Characteristic

 This method of the prestressed steelbox rahmen overhead bridge using vertical loading equals to a form of overhead bridge using temporary supporting point. We compare this method of overhead bridge construction with the method of overhead bridge construction using temporary supporting point in this chapter.

 

The side of mechanics

 

 Steelbox beam isn't united column in the existing style of structure of overhead bridge of style of simple beam, but we unites steelbox beam with column by the welding. so we solve problem of construction. A section of foundation becomes large in the method of overhead bridge construction using temporary supporting point, because many wrong negative bending moments result from upward loading. So we prevent downward loading in the middle span form negative bending moment. Though block of the positive bending moment is disadvantageous by the downward loading, we can design within allowable stress. because maximum bending moment of block of the positive bending moment doesn't reach half of maximum bending moment of block of the negative bending moment.

 

The side of an outward form

 

 We can construct same section through all the span in using this method, because reduced negative bending moment  by the downward loading results that section of column is reduced remarkably. Therefore we can secure space of girder in the block of the negative bending moment in using this method and reduce height and length of stairs.

 

Theoretical background

 

Procedure of construction

 

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

 

 Steel column of circular section enlarge reduction of material and mechanical effect. after this column is fixed in the constructed foundation like figure 1, steelbox beam that is made of the same section through all the span is set simply on the column. After that we calculate stress considering self weight of steel.

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

  STEP 2 : Loading of vertical load

 

 Many wrong negative bending moments result from upward loading in the method of overhead bridge construction using temporary supporting point. So we prevent downward loading in the middle span form negative bending moment. Therefore we can reduce section of box beam and section of column in the end of span, because of prestress effect from this method. so we apply vertical load in the downward direction and calculated stress before steelbox obtains prestress effect like figure 2.

(fig 2) Applied vertical loading diagram and bending moment diagram

  STEP 3 : Removing vertical load before unity of column and beam

 

 After unites beam with column by the welding, we remove vertical load like figure 3. So we can make overhead bridge like figure 4. If steelbox beam is reinforced with design and column is made good mechanical circular shape, At this time setting position of steel box beam on steel column can allow to change within 1cm allowable error, according temperature influences on shrinkage, expansion because of that, the method is economical on the side of reduction of material.

(fig 3) After steelbox is united beam with column by the welding, remove vertical load. At this time diagram and bending moment diagram

 

(fig 4) Prestressed steel box rahmen overhead bridge using vertical loading diagram and bending moment on live load.