Senior Manager (Design)
AFCONS Infrastructure Limited, India
Abstract: In complex Bridge construction projects, selection of correct method of construction / launching of superstructure plays a vital role, towards overall success of the project. The construction / launching method has a direct impact on cost, time and safety of the project. Inexperienced clients often deploy two separate agencies for (a) design of bridge structure and (b) for detailing a launching scheme. As a result the coordination between actual structure and construction is lost, resulting delays and cost overruns. Launching scheme and actual design (supers/sub structure) has a strong linkage with each other; in complex bridge projects, the scope of design and construction method can’t be segregated.
The recent advancements in bridge launching technology demand changes in the main design itself. In the present paper the author has described different method of launching (old and new), along with various Indian and International cases studies, and their prerequisites. The methods covered in this paper are :-
– In Situ Bridge Construction :- using Formwork-Staging; Moving scaffolding system
– Precast, Pre Stressed Bridge Launching :- using Cranes, Floating crafts, Push Launching, Launching Trusses (Under slung / Over slung type), Launching using Ground supported staging system, Cable cranes.
Paper covers interrelation between permanent, temporary structures with hints for engineers.
Bridges are the backbones of any Transpiration / Infrastructure Project like Railway, Metro or a Road. Bridge Engineering has a history of thousands of years, and finds it’s origin in Indian Mythology and old spiritual scripts like Ramayana / Mahabharata etc. In those days bridges were made from hip of stones / soil bunds / timber logs etc. Olden bridges were expected to merely act as a beam spanning across a gaps along river or a road; but as the growth of civilisation started taking place, the demand on all the branches of engineering increased, rapidly. For Civil Engineering, such increased demand called for more and more research about structural configuration and materials to be used for construction. From study of grow of engineering industry in past hundred years, we may observe that the development of Civil / Structural engineering is closely associated with the development of other branches of science / engineering like – Material Science, Mathematics, Communication, Electronics, Computing technology (computer science), IT etc.
As all the branches of engineering started growing, it was observed that the growth was rapid, but not parallel! As far as bridge engineering is concerned this fact resulted into a wide gap between imagination (i.e. conceptual planning and design) vs. the ground-reality (i.e. actual bridge erected at sites). The end results of this gap (rather a knowledge gap) and the solution to ‘bridge’ this gap is discussed in the further sections.
Bridge Projects in Olden Days
In olden days computing technology / simulation software etc. were not available. Engineers had to struggle hard to prove the adequacy of the concept, before starting the design and construction.
Fig.1 shows, photograph of ‘Firth of Forth Bridge, Scotland’, which is a steel cantilever type bridge. It was built in the 1880s designed by Allan Stewart and Sir Benjamin Baker. But both of them had to struggle hard to demonstrate by a Conceptual Model (as shown in Fig.1 (a) to the sceptical Victorians that a cantilever bridge would be safe. Fig. 1 (b) shows, bridge after actual successful construction.
Similar to above bridge, Fig.2 shows another interesting configuration of a Suspension cable bridge done out of India. In this structure the main pylons looks like a posture of a standing person, reclining backward, with both the arms folded, kept on waist. This bridge and many other similar bridges explain how human is getting inspired by ‘nature’ and ‘natural behaviour’. But definitely such bridges are the challenges for the Design and Construction.
Introduction to New Types of Bridges : A Challenges for Designers:
Unlike 50 / 60 years back, the computing technology, materials sciences have lead to converting even the complex shaped bridges in the reality. Fig.3 shows a beautiful String Cable stayed bridge designed by Famous Architect and Structural Engineer Santiago Calatrava with the inspiration behind this unique shape of bridge. During conceptual planning and the design of such modern bridges, often various challenges are faced. A few of these challenges are as listed ahead.
The various challenges that are faced are as below:-
– Complex Loading :– temperature, wind, erection, blast etc. and their combinations. Often the design codes and contract specifications are silent about the construction stage loadings.
– Complex Structural Geometry :– this includes a difficult /sensitive load transfer mechanism,
– Refined understanding about the Structural behavior – 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.
Advances in associated fields like :-
– Material Science (Concrete-steel-etc.)
– Electronic devices (strain gauges and other monitoring devices
– Surveying Instruments
– Increasing Project demands :- Time, Cost, Safety, Quality, Aesthetics
– New research towards :– Bridge execution / 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. A complete ‘Team Work’ is essential to solve these problems and to converge to effective solution. But, from the failures of many ambitious bridge projects, it can be observed that there was a clear absence of the team work (except a few cases). Typical failure of bridge may happen in case the designer don’t know how the contractor is going to construct that structure; on the other hand, even when the contractor does not know how that bridge is designed (boundary conditions, assumptions, standard design practices, loads etc.). The typical reasons of failures during construction of a bridge could be :– (a) Incapable Formwork, (b) Inadequate strength gain for concrete, (c) Lack of Synchronization between Hydraulic Jacks, (d) Inadequate bearings / temporary bracings, (e) Unexpected forces (wind / Blast) (f) misunderstanding between Designer-planner and the site personnel etc.
