125T STEEL LAUNCHING GIRDER LAUNCHED 250T RCC BOX GIRDERS ON 300M RADIUS RAIL ALIGNMENT..!
Sanatkumar P Rajamane, Sannidhi Technologies, Bengaluru
B. N. Sridhara, Senior Consultant and Advisor, Bengaluru
Abstract
In India, most of the launching girder(LG)s that are launching precast RCC box girders, (both segmental or single monolithic), are more than two span in lengths with a counterweight on their cantilever(rear) ends and always move on minimum three supportswith one spacing at almost a span apart. Their designs and operations are complicated hence demand not only trained but also alert supervisor to operate them. Their self-weight is close to their lifting capacities. Because of minimum three supports, these LGs cannot move on curved allignments. This paper discusses on a recently developed and designed new LG, weighing just 125MT that successfully launched 3.50KM long twin box girder viaduct (each girder of 22.50m long weighing 250MT). This LG is simple in design, less than two spans in length moves on only two supports. It isless complicated in operations. Because of the two closely spaced supports, the LG could also launch girders on curved alignments. Use of strand jacks made detailing for lifting and lowering arrangements simpler which reduced fabrication time.
1.0 Introduction
In India, most of the launching girder(LG)s that are launching precast RCC box girders, (both segmental or single), are more than two span in lengths with a counterweight on their cantilever ends and always move on minimum three supports.
The conventional (Fig. 1) LGs has the following components
i) Main Girder
ii) Lifting Arrangement
iii) Sliding Beams
iv) Counter weight
v) Supports
Main girder, may be of Box girder or twin plate girder or Lattice Girder, is designed to take the lifted RCC girder loads along with the impacts etc. Lifting arrangements consists of hoists with lifting hooks to lift RCC segments from trailers and help them get suspended to Sliding Beams. These Sliding Beams are then moved to the predetermined locations using winches. They act as hangers for suspending RCC precast segments till they are joined together and pre-stressed. After pre-stressing, the entire girder will be resting on bearings which are on the pier pedestals and Sliding Beams are relieved from all loads. Sliding Beams are also used as counter weights during the auto-launching of LG. Counter weights normally consist of RCC precast girders placed on the rear end of the Main Girder. They are designed to maintain not only the stability of LG but also to prevent the main girder from toppling during traversing.
Supports are 4 in number and play a very important role in transferring the self-weight of LG along with entire lifted RCC segment loads to piers or through RCC end blocks (of end RCC segments) to pedestals and ultimately to pier. The four supports are named as Front Support, the support next to it is called as Middle Support, then it is Rear Supportand last one is called Rear Trolley. (See Fig. 2)
1.1 Auto-Launching of Launching Girder
The following steps are involved in the auto-launch of conventional LG. It is to be noted here that, at all the supports, there are jacks and they are used to lift the Main Girder as and when it is warranted.
Step 1
i) Anchor all the supports and stress them to induce initial tension (about 2 – 5tons in each)
ii)Lay the rail tracks on the erected span
Step 2
i) Lift the Main girder at Rear Support with help of jacks there
ii) Withdraw jacks on Middle Support
Under this condition, LG load is shared by Front Support, Rear Support and Rear trolley only.
Step 3
i) Move the Middle Support to next span first segment (ie on to end block)
ii)Raise the jacks of Middle Support so that it touches and lifts Main Girder.
iii)Withdraw jacks of Rear Supportso that LG load is shared by Front Support, Middle Support and Rear trolley only.
Step 4
i)Move the Rear Support to the old position of Middle Support
ii)Raise the Jacks of Rear Support so that it touches and lifts Main Girder.
iii)ithdraw jacks of Middle Support so that LG load is shared by Front Support, Rear Support and Rear trolley only.
Step 5
i)Move the Middle Support to the front end block
ii)Raise the Jacks of Middle Support so that it touches and lifts Main Girder.
iii)Anchor Middle Support and Rear Support and stressed to induce initial tension (about 2 – 5tons in each)
Raise the Front Support (Which is telescopic) so that LG load is shared by Middle Support, Rear Support and Rear trolley only.
Step 6
i)Remove the anchorage of the Rear Trolley
ii) Start auto launching of with help of pushing jacks installed on the Middle Support
Launching Girder Moves Forward
Step 7
i) Remove the anchorage of the Rear Support
ii) Withdraw the jacks of Rear Support and move it to the front.
