A typical cross section of a railway bridge is shown in Figure 2.35. One can see the rail- way infrastructure that comprises the permanent way (track), the access ways beside the track, and the associated plant equipment that allows the proper function of the railway. The track carries the railway traffic and consists of the rails, the sleepers, and the ballast. When the track is on curves, it is superelevated to compensate for the effects of centrifugal forces.
Wind Z Wind Y
Stabilizing ropes
(a) (b)
Figure 2.34 (a) Torsional vibration due to wind and (b) temporary stabilization during erection.
Sleeper Cross girder Cross girder Section a–a Waterproofing Slab a a Ballast Ballast and sleepers Rail Rail Track Trimmer girder Main girder with
doubler plates
Main girder
Waterproofing layer Figure 2.35 Half-through plate-girder bridge.
The rails are mainly steel bars laid on the sleepers, and their weight ranges from 55 to 80 kg/m. In older railways, rails used to be bolted on the sleepers, but nowadays, continuous welded rails are preferred. The rails are pretensioned in order to remain stress-free and avoid buckling due to compression from braking–acceleration forces and thermal actions.
Sleepers pass under the rails and hold them at the right spacing. In high-speed routes, sleepers are usually of prestressed concrete, but when construction depth is very limited, shallow timber or steel alternatives are also used.
Ballast works as a resilient bed for the sleepers since it distributes the wheel loads onto the deck plate and allows for drainage. It consists of coarse stone, slag, or clinker about 50–65 mm with a density of 20–22 kN/m3 and is mechanically compacted. The usual bal- last depth under the sleeper ranges from 250 to 300 mm depending on the requirements of the local authorities. These values ensure a satisfactory load distribution and drainage. When construction depth is severely restricted, lower ballast depths may be adopted. In some bridges, ballast and sleepers were omitted by fastening the rails directly to the bridge deck, direct fastening. This method seems to solve the problem of the restricted construc- tion depth, but special detailing and experience are necessary. Moreover, track mainte- nance problems may arise when a rail is damaged. In general, a reduced ballast depth is the simplest solution and often the most preferable one.
Railway bridges are very demanding structures. The magnitude of both permanent and live loads may be several times larger than that of a typical highway bridge. As already noted, the construction depth is more difficult to optimize due to the additional depths of the ballast and the rails. Therefore, serviceability verifications (mainly deformation and vibration controls) are in many cases onerous, especially in multitrack railways. Designers should ensure that the track geometry remains and that the contact between the rails and the wheels is not lost.
Another important design aspect is the fatigue verification due to the dynamic nature of live loads. A railway bridge must be able to endure repeated actions whose magnitude depends on the annual tonnage of the traffic on every lane. Connections and especially the welded ones should be carefully designed and located where inspection, blast cleaning, and repainting is possible. It is true that in most railway bridges the inspection costs throughout the design life are comparable with the total cost of the structure. For these reasons, railway engineers are more concerned with the life cycle costs than with initial construction costs. From the design point of view, this is an important difference between rail- and roadway bridges.
Most of the cross sections described previously for the roadway bridges are also applied for railway applications. Modifications and special issues are discussed subsequently.
2.4.2 Half-through bridges
A popular solution is the bridge that is depicted in Figure 2.35. Two main girders are con- nected together through crossbeams. The crossbeams consist of composite cross sections and carry the tracks. The main girders are welded double T sections, and the connection between them and the crossbeams is rigid so that a stiff frame is formed. If the plate thick- nesses of the steel flanges are not sufficient, then doubler plates are welded. This is a practical solution but may lead to corrosion problems at the interface of the two plates, and therefore, noncontinuous welds are not allowed. If possible, doubler plates should be avoided and thicker plates should be chosen.
Rolled steel beams have better fatigue resistance than welded ones and are recommended to be used as crossbeams. One can also observe that longitudinal stiffeners at the web are
placed outside the bridge. This is done in order to avoid damage during track maintenance activities (relaying and reballasting operations).
Half-through bridges are usually preferred for railway underline bridges. The underline clearance is in such cases limited, and the half-through construction offers the most suit- able alternative. The spans are generally simply supported; this simplifies construction and replacement activities during traffic conditions. For spans less than 17 m, the top of the main girders need not be more than 100 mm above the rail level. This minimizes the construction depth that is the vertical dimension between the tops of the rails and the bridge soffit as much as possible. However, the main girder cannot be considered to provide a safe support (robust kerb) against derailment loads; additional parapets acting as robust kerbs are neces- sary. A main girder is generally considered as a robust kerb when the top of the main girder extends at least 300 mm above the rail level [2.22].
Half-through plate-girder bridges with a twin-track railway may be competitive for spans up to 50 m. For larger spans, half-through box-girder bridges are preferred. In the litera- ture, one can find a variety of half-through bridges. One is shown in Figure 2.36. For large spans, the longitudinal steel girders suffer from lateral torsional buckling. During construc- tion, additional bracing systems can be used but not at the final stage. For this reason, main girders are casted so that composite action is activated. This leads to an elegant structure, and excessive steel consumption is avoided. In cases of twin-track continuous bridges, slen- derness values 1/10–1/15 are feasible [2.43].
