composite bridges
2.5 CONSTRUCTION FORMS 1 General
Choosing the most appropriate structural system for a bridge project is not an easy task. The final decision is determined by many different parameters such as the available construc- tion depth, the soil quality, the seismicity of the area, future reconstruction activities, and maintenance. A discussion on the advantages and the disadvantages of each system follows.
Figure 2.41 Network arched bridges.
Cast in place slab Cast in place slab Lattice top chord Lattice bottom chord Upper precast slab Lattice diagonal Bottom precast slab At mid-span At pier Elevation
2.5.2 Simply supported bridges
Simply supported bridges (Figure 2.2) and bridges consisting of isostatic spans can be described as the simplest structural forms. Thermal effects, creep, and shrinkage do not cause any additional internal forces, and cracking of concrete is avoided since the deck plate is constantly under compression. Due to compression in the deck, many designers favor the implementation of an isostatic solution when prefabricated elements are to be used. Tension in the deck plate in hogging moment areas of continuous bridges may cause cracking in concrete or fatigue damages in reinforcement due to the movement of the joints between the precast planks during passage of vehicles.
Bridges with multiple isostatic spans facilitate erection and increase the prefabrication rate. Indeed main girders have smaller lengths and weights and can be placed on the piers with cranes of lower capacity. For larger spans, temporary falseworks must be used at intermediate positions. In some cases, isostatic systems are used during construction and at final stage are constructed as continuous ones. An example has been given in Figure 2.4.
Isostatic systems are also preferable when strong support movements are expected due to weak or compressible soils. Moreover, such bridges can be easier reconstructed or replaced, for example, after an earthquake. This is important in the case of urban areas where such activities must be conducted as quick as possible and with the less traffic disturbance.
2.5.3 Continuous bridges
Simply supported bridges can be cost-effective solutions mainly for small and medium spans. However, for longer spans, serviceability verifications are onerous and steel con- sumption becomes excessive. Therefore, stiffer systems are chosen. Continuous composite bridges (Figure 2.16) are associated with a limited deformability, increased redundancy, and redistribution capabilities. These are important structural characteristics that modern bridges should possess.
The structural topology of a continuous bridge often depends on the available posi- tions for placing the piers. Piers are constructed in positions of “healthy soil conditions,” and the trend is to reduce their number as much as possible. For the superstructure, the size of end spans should be equal to 80% of the internal spans so that bending moments at internal supports are approximately of the same magnitude. Obviously, this is not always feasible. Excessive hogging moments arising from the decompensation of long next to small spans can be reduced by lowering the structure at intermediate sup- ports after concrete hardening. This is an indirect way of prestressing concrete without longitudinal tendons. However, designers have additional means for reducing bending moments at supports. An example is depicted in Figure 2.36 in which a central V-shaped pier reduces the lengths of the main spans and contributes to the bridge’s performance both structurally and aesthetically.
Continuous bridges may be erected by launching. This is a sequential construction quite appropriate for box girders that starts from one end of the bridge. A new segment is added, and the whole deck including diaphragms and lateral bracing that rests on a system of guided rollers is pushed by hydraulic rams. A temporary launching nose is attached to the front of the first span to limit the weight and reach the next pier. The bridge may be launched downhill if a small inclination is present. Plate buckling of the girder webs must be checked since they are subjected to concentrated compression forces from the rollers during launch- ing operations; see Figure 8.31.
2.5.4 Frame bridges
In many bridges, longitudinal movements due to temperature, shrinkage, and any kind of hori- zontal support movement are supposed to be absorbed by expansion joints (Figure 2.43). These elements need to be replaced due to leakage of the joints. Moreover, expansion joints lead to discomfort for the drivers during passage of vehicles. Bearings should also be replaced during the design life of a bridge. Therefore, many designers tend to eliminate all movement joints and, if possible, all bearings as well. This is achieved through frame and integral construction in which super- and substructure act together in response to loading and imposed deformations.
In framed bridges, piers are rigidly connected with the main girders. This type of con- struction is suitable for long spans and in general where deformation, resonance, and fatigue requirements are difficult to fulfill. In the railway bridge of Figure 2.42, the lattice deck and the piers form a stiff composite frame. Through the trial and error method, bridge engineers try to optimize the stiffness ratio between horizontal and vertical components. For example, deck rotations should be avoided, and for this reason, the deck is more flexible than the piers. However, in seismic regions, an earthquake may cause plastic hinges in the deck, and this is not acceptable. Therefore, an adequate number of weaker piers should be placed capable of developing high-ductile plastic hinges (Figure 2.44).