Fig.17 (a) shows a typical failure of Bridge staging at Vietnam during construction stage itself. In certain cases, wrong sequence of pre-stressing exerts a huge / concentrated or even eccentric force for which often the supporting formwork / staging is not designed, leading to collapse. After the collapse of structure occurs, no one can trace out exact reasons, i.e. whether the structure was designed wrongly or executed wrongly. Unless thorough investigations are made, one cannot conclude in such cases. Fig. 17 (b) shows a typical collapse of segmental bridge, which was getting launched in place using a Launching Girder. All such failure cases explain that the work of a designer, planner and the execution (site) engineer shall be complementing to each other and not the contradicting. Thus each one of them must know the works of others (their counter-parts) to a sufficient extent.
Introduction to Various Bridge Erection Techniques : A Challenges for Execution
From the discussion till now it is clear that the complexity in Bridge Engineering is increasing, which cannot be handled alone by the bridge designer or a site engineer. The Designer must know the proposed method of construction, which the contractor wants to use for his construction site; 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 / Assumptions. In certain cases, the original designs /drawings are found to be un-executable due to actual site constraints (probably, which the Structural Engineer missed at design stage). In such cases the modification in design becomes necessary. If the contactor knows the design basis, then he can develop a executable proposal indicating desired changes in the design at a particular spot in the project, on the basis of which Designer can visit the site, review exact situation and revise the design suitably. 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 scheme, right from construction stage.
A few of the popular methods adopted for construction of bridge super structure are as listed ahead:-
Erection using Traditional Methods
– Using Formwork / Staging
– With land based hydraulic crawler mounted cranes (single / double cranes – for (i) span-by-span construction or even (ii) random span construction
Erection using Modern Methods
– Floating Cranes / Barges / Jack-up platforms
– Using Bridge Launching Girders, and push / incremental launching
– Cable Cranes
– CFT (Cantilever Form Traveler) / MSS (Moving Scaffolding System)
– Tower Carnes
– Ground Supported Staging (GSS)
Bridge Erection using Traditional Methods
Mostly the traditional methods viz. use of Formwork / Staging or use of direct cranes is suitable for simple bridges. Simply supported spans are mostly constructed using these methods. Use of Formwork / Staging consumes lot of construction time for erection of formwork, placement of reinforcements, casting. curing and strength gain and removal of formwork / scaffolds. Hence in modern bridge construction projects it not a preferred option.
Bridge erection using direct cranes is suitable for precast-components only, heavy cranes are required to lift precast girders / segments (ref. Fig.5). Even large space is required to position the crane. Access roads right upto the erection location is another requirement in this scheme. This option is rarely used, for one or two locations in the project. In bridge constructed in busy metro cities often this option is found to be unsuitable due congested busy roads. Open drains, cable trenches on the side of road, and other utilities may not always permit access of cranes to each span. Detailed explanation of these two methods (i.e. bridge construction using Formwork and Crane launching) are kept out of scope of present paper. But just a brief mention is made in present para for completeness of the matter.
Bridge Erection using a Floating Crane:
In case of long span bridges, Launching Girder (LG) is becoming more popular option by the contractors (ref. Fig.6 and Fig.7, which shows a typical Truss type Bridge Erection Launcher, used at Bang-Na-Trade Highway Project, Bangkok). But if the bridge is passing over a large water bodies like rivers, creeks then it is convenient to use floating cranes kept on the Barges / vessels / jack-up platforms. Fig.4 shows such typical floating crane. To use this methodology effective, points to ponder are as listed ahead.
Points to Ponder to use floating cranes for bridge launching:-
1) Duration of Project
2) Capacity of Floating Cart; weights of segments to be lifted
3) Availability and characteristics of water way (depth, width, tide levels, waves / currents)
4) Facts of actual Site :
– Location / distance of Casting Yard
– Wind Speed
– Tidal variations
5) Cost / Budget
6) Cooperation from Designers / Design capacity
Bridge Erection using a Launching Girder:
From Fig.7, which is showing Bridge Erection Launcher, used in Bangkok, it may be seen that the Permanent structure has good Aesthetic look, which is correctly utilized by the Designer of LG system to place the LG; it can be also seen that due to placement of LG inside the pier notch there is impact on the design and reinforcement detailing of such pier, to cater for LG loads. Also, from Fig.8, one may observe the necessity of stability of erection system during girder erection. Faulty system may lead to incorrect bridge geometry or even accumulation of secondary stresses in the girder body /distortion. Apart from these, there could be accidents leading to damage to structure or human life (as shown in Fig. 11, 17).
