Incremental Launching of Steel Truss Bridge Rajiv Surekha ...icipunecentre.org/papers/24-Rajiv Surekha.pdf · Incremental Launching of Steel Truss Bridge Rajiv Surekha General Manager,

Incremental Launching of Steel Truss Bridge

Rajiv SurekhaGeneral Manager,

Hindustan Construction Company Ltd

Nachiket TendulkarDeputy General Manager,

Hindustan Construction Company Ltd

209ARKEY CONFERENCE SERVICE CELL, PUNE

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AbstractFor launching superstructure, the spans are assembled in segments and tied together using temporary members behind abutment along the bridge alignment. Then the assembly of spans is pushed forward by jacks in steps. Depending on the design requirements, a temporary launching nose shall be added to reduce the erection stresses in the members. Subsequently, additional segments are added to the back of assembly as the launching progresses. At piers, special guides and bearings are placed to facilitate the push launching. The entire operation is repeated until the other abutment is reached. The force required for pushing keeps on increasing and therefore the abutment and the piers are to be checked for these forces. At the end of the launching operation, the temporary members connecting the spans are removed and the spans are placed on the permanent bearings.

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1. Preamble

The present scenario demands timely completion of

works, rather speedy execution of challenging projects,

in difficult environmental and geographical conditions;

the method to execute the Project plays a major role right

from the inception stage itself. A systematic and scientific

analysis along with “Out of the Box” thinking towards

execution of a project will reduce both time and cost of the

project.

Each project will have its own challenges depending

upon either environmental conditions or geographical

conditions. Hence, the design of each project will vary

based on the prevailing conditions. To execute such

challenging projects, the customary or standard method

may either be Time/Cost consuming or may prove

unfeasible. Here, a specifically derived method to

execute the project will pave way to complete the project

within the timelines.

In this article we have demonstrated how the right

methodology has been implemented to complete the

project within the specified milestones.

1. Project Scope

Construction of Composite welded steel truss girders

having span of 39 x 125m + 2 x 32.4m, thus total bridge

length of 4941.4m, for the Super-structure of Bogibeel

Rail-cum-Road Bridge over the river Brahmaputra near

Dibrugarh, Assam in India

The bridge has a navigational clearance of 10 m above

HFL. The whole bridge is on straight alignment without

any vertical gradient.

Fast Track Infrastructure Developments -- Challenges & Solutions

4. Environmental & Climatic Condition of Project

Site:

4.1. Climate

The climate of the project area is humid and tropical. A hot

and humid pre-monsoon from March to mid May, a

prolonged southwest monsoon or rainy season from mid

May to September, a pleasant post-monsoon or

retreating monsoon from October to November and a

cold pleasant winter from December to February are the

characteristics of the general climate.

4.2. Temperature

The hottest months are May to September with mercury

reaching 42ºC during June. The coldest months are

December, January and February when temperature

drops to 5ºC. The months, November and March, can

also be quite cold in some years. During the other

months, temperature is more or less moderate in nature

and pleasant to bear.

4.3. Hydrology:

Brahmaputra is a perennial river with maximum

discharge around 73000 cumec during monsoon. In the

off peak seasons, some part of the river bed remains dry

but it is not possible to identify which part as the stream

changes its course after every monsoon.

• Design Discharge : 73000 cumec

• Design HFL

* With Aggradations : 106.52m

* Without Aggradations : 102.92m

* Low Water Level : 98.3m

4.4. Design Limitation & Consideration:

Maximum wind speed 15 m/s [54km/hr] has been

considered during launching operation. If wind speed

goes more than the consideration launching operation

should be stopped and if wind speed exceeds 20m/s [72

km/hr] the truss will be anchored.

During launching process, weather forecast shall be

maintained at site office.


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Before each launching process a weather forecast shall

be obtained and forecast for 5 more days starting from

the day of launching operation shall be reported.

5. Factors for Selection of Launching Method

5.1. River Hydraulics

Brahmaputra is a perennial river with maximum

discharge around 75000 cumec during monsoon. In the

off peak seasons, some part of the river bed remains dry

but it is not possible to identify which part as the stream

changes its course after every monsoon. It is therefore

not possible to formulate a method that avoids use of river

bed during construction.

5.2. Construction Speed

Such long bridges over a river like Brahmaputra take a

few years in construction. The aim is be to minimize the

construction time by an appropriate construction

scheme. The method should allow erection in all seasons

(except in monsoon or in high wind situations) and

involve minimum time between erections in adjacent

spans.

5.3. Number of Spans

A total of 41 spans have to be erected. Therefore,

construction from both ends of the bridge may be

considered to save time. The chosen erection method

must justify, cost wise, the number of spans to be erected.

