RETROFIT DESIGN AND CONSTRUCTION FOR MAKING GERBER
GIRDERS CONTINUOUS ON THE METROPOLITAN EXPRESSWAY
Tokyo Branch Bridge Engineering Dept. Hidechika WATANABETokyo Branch Bridge Construction Dept. Tatsuyasu BESSHO

1. Introduction
2. Design outline (Design principles)

Gerber-hinged prestressed concrete (PC) bridges were apopular bridge type until around 1960s because they

The design load of the bridge at construction was TL-20

required relatively simple structural calculation andin accordance with the Japanese code of that time. In theallowed for longer span lengths (Figure-1).

present design study, strengthening of the structure was

The bridge in this study is a Gerber-hinged bridge withinvestigated along with the modification to the Gerber

PC box girders on the Metropolitan Expressway Shinjuku

hinges, based on the results of load bearing capacity check

Route (Route 4) located in Sendagaya, Tokyo. Being in(B live load) performed for the girders in accordance withservice for more than 50 years, the bridge shows corrosion

the latest code (Figure-2).

of the shoes in the Gerber hinges due to aging (Photo-1) andThe following principles were adopted for the design ofalso damage at notches due to stress concentration. To solve

the modification to the Gerber hinges to be continuous andthese problems, the Gerber hinges were modified and thestrengthening of the structure in this project:girders were made continuous by using external tendons in࣭Leave the existing shoes in the target Gerber hinges incombination with carbon fibers. This paper summarizes theplace, and cast mortar.

design and retrofitting of the project.

࣭ Use primarily the external tendon method which is atested and proven technique for making the girders
continuous.(Preliminary tensioning is made at a force of
20% of the final tensioning force by using a small-size
jack.)
࣭To strengthen the reinforcement which is interrupted atthe target Gerber hinges, check the current amount oftensile reinforcement, determine the required amount ofadditional tensile reinforcement, and apply an equivalentamount of carbon fibers.

Figure-1 A Gerber hinge in a box girder

3. Outline of the construction (External tendonplacing)
Although it was desirable to tension the external tendons
immediately after filling the Gerber hinge gaps, theexternal tendon placing had to be performed span by span,requiring a waiting time before application of stress. During
Photo-1 Damaged shoes found in a Gerber hinge

the waiting time, expansion or shrinkage of the main girder

Table-1Structural data of the bridge

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技報 第 19 号（2021 年）

Making Gerber

Making Gerber hinges continuous

hinges continuous

Figure-2Overview of the project

4. Concluding remarks

with the temperature fluctuations could cause gaps in theconstruction joints in the mortar filling the Gerber hinge

The project of making the Gerber hinges continuous also

gaps. To minimize the problem, the retrofitting work wereincluded various operations such as carbon fiber retrofit

started from the external tendons in the span with theand concrete spalling prevention which had to be carried out

Gerber hinges. Preliminary tensioning was made at a force

in parallel.

of 20% of the final tensioning force by using a small-size jackTheunder restricted operating

conditions on

Gerberhinges

weremadecontinuous,anda narrowstrengthening was made to carry the live load of the latestsuspended scaffolding (Figure-3, Photo-2). Full tensioning

code. It is hoped that this project will be a good example ofwas carried out after installation of the external tendons tomodification to the Gerber hinges in existing PC bridges,

the end spans was completed.

and will help future development of repair and maintenancetechniques.

Photo-3Project completedFigure-3Steps to full tensioning

Key Words㸸 prestressed concrete bridge, Gerber hinge,
external tendons, retrofitting.

Hidechika

Tatsuyasu

WATANABE

BESSHO

Photo-2Preliminary tensioning

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