TERM PAPER

METHODS IN CONSTRUCTION APPROACH BRIDGE OFCABLE STAYED BRIDGE

CE 5805 CONSTRUCTION EQUIPMENT AND METHOD

ADITYA NUGROHO

HT083276E

DEPARTMENT OF CIVIL ENGINEERINGNATIONAL UNIVERSITY OF SINGAPORE

2010

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Department of Civil Engineering

CE 5805 Construction Equipment and Method

1.0 INTRODUCTION

The most complicated and complex of cable stayed bridge construction is in the middlesection work. This construction activity in the sea needs fully attention with carefulness forwork safety issues and reliability of assembly. To avoid the faulty work in construction site,we need a construction method to ensure an engineer work in efficient and effective way.Construction method is a phase of the work on the construction process.

In this term paper, I would like to discuss methods of construction approach bridge in thecable stayed bridge. A cable stayed bridge is a bridge that consists of one or more columnswith cables supporting the bridge deck (Wikipedia, 2010). In the cable stayed bridge, anapproach bridge is a continuous pre-stressed box girder which connected with the main spanand consists of two parts side, which each of side have similar geometry (symmetrical).

2.0 CONSTRUCTION METHOD

2.1 Balanced cantilever

Method that usually applied for continuous girder is using balanced cantilever, cast in placefor each segment (see Figure 1), while part of V-pier is done by using a scaffolding and is partof pillar-V.

Figure 1Balance cantilever

The balanced cantilever method was developed to minimize the reference scaffoldingnecessary for the implementation of an in-situ casting. Temporary shoring is too expensive,especially for cases of high elevated bridge and the use of scaffolding across the river or sea isvery risky, if constructed at the mainland would result overcrowded of road traffic, thereforeuse of scaffolding is no longer economical. Thus, by applied construction method of balancecantilever would eliminate such difficulties.

Cast in place segmental methods is the method of operation which is still using the segmentalconstruction and the balanced cantilever, but each segment is cast in-situ. Formwork travelerwho depends on the previous segment is used to form segments. After the segment casted,after four days of concrete age, stressing done to keep segments in order to remain in place.

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Department of Civil Engineering

CE 5805 Construction Equipment and Method

Formwork traveler or traveler called it then pushed forward into the next segment and thesame process is repeated until the mid-span of the pier the other direction, forming acantilever structure.

Figure 2 belows illustrates an implementation of the box girder bridge using the balancedcantilever using traveler. To facilitate the implementation of a cantilever, a short plate with acounterweight on the other side which casted on the formwork which supported from theground in each pier. Ties while prepared to balancing the other side to ensure there is balance.Control must be ensured in every sequence of execution counterweight cantilever.

Figure 2. Implementation of segmental box girder bridge using the balanced cantilever cast inplace

There are many variations in the method described above. In the case of low elevated bridgemay be more economical to foundry end spans over scaffolding. When the bridge deck with afused monolithic pillars, ties or struts on the counterweight can be removed. Formwork forfoundry cantilever gantry was hold by the traveler. Gantry was pushed to move forward witha rail attached to the bridge deck that has been completed to the next segment. When thegantry is located in the right place, the new segment is formed, casted and in-stressing on thesegment that has been done previously. And the same process repeated until mid-span of thepier the other direction, forming a cantilever structure.

Ordering cantilever construction method is as follows:

a) Install and set the gantryb) Install and place the formwork according to the proper elevationc) Place the reinforcement and duck channels from tendond) Segment castinge) Install tendons withdrawal and do the stressingf) Remove the formworkg) Push the gantry at the next position and start a new cycle.

Modular formwork systems is the most popular. This system is flexible for a variety ofdifferent cross section and different length of casting. In this traveler there are mechanical andhydraulic systems are lightweight so as not to complicate assembly of the crane so that it canachieve the desired cycle time. Stressing steps:

a) The tendon is inserted into the sleeve pipe that had been prepared to concrete.

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Department of Civil Engineering

CE 5805 Construction Equipment and Method

b) The tendon is pulled by using the tip of where to jack in both directions longitudinalanchor who used both is anchor of life. Shortly after withdrawal of the tendon, has beenloss of prestress force in the form:

c) The abbreviation is elastic, the loss of voltage due to friction, and some moments deadload has been working as the impact of the arch tendon position. With thus jack stylemust already take into account matters concerning The voltage loss. Voltages restrictedlicense at steps devolution and taken together with the service given to the way pre-withdrawal.

d) If the bonded tendons are used, especially for corrosive environments, space blank in thepipeline around the tendon is filled with cement paste injection (grouting) after the tendonis pulled and before income working life. If so, then the stress due to live load iscalculated based on cross section as do the transformation in the way of pre-withdrawal.

e) In general, the anchor end after locked (turned off) have to be enclosed or protected withprotective layer.

Figure 3 Tendon layout for stressing longitudinal in segmental

Figure 4. a) Placing the anchor head and wedges, b) Positioning and insert strand the jack, c)Make withdrawals, d) The key holder wedge e) Jacking: to anticipate the possibilityof unbalance moment at each step of implementation, used here as the placement(temporary bearing) to restrain the load while the tensile force was arrested by thetemporary anchor.