5.4. Size and Self Weight

One normal span measures nearly 125m in length and

1700MT in weight. So, handling of such a massive

structure is of paramount importance. Simultaneously,

site work has also to be minimized. Assembly of

fabricated segments will influence the selection of

erection method.

5.5. Ground Conditions

Construction work carried out by providing temporary

supports from river bed is generally most simple, speedy

and economical. But given the river bed condition at this

site use of temporary supports is very uncertain. The

course of water flow, even in the off-peak seasons is very

unpredictable and it frequently changes with time.

5.6. Site Climate

The welding process for fabrication of the truss has to be

done in a controlled environment. The main trusses are

proposed to be fabricated in multiple units in workshop

and then brought to site. These units would be

assembled and connected at site in a controlled

environment before launching. No welding at site after

launching is allowed.

5.7. Construction Safety Requirements

The erection scheme adopted must provide a safe

operation for the construction personnel without

compromising on any safety aspect specified by the

governing body. It should also facilitate easy and regular

inspection and maintenance against possible corrosion

of the steel members.

5.8. Site Accessibility

Fabrication work would be primarily completed at

workshop. So, adequate transportation facility from

workshop to site would be required. Mobilization of

equipments for erection of truss, concreting etc. would

require the site to be easily accessible.

6. Methods of Superstructure Erection

Following are the commonly used erection methods

discussed herein for the selection of the most suited

method.

6.1. Cantilever Construction

By this method, first span of the bridge is assembled on

staging / falsework. After that, the second span is

erected, panel by panel, by cantilevering out from the first

span, using the first span as a counterweight.

Temporarily spans are made continuous for this purpose.

On the top chord, a crane is fixed to handle the panels.

Crane is moved forward after adding the panel. When the

cantilever reaches about 85% to 90% of the span,

temporary brackets are fixed to piers to receive and

support the cantilever tip. Length of these brackets could

be 10% to 15% of the span length. These steps are

repeated till all spans are completed.

Advantage: • Small weights are to be handled

Disadvantages:


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• Time consuming due to bracket installations.

• All joints are to be done at site.

• Some members would require strengthening during

construction.

• Not suited for welded type of connections since field

welding is generally not recommended.

6.2. Erection using Floating Cranes or Pontoons

A full span is fabricated and assembled in the yard. Then

the assembled span is transported to a floating pontoon

through jetties. Pontoon carries it to the destination for

erection. The span is lifted and erected over the piers

using large capacity-floating cranes.

Advantages:

• The whole span is assembled in yard thus ensuring

good quality.

• Welding is done in controlled conditions and can be

checked thoroughly for quality before erection.

• Low erection time.

• No members require strengthening during

construction.

Disadvantages:

• Bigger weights are to be handled.

• Special equipments such as pontoons, floating cranes

are required.

• Appropriate river hydraulics required.

6.3. Incremental Launching / Push Launching

The spans are assembled in segments and tied together

using temporary members near abutment along the

bridge alignment. Then the assembly of spans is pushed

forward by jacks in steps. Depending on the design

requirements, a temporary launching nose can be added

to reduce the erection stresses in the members.

Subsequently, additional segments are added to the

back of assembly as the launching progresses. At piers,

special guides and bearings are placed to facilitate the

push launching. The entire operation is repeated until the

other abutment is reached. The force required for

pushing keeps increasing and therefore the abutment

and the piers are to be checked for these forces. At the

end of the launching operation, the temporary members

connecting the spans are removed and the spans are

placed on the permanent bearings.

Advantages:

• Assembly and pushing of launching truss to be

controlled at one location during the entire operation.

• Lower erection time.

Disadvantages:

• Bigger weights are to be handled.

• Temporary bearings and heavy jacks are required.

• Some truss members will require strengthening during

construction.

• Truss members are subjected to fluctuation of stresses

during the erection.

6.4. Steel Launching Truss

This erection method uses a specially fabricated

launching truss. Launching truss spans the length

between piers and has front nosing for auto launching.

The launching truss is placed over the first span of the

bridge on temporary supports. The fabricated span truss

is rolled over the launching truss from the yard using rails

and trolleys. When the span truss is over the launching

truss, specially designed frame, erected over the piers,

picks up the span truss using lifting tackles on the frames.

Now, the launching truss is relieved of the span truss

load. Launching Truss is then moved over to next span.

Span truss hung on the frames is launched into span over

bearings. Frames are removed and fixed on the other

piers. Deck slab is cast over the erected span. New span

truss is rolled over the erected span and the launching

truss. These the steps are repeated to complete the

second span erection and to move forward the launching

truss over to the next span.This is repeated till all spans

are erected.

