BY Jayashree Mendes
The Britishers may not have been the best thing to have happened to India, but they did attempt to unite us. No, we are not talking about unity in the nationalist sense of the word; their attempts at unity was through building bridges that continue to stand the test of time nearly a century after they were built. Over the last 50 years, India has attempted to catch up in terms of adopting world-class technology (after all, structure marvels can only be created through design and engineering), and managed to see results quite lately. One does not need to go into the reasons as to why India chose to wake up late, but that is how it has been.
Satish Kumar Sharma, chief technology officer, HCC E&C, says, “There is huge scope for improvement in terms of the construction techniques used for building bridges and flyovers in India. Firstly, the conventional flyovers are simply supported, resulting in too many expansion joints, which affect the pleasure of driving on good flyovers. This problem can be optimised by the introduction of continuous deck method which will not only reduce maintenance cost but enhance the pleasure of driving. Furthermore, the design of the bridge should incorporate integral pier structures like Pier, Pier Caps and Decks to eliminate the bearings; this will help in reducing the maintenance cost and also provide a smooth riding quality.”
Building bridges and flyovers in India is not an easy task. What makes the task formidable is the space constraints that make the work challenging. Consider the Mumbai Metro work alone. Set on an extremely busy route, the work took longer than expected as the planners had to account for constant traffic and manage the logistics of transporting gigantic equipment by night so as to continue work.
With rapid development through availability of high strength steel and concrete, and new methods of building bridges and flyovers, not to forget the increasing span of these structures, a large number of continuous bridge is being constructed especially using precast segmental construction. If one has seen some of the amazing videos on social media that show the massive 580-tonne machine that the Chinese use to build bridges, it could help solve some of the problems of delays of these structures that are usually constructed on busy roads. The massive SLJ900/32 is basically a 580-ton, 300-foot-long and 24-foot-wide mega machine used in China to build bridges. Rather than use a stationary or crawler crane to lift the girder of a bridge off the ground and drop it into place, this mega machine drives the girder onto the previously placed girder, slowly extends its arms to the next support platform, pushes the girder towards the front of the machine, and then lowers it into place. If the pylons are properly installed, the SLJ900/32 can extend past the pre-made bridge segment to lay down a further segment. And, believe it or not, the workers on the site are mere onlookers to just ensure that the machine is functioning fine.
Steel holding on
In terms of new technologies, the inherent strengths of quality control, durability, and rapid production has made pre-cast in bridge building popular. It has the flexibility of off-site pre-cast making execution more prompt. Over the last few years, there has also been an improvement 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. In Limit State Design (LSD), the concrete structure is reviewed against the limit state of equilibrium, strength, internal stress, crack control, deformation and vibration & fatigue. A limit state condition defines boundary beyond which a structure is no longer suitable for said limiting criteria. This refers to a degree of loading to test structural integrity, fitness for use, durability or other design requirements. A structure designed by LSD is proportioned to sustain all actions likely to occur during its design life, and to remain fit for use, with an appropriate level of reliability for each limit state.
Sharma of HCC E&C says, “In India we follow the IS 456:2000 Indian Standard for plain and reinforced concrete and IRC:112 code of practice for concrete road bridges for LSD. With this method we are at par with European or any other international practices in bridge building. Nowadays for pre-cast we use high strength concrete (M75 and above) to achieve better economics and superior quality control. Similarly, high grade steel (Fe540 and above) are very commonly used for long span bridges.”
It is not uncommon in flyovers, under occasional, over loadings where full design load is repeated for a very large number of cycles, to see concrete undergo stress concentration, exhibit excessive cracking and eventually lead to failure after a sufficient number of load repetitions, even if the maximum stress is less than the static strength of a similar specimen. Such members undergo a process of progressive, permanent internal structural (micro-cracking) change in a material subjected to fluctuating stresses/strains, conventionally termed as “fatigue.” Substantial research and laboratory studies indicate that most existing concrete structures, designed under static load criteria offer much less resistance to many concentrated loads than, to repeated fixed point loads.
The roar of Equipment
Today, a wide range of new and modern equipment are available and used for the construction of state-of-the-art flyovers and bridges in India. Mohan Mathrani, chairman & MD, Mohanlal Mathrani Construction, primarily a road builder, says, “Building structures like bridges & flyovers call for speed of work and consistency. It is important that contractors invest in equipment and also test material that can go into construction.”
In terms of new equipment, Full Span Launching Truss can be used for launching large spans in one go, while multi-axle trailers can be used for transporting heavy and long segments from the casting yard to the site location. Heavy crawlers are being replaced by launching portal gantry to make good use of the working space available, and lifting strand jacks are used to avoid deploying ground based cranes which occupy a lot of space.
Recently, Bhopal-based Dilip Buildcon won a contract worth Rs 546 crore from MoRTH (Ministry of Road Transport and Highways) to build a 640-metre long, eight-lane, cable stayed bridge, across the Zuari river in Goa. The company has partnered with Ukrainian firm Mostobudivelnyi Zahin (MBZ) for its technical expertise to construct the cable bridge. MBZ has vast experience in construction of cable suspension bridges, will be the technology partner to the project. Rohan Suryavanshi, head, strategy & planning, said that there are around 4-5 cable suspension bridges built on a large scale in India, and this would be the second-largest cable bridge length-wise in the country, after Vidyasagar Setu in Kolkata. “With this contract, we are now present in 12 states.”
Considering the numerous developments and giving importance to durability aspects, today it is possible to achieve target design life of 100 years for concrete bridges. With increase in span length of the bridges, more number of continuous bridges (three of four span units) is being built, which has led to the development of high capacity bridge bearings, POT bearing, POT-PTFE bearing sand spherical bearings. Also, modular expansion joints have replaced the widely used strip seal expansion joints in many bridges.
There is yet another point to note. A large number of highway bridges and flyovers are showing sign of distress all over the globe. Distress diagnosis is an important step to implement proper repair scheme to achieve the designed life of the bridge. In view of the large number of distressed bridges, monitoring the health of important bridges through instrumentation during the construction stage will help in early detection of flaws and suggesting suitable strengthening scheme.