In the field work unbalance moment is caused by several things, namely: (i) wind pressurethat does not balance (ii) severe form traveler is not balanced, especially at the time manouver(iii) self weight of concrete is not the same as (iv) burdens/loads of implementing

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Department of Civil Engineering

CE 5805 Construction Equipment and Method

2.2 Closure

If the unbalance moment that occurred beyond the capacity of placement, thus the temporaryanchor begin work, namely the one hand interested in the conditions endured anchor and theother side of the oppressed borne placement.

Should in practice, the anchor capacity is not fully utilized, given the presence of otherpossibilities that are unexpected. If the implementation steps for each segment is finished (fullcantilever), the next stage is the closure, namely joining the end of the cantilever with theother cantilever across. There are several methods that can be applied to closures, which inprinciple all methods must refer to the requirements, that is loads which will be used is not toolarge (preferably made equal to the reaction at the end of the cantilever closure) and theplacement of charges made in a manner so as not to cause deformation of the closure.

Example placement of the load carried on the bridge is shows in Figure 5, here the burdenPft1, Pft2, Pw1 and Pw2 is the same that is equal to the weight closur divided in two.

(a) Form traveler is shifted and debark

(b) Imposition prior to closure

(c) Formwork & locking devices fitted

(d) Casting of concrete & water released

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Department of Civil Engineering

CE 5805 Construction Equipment and Method

(e) Casting of concrete finished & blank water

Figure 5 Stages of construction closures

This condition is well balanced before casting closures or closures already completed. Tomaintain a balance and no deformation in the molding of the closure, Pw1 and Pw1 shouldconsist of load water and the tank, so that at the time of execution, here concrete is foundryparallel with water removed with the same weight. If the casting is done gradually, then thebalance would be maintained both before, during and after casting.

As ballast, P1 and P2 can use the form traveler to dismantle the inner form-work to reduce theweight to be balanced with Pw1 and Pw2. Figure 3 illustartes the complete stepsimplementation, that is the sequence of segment-making that apply to each pillar. Here everysegment making consists of four steps, namely the transfer form-traveler, making the form-work and reinforcement, casting and stressing of prestressed cables.

3.0 TYPE OF CONDITION AND APPLICATION IN THE PROJECT

The balanced cantilever method is chosen according to the needs of long-span of approachbridge. This method is suitable for the work done at sea or valley with the 120th meter span.Box girder casting method is to use a form traveler, which consists of the main stimuler trustthe system, the system bottom basket, suspension systems, form work systems, anchoringsystems and motion systems.

The system consists of a side form work formwork, form work and the diaphragm innerformwork. Formwork is ready for use after all activities perangkaian completed. The processwas conducted in stockyard semifinish rebar and rebar finalization process conducted at jobsites. Rebar placement conducted alongside a step by step with the formwork and the castingprocess. The process of rebar placement done after the formwork installed.

Casting segmental box girder that will be used is foundry cast in situ. Casting rebar and rebardone after duct installed properly. Casting is done by using a concrete pump with the help ofthe pipe. Stressing job is a very important job for the long-span continuous employment.

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Department of Civil Engineering

CE 5805 Construction Equipment and Method

Figure 6 Balance cantilever construction

In summary, the balance cantilever for construction of approach bridge have severaladvantages such (i) bridge could be erected without any scaffoldings from the ground, (ii)erection need not to be affected by the land condition (river, sea, valley) or road, railwaysurrounding the erection spot, (iii) bridge could be erected safely and only required a smallnumber of workers, because of the repetition works, (iv) arrangement of the PC tendons andthe bending moment distribution are similar. Therefore, the rational design could be done, (v)can be applied for the span length of 40 m-300 m and (vi) can be applied for the certain curvealignment.

4.0 REFERENCES

Bridge Engineering Handbook. Ed. Wai-Fah Chen and Lian Duan Boca Raton: CRC Press,2000

Balance Cantiveler Bridge Section for Suramadu Bridge. Directorate of National Highwayand Bridge, Ministry of Public Works Indonesia. 2008

Design and construction of bridge approaches. Harvey E. Wahls. Published 1990 byTransportation Research Board, National Research Council in Washington, D.C

http://personal.cityu.edu.hk/~bswmwong/pl/pdf/bridge_construction.pdf

http://en.wikipedia.org/wiki/Cantilever_bridge

http://www.bbr.com.sg/BC-cantilever.html

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Department of Civil Engineering

CE 5805 Construction Equipment and Method

Appendix

Work sequence in the model of the cantilever-method bridge is shown in the followingdrawing figure. The work sequence is based on the concept design of cantilever-method. Cast-in situ method and Pre-cast segment method are used in the cantilever bridge. Cast-in situmethod has the advantage that the accuracy of the girder could be kept since during theconstruction, the deflection and/or the pre-camber control could be easily done.