Piers at side spans experience higher deformations and loadings due to temperature effects, creep, and shrinkage. This is because of their larger distance from the neutral displacement
point of the deck. This is defined as the stiffness center of the deck in which horizontal
displacements are equal to zero. In Figure 2.45, one can see the estimation procedure of this point whose coordinate depends on the stiffnesses of the piers and the bearings if any. Moreover, the maximum values of deformations and forces arise at the abutments. For this reason, most of the designers avoid the rigid connection of the deck with the abutments by the use of bearings and expansion joints.
Expansion joint
Main girder Backfill
Backfill
Section a–a
Surfacing Expansion joint
Bearing Approach
slab
Approach slab Abutment
a a
Plan view
Figure 2.43 Simple support detailing with expansion joint. Earthquake
force
Strong piers Plastic hinge (ductile)
Weak Weak
Concrete pier with confinement reinforcement for high ductility Figure 2.44 Frame bridge with weak and ductile piers at side spans.
Horizontal deformation of pier i: d l l i i i i i i i x x H K H K x x = ◊ -
(
)
= fi = ◊ ◊ -(
)
0 0 It is H x K x K i i i iÂ
= fi0 0=ÂÂ
◊ (2.1)Piers in bridges crossing over valleys may be extremely high. Piers with a total height larger than 200 m are not peculiarities. Obviously, such slender structural elements are associ- ated with an increased buckling risk. It is worth noting that architects favor “very thin” piers that do not attract the observer’s attention from the surrounding environment. Framed bridges ensure for the piers a reduced buckling length due to their rigid connection with the deck. This is an additional advantage that designers always consider.
2.5.5 Integral and semi-integral bridges
Bearings and expansion joints can be omitted by connecting the main girders rigidly with the abutments. The elements of the superstructure act together with those of the substruc- ture, and thus, a rigid frame is formed. Integral bridges are very robust structures that can reach spans up to 100 m with impressive slenderness values (min h/L ≈ 1/50). This made them very popular both for high- and railway applications in many European countries.
One can find a variety of end connections for integral bridges. A typical one is shown in Figure 2.46. Steel or concrete piles are connected with each other through a reinforced concrete crossbeam (pile cap). On the upper side of the pile cap, the main girders are placed on the pile cap through the use of temporary bearings, for example, steel plates. Thereafter, the endscreen wall is completed after the deck steelwork has been erected and the deck slab cast. In this way, bending moment continuity is achieved. Moreover, the encased part of the steel girders has to be well anchored. Therefore, girders are equipped with shear connectors, hoops, and/or tie beams. Holes in the webs are also needed for the reinforcement continuity of the endscreen wall.
x
[δ]
δi
Neutral displacement point
Hi ki x0 k1 k2 k3 ki kn
The piles have to be flexible enough in order to absorb the horizontal deformations of the deck due to thermal expansion and contraction, concrete shrinkage, etc. Therefore, they are positioned in a straight line. The soil pressures due to the aforementioned displacements should be carefully calculated in order to achieve an economic and safe design. This is a difficult task because soils are often inhomogeneous. To further complicate matters, second- order theory deformations due to buckling of the piles may arise.
When the endscreen wall does not provide support to the main girders, the bridge is defined as semi-integral. In such cases, horizontal deformations are accommodated by con- ventional bearings that are placed on footings or piles; design is easier and uncertainties due to soil conditions do not need detailed consideration. An end connection of a semi- integral bridge is depicted in Figure 2.47. The main girders are placed on the bearings and grouting of the joint between them, and the retaining wall with nonshrinkable mortar follows. The moment continuity is ensured by means of high-strength anchor bolts that connect the upper flanges with the backfill and the lower flanges with the retaining wall.
Approach slab
Approach slab
Final concreting phase
Temporary bearing Pile cap Backfill Endscreen wall Endscre en wall Pile Figure 2.46 Fully integral bridge on piled foundation.
Backfill Backfill Headed studs Bearing Flush end-plate Fully precast deck plate Grouting after assembly
Hole for grouting (e.g. Φ100)
Prefabricated footing Grout
High strength anchor bolts Load introduction plate
Retaining wall Prefabricated
retaining wall Figure 2.47 Semi-integral bridge.
Prefabricated footings and back walls are used when old bridges need to be replaced and traffic disturbance must be minimized. Erection time can be further reduced with fully precast elements for the deck plate.
It is worth mentioning that in cases of skew bridges, soil pressure tends to cause plan rotation of the deck. For this reason, integral construction should be considered for skews up to 30°.
In seismic regions, abutments have a major contribution to the seismic resistance, and the bridge should be designed to remain elastic during the earthquake event. EN 1998-2 pro- vides limit values for the displacements at the abutments so that the soil or the embankments behind them are not severely damaged.
Interesting information and experiences from integral and semi-integral bridges are given in numerous papers of [2.3].
2.6 ERECTION METHODS