What Are Precast Segmental Bridges. Types and pros and cons of Precast Segmented BridgesArticles > What Are Precast Segmental Bridges. Types and pros and cons of Precast Segmented Bridges
What Are Precast Segmental Bridges:
Precast structures are completely constructed before positioning them. A precast structure is constructed from pre-stressed concrete and the final form of the structure is then positioned where it has to be used. Precast concrete is made up of fine stone gravels, cement, water, and admixtures. The mixture is prepared in the factory's batch plant according to the concrete mix design specified by the civil. The approved batch of concrete is transported to the molds by means of a dispensing machine using an overhead crane. Discussing the construction of the bridges, all the bridges that are constructed with CFST or concrete materials are constructed in two ways. One is the “Single Span Construction Method”, while the other is the “Segmental Construction Method”. In a single span bridge, the whole span of the bridge is constructed at once. The whole span of the bridge is in consolidated form, and no part of the bridge span is separated from the other part. The span of the bridge does not include piers and abutments. The span of the bridge consists of three parts i.e. beams, deck, and parapets. Abutments, piers, foundations, and arch are not included in the span of the bridge. As we mentioned, a CFST or concrete bridge can be built either in the form of a single span as well as in a segmented form. In a segmented span bridge, there are many spans of the bridge which are placed on the piers. The pier to pier connection is built by means of segmented spans. In other words, the span of the bridge has been divided into many segments or parts. It is done because of the convenience of lifting the beams and slabs in parts. It is very difficult to even for the heavy machines to lift a consolidated beam. Therefore, the beam is pre-casted in segments on the ground, then these segments are lifted above the top of the piers.
Advantages of Precast Segmental Bridges:
American Segmental Bridge Institute has a monopoly in the field of constructing segmental bridges. It gives preference to the segmental construction of the bridges due to the following reasons.
- It is easy to lift and shift the segmental parts of the precast structures. Even the heavier machines fail to lift the beams and sections of the bridges in consolidated forms.
- There is a minimum impact on the traffic and environment during the process of construction.
- Segmental construction is economical as compared to composite construction.
- A break in the procedure of construction does not influence the quality of construction. If some segments are prepared, while others are to be prepared due to some environmental condition or financial problems, the quality of construction is not impacted. The remaining work can be done at any time. No rush to complete the work as in other types of construction.
- Scaffolding and Shuttering are not needed for a segmented construction of bridges, as the precast construction is carried out on the ground. The readymade structures are, then lifted up to the top of the piers and abutments.
- It is easy to keep concrete moisture in the segmented construction of bridges.
- Pre-fabricated segments provide better quality control.
- The pre-stressed structure can be fully loaded immediately.
- The pre-stressed cables used in the structure can be inspected and checked the time by time.
- These bridges are low weight due to thin bridges sections.
- The maintenance cost is very low in this method.
- Less time taking.
Disadvantages of Precast Segmental Bridges:
Despite the facts mentioned above in the heading of “Advantages of Segmental Bridges”, the disadvantages of this method can not be avoided.
- High construction loading or high technology is needed during the construction of segmental bridges.
- Lifting and positioning the heavy segments is troublesome and risky for the lives of the workers. Thus, skilled labor is needed in such constructions.
- The pre-stressing of the material needs extra costs.
- Land sliding and erosion may affect the solidarity of the bridge.
Types of Segmental Bridges:
There are four different types of segmental bridges.
- Balance Cantilever Bridges:
Such type of bridges is built by projecting cantilevers (bridges beams) horizontally in the space which are supported at the dorsal end. In small cantilever bridges, the cantilevers are in the form of beams, while in large bridges cantilevers are made in the form of trusses made up of steel/iron.
The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1,500 feet (460 m), and can be more easily constructed at difficult crossings by virtue of using little or no falsework.
- Progressive Placement Bridges:
In this method, construction starts at one end of the bridges, and the constructed portion is erected ahead with the continuous process of construction. This type of construction is suitable in environmentally sensitive regions.
- Span-by-Span Constructed Bridges:
Span-by-span erection is another successful method of constructing bridges and being a very common and cost-effective method. In this method, precast segments of the bridges are lifted to the top of the piers one by one. This method is used in long bridges.
- Incrementally Launched Bridges:
The incremental launch is a very fast method of constructing bridges in civil engineering. In this method, the complete construction of the deck of the bridge is carried out from one abutment only. This method is highly mechanized and uses pre-stressed concrete.
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