The existing types of overhead bridge

Steel box beam is constructed on upper column in a placed state as simply supported beam. (Fig. 1)

Because it is simply supported beam, the steel box section is designed as maximum positive moment occured at the middle of span. so depth of beam is very large and it is unshapely.

It has so large deformation because simply supported beam.

When the beam contract with column, it is contracted through previousely boiled bolt hole at beam and column after column is fixed at foundation. so it is cause of poor construction that column leans or wide the bolt hole by oxygen.

 

fig. 1 Simply supported overhead brige

The other structure of existing overhead brigde is double steel box section and rahmen structure consist of four column, it has two column at one side. (Fig. 2)

Because beam and column section are designed as double steel box section, it is more disadvantageous then single steel box section at material using and production.

When the beam contract with column, a upper part of column and beam are combined and place it on column after part of column is fixed at foundation. this method is completed by welding so there are many poor construction cases that cutting the column by error of placed in field.

It need more material and expensive than simply supported beam.

 

Figure 2. Double steel box rahmen overhread brigde

 

Characteristics

 

The side of mechanics

 

 The specific character of this method is unification of steel box beam and column by welding. The existing method of simply suported overhead bridge structure is not.

 The method, prestressed steel box rahmen overhead bridge using temporary supporting point, reduce a third of maximum positive moment compare with existing method and two thirds of both side negative moment of steel box beam compare with existing simply supported beam so the hight of middle span section is reduced compare with existing that. This effect of decreasing moment not only make it possible construction that is varied the section at middle and end of span with linear and curvature but reduce 30% in amount of steel compare with existing simply supported beam.

 In serviceabillity, because the maximum deformation reduce 20% when use the same amount of steel by existing method of simply supported beam, it is advantageous. In construction, it is useful in problem in temperature by contract beam and column.

 

The side of an outward form

 

 Compare this method with existing doube steel box rahmen overhead brigde, existing method is similar to this new method in mechanics. Section of beam is double steel box and there are two columns at the end of overhead bridge in existing method. But this new method reduce the materials and advantageous in construction by use the single steel box, reduce the number of column to one and large variation at upper section of contract with single steel box beam compare with lower section.

 

The side of structure

 

 In this method, after steel box beam is placed on column, the temporary column that can control the hight as equal to both sides of beam and upper column is instituted at 1/81/6 point from end of both sides of beam. It make negative moment so it have prestressing effect that reduce the positive moment by dead load and live load after contract beam and column. In part of beam, the effect of prestress is harmful but it is very short.

Construction Method

 

Step of construction

 

  STEP 1 : Camber and loading of self weight of steel member

 

Sigle steel box beam is placed on column as linear or curvature varied section

(fig. 1) Shape diagram and bending moment diagram about the set single box beam on the existing column

  STEP 2 : Set the temporary supported point and loading of upword load

 

Set the temporary column that get the prestressed effect by control the hight and loading of upword load

(Fig. 2) Shape diagram and bending moment diagram about creat the negative moment by temporary spported point

  STEP 3 : Remove the temporary supported point after contract beam and column

 

Remove the temporary supported point(fig. 3) after contract beam and column by welding and complet the construction of overhead bridge(Fig. 4)

(Fig. 3) Shape diagram and bending moment diagram about remove the temporary supported point after contract beam and column by welding

 

(Fig. 3) Shape diagram and bending moment diagram about completed prestressed steel box rahmen overhead bridge using temporary support