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The Indian Concrete Journal March 2017 73

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Challenges in design and construction of temporary bridges across water bodies

Vivek G. Abhyankar

introductionConstruction of Jetties / Quay walls / Bunds / Bridges and similar works in deep rivers or sea shore is a very challenging task, because of lack of accessibility, all along the length of the new structure to be built. In such cases, contractors often use floating barges, Jack-up platforms (as applicable) for accessing such locations. But such floating devices can be used only if the depth of water is sufficient all along the alignment; if the water level varies as per high/low tide, then use of floating crafts may not be the right option. In such cases to solve such problems for access often a ‘Temporary Bridge’ is considered as a solution, which provides an access all along the new structure to be built. Thus a temporary Bridge is nothing but a short-life bridge, which is used as access to the proposed structure (Jetty /Quay / Permanent Bridge etc) for workers and construction equipment and material to each location; and often runs parallel to the new structure till the construction of new structure is complete.

Being temporary in nature, often the responsibility of Design and Construction of temporary bridge is left to the contractor. Also during ‘tender-stage’ of the project, contractors rarely make a detailed provision of such bridge; hence after award of the project an attempt is made by contractors to optimise the cost of the bridge to the maximum possible extent. Due

to this many options are explored and corners are cut in the design stage. Though the maximum design loading on the main bridge and temporary bridge is practically the same, cost of temporary bridge can’t be more than 5 to 6 percent of the main bridge (or as budgeted during tender stage); also the construction time spent on it is a small fraction of the complete project. All the above stated facts increase challenges and constraints in the design and detailing of the temporary bridges.

In this paper the author has attempted to present key features of ‘Temporary Bridges’ with a few hints / tips to the users and at a few places case studies are also given (note :- The temporary bridges described here are different than the standard ‘Bailey Bridges’ often used in hilly roads by the military forces).

confiGurAtionUsually a temporary bridge has steel piles driven in sea / river bed to support the steel beams and concrete / steel deck, on which the construction vehicles / workers / cranes etc. operate. Usually piles are vertical (racker piles are not preferred due to difficulty in construction). The piles are formed using steel liner plates (mostly 1000 mm to 1200

The Indian Concrete Journal March 201774

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mm dia and 10 mm to 12 mm Thickness). Piles are braced using diagonal braces in both the directions. For design of temporary bridges large scour depth of the piles (as that of the bridge/ jetty) can’t be considered as it’s used for very limited time duration. Hence apart from operating loads the design has to be also checked for stability (under self-weight) and actual live load even with a consideration as if ‘any one of the pile in a span has got sunk / tilt (hence ineffective). The piles are driven using vibro / drop hammers. For driving steel piles ‘guide-frames’ are required so as to control the drift of the piles due to water currents. A steel shoe / shear lug may be required at the base of the steel liner pile to increase its strength against driving forces. Figure 1 shows a typical temporary bridge being constructed in a Ganga river and pile driving in progress. For the design of such steel liner piles API-RP.2A code may be referred (but slight relaxation may be given on the member sizes and stresses, considering temporary nature of the structure).

On the top of steel-piles cross beams and seating girders are placed to support the load from main longitudinal beams. Usually longitudinal beams / trusses are used to directly support the load of crane track. On the top of such longitudinal beams either concrete planks are used to act

as a deck slab or alternatively secondary beams with the steel plate / grating on top are used. Figure 2 shows typical components in a temporary bridge in section and elevation.

The spacing of piles in the longitudinal direction depends on (i) construction loads, (ii) Size / Stiffness of Longitudinal beams / trusses, (iii) reach / access of construction equipment / cranes to drive piles in front (iv) clear water-way required /specified by agency (v) and of course the capacity of the liner piles.

Many times at abutment locations the construction agency encounters with soft / clayey strata. In such case the firm abutment can be formed by using ‘Gabion blocks’ / soil bunds. Thus upto 5 to 6 m depths the bunds are used; upto 10 m depth the temporary bridges are used; about 10 m depth of water barge / jackup may be used (as such these are not the hard and fast rules but the general observations).

