200703161436A Study of PSM Construction Method (2/2)


A Study of PSM Construction Method (2/2)

A type of modern viaduct construction method proposed

for

The Taiwan High Speed Rail Project

 

3. ANALYSIS AND COMPARISON

3.1  DESCRIPTION OF APPROACH

According to the Construction and Operation Agreement (C&OA) between the Ministry of Transportation and Communications and the Taiwan High Speed Rail Corporation (THSRC) THSRC is to build and operate the Taiwan High Speed Rail for a period of 35 years. It consists of a 5-year construction and a 30-year operation. THSRC will lose NT$ 100,000,000 per day approximately if the Project is delayed. Therefore, it is expected to complete the Project within 5 years and then to start operation immediately. In other words, selecting a faster and cheaper construction method is a target to be pursued by both Project owner and contractor. For avoiding of a verbose writing the only topic of viaduct superstructure construction method will be discussed in this paper.

3.2        ANALYSIS ITSELF

There are only four possible types of construction method to be used for viaduct superstructure in Taiwan High Speed Rail Project under the conditions of BOT and Design/Build contract bases. They are Precast Span Method (PSM), Segmental Cantilever Construction, Bent Support Shoring Method and Movable Scaffolding System (MSS).

For the consideration of saving cost and time both PSM and MSS can be adopted in the long distance viaduct that has same span length and especially located in a big plain area because their facilities can be used and routine work can be performed over and over. On the other hand Segmental Cantilever Construction and Bent Support Shoring Method only can be used in the special length bridges, which will overpass the big road, river or valley in the mountain. Therefore, only PSM and MSS will be discussed and compared on the same basis in this paper.

On the other hand due to the reason of difficulty for renting private land in the vicinity of Chainage 55K+500 of contract C215, we are considering setting up only one precast yard adjacent to Chainage 44K+775 instead of original plan of two precast yards in both Chainages above mentioned. Therefore, we have also to review and check whether the new schedule for one precast yard still can be matched with original progress requirement.

Finally optimal girder length shall be studied and decided in accordance with the special situation of contract C215.

3.2.1  PSM

l        DESCRIPTION OF CONSTRUCTION METHOD

(Please refer to Section 1.4.1)

l        COST BREAKDOWN FOR PSM

(Table 3. is Based on a precast yard with two production lines @30m girder and serving a 30 Km length, which total deck area is 30000m×12.9m = 387,000m2 and 4 years construction period)

Table 3.  Cost Breakdown for PSM

Therefore, the unit total cost of PSM for superstructure is NT$8,130.9/ m2 if the cost of viaduct substructure is not considered.

3.2.2  MSS

l        DESCRIPTION OF CONSTRUCTION METHOD

Movable Scaffolding System (MSS) is a quick, reliable and economical solution in modern bridge construction. It can also easily be adapted to any viaduct cross section (single box, double box, double T, etc) and span configuration. This flexibility provides the contractor the opportunity to re-use the equipment from one project to another. (Refer to Picture 4. MSS Form-travellers and Launching Girders on this page)

A typical construction cycle with the underlane MSS will be as follows :

After concreting, curing and tensioning of cables, the Main Girders are lowered by the main jacks at the rear Suspension Gallow and on the front Pier Bracket.

The joints in the middle of the Transverse Beam system are released, and the Main Girders are moved transversely into a position where the Transverse beams can pass the piers.            

            Picture 4. MSS Form-travellers and Launching Girders

The MSS is ready for launching. The launching operation to next concreting position is to be carried out. The two Main Girders are moved independently to the next span.

During the launching operation, the Suspension Gallow is moved to its next position.

The two girders are moved transversely and joined in the middle of the Transverse Beam system.

The Main Girders are raised to the concreting position by the Main Jacks.

Adjusting the formwork by means of the screw jacks and the adjustable supports.

For box girder bridges : After placing the reinforcement and tendons of the bottom slab and webs, the Internal Formwork is moved to its next position.