Advantages:

• Number of spans will justify the initial investment on


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launching truss

• Whole system is safe for all operations

• Finishing work on the erected span can be done as an

activity parallel to fabrication of span trusses

• No members require strengthening during

construction

• The whole span is assembled in the fabrication yard

ensuring good quality

• Welding is done in controlled conditions and can be

checked for quality before erection

• Low erection time

• Independent of river hydraulics

Disadvantages:

• Bigger weights are to be handled

• Special equipments are required

• Large deflection at cantilever tip of the launching truss.

After considering all above factors we have concluded

that incremental launching / Push launching would be the

best option for the launching of span truss.

7. Incremental Launching

The superstructure is proposed to be launched by

“Incremental Launching” from the embankment on A1

side. Entire project standard spans, 39 nos of 125m, are

divided into 4 launching trains consisting of 3 trains of 10

spans and last train of 9 spans. This is done to keep the

launching force within limit of 900 ton. Required

Launching Force 1800.000 t x 10span x 5% = 900.0 t

The components of the truss like Nodes, Bottom chord,

Top chords, Vertical, diagonals etc are fabricated in a

custom built workshop. These fabricated units are

transported to Lay Down sheds (sheds 1and 2) wherein

the complete profile of each truss are marked on floor.

The components are assembled in Lay down condition to

make modules of about 30 m each. All welding, tests etc

are completed in these sheds. Each of these modules

(weighing about 110 to 135 T) are transported by two

gantries operating in tandem to the vertical assembly

yard (Sheds 3 and 4) wherein all these modules are

assembled to true line and level. The joints between the

two modules are welded here along with other

components like cross beam, bracings etc. The whole

truss is assembled on sledges of varying height to obtain

precamber.

These sledges will also facilitate longitudinal sliding of the

completed trusses in the yard. The sledges supporting

the truss will slide over the launching tracks which in turn

are supported by specially designed foundations.

Once the first truss (for span P39-P40) is completed, the

truss will be jacked up on another set of specially

prepared foundations (-temporary foundation) to verify

the camber (when the truss is supported on four end

supports). Once the camber checks are completed a

prefabricated Nose will be fixed to the front end of the

truss. The purpose of the Nose is to reduce the cantilever

bending moments and the reaction on bearings due to

cantilever.

During this process of fixing of nose, a launching bearing

will be mounted on -temporary foundation. For further

operations of launching, -temporary foundation of viaduct

will be used as support.

After the Nose erection is complete, the truss is pulled

forward with the help of a pulling device. This pulling

device is mounted on a Pier P1 and it will pull the truss

and the nose forward. As the truss moves forward, a

sequential jacking of the truss at temporary foundation is

done to control excessive reactions on the sledges.

Launching is continued till the nose takes reaction on the

launching bearing mounted on A1. In order to negate the

deflection of the nose tip due to cantilevering, the bottom

of the nose is profiled so that the nose lands freely on the

launching bearing in spite of cantilever deflection.

In order to satisfy the strength requirements during

launching process, the members of trusses (like bottom

chords, diagonals etc) are strengthened. For some

trusses, “A” struts are installed at specified locations to

minimize the stresses in the bottom chord.

Launching is continued further till the rear end of the first

truss reaches the foundation for camber checking. At this

time the second truss is moved forward and connected to


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to the rear end of the first truss with a connection piece

welded between the two trusses. . The process is

continued till the 10th truss reaches at pier P31 of viaduct.

At this stage a “Tail” (which is similar to Nose) is attached

at the rear end of the 10th Truss. This is again to reduce

cantilever moments at the rear end. Pulling device of P1

will be disconnected and pulling device of P11 will be

activated. Now the whole system of 10 trusses will be

pulled forward till the rear end of truss 10 reaches at Pier

P10. Pulling device of P11 will be deactivated and further

pulling will be done from P21. The process is repeated till

the first train of 10 trusses reaches at P40.

At the end of this stage, the nose is between P40 and A2

and the tail is between P29 and P30. Nose is dismantling

with the help of land based crane between P40 and A2.

Tail is dismantle with the help of a floating crane/or by

other devices like derrick etc operating on the launched

truss and tail.