Step 8.
i) Further auto launch the LG till Front Support reaches its final position
ii) Open the telescopic legs of the Front Support and lower it to rest on pier
1.2 Chances for Human Errors
Because of the various operations involved in the different steps of auto launching sequence,there are amplescopes for human errors. To handle this auto launching operation, one need not only a trained technical person but also his/her alertness is most important at all the time during the operation. Following are the few important operations which are sources for human errors.
i) Different supports are to be anchored at different point of times.
ii) Withdrawal and opening of Jacks at the Middle Support, Rear Support at different stages of auto launching of LG
iii)Shifting of Rear Support before the Front Support reaches its final position
1.0 Proposed New Configuration
To avoid human errors completely the following newconfiguration has been developed which works most efficientlywith extreme safety. (Fig.3)
In the new configuration, only two main supports exist namely End Column and Trestle Support. There are three sub-supports on trestle top named as A, B and C.
2.1 Design Concept
The Main Girder is balanced about point B with help of counter weight CW. Whenever load on span C-D increases, the point D of girder starts deflecting down and the girder touches the point C of Trestle Support. At this condition, the restoring moment due to CW increases due to increase in the lever arm by amount an amount equal to trestle width thus automatically the stability is restored. On the other hand, if load on cantilever side of the girder increases, the cantilever end of the girder starts deflecting down and the girder touches the point A of Trestle Support. At this condition, the restoring moment due to self-weight of the girder between C and D increases due to increase in the lever arm by an amount equal to trestle width thus automatically the stability is restored. Hence, due to this new support condition, the stability of the entire LG is always maintained (ieToppling of Main Girder will never happenand it is always in safe mode).
2.2 Support Conditions
In the new configuration, at any given time, the Main Girder is always supported on two supports. It is explained in the following sections.
a) Main Girder supported at ' B ' and 'D' only:
The Main Girder is supported at 'B' and 'D', and supports 'A' and 'C' are inactive (Fig.4).
Under this support condition, the entire Main Girder,counter weight CWand other mounted fixture weights will be supported on supports B&D(end column) only. This condition is used for launching the RCC segments/girder in the new span. Here, CW helps in reducing the span moment in the span B and D which results in the reduction of size of Main Girder in turn the economics of entire system. As explained in the previous section, there is no question of overturning of the LG.
With this support condition, the LG is moved forward to next span. When launching of the RCC segments/girder is over, the support conditions on trestle top are modified. The bolts at support B are loosened and with the help of flat jacks the Main Girder is lifted up. Thick plates are inserted at support A and C. Now, flat jack loads are withdrawn which makes the Mai Girderto sit on support A and C. With the help of bolts the Main girder is secured to the trestle (Bolts are loosely connected so that vertical movement/deflection of the Main Girder is not prevented. Now the girder is ready to move to next span without any stoppage in between till the end column sits on the next pier top.
Once the end column reaches its final position on next pier top, the auto launching operation is halted. The support conditions at the trestle top are modified to original condition ie, support 'B' active and support 'A' and 'C' are inactive.
Now the LG is ready to launch RCC girder in new span.
3.0 Advantages of New Configuration
- No anchring of supports required
- No frequent change in support conditions for Main Girder during auto launching of LG
- No instability problem at any loading condition
- No instability problem at any support condition
- No complication in operation
- Auto launching is simple and fast (80% Time saving !!)
- No highly skilled or trained person required for operation of LG
- Very few men are required for operation
- LG can travel along curves
- LG can launch two girders in each span side by side
- More cost effective compared to convetional LG
125T Steel LG Launched 250T RCC Box Girders on 300m Radius Rail Alignment
The above launching girder is designed based on the new configuration and has successfully launched RCC segmental girders. The salient features of the project are as follows.
| Project: |
Mass Rapid Transport System (MRTS), Chennai |
| Description: |
Construction of Elevated Rail Link Between Velachery Station to Saint Thomas Mount Station |
| Length of Viaduct: |
3.50KM
|
| Pier Spacing: |
22.50m
|
| PSC Girder weight: |
250 MT - Two girders side by side |
| LG weight: |
125 MT |
| Auto-launching Time |
30minutes maximum…!!! |
5.0 Acknowledgment
The authors are thankful to M/S Vijay Nirman Co Pvt Ltd, its director Mr. Y.V. Ramana, for giving us an opportunity to work on this project. The authors also record sincere thanks to Mr. NageshwaraRao (project Incharge, Vijay Nirman Co Ltd) for all his technical inputs and co-operation.