When placing of reinforcement and tendons is finished, the MSS is ready for concreting of the next span of the superstructure.

During the pouring period, the rear pair of Pier Brackets are dismounted, moved to the next front pier and re-installed.

l        COST BREAKDOWN FOR MSS

(Table 4. is Based on the slab area of 11 spans @35m : 11×35m/span×12.9m = 4,966.5 m2 with 4 years construction period)

            Table 4.  Cost Breakdown for MSS

Therefore, the unit construction cost of MSS is NT$9,910.6 per square meter, if the cost of viaduct substructure is not considered.

3.3  COMPARISON

3.3.1  COMPARISON BETWEEN PSM AND MSS

PSM is a method with higher equipment expenses and lower production rate in early construction stage, but it will be better and faster when commencing to produce. According to past experience the final progress rate can be one span per day. The normal length of each span is about from 25 M to 45 M. In the mean time all works can be performed only on the viaduct and precast yard, so there is no need for additional construction land and no interference to any activities under the viaduct. Another benefit of construction quality can be easily controlled in the precast yard.

MSS can produce girders within a short time from commencement but it has only a tenth production rate of PSM. The normal length of each span is about from 30 M to 45 M. Hence it needs more construction equipment when performing a long viaduct.

Anyway both PSM and MSS are a quick, reliable and economical solution in modern viaduct or bridge construction. Which one is a better option will depend on the length of viaduct, the length of each span and contract duration time.

Finally and obviously the cost of PSM will be cheaper than that of MSS if the project is bigger enough, which the length of viaduct is 30 Km at least and under the condition of such short construction time 4 years. Therefore, PSM is a better construction method to implement the viaduct of Taiwan High Speed Rail Project.

3.3.2   COMPARISON BETWEEN TWO DIFFERENT LOCATIONS OF PRECASTING YARD

There is a viaduct section with a 20 Km length from Chung-Li to Hu-Kou in contract C-215 of the Taiwan High Speed Rail Project. The PSM construction method was considered on the viaduct superstructure due to this fast schedule. Precast Span Method is a type of modern construction method for the viaduct superstructure – box girder which is built in the Precast Yard and transported to job site on the completed viaduct.

Based on the reasons of difficulty for renting a piece of private land in the vicinity of Chainage 55K+500 and easier and cheaper for renting a piece of government-owned land in the Taoyuan Station Area adjacent to Chainage 44K+775, we are considering setting up only one precast yard in the latter location instead of original plan of two precast yards in both Chainages above mentioned. For the reason of assessment of two different plans following are the analysis and comparison in the point of view in Methodology, Productivity, Risk and Cycle Time. In the mean time we have also to review and check whether the new schedule still can be matched with original requirement.

l        THE DIFFERENCE BETWEEN ORIGINAL PLAN AND REVISED PLAN

        Original Plan : Two precast yards were supposed to be provided approximately at both Chainages 44K+775 and 55K+500. Each yard has one production line without girder storage place. Two sets of transporters and launching girders are equipped.

        Revised Plan : Precast yard is set only approximately at Chainage 44K+775, which has three production lines with storage yard of nine girders. Two sets of transporters and one launching equipment are employed.

l        METHODOLOGY COMPARION

The detailed comparison of original plan and revised plan is shown in Table 5 on this page.

                    Table 5.  Summary of Methodology Comparison

l        PRODUCTIVITY ANALYSIS

         Table 6.  Productivity Analyses of Original Plan and Revised Plan

l        RISK ANALYSIS (RISK IDENTIFICATION & MITIGATION)

  Production system stops due to uncertain incident.

(Original plan) :

All works must shut down until the system recovers because there is neither stock yard nor other production lines.

(Revised plan) :

Since other two production lines still work, casting can be carried out at 1 girder/day. Furthermore, erection can be maintained at 1.5 spans/day, because there are nine girders in stock. Girders for stock can be produced on Sundays or holidays.

  Transporter breakdown

(Original plan) :

When transporter breakdown, erection work must stop till completion of repair since there is no spare transporter. Casting work must also pause since a girder can not be removed from the form.

