What Is a Box Girder?

definition

In the analysis of box girder, the main loads acting on the box girder are dead load and live load. Dead load acts symmetrically, and the cross section of the box beam is basically symmetrical, so dead load generally does not produce eccentric effects; live loads can be symmetrical or asymmetrical eccentric effects, and must be considered separately. Under the effect of eccentric load, the box beam will produce four basic deformation states of longitudinal bending, torsion, distortion and lateral deflection (as shown in the figure below).

The main calculation methods and publicity are as follows:

1 The bearing capacity of the foundation of the storage and beam platform is insufficient, and the surrounding drainage system is not smooth.

Take the standard construction of 20m box girder as an example

1 continuous box girder

1.1 Equal-section box beams In China's prestressed concrete continuous beams, the most commonly used are constant-section and variable-section box beams. Constant-section continuous beams are mainly applicable to the following situations:

(1) The span is generally 40 ~ 60m (80m span is also available in foreign countries), the structure is simple, and the construction is fast.

(2) It is advisable to use equal spans for the layout of the facade, and it can also be arranged without equal spans. The ratio of the side span to the middle span is not less than 0.6, and the high span ratio is generally 1/15 ~ 1/25. Construction, frame-by-frame installation, mobile formwork construction, and push-up construction.

1.2 Variable cross-section box girder Variable cross-section box girder is mainly suitable for large-span prestressed concrete continuous beam bridges, and the bottom curve of the beam can adopt arcs, quadratic parabola, and polylines. In order to meet the stress requirements of each section in the beam, the thickness of the bottom plate, top plate and web of the section can be changed. In terms of aperture arrangement, the ratio of the span of the side hole to the middle hole is generally 0.5 ~ 0.8. When the ratio of the side span to the middle span is less than 0.3, the side hole bridge abutment support must be made into a compression type to bear the negative reaction force. . Combined with an example, the analysis found that when the ratio of the side span to the middle span is 0.5 to 0.54, there is still enough positive pressure at the top of the transition pier but no negative reaction force. When it is less than 0.3, the beam end is close to the force. Fixed end. The ratio of the beam height to the maximum span of a variable-section box girder. The mid-span cross-section is generally 1/30 ~ 1/50, and the fulcrum section can be selected from 1/15 ~ 1/20.

(1) Mid-span section: h == (1/30 ~ 1/50) L

(2) Section of fulcrum: h-branch = (1/16 ~ 1/25) L (3) h-medium / h-branch: 2.0 ~ 3.0

1.2.1 Relationship between the number of box rooms in cross-section and the width of box girder:

Single box single room: <18m

Double box single room: about 20m

Single box double room: about 25m separated double box:> 25m

In general, box girder of equal height can adopt straight web or inclined web, and box girder of variable height should use straight web.

1.2.2 Floor thickness of the floor is gradually thickened to the root with the increase of the negative bending moment. The thickness of the root is generally 1/10 ~ 1/12 of the height of the root beam to meet the pressure requirements of the construction and operation stages, and it is damaged The stage keeps the neutral axis as far as possible within the floor. The thickness of the mid-span bottom plate is generally 20 ~ 25cm to meet the requirements of the mid-span positive bending moment change and the configuration of prestressed steel bars and ordinary steel bars in the board.

1.2.3 Roof thickness The thickness of the roof must meet the requirements of the transverse bending moment and the requirements of the longitudinal and longitudinal prestressed reinforcement.

The length of the cantilever plate is an important factor for adjusting the internal bending moment of the top plate. Generally, half of the web distance can be taken. When the transverse prestress is configured, it should be extended as far as possible. The cantilever length of the top plate is 3 ~ 5m, its root thickness is 60 ~ 70cm, and its end thickness is 15 ~ 20cm. If the box girder is arranged with transverse prestressing, its end thickness will be limited. The transverse prestress is generally flat anchored. The largest type of flat anchor is 15-5, and the minimum distance between the anchor midpoint and the edge of the concrete is 9cm. 1.2.4 Web thickness The web is mainly subjected to section shear and main tensile stress. In the prestressed continuous beam bridge, the offset of the bending beam to the load shear force makes the internal shear stress and the main tensile stress of the beam smaller. In a variable-height continuous beam bridge, changes in section height can also reduce the principal stress value. Therefore, in addition to the above-mentioned stress factors, the minimum thickness of the box girder web considering the arrangement of prestressed steel bars and concrete pouring is generally: when there is no prestressed beam pipe in the web, 20cm is used, sometimes 23-30cm; Use 35cm. In large-span prestressed concrete continuous beams, the web span should be gradually widened from the mid-span to the fulcrum to support the larger shear force at the fulcrum. Generally, 30 to 80 cm is used, and some reach 1 m.

