What are the design ideas and engineering characteristics of overpass in Changyi section of expressway?

With the development of social economy, expressway construction, as one of the symbols of national modernization level, has developed extremely rapidly in recent years. Based on the characteristics of fully enclosed traffic control of expressway, various overpasses and underpasses are widely used in expressway construction. This kind of bridge has a small load (generally H- 10, H- 15) and a wide deck, but there are a large number, especially in hilly areas like Hunan, where there are a large number of overpasses, tractor roads, township roads and footbridges. At 7 1.5 1 km section of Changyi Road, there are only 43 such overpasses. In order to make the overpass skillfully combined with the terrain, beautiful and generous, low cost and convenient for construction, the owner and the design unit worked closely together to study and discuss various schemes according to the position of each bridge, and finally chose the best scheme. This paper briefly introduces the selection of overpass scheme and its engineering characteristics.

Second, the principle of overpass scheme selection

According to the characteristics of overpass bridge on expressway and the application of Changyi section of Changchang Expressway, several principles that should be considered in the scheme selection of overpass bridge are summarized as follows:

1. Fully consider the terrain according to local conditions.

Changyi Highway is located in hilly area, and most overpasses are located in cutting section. In addition to ensuring the necessary clearance under the bridge, the selection of the height of the overpass building also needs to be combined with the terrain to reduce the cutting or filling amount of the bridgehead wiring, and finally achieve the purpose of economy and practicality. If the terrain at both ends of the bridge is low, the beam bridge is mainly used; Slightly higher, mainly using half-through arch rib bridge; If it is higher, inclined leg rigid frame, two-way slope arch and arch frame should be adopted.

2. Reasonable selection of bridge span follow-up construction coordination

At present, large pavers are mostly used in expressway pavement construction, but pavement construction is generally later than overpass construction. In the past, middle piers were used to reduce the span and section height of bridges, which hindered the passage of pavers and directly affected the pavement construction progress. There are also hidden dangers for safe driving in the operation stage. Therefore, when selecting the scheme, the middle pier bridge should be used as little as possible, even if it is set, the pier size should be small.

3. On the premise of reasonable structure and convenient construction, try to make the bridge type elegant and diversified, reflect the flavor of the times, and make appropriate light decoration after the acceptance of the main project.

Because of the good alignment and wide and smooth pavement, the expressway is monotonous and drivers and passengers are prone to fatigue. The sense of space in the driver's field of vision plays a great role in driving safety and comfort. In order to create a more comfortable driving environment, in addition to proper green decoration, all kinds of overpasses are beautiful and generous, and the differences in bridge types, color changes and coordination with the environment can better enjoy the aesthetic feeling of drivers and passengers during high-speed driving. Therefore, under the premise of reasonable structure and convenient construction, make the bridge type elegant and diversified as far as possible to reflect the flavor of the times, and carry out appropriate light decoration after the acceptance of the main project, which can greatly improve the driving safety and comfort, and improve the social and economic benefits of the expressway.

4. Economical and practical, with reasonable cost.

In the choice of bridge type, on the one hand, we should start with "lightness" and reduce the volume of masonry; On the one hand, it does not climb materials. Therefore, the overpass on Changyi Road mainly adopts reinforced concrete structure. Among the 43 overpasses in the whole line, only the upper structure of pedestrian overpasses in two service areas adopts steel structure.

Third, the application of overpass

According to the basic principles of the above selection scheme, we adopted more than 8 kinds of arch beam reinforced concrete superstructures and more than 2 kinds of steel structure superstructures in the design of 43 overpasses across the line. The design characteristics and overall effect after completion are summarized as follows:

1. Reinforced concrete inclined leg rigid frame bridge

The upper structure is stressed as a whole, statically indeterminate structure for many times and cast-in-place construction as a whole. The upper part is 48 meters long, the middle span is 25.8 meters, the distance between the two inclined leg support points is 33.8 meters, the main girder is heightening type II, and the height difference from the bridge deck to the top of the pavement under the bridge is 6.89 meters. The simplicity of components, the change of cross-section and the full combination with the terrain make people feel beautiful and harmonious as a whole. At the same time, the opening section of the bridge is convenient to construct, the formwork support is easy, it can be reused, and the cost is lower than other bridge types. Therefore, the number of seats in this bridge type is the highest in the overpass. However, due to the weak stress on the lower edge of the opening section, in order to make the stress more reasonable and the bridge type lighter and more beautiful, on the premise of fully considering the construction difficulty, the integral box section can be considered in future design.

2. Reinforced concrete arch bridge with slab columns on the arch, solid slabs on the arch and solid slabs on the main arch.

This bridge is a traditional arch bridge, but due to the thin-walled plate structure on the arch and the addition of transverse cantilever lines in the main arch, its appearance is light, beautiful, generous and decent. Because of its high building (the height of the road surface under the bridge or even the bridge deck is 7.98 meters), it is easier to set this bridge type on the high cutting side. However, the arch structure is characterized by the large volume and high cost of the two abutments. Therefore, in the future design, if the basic conditions permit, the cost can be appropriately reduced by setting a separate foundation. This type of bridge is easy to build, simple in formwork and easy to operate. This type of bridge is also widely used, with 8 bridges on the whole line.

