Height of innovation
Construction of bridges and flyovers are seeing efficient building processes while keeping costs at a minimum. BY TEAM CW
In a bid to ensure smooth flow of traffic across the Bengaluru city, the state government has decided to build 200 bridges entailing an investment worth Rs 1,000cr. This is good news for infrastructure companies. However, construction of bridges turns complicated because most times concessionaires face unexpected difficulties during construction especially in challenging terrain conditions and most often during concreting.
The formwork systems are among the key factors that decide the success of a construction project in terms of speed, quality, cost, and safety of workers. Considering that clients are looking at completing projects in the shortest time possible to minimise the cost, an effective way to expedite works is to have the structure completed in the shortest time, using a system of efficient and appropriately designed formwork. The design and use of the right formwork system, as well as stipulation of an effective resource planning strategy to control and maximise the use of the formwork, are crucial to the overall success of a project. Depending completely on speed often contradicts the achievement of other quality targets. Problems such as misalignment, misplacement, deflective concrete or holding up other works result serious interruption.
In an interview, Raj Lakhani, managing director, PERI India, had said, “Considering that most projects are time bound, formwork activity will be critical for achieving faster construction. Infrastructure players know that formwork is supposed to take care of heavier loadings from concrete, high concrete pressure, high tolerances, wind loads and so it is to be strong and sturdier, as it is subjected to high risk. Adding to this, different trades likes rebar, MEP, and concreting need to work on these temporary supports. They are not aware of the danger of working and all this needs to be taken into account in formwork.”
With the increasing span of bridge structures, large number of continuous bridge is being constructed using precast segmental construction. There have been improvements in code of practices in India with the introduction of limit state design method, special vehicle (SV) loading and fatigue load for bridge design. This has led to the design of bridge bearing as well. Arun Karambelkar, president & CEO, HCC, speaking about the technicalities of the Bogibeel rail-cum-road bridge on the river Brahmaputra in Assam, says, “When you look at the Brahmaputra, the course of the river is about 5-10kms. Bridging it calls for first finding out the closest points with the least width. This is a 5.5km rail-cum-road bridge. HCC had to build two tracks with a road atop. We first built this huge truss with a mammoth cross section, which was a fully welded steel structure. It was then made into a complete structure. We created infrastructure to assemble the structure and built a fabrication unit next to it. Each truss is then pulled to fit next to the other and each has a span of 125m. A single portion weighs 1,700 tonnes. And we have the capability to pull 10 such trusses in one go, which means 17,000 tonnes. Each truss is pulled like a train to the end and that way the entire bridge is constructed. We had set up high-tech factories to build the trusses or the superstructure fabrication facilities.”
The quality and reliability of the concrete is crucial to safe construction and lifespan of the bridges. To ensure the same, different ingredients of concrete has to be mixed homogenously to produce a perfect mix of concrete and has to be place in short span of time. Bridge concreting is done using concrete mix up to M80 grade and critical requirements like casting blocks need maximum strength in minimum possible time for pre-stressed or post-tension casting.
Some of the other issues and challenges in concreting of bridges are stiff consistency concrete producing, placing concrete girders using cranes (e.g. Metro projects) along heavily crowded roads and transportation and laying of concrete pipes on the sea or river for concrete placement. To overcome these concreting difficulties, Schwing Stetter India only manufactures different types of advanced concreting equipment and provides various concreting solutions to its customers. The most commonly used equipment to place concrete for bridge projects are: Batching plants CP30 and M1, which provide a homogenous mix of concrete. Concrete pumps like BP350, SP1800 and BP3500 which are designed with two valve technologies; Flat Gate Valve (FGV) and Rock Valve, which enable smooth conveying of concrete to long distances and also helps to pump stiff mixes for casting.
Transit mixers are used to transport concrete for bridge constructions. Most of them come with modern design, simple handling and are cost effective. Separate placing boom — 28m to 35m — and boom placers are used for faster completion of concrete placement.
The advanced shoring method or movable scaffold system (MSS) has been developed for multi-span bridges over difficult terrain where scaffolding would not feasible. A launching girder moves forward on the bridge piers to allow placing of the cast-in-situ concrete. The method – both underslung and overhead – is adaptable for a range of spans. Conventional scaffold is a suitable choice for single-span bridges. For bridges that are longer and have multiple spans, the scaffolding needs to be moved between the different sections during construction. This has developed the advanced shoring technique. The construction method uses a movable supporting beam, gantry, for the falsework that reaches over at least one span but usually over the length of two spans. With the supporting beam in place, transverse beams along the gantry secure the formwork and working platform and the building process can be carried out efficiently. With special roller bearings and launching jacks the gantry can easily be moved forward along the bridge as the construction proceeds. The travelling gantry system is most suited for spans of 30m to 60m.