2.4.3 Plate-girder bridges
Plate-girder bridges are chosen for railway applications when the construction depth is not critical. The longitudinal girders consist of welded cross sections, and the concrete deck’s geometry is similar to that employed for roadway bridges. Figure 2.37 shows some common cases. Deck plates with precast planks and in situ topping offer a fast and simple construc- tion method (case A). Fully precast concrete deck slabs with dry joints are the best solutions for reconstruction activities, but experience on durability issues for railway bridges is lim- ited; see Figure 2.10. In situ concrete deck slabs (cases B and C) is applied in more complex geometries or when transverse prestressing with tendons is necessary. In case B, the deck plate is designed to provide a robust kerb; thus, additional support parapets are avoided. A modern solution is depicted in case D. Prefabricated composite girders arrive on site, and the final deck concreting follows, VFT construction method [2.43]. Such cross sections seem to be very effective since it makes the limitation of deformations less laborious, especially in continuous and integral bridges.
Composite bridges with multiple girders can be applied for spans up to 50 m. For larger spans, twin-girder bridges are more cost-effective and easier to construct. Twin-girder bridges are less redundant structures than those with multiple girders, and therefore, fatigue
Composite girder
Reinforced concrete plate
Elevation
Cross section
resistance seems to be their weak point. However, inspection and repair are less compli- cated, and with a rigorous maintenance policy, this risk is minimized.
In Figure 2.37, one can see that some plate-girder bridges are equipped with lower bracing sys- tems. These may be trusses or stiff frames. Lower braces increase the redundancy of the structure and its torsional stiffness. Indeed the cross section behaves as a closed one, and expensive solutions with steel boxes are avoided. In case C, a concrete slab has been chosen to serve as lower bracing. This activates also a double composite action that is advantageous in continuous systems.
2.4.4 Box-girder bridges
Closed or opened box girders are used in a similar way as for the highway bridges. Due to their increased flexural and torsional strength, they are mainly preferred for long spans. For small and medium spans, they are less competitive, and simpler solutions are obviously cho- sen. One of the main disadvantages of the boxed sections is the large areas that have to be repainted due to corrosion; this significantly increases maintenance costs. Moreover, repaint- ing is time consuming, and this can be very dangerous under load conditions. In coastal regions, corrosion is considerably accelerated by airborne salt, and this has been detected as the main reason for many damages. A solution to this can be ship-bottom-shaped cross sec-
tions; see Figure 2.38. This configuration helps rainwater to wash away the airborne salt and
Figure 2.38 Ship-bottom-shaped cross section. Precast planks Lower bracing Lower slab VFT- composite beams B D A C
Figure 2.37 Plate-girder railway bridges. (A) With precast planks, (B) with lower steel bracing, (C) with
decreases the repainting frequency. Ship-bottom-shaped cross sections in combination with special types of weathering steel can enhance the salt corrosion resistance considerably [2.19].
2.4.5 Filler-beam bridges
Encasing steel beams in concrete is beneficial from many points of view. Steel beams do not require coating, a composite action can be achieved without the use of headed studs, local buckling of steel plates is avoided, and a better stiffness with less steel is achieved. In Figure 2.39, a filler-beam bridge is shown. These are small-span bridges and are used both as simply supported (max. span ≈ 15 m) and continuous systems (max. span ≈ 30 m). Transverse reinforcement passes through holes at the webs of the steel beams; thus, shear connection is ensured. EN 1994-2 offers detailed guidance on the design of filler-beam deck bridges.
2.4.6 Pipe-girder bridges
For small and medium spans, the cross-sectional configuration of Figure 2.40 may be seen in certain cases as an attractive solution. Steel pipes do not suffer from lateral torsional buckling, and therefore, horizontal bracing systems during concreting are omitted. This has a positive effect on both the speed of construction and the appearance of the bridge. Steel pipes can be filled with concrete at hogging moment areas so that a double composite action is achieved. At midspans, pipes can be filled with low-density mortar so that the noise level is improved [2.34]. Finally, the tubular shape of the pipes increases the corrosion resistance since it minimizes the accumulation of airborne salt especially in coastal regions. A pipe-girder cross section has been successfully designed by the authors for a 25 m simply supported one-track railway bridge.
2.4.7 Arch bridges
In arch bridges, nonsymmetrical vertical loads result in strong bending moments both to the arch and the stiffening girders. Therefore, in cases of railway applications, cross sections of excessive size and weight may arise. An alternative solution is network arches in which inclined hangers form internal trusses (see Figure 2.41). This develops a diaphragmatic action between the arch and the stiffening girders, a characteristic that is missing from a common arch bridge. The hangers in the networks are positioned in such a way so that
Figure 2.39 Filler-beam bridge.
At mid-span At support
bending moments in arch and stiffening girders are practically negligible and only normal forces arise. Network arches are stiffer than regular ones and with more slender structural elements [2.9]. Therefore, deflections and vibrations are easier to control.
2.4.8 Lattice girder bridges
Composite lattice girder bridges may be a viable option for long spans with no underclearance limitations (Figure 2.42). The steel structural elements of the girder are usually closed cross sections with adequate resistance against buckling. The top chord is rigidly connected with the deck slab. In Figure 2.42, this is achieved through an additional boxed beam encased in concrete so that longitudinal shear forces are closer to the gravity center of composite upper chord. In this way, local forces and bending moments in the connection are minimized [2.33].
Lattice girder bridges may be equipped with lower bracing systems or concrete slabs in order to develop closed box behavior. At hogging moment areas, a thicker concrete slab increases the flexural resistance and stabilizes the bottom chords laterally in plane. It is worth mentioning that regulations for composite lattice girders are not given in EN 1994-2. Attention should be paid to the rheological and the temperature effects. In order to neu- tralize members’ shortening due to shrinkage of concrete, longer segments may need to be erected. The use of prefabricated elements has reduced creep- and shrinkage-induced defor- mations and is in such cases to recommend.
2.5 CONSTRUCTION FORMS