With the fast paced technical innovations and increased demand for speedy constructions incorporating global outlook, bridge construction process evolves structurally and aesthetically. Veena kurup tracks the transformational path witnessed in bridge engineering.
The infra focused growth strategy estimating to regain the double digit progressive dream of Indian economy is being widely discussed since the advent of Twelfth Five Year Plan period. This increased focus towards infrastructure development has brought in numerous advancements in the construction segment, particularly in the engineering strata. Similar significant progress is also being witnessed in the bridge engineering spectrum since past 50 years, owing to the developments in peripheral branches of engineering.
Today, many new complex bridge projects are also being implemented in reality, wherein India has now stepped into the superlative league and is in the process of constructing the world’s highest rail bridge and longest rail-cum-road bridge. Additionally, no effective railway stretch can be effectively functional unless bridges are erected over rivers, creeks and mountains, which prevail as the barricades to be crossed. India plush with distinctiveness in the terrain regions involves numerous ghats, mountains, and major rivers like Ganga, Brahmaputra, Yamuna, Tapi, Son, Godvari, Narmada and so on. However, the country’s core infra executioner in developing roads and bridges – National Highway Authority of India (NHAI) along with the state Public Works Department (PWD) and Indian Railways are constructing numerous road, rail-over and rail-cum-road bridges over these rivers and even through the hilly terrains.
However, this natural geographic distinctiveness existing in India is one amongst the major barricades faced by the developers and contractors involved in executing these bridge projects. “Due to the varied terrain conditions, the bridge designs and engineering processes being practiced in India often does not go hand-in-hand with the prevalent construction technology utilized in India,” says a senior project engineer from Ircon International. Similar views are being uniformly upheld by industry analysts that, unless the construction methods are excelled, there is no point in implementing any modern theory/software in executing the bridge construction process.
Evolutionary Pace
Since the mythical ages, bridges have played a vital element in connecting the human civilizations. With the evolution of numerous civilizations supplemented with reformations in construction technologies, the bridge construction process is undergoing a constant change. A transformation from structures made of heap of stone blocks, bunds and timber logs to concrete and today cable stayed and sustainable bridges can be trailed.
The emergence of long span cable stayed bridged with hybrid decks using steel girders, concrete slabs and high tensile steel cables is increasingly noticeable with the dawn of 21st century. The recent report disclosed by ASA & Associates titled ‘Engineering Sector in India’ states, “The cantilever method of construction is the latest and most economical and popular method generally adopted for the construction of long span precast or cast in-situ pre-stressed concrete segmental bridges.”
The cantilever construction module involves erecting bridges by utilizing cantilevers, which are structures that project horizontally into space and are supported on only end. For small foot bridges, the cantilevers can be erected as simple beams, but for large structures, the design codes are outlined to handle road or rail traffic use trusses built from structural steel, or box girders built from pre-stressed concrete. German engineer Heinrich Gerber was
one amongst the engineers who patented the hinged girder cantilever bridge concept. The Gerber headed engineering marvel, the Hassfurt Bridge in Germany opened the doors for the popularity of cantilever construction technology. However, engineers in the nineteenth century discovered an added impetus to the designing pattern, wherein it was explored that a bridge which was continuously balanced across multiple supports can distribute the load, tension and compression at different modules and can thus increase the structures overall lifespan. This was considered as an added boon in lowering the stress on the girder or truss and meant that longer spans could be built.
Later followed an era of suspension bridges, with the erecting of the Humber Bridge in 1970. The bridge was considered a super-structure due to its record breaking spans of 1,410 m. As a suspension bridge, one of the main challenges the designers Freeman Fox encountered was the liability of the road to oscillate under extreme conditions, be they wind or high loads. To overcome this, the architects created streamlined hollow box sections, the upper flange of which, when covered with a mastic asphalt, provides the roadway.
“The streamlined shape is not only aerodynamically stable, but greatly reduces the direct loads arising from wind, thereby relieving the towers of very considerable lateral forces with consequent further economy,” explained the Humber Bridge Board.
The recent infra-focused critical survey over the bridges being constructed in various countries (involving the emerging as well as developed nations) in the last two decades, outlined by the World Council of Civil Engineers indicates a common trend in adopting pre-stressed concrete cellular box girders decks for urban flyovers and cable stayed bridge decks for challenging terrains equipping to incorporate longer spans. This development is further fast tracked with the advancements in secondary segments which outline the project designs – involving science, mathematics, communication, electronics and even information and technology services.
Steel-concrete composite bridges occupy the middle ground between concrete and all steel solutions, wherein composite bridges are often opted in the span range where neither concrete nor steel is the obvious choice of material and where the composite bridge offers technical and economical advantages. However, composite bridges challenge the tendency of both designers and contractors to have a preferred choice of material and are demanding in this respect.
Additionally, with the increasing focus towards replicating globally renowned structures and in scaling the engineering spectrum to an international level, an accelerating inclination is witnessed towards cable stayed bridges. “The cable stayed bridges are economical, structurally and aerodynamically efficient and aesthetically superior in comparison when compared with other type of bridges,” shares a renowned contractor.
Cable-stayed bridges evolved as an arithmetic phenomenon in the technical era and have emerged as more efficient and longer in span as a result of advances in computer technology since the 1980s and 1990s. The recent report titled ‘Transportation in New Millennium’ outlined by the International Committee on Construction of Bridges and Structures stated that the cable-stayed bridges represent an efficient alternative to suspension bridges, particularly when spans are in the 300 m – 500 m range. The record spans, such as the Tatara Crossing Bridge now stands in the 900 m range. But a major concern and debate amongst the
engineers relating to the cable-stayed bridge construction is its post-maintenance and protection measures for the stay tendons.