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 now days as shown in Fig. 10, for a steel Arch Segmental Bridge in China. 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 (i.e. critical combination of self wt, Live load, wind / earthquake etc.).
Push / incremental launching is another interesting method, in which the entire precast bridge structure is pushed from one abutment to another abutment. No external LT is used, except a nosing. In such method the super structure experiencing ‘cycles of stress reversal’, this has to be catered for in advance.
Bridge Erection using a Suspended Cable Crane (Cableway):
This is a unique and challenging technique of bridge construction. Fig.10 shows a photograph of steel segmental arch bridge construction, in China, using cable crane (or alternatively called as a cable way). In India, for erection of Chenab Railway bridge over Chenab river, M/s Afcons is planning use similar technique. Cableway is also- being used on the Concrete Arch Bridge at world famous Hoover dam. In this case the design and – construction must go hand-in-hand to reach to successful conclusion. Issues like maintaining tension in the cable, limiting sag due to dead loads, wind velocity for the stability of ropes under self wt and lifted weight condition (i.e. operating and idling conditions), safety from falling objects at night, possibility of any object getting hit to the cable etc. are the critical issues in such method. All these issues must be addressed prior to the start of design (at conceptual stage itself). Without coordination with the contactor, it simply cannot be done !
As mentioned earlier there are many other innovative methods like CFT / MSS / erection using tower crane. They can be used only if there is close interaction between the designs – detailer – contractor is established. The same are briefly explained ahead.
Erection / Construction of Balance Cantilever Bridges using – CFT (Cantilever Form Traveler) or Segemnt Erector or LG
A Cantilever Form Travel (CFT) is commonly used method for the construction of Balance cantilever bridges. This is a method of ‘in-situ’ construction. In recent days to save time, instead of in-situ construction the balance cantilevers are even erected using a ‘Precast-Segment erectors’ Fig. 12 and 13 ahead show these two methods respectively. In both these methods the central fixed-pier and the post-tensioning cable configuration has to be designed to sustain the forces generated during the erection stages. The balance cantilever bridge gets ‘Tension’ at top during construction due to only self-weight. But as soon as at the end of the construction the tip of balance cantilever rests on the far-end pier, the structure starts acting like a ‘propped cantilever’ and the force pattern get completely changed i.e. top tension in cantilever changes to bottom tension in the propped cantilever due to self-weight. Thus the Designer has to design the PT-cable configuration so as to take care of tension occurring at top during construction and at bottom (at mid span) in service stage.
Another innovative technique sometime used for the construction of ‘long-span’ balance cantilever bridges is using a Launching Girder (ref. Fig.14 below). In such erection method there is no additional ‘bending moment’ generated in the bridge due to weight of formwork like in case of CFT (or at least can be minimized). But the contractor need to invest on additional cost of LG and even the piers and segments shall be designed / detailed support the legs of LG. But the construction time can be reduced. So while selecting any of the above three methods, thorough comparison may be made.
MSS (Moving Scaffolding System)
Moving Scaffolding System (MSS) is another upcoming method of bridge construction (Fig.15). It includes a bridge formwork mounted on a moving girder at the construction site itself. Thus the girder moves ahead to each span one by one and keep casting the same at it’s desired place (in-situ construction). Basically this is a slow method, also imposes additional loads on the Piers. The moving girder get’s locked with the piers for the period, till concrete gains specified strength and the post-tensioning is done. This is a cumbersome / bulky method of construction; it is suitable only for fairly straight spans with nominal gradient. While designing the bridge using MSS the designer must first consult the contractor and client.
Erection using Tower Crane
Yet one another method of bridge erection is using tower cranes. Normally tower cranes are used for the construction of pylons in cable stayed bridges due to the large heights. But rarely tower cranes mounted on track are used for erection / servicing of the bridge superstructure as well. But as often load carrying capacity of tower cranes is very less as compared to crawler cranes, they can be used for erecting light-weight units only (like bearings, cables etc) instead of full superstructure.
