Chapter 4: Erection Methods of Cable Stayed Bridge

7/30/2019 Chapter 4: Erection Methods of Cable Stayed Bridge

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CHAPTER 4

ERECTION METHODS

4.1 GENERAL

The erection procedure depends on the structural system of the bridge, the site

conditions, dimensions of the shop-fabricated bridge units, equipments and other

factors characteristic of a particular project. The techniques and methods of

erecting cable-stayed bridges are as varied and numerous as the ingenuity and

number of erector contractors. It is common practice for the design engineer to

suggest a method of erection, because erection method not only affects the

stresses in the structure during construction but may have an effect on the final

stresses in the completed structure.

4.2 ERECTON METHODS

The methods of erection for cable stayed bridges can be broadly described by

three general methods as follows:

4.2.1 Staging Method

The staging method of erection is most often used where there is a low clearance

requirement to the underside of the structure and temporary bents will not

interfere with any traffic below the bridge. Its advantage is its accuracy in

maintaining required geometry and grade and its relatively low cost for low

clearance. The staging erection method is explained through a case study as

follows.

Erection Methods

Staging Method Push-out Method Cantilever Method


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impractical. In this method, large sections of bridge deck are pushed out over the

piers on rollers or sliding Teflon bearings. The deck is pushed out from both

abutments toward the center, or, in some instances, from one abutment all the

way to the other abutment. Assembling the components in an erection bay at one

or both ends of the structure and progressively pushing the components out into

the span as they are completed can simplify construction and reduce costs. The

push-out erection method is explained through a case study as follows.

4.2.2.1 Push-out method: Julicher Strasse Bridge

Here the erection problem was that the federal railway operation, which consisted

of six electrified tracks under the eastern side span and the marshalling yard

under the center span, could not be interrupted. The push-out concept was

selected as the most feasible for the site conditions.

An area behind the west abutment of approximately 61 m by 39 m was utilized

as the assembly shop. Erection units were approximately 16 m in length and

were assembled from six subunits and, as much as possible, were

automatically welded at the assembly site.

The erection procedure is shown in Fig. 4.2. It should be noted that in the final

position the reaction load of the towers is borne by the permanent piers VIII andXI. However, during the push-out operation the tower reaction must be

resisted by a lateral-beam diaphragm which in turn transmits the load to the

longitudinal box girders. For this reason the cable stays are only partially

tensioned. The jacking mechanism at the saddle is used to compensate for the

cantilever deflection of the leading edge of the pushed out section.

When the leading edge of the bridge reaches pier VIII [Fig. 4.2(d)], the bearing

is elevated approximately 100 mm by jacks. As a result of this action the

bearing pressure at pier VII is relieved. As the structure is pushed out farther,

the bearing pressure at pier VIII will increase. It was determined that the

allowable bearing pressure was reached when the leading edge extended

approximately 7.3 m past pier IX. At this point the bearing at pier VIII is


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lowered to its original position. This procedure is then repeated until the

structure is in its final position.

4.2.3 Cantilever Method

Because of their self-anchored cable systems, the cantilever method has been

widely used for the girder erection of cable-stayed bridges. The cantilevermethod is considered as the natural and logical solution for constructing the

cable-stayed bridges of large span, where new girder segments are installed and

then supported by new cable stays in each erection stage, and the construction

process keeps going stage-by-stage until the bridge is completed. Since no

auxiliary supports are needed for constructing the bridge girder in the cantilever

method, a lot of construction cost and time can be saved. There are two basic

alternates in the cantilever method. One is named herein the single cantilever

method and the other is the double cantilever method. In the former the side

span girders of the bridge are erected on auxiliary piers and afterwards the

stiffening girder in main span is erected by one-sided free cantilevering until the

span centre or the anchor pier on the far end is reached. In the latter, the bridge

girder is erected from both side of the tower towards the anchor piers and the

Figure 4.2: Push-out method: Erection sequences of Julicher Strasse Bridge


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main span centre by double-sided free cantilevering. The cantilever erection

method is explained through the erection procedure of a harp type cable stayed

bridge as follows.

4.2.3.1 Cantilever method: Harp type cable stayed bridge

The Pylons are erected.

The first pair of girder segments B1 is installed.

The stayed cables C1 are installed and stressed initially to elevate the girders

and relieve their bending moments.

The pair of girder segments B2 is installed.

The stayed cables C2 are installed and stressed.

The girder segments B3 are installed.

The stayed cables C3 are installed and stressed.

Girder segments B4 is installed and the bridge is closed at the main span

centre.

Figure 4.3: Cantilever method: Erection sequences of a harp cable stayed bridge

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Chapter 4: Erection Methods of Cable Stayed Bridge