies the standard design loadings for railway bridges in bridge rules. For the highway bridges, the Indian Road Congress has specified standard design loadings in IRC section II. The following few pages brief about the loadings to be considered. For more details, the reader is referred to the particular standard.
Railway bridges: Railway bridges including combined rail and road bridges are to be designed for railway standard loading given in bridge rules. The standards of loading are given for:
- Broad gauge - Main line and branch line
- Metre gauge - Main line, branch line and Standard C
- Narrow gauge - H class, A class main line and B class branch line
The actual loads consist of axle load from engine and bogies. The actual standard loads have been expressed in bridge rules as equivalent uniformly distributed loads (EUDL) in tables to simplify the analysis. These equivalent UDL values depend upon the span length. However, in case of rigid frame, cantilever and suspension bridges, it is necessary for the designer to proceed from the basic wheel loads. In order to have a uniform gauge throughout the country, it is advantageous to design railway bridges to Broad gauge main line standard loading. The EUDLs for bending moment and shear force for broad gauge main line loading can be obtained by the following formulae, which have been obtained
from regression analysis:
For bending moment:
EUDL in kN = 317.97 + 70.83l + 0.0188l2 ? 449.2 kN (7.1)
For shear force:
EUDL in kN = 435.58 + 75.15l + 0.0002l2 ? 449.2 kN (7.2)
Note that, l is the effective span for bending moment and the loaded length for the maximum effect in the member under consideration for shear. ‘l ‘ should be in metres. The formulae given here are not applicable for spans less than or equal to 8 m with ballast cushion. For the other standard design loading the reader can refer to Bridge rules.
Highway bridges: In India, highway bridges are designed in accordance with IRC bridge code. IRC: 6 – 1966 – Section II gives the specifications for the various loads and stresses to be considered in bridge design. There are three types of standard loadings for which the bridges are designed namely, IRC class AA loading, IRC class A loading and IRC class B loading.
Fig.9 IRC AA loading
IRC class AA loading consists of either a tracked vehicle of 70 tonnes or a wheeled vehicle of 40 tonnes with dimensions as shown in Fig. 9. The units in the figure are mm for length and tonnes for load. Normally, bridges on national highways and state highways are designed for these loadings. Bridges designed for class AA should be checked for IRC class A loading also, since under certain conditions, larger stresses may be obtained under class A loading. Sometimes class 70 R loading given in the Appendix – I of IRC: 6 – 1966 – Section II can be used for IRC class AA loading. Class 70R loading is not discussed further here.Class A loading consists of a wheel load train composed of a driving vehicle and two trailers of specified axle spacings. This loading is normally adopted on all roads on which permanent bridges are constructed. Class B loading is adopted for temporary structures and for bridges in specified areas.For class A and class B loadings, reader is referred to IRC: 6 – 1966 – Section II.
Foot Bridges and Footpath on Bridges – The live load due to pedestrian traffic should be treated as uniformly distributed over the pathway. For the design of footbridges or footpaths on railway bridges, the live load including dynamic effects should be taken as 5.0 kN/m2 of the footpath area. For the design of footpath on a road bridges or road-rail bridges, the live load including dynamic effects may be taken as 4.25 kN/m2 except that, where crowd loading is likely, this may be increased to 5.0 kN/m2.The live load on footpath for the purpose of designing the main girders has to be taken as follows according to bridge rules:
(i) For effective spans of 7.5 m or less – 4.25 kN/m2
(ii) The intensity of load is reduced linearly from 4.25 kN/m2 for a span of 7.5 m to 3.0 kN/m2 for a span of 30m
(iii) For effective spans over 30 m, the UDL may be calculated as given below:
Impact percentage curve for highway bridges for IRC class A and IRC class B loadings
The dynamic effect caused due to vertical oscillation and periodical shifting of the live load from one wheel to another when the locomotive is moving is known as impact load. The impact load is determined as a product of impact factor, I, and the live load. The impact factors are specified by different authorities for different types of bridges. The impact factors for different bridge for different types of moving loads are given in the table 7.1. Fig.7.11 shows impact percentage curve for highway bridges for class AA loading. Note that, in the above table l is loaded length in m and B is spacing of main girders in m.
Thermal forces – The free expansion or contraction of a structure due to changes in temperature may be restrained by its form of construction. Where any portion of the structure is not free to expand or contract under the variation of temperature, allowance should be made for the stresses resulting from this condition. The coefficient of thermal expansion or contraction for steel is 11.7 x 10-6 / oC
Wind load – Wind load on a bridge may act
- Horizontally, transverse to the direction of span
- Horizontally, along the direction of span
- Vertically upwards, causing uplift
- Wind load on vehicles
Wind load effect is not generally significant in short-span bridges; for medium spans, the design of sub-structure is affected by wind loading; the super structure design is affected by wind only in long spans. For the purpose of the design, wind loadings are adopted from the maps and tables given in IS: 875 (Part III). A wind load of 2.40 kN/m2 is adopted for the unloaded span of the railway, highway and footbridges. In case of structures with opening the effect of drag around edges of members has to be considered.
Racking force – This is a lateral force produced due to the lateral movement of rolling stocks in railway bridges. Lateral bracing of the loaded deck of railway spans shall be designed to resist, in addition to the wind and centrifugal loads, a lateral load due to racking force of 6.0 kN/m treated as moving load. This lateral load need not be taken into account when calculating stresses in chords or flanges of main girders.
Forces on parapets - Railings or parapets shall have a minimum height above the adjacent roadway or footway surface of 1.0 m less one half the horizontal width of the top rail or top of the parapet. They shall be designed to resist a lateral horizontal force and a vertical force each of 1.50 kN/m applied simultaneously at the top of the railing or parapet.
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Seismic load – If a bridge is situated in an earthquake prone region, the earthquake or seismic forces are given due consideration in structural design.Earthquakes cause vertical and horizontal forces in the structure that will be proportional to the weight of the structure. Both horizontal and vertical components have to be taken into account for design of bridge structures. IS:1893 – 1984 may be referred to for the actual design loads.
Forces due to curvature - When a track or traffic lane on a bridge is curved allowance for centrifugal action of the moving load should be made in designing the members of the bridge. All the tracks and lanes on the structure being considered are assumed as occupied by the moving load.