(Revised plan) :

Another transporter is available. There is minor impact on erection rate. Erection work still can continue.

  Erection work stops due to delay in either pier construction or Type-2 bridge construction.

(Original plan) :

To catch up the schedule, casting bed (production line) shall be added. It takes time to set up the equipment.

(Revised plan) :

Temporary platform shall be constructed adjacent to the superstructure to allow transporters.

  Impact on follow up work (parapet, cable trough, etc.)

All follow up works except expansion joint installation shall be carried out simultaneously during PSM erection for both original and revised plans. The impact on the follow up works is not very significant for both.

l        CYCLE TIME ANALYSIS

        Original Plan : (Production Cycle Time : 2 days per girder)

(Work at casting yard)

Activities

Time (Hrs)

Girder production per girder

32

Steam curing

14

Total

46 ≈ 48

(Transportation and erection)

Activities

Time (Hrs)

Load

3

Transportation (longest destination)

5

Erection

4

Return (longest destination)

3

Total transportation cycle time

3+5+3 = 11

        Revised Plan : (Production Cycle Time : 2 days per 3 girders)

(Work at casting yard)

Activities

Time (Hrs)

Girder production (No.1 girder)

27

Steam curing (No.1 girder)

15

Move to storage yard (No.1 girder)

3

Total

45

 

Girder production (No.2 girder)

27

Steam curing (No.2 girder)

15

Move to storage yard (No.2 girder)

3

Total

45

 

Girder production (No.3 girder)

27

Steam curing (No.3 girder)

15

Move to storage yard (No.3 girder)

3

Total

45

(Transportation and erection)

Activities

Time (Hrs)

Case-1 (Max. cycle up to TK57+494)

Load

4

Transportation (To TK57+494)

5

Erection

5

Return (From TK57+494)

2

Total transportation cycle time

5+5+2 = 12

 

Case-2 (Max. cycle up to TK64+114)

Load

4

Transportation (To TK64+114)

7

Erection

5

Return (From TK64+114)

3

Total transportation cycle time

7+5+3 = 15

3.3.3   SELECTION OF OPTIMAL GIRDER LENGTH

PSM is a construction method for viaduct superstructure with aim of achieving the construction efficiency. This method utilizes heavy-duty carrier and launching equipment to erect the precast box-girder regularly onto the piers. The length of the precast box-girder is usually between 25M and 40M, and different girder lengths are allocated carefully together to avoid the conflicts of the pier locations against existing roads and ditches.

The length of typical precast girders, which are used overpassing the land, the rivers as well as escaping the utilities underground, is evaluated generally. It is necessary to estimate the weight and cost of carrier and launching equipment precisely in order to be the basis of evaluation. The layout of piers not only takes the production plan of precast box-girders into account but also will not be conflicted with existing roads and channels because of utilization of two different lengths of girders.

Anyway, a longer precast box-girder may reduce the number of piers and time bottlenecks caused by three-span continuous beam, cast-in-situ bridges in between PSM construction. In the other hand the girder length is proportional to the girder depth which may affect the required clearance under viaducts due to unchangeable vertical alignment.

In addition to the reason of lower price of carrier and launching equipment for shorter girder there is another consideration, no big rivers, on the viaduct section 2 of contract C215. Therefore, 30M and 25M of girder lengths will be used in the PSM construction method.

4. RESULTS

4.1        RESULT FROM COMPARISON BETWEEN PSM AND MSS

According to the analysis and comparison from section 3.2 the unit construction cost of PSM and MSS are NT$8,130.9 and NT$9,910.6 per square meter, respectively, if the cost of viaduct substructure is not considered. In the other hand we know that the production rate for PSM can be one girder or one span per day but for MSS the production rate of one girder or one span needs 10 days approximately. In other words the PSM is only 80% of the cost and 1/10 of the time required by MSS method to produce the girders with the same length. Therefore, PSM is a cheaper and faster construction method, which is surely selected as the main construction method for the long viaducts in the Taiwan High Speed Rail Project.

