ERECTION METHODS 1 General

The final choice of an appropriate erection procedure is of great importance for achiev- ing the desired geometry, construction speed, and cost-efficiency. The erection method defines the loading history of the bridge and has a primary influence on the evolution of stresses and deformations. For the majority of medium- and long-span bridges, the cross sections, the bracing, and the strengthening members mainly depend on the internal forces that emerge during erection. This means that the static design for the transient situations of erection may be more crucial than for the final-stage design; usually it is more laborious as well. Experienced designers discuss all the possible alternatives together with steel fabrica- tors and contractors. The risk assessment of the proposed alternatives is a demanding task, and detailed safety plans need to be prepared. The erection of bridges is a complicated issue and cannot be covered in few paragraphs. However, a brief introduction follows. Interesting information on erection techniques is found in [2.48].

2.6.2 Lifting by cranes

Lifting steel members with cranes is the most economic method for the erection of small- and medium-span bridges. Mobile cranes can lift 50 tones at 50 m radius or 100 tones at 28 m radius. For lifting heavier elements, cranes can be used in tandem, but this may increase the costs considerably. Cranes are used in tandem when erection has to be carried out during a limited period of time and I girders are lifted in pairs. Similarly, lifting of complete composite members may be preferable. This may be the case for the VFT beams described in Section 2.3.5.

The most common type of cranes is the road-mobile ones; see Figures 2.30 and 4.26. Other types are the rail-mounted cranes or those on a floating vessel. In all cases, the advice of a specialist contractor is necessary so that important factors are considered, for example, overhead electrification equipment, access to site, and exposure to wind.

2.6.3 Launching

This method is based on the concept of assembling the bridge’s steel part and launching it forward on rollers or sliders to its final position; see Figure 8.31. A tapered launching nose is connected to the main girders so that stressing and cantilever deflections can be kept to

a minimum. The launching nose is a light truss construction with a length approximately equal to 30% of the length to be crossed. Rollers and sliders are placed on the piers and on temporary piers in the case of very long spans. The slide path generally runs parallel to the lines of the substructure, and temporary piers are located where stable solid exists. It is important to note that during launching laborious stability investigations have to be conducted for all the possible launching phases since plate buckling due to shear, bend- ing, or concentrated support reactions may occur; see Figure 8.1. Moreover, the system needs to be robust against pulling forces that are produced from the launching devices. These devices consist of a system of hydraulic jacks for heavy structures or winches for the lighter ones.

Rollers are usually ball bearings constrained in a channel (Figure 2.23) and are used for light superstructures. Sliders are devices composed of different materials with sliding inter- faces. These may be phosphor/bronze or a PTFE sledge on stainless steel. Sliders during launching may exhibit a friction coefficient up to 8% and are used for heavier structures.

Launching operations are usually conducted for the pure steel girders; thereafter, rein- forcement is placed and concreting follows. Launching the steel frame with its slab rein- forcement on it has been applied in few bridges where steel cage handling had to be avoided. However, this alternative should be treated as a nonstandard solution. In cases of bridg- ing very busy roads or railways, launching steel–concrete composite frames has been seen by some engineers as an attractive option. Launching the completed composite structure is sometimes preferred when casting with mobile formwork (Figure 2.3) is considered as expensive, more time consuming, or causing under clearance problems. Obviously such an erection method leads to an increased steel consumption since it is more difficult to ensure stability. Moreover, stronger launching devices are needed. The picture in Figure 2.48 shows the world’s longest composite bridge ever constructed with composite launching.

The launching method, known also as incremental launching, is usually chosen for mul- tiple continuous span bridges with constant height girders and when lifting is seen as impos- sible, for example, bridges crossing deep valleys. It is a complicated procedure and more expensive than crane lifting and carries higher risks. An additional drawback is the need of an assembly area behind the abutments. However, launching is associated with a high erec- tion speed that ranges from 30 to 50 cm/h.

2.6.4 Shifting

The steel structure is fully constructed on temporary supports alongside its final position. Then it is moved transversely trough rollers and jacked down onto its permanent bearings (Figure 2.23). The main advantages of this method are the brief traffic interruption and the reduced need of stability strengthening since the structure remains always in its final struc- tural configuration. Many bridge engineers are fond of combining shifting with launching. In urban areas, it may be difficult to find a sufficiently wide area next to the final position.

2.6.5 Hoisting

Lifting devices attached to the cantilever parts of the bridge hoist up vertically central parts to their final level (Figure 4.26). This is a rarely used method that is mainly appropriate for bridges crossing waterways. The hoisting operations are performed by cables drawn by launching jacks or winches with pulley blocks. Heavy and very large beams can be erected in few hours with this method. The wind speed during erection must be very low.

2.6.6 Segmental construction

This is the standard construction method for box-girder frame bridges crossing deep valleys, also known as the cantilever method. Steel segments are transported and hoisted up to their final level. The next segments are suspended in place by cranes and on-site welding follows; see Figure 2.49. The construction is repeated until the span is completed. The segmental construction may begin from different starting points, usually central piers. The length of the steel segments usually varies from 3 to 6 m. For equilibrium reasons, segments at sup- ports are obviously rigidly connected with the piers.