Simultaneously the second trains of trusses T11 to T20

are launched in similar procedure till the tip of the nose is

about 10 mts from Pier P30. Since the previously

launched truss T10 is already in place, the nose cannot

cross beyond P30. To facilitate further launching, a pier

bracket is attached to Pier P30 on A1 side. A launching

Bearing is mounted on this pier bracket. Second train of

truss along with nose is launched in stages over this pier

bracket. Part of the nose beyond the launching bearing

on Pier bracket is dismantled with floating crane (or a

similar device operating from the previously launched

truss). After dismantling part of the Nose, further

launching is done in stages and simultaneously part of

the nose is dismantled. This process is repeated till the

Truss T11 reaches the final bearing locations. Repeating

the above process, the third and fourth trains of trusses

(10 Trusses for train 3 and 9 trusses for train 4) are

launched.

Due to the effect of continuity, there are locked in stresses

at the connection between two trusses. By a systematic

procedure of jacking up and jacking down of some of the

supports, the stresses in the connections are brought to a

minimum values. Under such reduced stress level in the

connections, the connections are gradually cut with flame

cutting and the ends of the trusses are finished to the

required specification. Since the launching levels are

higher than the required final levels of trusses, all the

trusses are jacked down systematically to lower the

trusses on final bearings.

After the truss is placed on bearings, stringer trolley shall

place the stringers and rails over it. With the help of

shuttering trolley deck shuttering trusses are placed.

Cantilever moving trusses are used for concreting of

cantilever deck portion.

In the following sections the temporary installations

required for launching are explained


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8. Installation of Enabling Structures for

Launching

To execute the launching process, following equipment /

machine and temporary steel structures are required

8.1. Assembly / Launching yard

Assembly of span truss shall be carried out in two phases

a) Assembly yard (Bed 1 & 2) – Laydown position

In Assembly yard laydown position, the components will

be assembled to form the modules namely P, Q, R, S & T.

b) Assembly yard (Bed 3 & 4) – Vertical position

In Assembly yard (Bed 3 & 4), all assembled modules

shall be assembled in vertical position to make the truss.

Fabrication and Assembly yards are designed to cater

2500 ton of structural steel per month. i.e. approx 1.5 nos

span per month.

8.2. Launching Track at Assembly yard

In order to shift the completed trusses to the launching

position, launching track (sliding) shall be provided from

the assembly yard (Bed 3 and 4) to the launching

position.

8.3. Launching sledge at Assembly yard

In order to shift the completed truss in the vertical

assembly yard, the truss is erected on sliding blocks

called as “Sledges”.

Due to the pre-camber to be provided to the truss, the

sledges shall be made of different heights. There are in

total 22 sledges for each truss. The sledges are located

under every node.

8.4. Foundation for Camber Check

In order to check and verify the pre-camber, to be

achieved after final assembly of each truss, four

foundation blocks with facility to jack up the whole truss

are provided at each corner of the truss. These

foundations also accommodate hydraulic jacks to lift the

whole truss. These foundations also support the

launching track.

In order to check the camber of the span truss 4 jacks (4 x

500 t jacks with tilt saddle) will be assembled at the

jacking points of the foundation. In order to safely transfer

the weight of truss, an RCC foundation of 4.5x5.1x1.5 m

is adopted. To minimize any settlements and to distribute

the loads, a rubble soling is also provided below the

foundations.

8.5. Launching bearing at Assembly yard

(Temporary pier)

Launching bearing is installed at temporary pier to

support the truss during launching operation.The

launching bearing is designed to distribute the reactions


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uniformly of the bottom chord of truss uniformly. The

launching bearing at the temporary pier is also designed

for jacking of the bearings to control the reactions at

various stages of launching as defined in launching

operation.

8.6. A-Struts (For temp. strengthening of truss)

In order to reduce the stresses due to the local bending of

the bottom chord vertical temporary beams will be

arranged between the bottom chord and top joint of both

sides of the truss. Due to its geometry these members are

called “A” -struts. The “A” – struts are placed in the zones

of the maximum vertical forces due to launching.

8.7. Temporary Connection Truss

For the launching process it is necessary to connect the

single span truss girders to form a continuous beam. At

the connection point the last assembled truss (i.e. Tn-1)

will be connected to the already launched segment's

truss (i.e. Tn).

8.8. Launching Nose / Tail

In order to reduce the reactions and bending moments in

the members of truss during full cantilever, a nose is

attached to the front end of the first truss of each

segment. Similarly for the same reason, a tail is attached

to the rear end of the last truss of each segment.

8.9. Pulling System

8.9.1 Pulling Arrangement at yard

The pulling device at Assembly yard will be used to shift

the Truss from assembly bed 3 to the connection point for

launching at assembly bed 4.

This pulling device has to pull a single truss (~1800 t) to

the connection point. A steel transverse beam between

the two launching tracks will be assembled.

Two strand jacks of 200T capacity each will be installed at

the locations.