Construction of temporary bridge

Temporary bridges are often constructed using “End-on” method, which means that the crane and piling rig stand

Figure 1. A typical view of temporary bridge being constructed in Ganga river

(a) Crane launching the concrete deck (b) Crane driving forward piles


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on the shore line and drive first row of piles in the water. As soon as first row of piles is driven, the crane places steel beams / trusses on the top of the pile and abutment, followed by the deck elements (concrete planks or steel plates, handrails etc.); thus the first bay / span is read and the crane / rig marches on it to drive second row of piles, followed by placing the deck elements. Thus the Temporary

bridge construction progresses from shore towards inside the water.

During construction proper care has to be taken to avoid any over stressing of structural members / falling of objects in the deep sea, striking of objects with other members (during swinging with crane) etc. Often during construction

Figure 2. (a) Typical cross section of a temporary bridge (b) Typical longitudinal view with different components

(a)

(b)

Main Beams

Crane

Handrails

Decking ( IN RCC OR STEEL)

Secondary / Cross Beams

Steel Liner Piles

H.HFL

HFL

Seating Girders

Cross Bracings


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the stability of crane (forward lifting / side-ways lifting and swinging movement) has to be checked by structural designer and ensured at site. Figure 3a shows a typical position of cranes and also cross bracings between piles which plays most important role in the stability of spans of temporary bridge. Sometimes to support the main beams over piles / cross beams, friction camps or pile muffs are used [1].

Figure 3a. Typical cross section of crane operating on a 'Temporary Bridge'

Crane lifting load in transverse direction Crane lifting load in longitudinal direction

Handrails

Steel Decking

Steel Liner Piles

Concrete Plug

Soil bunds, sheet pile bunds versus tempo-rary bridge

Some times to facilitate construction work in water, or for providing access to equipment inside the water-way along the length, contractors construct a soil bund; which is a popular and affordable option for construction. Figure 3b shows a typical soil bund with crane operating on it. But bund construction takes a large time, also it virtually blocks


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the flow of water, hence cross drains / pipe-culverts are required to allow passage of water below the soil-bund. Even the maintenance of soil bund required is high as compared to the temporary bridge. Also a bund can only ‘partially block the river’ unlike the temporary bridge which can span across for full width of the river / water body (especially during rainy seasons). In wider and deeper rivers, the construction of soil bunds is time consuming as well as costly affair because of the depth and material consumption (typically beyond 6 to 7m depth).

Often clients ask for Steel Sheet-piled bund, especially in case of deeper water channels. The construction of steel-pile bunds also virtually blocks the water-way in rivers. It also consumes lot of time and cost for construction. In steel-piled bunds, two walls opposite to each other are constructed by driving the steel sheet piles, then both these walls are tied to each other using wire ropes / ties (if required in design). Later the space between both the walls is filled using soil fill / gravels / stones. It may be clear that to retain the soil, a cross-wall in sheet pile is also required at the end of the bund ending inside water. Figure 4 shows a photo of a typical steel sheet-piled wall.

Analysis and design of temporary bridge, forces acting on the bridge

Apart from vehicular loads, cranes and workers, the temporary bridge is subjected to wind and wave or current forces. Temporary bridge is neither intended nor designed to resist forces arising due to extreme nature events like seismic / tsunami being a temporary in nature (and a provision is to be made in the insurance for such acts of nature).

The analysis of temporary bridge as a 3D space frame is beneficial to optimise the member sizes, but normal 2D plane frame analysis could also be sufficient depending on the magnitude of loads and importance of this structure. Figure 5 shows a typical space frame model of a temporary bridge / load out jetty. The bracing for the piles has to be placed above high tide level so that they are accessible for maintenance, and do not cause hindrance to the floating bodies (like tree trunks etc.) during floods in rainy seasons.

