The new method of preflex beam

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   The abstract of Preflex Beam

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 The Preflex composite girder is well-known and constructed worldwide. It was initially proposed by the Belgium designer of structure , A. Lipski in 1949. From 1951 to 1954 A. Lipski and professor L. Bases, Brussel Univ. jointly researched and developed the Preflex composite girder bridge. It makes up for the weak point of concrete which is brittle and weak in tensile. Now the superiority of Preflex composite girder bridge is guaranteed worldwide. The pre-compressed casing concrete covering tensile flange can counterbalance tensile stresses due to dead and live load. This method is an economical method of construction and its characteristic is as follow.

   The safety and durability

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 The Preflex composite girder is safer than other steel girder or concrete girder under service load because maximum bending tensile and compressive stresses occur when it is manufactured.

 It has bigger rigidity, corrosion resistance and durability because its lower flange is covered with high strength concrete and the whole steel girder is covered with concrete.     

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   The workability and availability

 This method can reduce a term of work for using Preflex girder manufactured in factory and can reduce construction price because we can construct slab without additional support system.

 If you apply the method under following conditions, you can take advantage of its merits because of relatively lower height of girder.  

 ¨ç In case of limited height of girder for changing work to repair river.

 ¨è For reducing maintenance expenses or getting easiness of construction.

 ¨é When we construct grade separation, subway and pedestrian over bridge that must make space under bridge, Preflex composite girder is applied in form of simple beam or continuous beam and nowadays Preflex composite girder of block form constructed is using.

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   The comparison with a bridge that is using other method of construction.

 It is the one of the best advantages that we can reduce height of girder. It is widely known that it can has 45 of span-height ratio and 50 of the least height of girder.

 We can say that Preflex coposite girder is safer from fatigue than common plate girder because the range of stress the former is smaller than that of latter.

 Because it has bigger rigidity than common plate girder, deflection and vibration is very small. And the maintenance expense is cheaper because it is covered with concrete completely.

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2nd prestressing for simple and continuous preflex composite beams

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2nd prestressing for simple preflex composite beams using PC steel bar

 An outline of the above method is as follow.

 In advance set Preflex composite girder beam manufactured in factory or field between abutment or bridge pier. And after connecting end of beam with PC steel bar by using anchor plate, equip hydraulic jack 1/3§¤point from end of bridge on either side. By use of this hydraulic jack tense the PC steel bar then cast and cure concrete when bending occurred at beam. Next remove the PC steel bar so give additional compressive stresses to lower part of beam at range of the positive moments.

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2nd prestressing for simple preflex composite beams by lifting temporary support

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  2nd prestressing for simple preflex composite beams by shifting down-up support

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 First convert hinge support to fixed ends on simple beam structure system and lift down roller support to give additional compressive stresses to lower flange. In this condition cast slab concrete. After the slab concrete is cured, lift up the roller support for giving compressive stresses corresponding to negative moments of fixed ends to slab concrete.

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The manufacturing method of inner and outer preflex girder for manufacturing continuous preflex composite beams

 In the first place, manufacture Preflex girder in lots according to inner and outer span to make Preflex beam of continuous beam. The next give secondary prestressing by Down-up method after connecting inner and outer Preflex girder according to each span. The progress of construction Preflex composite girder is as follow.

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  2nd prestressing for 2 spans continuous preflex composite beams by shifting down-up support(type separated beam from pier)

 First connect two external beams on the inner support. And cast slab concrete in condition that both ends of external beams are lifted down. After the slab concrete is cured, lift up the both ends of external beams for giving compressive stresses to slab concrete corresponding to tensile stresses of inner supports.

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2nd prestressing for 2 spans continuous preflex composite beams by shifting down-up support(type interated beam from pier)

 The moment that two outer beams are connected make them be in a body with bridge pier. Then cast slab concrete in condition that both ends of external beams are lifted down. After the slab concrete is cured, lift up the both ends of external beams for giving compressive stresses to slab concrete corresponding to tensile stresses of inner supports.

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2nd prestressing for 3 spans continuous preflex composite beams by shifting down-up support(type separated beam from pier)

 First connect two outer beams with inner support respectively. Then cast slab concrete in condition that both ends of external beams are lifted down. After the slab concrete is cured, lift up the both ends of external beams for giving compressive stresses to slab concrete corresponding to tensile stresses of inner supports.

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2nd prestressing for 3 spans continuous preflex composite beams by shifting down-up support(type integrated beam from pier)

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 The moment that two outer beams are connected make them be in a body with bridge pier. Then cast slab concrete in condition that both ends of external beams are lifted down. After the slab concrete is cured, lift up the both ends of external beams for giving compressive stresses to slab concrete corresponding to tensile stresses of inner supports.