8.9.2 Pulling arrangement at pier

10 (9) trusses form a launching segment. In order to pull

this launching segment a pulling device is required. This

operation will be done with the use of multi strand jacks

that are connected to the substructures (piers). These

multi strand jacks will be used to pull strands which are

anchored at the launching segment.

The strand jacks are placed at the piers P1, P11, P21 and

P31 at vertical beam. The strands are anchored at the

anchor beam. In order to limit the strand length the

anchor beam will be shifted after each launching stage of

125 m. The anchor beam is located at the first cross beam

of the truss.

Pulling jacks used for launching

8.10. Launching Bearing or (sliding bearings) at

pier

The launching bearings (or sliding bearings) are

arranged under both side of the bottom chord of the truss.

The bearings are placed on a spherical plate in order to

divide the support reactions equally to the two webs of the

bottom chord and to take over the rotation angle of the

superstructure due to the launching.

During launching process, sliding plates will be inserted

between the bottom flange of the superstructure and the

40 mm steel plate with the welded stainless steel plate on

top. The sliding plates consists of steel plate and 6


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6 numbers of 5.5mm thick PTFE. The PTFE is

chambered in the steel plate.

For the launching process sliding plates will be inserted

with the PTFE on the bottom side of bottom chord of span

truss. At this PTFE will be connected to the stainless steel

of the launching bearing. Because the friction between

the sliding plates steel surface and the contact area of the

stainless steel / PTFE is much higher the whole

superstructure can be launched forward.

The sliding plates will be inserted at one end of the

launching bearing. With the launching they will be moved

forward to the other end of the launching bearing. There

the sliding plates will be taken out again and carried again

to the end of the launching bearing. The weight of one

sliding plate is around 10 kg.

In order to lower the friction coefficient the PTFE shall be

applied with soft soap.

Both ends of the stainless steel plate are rounded so that

sliding plates can be easily inserted.

8.11. Cantilever Bracket at Pier P10, P20 and P30

for Nose dismantling

In order to launch the launching segment 2, 3 and 4 to

their final position a bracket is required at the piers P10,

P20 and P30. The launching bearing will be located on

the bracket with an offset of 9000 mm to the pier axis. As

we pull the segment over the launching bearing on the

pier bracket, a section of the nose beyond the launching

bearing will be dismantled with a crane or suitable

system. Launching will continue further till next section of

nose clears the launching bearing. This process is

repeated till the front truss of segments 2, 3 and 4 reach

their final position.

1. Launching Procedure

Step by step procedure of launching is explained along

with relevant checks etc. as below

1. After assembling the truss T40, camber checking is

done.

2. Nose is assembled and erected over the temporary

supports initially. After complete Nose is assembled it is

supported over Launching Bearing (LB).

3. Launching Bearings (LB) is installed at temporary

Foundation, A1 and P1 and all the pier locations.

4. Anchor beam is installed in truss (span) 40 and pulling

device (multistrand jacks) are installed over Piper P1.

5. With pulling device truss is launched (pulled) in steps

as per the design and deflection is monitored.

6. After certain interval of launching, pulling is stopped

and LB is lifted by certain height.

7. This procedure is continued in steps till the truss

reaches the Pier P1.

8. After above steps, truss (span) 39 is launched upto

connecting point.

9. Simultaneously camber check of truss (span) 39 in

completed. Temporary connection of span 40 and 39 is

done by welding.

10. Anchor beam is installed in truss (span) 39.

11. LB at temporary pier is lifted and truss train is pulled in

steps as mentioned above.

12. The procedure of pulling and lifting is continued in

steps till the truss train reached Pier P2.


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13. The above steps are repeated till the truss train of 10

spans are launched completely.

10. Project Duration and present status

The contract envisages completion of project within

48months

Time cycle for

a. Fabrication and assembly of one truss = 20 days

b. Launching one span = 3 days

c. total days for one span launching = 23 days

for 34 spans as per contract, time required = 34 x 23 days

= 782 days

Considering 25 working days per months, time reqd = 32

months

Initial mobilisation and set up period = 9 months

Finishing Activites = 6 months

Total duration = 32 + 9 + 6 = 47 months

During the actual execution, the scope has increased by

additional 5 spans and weigth of the span has also

increased.

Accordingly Client has granted extension of time upto

June 2018.

Presently 34 spans are Assembled and launched out of

39 spans.


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Incremental Launching of Steel Truss Bridge Rajiv Surekha ...icipunecentre.org/papers/24-Rajiv Surekha.pdf · Incremental Launching of Steel Truss Bridge Rajiv Surekha General Manager,