In case the temporary bridge is to be constructed in the creek portion, then reversible water load is experienced and has to be catered to in the analysis and design by the structural engineer, especially if the geometry is unsymmetrical in cross section.

Figure 4. A Typical steel sheet-piled wall bund

Figure 3b. Typical cross section of 'Temporary Soil Bund' in water and crane operating on it

Typical cross section of a soil bund

Surface protection

on the soil bund

Compacted soil fill

Cross drainage pipes

(culvert)


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It is worth nothing here, that a simple analysis software is sufficient for analysis; simple line model is sufficient and no sophisticated model using F.E.A etc. is needed. Often the young generation of the structural engineers get carried-away by the easy availability of such advance analysis tools, but the end results of detailed analysis model is no way different that simple 2D / 3D line model. The Deck slab (precast concrete or cast-in-situ or steel) is usually not modelled in analysis software, but simply designed and detailed manually.

More than analysis and design, the detailing of components for easy flow of forces in primarily important in the Temporary bridges. The main beams are usually placed on the ‘seating girders’ passing though the piles, but in rare cases friction clamps can also be used to support the beams, provided the load is primarily static in nature or lighter (viz. Walkways). A typical friction clap during fabrication stage and during use is as shown in picture below (refer Figures 6 and Figure 7).

Figure 6. (a) A typical view of friction clamp at analysis stage (b) and fabrication stage

YZ

X

Max Absolute2

N/mm<=128

122

232341451

560

670

779889

99811081217

1327

1436

15461655>=1765

Figure 7. A typical schematic drawing of friction clamp and actual photo of clamp installed on a pile

Connector


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Figure 8. Typical view on temporary bridge placed close to a steel sheet-piled cofferdam

Usually the forces acting on the temporary bridge are as below.

a. Self Weight

b. Live Loads

• Vehicle Loads - Mainly Construction equipments (like Cranes, Transit mixers, Trailers / Dumpers), LMV

• Worker’s Gang / Gang Load

c. Water Load – current / wave forces

d. Wind Load

e. Small floating vessels Birth and mooring loads in case of load out jetty. Figure 5 shows that the front row steel piles are slight raised above the bridge, for mooring the vessels.

Planning for temporary bridge

While planning for the temporary bridge, site selection is most important step. The site should be so selected that the bridge is as close as possible to the proposed structure, casting yards, batching plants, access roads (connectivity), yet the length of the temporary bridge should be kept to minimum. The water currents / wave forces, depth of water should be minimal at the site selected to build a temporary bridge. In case of a river or a creek temporary bridge is not selected in location where the water channel has a curvature to avoid eddy currents formed in water. Commonly, the temporary bridge will be constructed within six to seven months times and will be used for about two to four years duration. The dismantling of the bridge also takes about two to three months time.

While finalising the site for temporary bridge the engineer / construction team must hunt for a land which can act as a firm (unyielding) abutment. If the casting yard (i.e. a place where Precast units to be used for the main bridge are cast) is close to the temporary bridge then it gives maximum benefits during construction.

Sometimes in the river bridges, ‘well foundation’ is proposed by the Designer. In such projects, care shall be taken while finalising the location of piles of the temporary bridge. Often in deep waters designer specify use of ‘well-foundation’ below the bridge instead of pile foundation. In such cases the piles of temporary bridge shall not be located ‘too close to the well’. Refer Figure 8 where a steel sheet pile cofferdam for well sinking is getting ready using access for equipment from a closely spaced temporary bridge in US motorway construction.

Some times during ‘sinking operation’ of wells the contractor attempts for perform jetting/air-blowing of well-base (especially if the well has got struck-up). In such situations the well may suddenly sink in the strata due to air-blow resulting into a sudden draw-down of adjoining peripheral soil. In case the piles of temporary bridge are falling in the ‘draw-down region’ then they are expected to experience a large lateral force (which is never accounted in the design). Refer Figures 9 and 10 which show the phenomenon of sudden draw down during air jetting and a large force acting on piles due to which it got sheared-off.