3. Ordinary reinforced concrete slab arch bridge with prestressed hollow slab bridge on arch

The arch length of this bridge type is 47.76m, the clear distance of the main arch is 38.00m, the building height is 7.88m, and the camber is 1/8. In order to lighten the bridge type and reduce the width of the main arch, the scheme of increasing the cantilever front edge line on the arch and installing longitudinal prestressed steel bars in the range of 16.4 1m instead of the columns on the arch is adopted. Due to the reduction of columns on the arch, people's horizons are broader and more conducive to driving safety. However, compared with other bridge types, this bridge type has higher cost and slightly higher construction technical requirements, so there is only one bridge on the whole line.

4. Hollow slab slope arch bridge with ordinary reinforced concrete slab arch.

This bridge is an improvement of the third kind of bridge. In order to combine the roadside topography and avoid wiring filling, the buildings on both sides of the arch are designed in a slope shape. The height of the mid-span building is 7.53 meters, and the main abutment part can be reduced to 5.67 meters, thus reducing the prestress on the three types of bridge arches, simplifying the construction and reducing the cost. The length of the hollow slab in the hollow part of the arch part is reduced to 1 1.6638. It can better highlight the full combination with the terrain, and the cost is about 29% lower than that of the three-bridge type. Therefore, a * * * type bridge is set up in this section, which is one of the most widely used bridges.

5. Ordinary reinforced concrete T-shaped rigid frame bridge

The length of the bridge is 48 meters, and a thin-walled pier is set in the middle of the bridge span. In order to save cost, the main girder adopts hollow variable cross-section. The building height is the lowest among all bridge types (7.07 meters from the road surface to the bridge deck), which is suitable for sections with less cutting. Pier masonry is small in size and low in cost, but it is difficult to recycle the internal formwork of hollow beam. This bridge type is light in appearance and is an ideal bridge type, so it is widely used on this road (***5), and its cost is equivalent to that of the first bridge type.

6. Prestressed reinforced concrete three-span continuous beam bridge

* * * There are two three-span prestressed reinforced concrete beam bridges along the line. The main girder has a box section, with a beam height of1.48m, a bridge deck width of 7.00m, a design load section of H- 10, a height difference of 6.7m between the bridge deck and the overpass pavement, a total length of 58.00m and a mid-span of 320m. Because of the low wiring at both ends, this bridge type is very suitable. The middle piers are1.20m in diameter, all of which are located outside the side ditch of the main line. In order to avoid large mid-span bending moment, the side span is longer. The box girder of this bridge has a high cross section and a large volume, but the space in the box is about1.20m, which is convenient for construction and the formwork can be reused.

7. Prestressed reinforced concrete double-span continuous beam

This bridge is used on the interchange ramp, with a total width of 19m, without abutment separation and pier separation from the upper part. The widths are 7.75m and 1 1.25m, respectively. The beam length of the upper structure is 2×28. 10m, and the middle pier is made of thin-walled reinforced concrete. In order to facilitate the paver to walk under the bridge, the thickness of the pier is only 0.70m, the height difference between the bridge deck and the road surface is 7.07m, and the beam adopts the lower section, which reduces the earthwork volume of the ramp and reduces the cost. The bridge layout is reasonable and the overall appearance is harmonious.

8. Ordinary reinforced concrete three-span continuous beam bridge

The bridge is located in the interchange, which spans the main line and the ramp. The length of the bridge is 27.0+24.0+27.0 = 78.0m, and the width of the bridge span is 3.0m. On the one hand, the balance and symmetry in the characteristics of ancient buildings in China are considered, on the other hand, it has no influence on the main lanes and asymmetric addition and subtraction lanes. After comparing several different schemes, the scheme with small middle span and symmetrical increase on both sides is finally chosen, which is suitable from the effect point of view. The second feature of the bridge is the choice of section type: because the bridge is narrow (with a total width of 3.0m), if it is difficult to construct with a box section, T-shaped section is adopted, and the elevation of the whole bridge is raised to make it more lively and light. Although one of the side spans has been lengthened, the bridge is at the starting point of the interchange, with a wider view and a panoramic view of the whole interchange. There are two bridges of this type, and the other one is located on the ramp after entering the toll station, with a slightly smaller span and a larger middle span. The two side spans mainly span the cutting slope and play a balancing role.

9. Half-through reinforced concrete ribbed arch bridge

This bridge is located in the suburb of Changsha, and the main line of Changyi Road intersects with the old road. In order to ensure the clearance under the bridge and reduce the earthwork quantity of wiring, the height difference between the bridge deck and the main road surface is only 6.40 meters, so the half-through arch structure is adopted. This bridge is 50.0 meters long, with a deck width of 12.0 meters. The double ribs are symmetrically arranged, with a rib width of 0.80 meters, an I-shaped cross section and a tie bar spacing of 4.0 meters. The deck is assembled with precision rolled rebar, stainless steel sleeves and flexible plates. The bridge has a beautiful appearance and prominent lines. After the main body is completed, decorate it with appropriate paint. From a distance, it hangs in the sky like a gorgeous rainbow, which makes people feel relaxed and happy. It also shows the bustling scene of entering the city, which is the first scene to enter Changsha.

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