The other factor to be outlined as a core element in the bridge designing process is the length of the barrier to be crossed (be it waterway or hilly terrain) and the amount and type of traffic as well as forces of nature (wind, tide, flood etc) as core factors deciding the bridge designs. “A bridge needs to be strong enough to support its own weight plus the load of passengers and vehicles travelling on it against the pull of gravity,” says a senior executive engineer from PNC Infratech Ltd. Balancing compression and tension is the other major objective in erecting a well-built bridge structure.
Design Elements
Though the bridges distinctively in material used and construction processes, numerous components like the basic designing concept remains vital to any structural patterns. The prime facets which decides upon the bridge designing pattern is relied upon the foundation base and the superstructure. The structure consists of the components that actually span the obstacle the bridge is intended to cross which includes: bridge deck, structural members, parapets (bridge railings), handrails, sidewalk, lighting and some drainage features.
The deck is the roadway portion of a bridge, including shoulders. Most bridge decks are constructed as reinforced concrete slabs, but timber decks are occasionally used in rural areas and open-grid steel decks are used in some movable bridge designs (bascule bridge). With the increasing momentum gained in utilizing advanced composites made of resin-impregnated strong fibers or fiber-reinforced plastic (FRP) has become one amongst the prominent construction materials in the 21st century. “This class of light, durable materials could provide sizable benefits to the global infrastructure, wherein the speed and ease of installation of composites as compared with conventional materials will make the use of FRP particularly competitive with respect to construction costs,” adds the project engineer from Ircon.
FRP composites are today even used as stand-alone structural members, as reinforcement for pre-stressed and non-prestressed concrete, or in combination with other structural materials for new construction or repair and rehabilitation puroposes. Additionally, industry majors anticipate composite bridges made with such new raw materials to afford the opportunity in embedding sensors, actuators and inspection monitors in the assessing the sustainability of the structures being erected. However, concentration needs to be emphasized on conforming the grade of the approach maintained on the bridge surface to avoid any bump or dip, especially in the road over bridges.
To combat the problems of cost escalation and increasing urge in maintaining the project execution expenses, continuous decks are preferred as viable options in the superstructure, for ensuring speedy construction. However as per the constructional review put forth by the World Council of Civil Engineers, Indian designers are still obsessed with differential settlement of foundation and prefer simply supported decks, with attendant riding surface problems. The other issues outlined as major cause for bridge collapse or constructional failure were the unsatisfactory operation of the drain pipes embedded in the deck slab. “Due to the prolonged monsoonal climatic conditions, the pipes get clogged in the absence of regular cleaning; water stagnates, which ultimately leads to corrosion of steel reinforcement. The international practice proposed is to provide longitudinal slope from the centre both
ways, collect the rain water from behind the abutments and drain away the so accumulated water,” outlines the report.
The other major component to be considered in the bridge construction process is the seismic and soil assessment of the region outlined for bridge construction. In India, there are four codes/standards/guidelines dealing with seismic design of bridges. These are IS:1893-1984 (Bridge Rules), IRC:6-2010 and IITK-RDSO Guidelines. However, the industrial associations (involving both national and global civil engineering bodies) has already proposed the authorities to revise IRS and IS:1893- 1984 codes. The current provisions observed for assessing the structures is IRC and IITK-RDSO guidelines, which are based upon the seismic zone map and zonal terrain factors.
Global engineering associations have put forth these essentialities outlined and assessed through their reports before its Indian counterparts, as the country now prevails as one amongst the most favorable infra investment destinations. “We will be reviewing and looking forward to implement the advanced codes, practices and execution modes being followed globally and estimates in scaling the construction level utilized for executing such projects,” states the Indian Civil Engineers Association in its statement on the report outlined by World Council of Civil Engineers.
Future Prospective
However, industry analysts estimate that the bridge construction will continue to grow and chip in sizeable investments from the highway, railway and metro rail sectors in India. In the roads sector, the National Highways Development Programme’s (NHDP) plan to award 17,000 km of national highways by 2015, seven mega highway projects (each covering 400-700 km stretch) and numerous flyover projects under the New Improved Jawaharlal Nehru National Urban Renewal Mission is expected to open up more such robust avenues for the bridge construction spectrum. Additionally, the investment plan of Indian Railways is likely to put bridge construction activity on a fast track. By 2017, almost 4,600 bridges will be constructed under the `800 billion Dedicated Freight Corridor project. Further, the Indian Railways plans to construct over 10,000 new bridges and invest `70 billion in the deployment of modern technology for rail bridges by 2020. The metro rail network is set to expand by over 800 km, of which 90% is expected to be elevated. Expansion of the three operational systems and development of eight new systems is under way. In addition, 70 km of an elevated monorail network is planned to be operationalised.
However a major hurdle in achieving these targets are the geological surprises, inadequate and frequent financial crunches, delays in securing approvals and complexities in the construction of sub/superstructures etc which still needs to be addressed. Moreover, the global analysts feel that Indian contractors need to use innovative technologies and methods to enhance productivity and timely delivery of project. Further, the implementation and practice of utilizing automated bridge construction management systems, monitoring modules, sensors etc., still remains unexplored.
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