Ground Supported Staging (GSS)
Sometimes the launching girder cannot be used for launching bridge superstructure in cases like – sharp radius of curvature in plan, large gradient (longitudinal slope) and super-elevation, continuous spans etc. In such the segments can be erected using Ground Supported Staging System (GSS). GSS system can be used only if the ground below has sufficient load bearing capacity and availability of separate crane / EOT for placing the segments from trailer to the GSS. If these two re-conditions are satisfied then GSS proves to be most comfortable and fast system of erection. Weight of GSS system is significantly lesser than the LG. Ref Fig.16 for GSS.
Construction Failures During Bridge Launching / Construction
All Bridge erection methods explained till now have certain advantages and limitations; to use any selected method effectively one need to fulfill certain prerequisites (already discussed in respective method above). But as explained at the beginning of this papers, the consultant may not know (or wants to know) the construction method, and on the other hand the contractor / bridge erection agency does not know the basis of the designs. In such cases there is a likelihood of certain pre-requisites may not get fulfilled, leading to catastrophes as shown in Fig. 17.
A Few Case Studies
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. Following are some of the projects where the permanents designs and enabling works have affected each other. The author of this paper has experienced many live case studies; amongst which two cases are presented here as a sample:-
- a) Allahabad-Nainy Cable Stayed bridge project (example of a project where Permanent Design of Bridge affecting the design of Formwork)
This project was done by Joint venture (JV) of HCC and Hyundai in Uttar Pradesh India, in Yr. 2002-03. In this project a 20m wide bridge deck was supported with 1400mm wide and 3500mm deep two Pre-stressed rectangular girders (which were monolithically cast with the deck slab). Each span was 60m long. There were 4 modules to be cast. The longest module had nine continuous pres-stressed spans. Every 60m span was cast along with the 1/5th (i.e. 12m) of next span i.e. at theoretical point of contra flexure.
During the application of pre-stress as the 60m portion of the girder used to get ‘hogged–up’, the remaining 12m portion used to get dipped-down, because of the continuity in the construction, This resulted into enormous increase in the load on the formwork / staging in the 12m portion (which was temporarily acting as cantilever, till the remaining 48m was cast in next pour). This effect was predetermined and the Formwork and Staging was designed accordingly. Also the pre-camber was pre-determined and set in the staging during erection work.
- b) Longest Railway Bridge at Cochin for RVNL
(example of a project where Erection Method influenced the design of Bridge) :-
This bridge was constructed by Afcons Infrastructure Ltd. for Rail Vikas Nigam Ltd. in Yr. 2007-08. In this project, Rail Vikas Nigam Limited (RVNL) had initially proposed a 600Mt single box girder per span. Launching such a heavy structure was not possible inside the crek area. Also the SBC of soil at casting yard was very less. Considering these limitations the contractor proposed Two-Pre-stressed concrete I girders at each span. This Modification was readily approved by the client considering the practical limitations. NRS Asia ‘Over-head type’
Launching girder was used for erection. The entire project was completed ahead of the schedule, within the budgeted cost. This example describes how the enabling works forced to modify the permanent structure, yet leading to ultimate success of the project.
From the discussion till now, which has covered (a) the olden method of construction of bridges, (b) the challenges in new era (like complex loadings, geometry, codes, new materials etc), (c) upcoming methods of bridge construction / erection, and from various case studies the interrelation between the Paramagnet design and the Erection Design of Bridge must be clear. Many hints are embedded for the Designer as well as contractor at each method of launching, one can use them while working on live project.
At the end, the main fact to be remembered is ‘Bridge Design and Construction’ is not a single agencies responsibility; only ‘Team-work’ can lead to a overall successful project!
The author is thankful to the Afcons Infrastructure Limited and entire site team for making all the relevant data available for this paper. Author is thankful to authors of various books / papers / techniques listed in the References below. Author also wishes to express his thanks to the Organizers of Structural Engineering World Conference 2015 for considering present paper.
– Dhanajay Bhide, “Syzygy between Design and Construction of Bridges (Part-I of II)” Indian Road Congress Journal, July’2015.
– Dhanajay Bhide, “Replcament of damaged Suspension Span of Versova Bridge Across Vasai Creek on NH-8”, Indian Roads Congress, Jan-March’15.
– Vivek G. Abhyankar, “Nad-al-Sheba Race Course Development Project : Construction of three Bridges” Indian Society of Structural Engineers (ISSE), Mumbai, Vol-13/2, April-May-June’2011 issue.
– Vivek G. Abhyankar, “Construction Longest Railway Bridge Project at Cochin” Indian Society of Structural Engineers (ISSE), Mumbai, Volume 11-3, Jul-Aug-Sep
– M. K. Hurd, “Formwork for Concrete”, ACI, seventh edition.
– Marco Rosignoli, “Bridge Launching”, Thomas Telford Publishing (January 1, 2002)