4.2        RESULT FROM COMPARISON BETWEEN TWO DIFFERENT LOCATIONS OF PRECAST YARD

The proposed location of the precast yard in revised plan is only one in the beginning point of this section which is worse than two precast yards are in the both center area and beginning point of this section in original plan if we just consider the distance of transportation. On the other hand the cost in the revised plan with only one precast yard is obviously cheaper than the cost in the original plan with two precast yards no matter what in the point of view in Methodology, Productivity, Risk and Cycle Time.

According to the analysis and comparison in the section 3.3.2 the revised plan with only one precast yard and 3 production lines in the Taoyuan Station Area adjacent to Chainage 44K+775 is a better selection than the original plan with 2 precast yards and 2 production lines.

4.3        RESULT FROM COMPARISON AMONG DIFFERENT GIRDER LENGTH

According to the analysis and comparison in the section 3.3.3 30M and 25M of girder lengths will be a better solution in the PSM construction method of the viaduct section 2 of contract C215 that can be completed on planned schedule and with lower cost.

5. CONCLUSIONS

According to the results in section 4 we recognize the importance of PSM construction method to THSR project. It is expected to complete the Project within 5 years and then to start operation immediately. In other words, selecting a faster and cheaper construction method is a target to be pursued by both Project owner and contractor. Therefore, PSM is a better construction method for viaduct superstructure when comparing the other construction method on the basis of a certain long distance and a shorter time schedule.

Secondly, the selection of one precast yard in the beginning point of viaduct section 2 in Contract C215 is a good alternative when considering and comparing the various positive and negative factors. The 30M and 25M of girder lengths will be used on the special conditions of no bigger roads and rivers within the viaduct section 2 of Contract C215.

Finally, since the PSM method has never been used in Taiwan before, for the reason of having good performance during construction the writer would like to emphasis following three important concepts :

l        Discovering the blind spots of PSM continuously, the engineers have to prevent these in advance.

l        The engineers should be familiar with the application of machinery in construction, especially highly automated viaduct construction.

l        Besides these, the engineers must recognize that good and regular maintenance for machinery during construction period will promote the performance of overall project.

6. REFERENCES

6.1  THSRC, Taiwan High Speed Rail – Background, issued on 29th September 1999.

6.2  THSRC, Volume 26 – Particular Specification, Contract No. C215, issued on 5th November 1999.

6.3        FERROCEMENTO – RECCHI GROUP, Prestressed Concrete “Speedy Giant” Prefabrication – 14th Annual Meeting, International Bridge Conference in Pittsburgh, Pennsylvania, USA, issued on 2nd ~ 4th June, 1997. Picture 1, Picture 2, Figure 3, Figure 4 and Figure 5 are copied from this report.

6.4        Mr. Charles P. Hung, the Agent of FERROCEMENTO – RECCHI in Taiwan. The picture in cover page and Picture 3, Picture 4 are given by Mr. Hung.

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I, a sincere and diligent man, was born in Nanjing, China and grew up in Kaohsiung and Taipei, Taiwan.
I graduated from a certain university in Taiwan and earned a master degree in civil engineering from an American University. I have ever been a civil engineer for more than thirty-eight years which including 5 year experience of bridge maintenance in the Kingdom of Saudi Arabia. Fortunately I had joined some of very famous bridge construction projects in Taiwan, such as Guan-Du highway bridge in Taipei and Bih-Tan bridge of second Freeway in Hsin-Tien city. In the years from 1997 to 2009 I had worked with Taiwan High Speed Rail, which has ever been the biggest BOT project in the world. Recently since May of 2010 until the end of 2013 I joined in China High Speed Rail project with a Germany consultant company (DB International) in Zhejiang province.
I am a man who will try hard to succeed and will not give up when I set a goal.
Reading, walking, playing basketball and listening music are my major hobbies at my leisure. In the mean time, I like to make friend with and talk various topics with the person who is from any place of the world.

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