Construction Engineering Assignment research paper
The balanced cantilever construction method was developed primarily for long spans, so that construction of the deck can be carried out without the use of extensive falsework.
The span-by-span method was developed for long viaduct structures with relatively shorter spans.
In the first stage, the end span and a section of the penultimate span up to its nearest point of contraflexure is cast. The supporting formwork is then moved forward and the section of deck up to the nearest point of contraflexure of the next span is cast. This second cycle is repeated until the final section of the end span (from the point of contraflexture to the end support) is cast. The superstructure construction therefore proceeds in one direction span-by-span. To speed up the construction it is also possible to commence construction somewhere in the middle of the viaduct and work towards the ends.
The formwork is generally supported on a moving gantry system. This in effect provides a type of factory operation transplanted on the job site. It has many of the advantages of mass production commonly associated with precasting as well as the added advantage of permitting versatile adjustments on site. The gantry may be supported from the piers or from the edge of the previously completed cantilever. The formwork gantry can be either above-deck or below-deck. For the above deck gantry, the formwork is suspended from steel rods. After concreting and post-tensioning, some forms are released and the gantry rolled forward by means of outriggers on both sides gantry’s superstructure. For a belowdeck gantry, a similar procedure is followed.
In some instances the formwork may be supported from the ground. To date all cast insitu span-by-span construction has been carried out using formwork supported from the ground generally.
Incremental Launching Method
Using this method, constructors cast the segments of the bridge deck in place in short lengths of between 10 to 30m, on stationary formwork located behind one abutment. Each unit is cast exactly against the previous one. After the concrete has reached sufficient strength, the new unit is post-tensioned against the previously cast segment. The assembly of segments is later pushed forward to enable -the next segment to be cast.
The alignment of this kind of bridge should be straight or curved with a constant rate of curvature. It is for horizontal and vertical alignments. The deck should have a constant depth that is almost always 1/12 to 1/16th of the longest span.
The main problem associated with this method is to ensure the resistance of the superstructure at all stages of construction. The methods adopted for this purpose are:
a) A first stage of prestress is applied concentrically to the entire cross section and in successive increments over the entire length of the superstructure.
b) To reduce the large hogging bending moments as the leading edge reaches the next pier, a fabricated structural steel launching nose is attached to the leading edge. An alternative method adopted by the French is to provide a tower and stay system in the forward part of the superstructure.
c) To construct long spans temporary piers are provided to keep the cantilever moment to a reasonable magnitude. The longest span built by this method is 168m. Two temporary piers were used in this case (Russell, 2009).
After launching is complete, and the opposite abutment has been reached, additional prestress is introduced to accommodate the moments of the structure in service.
Precast Concrete Construction
The most common form of precast concrete bridge construction is with the use of the conventional precast bridge beams. Different types of precast concrete decks and their structural advantages have been dealt with. The construction of these types does not in general require special techniques.
Precast segmental construction is widely used for the construction of long bridges and viaducts with medium to long spans. In such kind of construction, segments are made in a casting yard not far from the site and then transported to their final assembly. At the beginning stages of segmental precast construction development, the joints between the units are wet poured grout or dry mortar pack. These kind of joints enabled the units to be adjusted ensure that the bridge keeps the right alignment. Modem segmental construction uses the match casting technique. In this method the segments are cast against each other in the same relative order they will have in the final structure. No adjustment is therefore necessary between the segments at assembly. The joints are either left dry or skimmed with a layer of epoxy resin. This method enables the prestressing to proceed as soon as practicable after the assembly of the segments. For erection of the precast segments to proceed smoothly, a high degree of geometry control is necessary during match casting to ensure accuracy.
There are three methods for the construction of precast concrete segmental bridges. These are:
a) Balanced cantilever method,
b) Span-by-span method,
c) Progressive placement method.
Balanced Cantilever Method
This method of constructing precast unit bridges is essentially the same as insitu construction by the same method. The difference lies in the method of assembling the segments. In the case of insitu construction: the segments are cast insitu.
Whereas, in precast concrete, construction the units are transported from the casting yard and placed in position with the help of mechanical means. There are three popular methods for placing the units: crane placing, beam and winch method and use of launching gantries (Baxter, 2001).
Crane Placing – Truck and crawler cranes can be used on land where feasible. Floating cranes can be used over navigable water.
Beam and Winch Method – Where access is available under the deck the segments can be transported to the site and lifted into place by hoists attached to the previously erected segments.
Launching Gantries – There are basically two types of launching gantries. The first type of gantry is slightly longer than the maximum span length. Its rear support reaction is applied near the far end of the last completed cantilever and the front support is placed on the next pier (off which the segments will be placed). After completion of the cantilevers, the gantry moves forward to place the diaphragm segment at the next pier. The gantry then positions itself for the next cycle. The second type of gantry has a length of twice the typical span. The reaction of its legs is always applied above the piers.
As previously described this method was initially conceived as a cast insitu method. A modified form of that method is used for precast segmental construction. The segments then prestress to provide continuity with the preceding span (Patterson, 2010).
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