At the time of Tender stage of the project, construction agency must first visit the site along with the structural engineer and a planning engineer and decide on various features of temporary bridge including location, height, width, length, span-length, bracings height etc. on preliminary level; in case if the bracings are needed then make suitable provision in the budget to avoid post-tender conflicts.

Maintenance of temporary bridges

Safe, easy operations and routine maintenance of Temporary bridges is required. The bridge components are to be protected from rusting of steel sections, proper handrails and lighting arrangements should be provided at site so as to make safe use of bridge during nights also. If the bridge is long, then the designer should plan for a parking /cross-over bay in the bridge.

When temporary bridge is used, the contractor should make a provision of one or two small floating crafts and divers for rescue operations in case of hazards.

Author was involved in design, planning and construction of temporary bridges / jetties various projects like river bridge over Ganga river (Munger), Mahanadi river (Bhubaneswar)


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Figure 10. (a) Temporary Bridge that suffered due to sudden down-ward drag (sucking by air jetting) (b) Steel liner pile got sheared off at the join location due to lateral sucking force (presence of bracing could save the entire structure)

(a) (b)

Figure 9. Typical draw-down force contours due to well bottom air-jetting


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Er. Vivek G. Abhyankar, C.Eng (Ind), a Gold Medallist in Structural Engineering, is an Assistant GM (Design) at AFCONS Infrastructure Ltd, Mumbai. He has seventeen years of experience in planning and design, detailing of various enabling and permanent works in reinforced concrete and steel. He is a life member of various institutes, professional trainer, visiting faculty for graduate and post-graduate students in structural engineering, guide for AMEI projects in civil engineering. He has written various technical papers, contributed to two books, has been a guide to various technical thesis, technical trainer and a certified internal auditor.

and load-out structure in sea of Al-Ruwais(in Abhu-Dhabi), Cuddalore Jetty project, Airoli Bridge Project, Dabhol project, Railway Steel bridge over Zuari and Mandovi river. These structures gave lot of understanding and learning about the behaviour of temporary bridge and various other aspects involved in planning-design-procurement-construction and approval of temporary bridge, which is covered in above paper in nutshell.

Summary and conclusion

Temporary Bridges are the Life lines in major marine / river projects. Unfortunately such subjects are not taught in our engineering curriculum. There are no formal books, manuals available on the subject. Hence the young engineers coming out from the colleges have to struggle when they are required to plan and construct a Temporary Bridge at their site. Often the cost cutting and negligence by the construction team proves to be detrimental and could turn-out to be a catastrophe. Temporary bridge is not a mere science but it is an art; enhancing the performance of a typical temporary bridge in night time by use of ‘radium bands’, ‘use of rubber tyres as fenders to protect the structure from hitting a flowing bodies in rainy season’ etc. are the practical ways every engineer can device on his own at site as per the problems faced. The current paper is written with an intension to give

a wider forum to this topic. It is practically not possible to cover each and every aspect of it in detail in ten pages. Still an attempt is made to touch upon most of the areas.

It may be clear from discussion till now that how temporary bridge serves as an important lifeline in a typical bridge construction project over a river / marine structures. One can learn and master various aspects of planning, design, construction and maintenance of this subject only by thorough experience and involvement.

Acknowlegement

The author is highly obliged to his seniors, colleagues, friends, sub-ordinates, his organisation for giving this opportunity to write on this neglected topic. The author is also thankful to the various organisations / institutes referred for preparing the paper and will be very happy to respond to the technical queries related to Temporary Bridges.

Bibliography

1. ‘Design and Construction Marine Friction Clamps’ (inclusive of pile muffs) – discussion by authors of present paper on ‘Structural Engineers Forum’ – www.sefindia.org

2. Technical workshops / lectures delivered by author on ‘Temporary Bridges’

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