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Monday, 06 May 2013
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Sanjay Londhe_Expert Speak_ProjectsToday

Innovation and technology in bridge construction
Sanjay Londhe, Director,
Ashoka Buildcon Ltd., traces the evolution of bridges over a thousand years up to present times.

 

 

Bridges are the backbones of any transportation and infrastructure project (railway/road). Bridge engineering has a history of thousands of years, and finds its origin in Indian mythology and spiritual scripts like the Ramayana and Mahabharata. In those days bridges made of a heap of stone blocks, bunds and timber logs were the only options possible. The old bridges were expected to merely act as a beam spanning across the gaps along the road and railway alignments.

 

  Bridge Constructions_Ashoka Buildcon_Expert Speak

But as the growth of civilisation (society and cultural development) started taking place, the demand on all the branches of engineering (industry) increased exponentially. The increased demand called for more and more research that started taking place. From study of engineering growth in last hundred years, we may observe that the development of civil and structural engineering is closely associated with the developments that took place in the peripheral branches like material science, mathematics, communication, electronics, computing technology (computer science), IT, and the newly emerging branches of engineering.

 

As all the branches of engineering started growing, it was observed that the growth was rapid, but not parallel. This fact resulted in a serious problem – or a gap between imagination (conceptual planning and design) and the ground-reality (i.e. actual bridge erected at sites).

 

In olden days the computing technology and simulation software etc. were not available. Engineers had to struggle hard to prove the adequacy of the concept, before starting the design and construction. The photograph of the Firth of Forth Bridge in Scotland, which is a steel cantilever bridge, was built in the 1880s by Allan Stewart and Sir Benjamin Baker. But both of them had to struggle hard to demonstrate by a conceptual model to the sceptical Victorians that a cantilever bridge would be safe.

 

Introduction to new bridges Unlike 50 to 60 years ago, computing technology and material sciences have led to converting even complexshaped bridges into reality. Below shows two a beautiful string cable stayed bridges designed by the famous Spanish architect and structural engineer Santiago Calatrava. During conceptual planning and the design of such modern bridges, often various challenges are faced. A few of these challenges are listed below:

 

 
  • Complex loading: temperature, wind, erection, blast etc. and their combinations.
  • Complex structural geometry: difficult load transfer mechanism.
  • Refined understanding about the structural behaviour: failure theories, overall performance-based design, reliability analysis as compared to limit state vs. working stresses.
  • Changing ‘codes/standards’ requirements: QAQC, crack width, exposure, non-linearity etc.
  • Advances in associated fields like material science (concrete-steel etc.), electronic devices (strain gauges), monitoring and surveying instruments.
  • Increasing project demands (time, cost, safety, quality, and aesthetics).
  • New research towards bridge execution and testing.
  • HSE (health safety and environment) requirements.
  • Construction stage analysis.
  • Structural health monitoring systems.
  • Life expectancy etc.

 

The architect, structural designer, planner or execution engineer cannot overcome all these problems by working in isolation. Complete team work is essential to solve these problems and to converge to an effective solution. But, from the failures of many ambitious bridge projects, it can be observed there is clear absence of team work (except in a few cases). Typical failure of a bridge may happen in case the designer doesn’t know how the contractor is going to construct that structure and, on the other hand, even if the contractor does not know how that bridge is designed (boundary conditions, assumptions, standard design practices etc.). The typical reasons of failures are insufficient formwork, inadequate launching girder, improper execution, unexpected forces (wind/blast) and misunderstanding between designerplanner and site personnel etc.

 

The work of a designer, planner and the execution (site) engineer shall be complementary to each other and not contradictory. Thus, each one of them must know the works of the other (their counterparts) to a sufficient extent.

 

Bridge erection techniques:

From the above facts it is clear that the complexity in bridge engineering is increasing, which cannot be handled by the bridge designer or site engineer alone. The designer must know the proposed method, which the contractor wants to employ for the construction site of the said bridge; and, on the other hand, the execution person and planner must know briefly, how the designer has designed the structure, with its limiting conditions, boundary conditions and assumptions. In certain cases, the execution of design, as it is, become difficult due to site-specific conditions (probably, which could not have been addressed by the structural engineer at design stage). In such cases the modification in design becomes necessary. If the contactor knows the design basis, he can develop a better proposal indicating desired changes in design at a particular spot in the project. (On the other hand, even if the designer knows the constraints at site, in which the site engineer is supposed to work, he can develop better design right from construction stage).

 

A few of the popular methods adopted for construction of bridge superstructure are given below:

 
  • Erection using traditional methods: using formwork and with land-based hydraulic cranes (single/double cranes for span-by-span construction or even random span construction.
  • Erection using modern methods:
    floating cranes, barges and jack-up platforms, using bridge launching girders, cable cranes, CFT (cantilever form traveller) or MSS (moving scaffolding system), and tower carnes.
  • Bridge erection using a floating crane

 

If the bridge is passing over a large water body it is convenient to use floating cranes kept on the barges, vessels and jack-up platforms.

 

Points to ponder:

Duration of project, capacity of floating crane, availability and characteristics of waterway, facts of actual site like casting yard location, wind speed and tidal variations, cost and budget, and cooperation from designers and design capacity.

 

Bridge erection using a launching girder:

In case of long span bridges, launching girder is commonly used by the contractors.

Apart from the regular bridge erection techniques like formwork, staging, launching girders and advanced technique of floating cranes, a suspended erection crane/cableway is also used nowadays. In such cases the permanent structure has to be designed keeping in mind the construction/erection and dismantling methodology. The construction scheme/methodology imposes large loads on the permanent structure, which are often greater than the design loads even.

 

Bridge erection using a suspended cable crane (cableway):

This is a unique and challenging technique of bridge construction. As mentioned earlier, there are many innovative methods like CFT, MSS and erection using tower crane. From the discussion till now it may be clear that the permanent design and erection method (enabling works) have very close relation to each other.

 


 

 
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