1.2.5 The main function of the diaphragm is to increase the transverse stiffness of the section and limit the distortion stress. For single-box single-chamber sections, the trend is not to have a center beam.

2 Prestressed concrete continuous rigid frame bridge

Continuous rigid frame bridges are generally used on long-span, high-pier bridges. The structural features of the bridges are that the middle piers are consolidated with piers and the lower structures are generally flexible piers to reduce prestressing tension and temperature changes due to the main beam. , Internal shrinkage, creep, and other secondary forces generated by the restraint of the pier. The continuous rigid frame bridge works close to a continuous beam bridge when the bending stiffness of the pier is small. Compared with continuous girder bridges, the top of the pier and the beams have remained consolidated during the cantilever construction and in use. The main advantages of continuous rigid frame bridges are that they can reduce the troubles of supporting and maintaining large bridges, and reduce the amount of materials used in piers and foundation engineering. The following content mainly introduces the mechanical characteristics, applicable scope and some structural features of continuous rigid frame bridges commonly used in medium and long-span bridges.

2.1 Mechanical characteristics and scope of application

In terms of stress, the superstructure is still characterized by continuous beams, but the influence of elastoplastic deformation due to the bridge piers' stress and concrete shrinkage, creep, and temperature changes on the internal forces of the superstructure must be taken into account. The bridge piers require a certain degree of flexibility, and the bending moment is reduced, but the rigidity of the rigid frame is still at the joint between the piers and beams. Due to the participation of the pier, the working conditions of the continuous rigid frame bridge and the continuous beam bridge are different. The continuous bending moment caused by the live load of the continuous rigid frame bridge is smaller than that of the continuous beam bridge with the same span. When the pier reaches a certain height, the internal forces of the two superstructures are not much different. Comparing the bending moments of the upper structure of the three-span continuous rigid frame and the three-span continuous beam, the dead and live moments at the roots of the two beams are basically the same; when the bridge pier is 40m high, the dead and live loads are in the middle of the beam span The difference in bending moment is less than 10%; the dead load and live load bending moment of the continuous rigid frame bridge pier decreases with the height of the pier, but the rate of decrease is small when the pier reaches more than 40m; the dead load in the continuous rigid frame beam The axial tensile force of live load decreases with the height of the bridge pier, but the reduction rate is small when the pier reaches more than 30m.

When the design span exceeds 100m, prestressed concrete continuous rigid frame bridges can be used as a comparison option for continuous bridges.

2.2 Aperture arrangement

The continuous rigid-frame bridges built at home and abroad have a span-to-span ratio of 0.5 to 0.692. Most of the ratios are between 0.54 and 0.56, which is smaller than the range of variable cross-section continuous beam bridges from 0.6 to 0.8. Theoretical research analysis proves that due to the consolidation of the piers and beams, the length of the side span has little effect on the adjustment of the dead load moment in the midspan, and when the ratio of the side and main span is 0.54 to 0.56, it can not only make the inside Dead load eccentric bending moment, and can be supported on the side pier with a guide beam at the cantilever end of the side span, and the side span can be closed, thereby eliminating the floor stand, and the construction is very convenient.

2.3 Girder section size

The section form of the main beam of the continuous rigid frame bridge is mainly a box section, and the section size is basically the same as that of the continuous beam. Due to the connection of the piers and girder of the continuous rigid frame bridge, the live load moment at the mid-span is smaller than that of the continuous beam bridge with the same span, so the mid-span beam is slightly smaller than the continuous beam bridge. For equal-section beams, according to the actual construction statistics, the relationship between the height of the main beam and the maximum span: max0.0520.202 () hlm 9m, bridge deck width is 15 18m, wide bridge can use separate single box. The thickness of the top plate is 0.25 0.28m, the mid-span thickness of the bottom plate is 0.25 0.30m, and the mid-span thickness of the web plate is about 0.5m.