First, about the load standard.
For those bridges that are in good use and meet the automobile -20 standard, they can be directly used or widened in the process of rebuilding into first-class and second-class highways. Otherwise, it should be checked by investigating the geological conditions of the original bridge and using the original bridge design and completion data. In the absence of the above information, necessary dynamic and static load tests should be carried out to make the bridge reach the minimum automobile -20 standard through reinforcement. Generally, it can be directly used for automobile-13 load standard, automobile-15 load standard, reinforced concrete rectangular slab with a span of 8m or less, and automobile-15 arch bridge. The substructure, especially the gravity pier and abutment, has been compacted and stabilized after years of traffic and flood tests due to its heavy mass and load. The proportion of vehicle load increased by raising the standard to the total load is relatively small, and it can also be used directly.
Second, about flood control standards.
Due to the low grade of the original highway or the low flood control standard of the river itself, the flood control standard of some bridges is low. While improving the highway grade, the flood control standards of some bridges are relatively reduced, and even some bridges that meet the standard requirements can not meet the specification requirements. If the third-class highway is changed to the second-class highway, the design flood frequency of large and medium-sized bridges will be increased from1100, and that of small bridges and culverts will be increased from 1/25 to 1/50. When the second-class highway is changed to the first-class highway, the extra-large bridge will increase from1100 to 1/300, and the small bridge and culvert will increase from 1/50 to100. This is a universal problem that cannot be ignored. The solution is to raise the elevation of beam bottom to meet the flood control standards of large and medium-sized bridges with flood discharge channels. For arch bridges that cannot be directly raised, the upper part can be removed and the lower part can be raised. For the small bridges and culverts on the drainage ditch, consider lengthening to improve the standard. When the super-large bridge is changed from a second-class highway to a first-class highway, the length and height of the bridge should be determined according to the 300-year return standard for new bridges, and the original bridge can be raised and lengthened to meet the requirements.
Third, about the bridge type.
In the process of highway reconstruction, there are usually the following treatment methods:
(1) Pile pier
Generally, the width of reinforced concrete is still used, and the capping beam can be connected or not. If the cantilever of the old bridge is short and not widened much, the capping beam will not be connected. When the cantilever of the old bridge is long and wide, the cover beams should be connected together to improve the cooperative stress of the structure. When using friction piles, it should be noted that the settlement of the old bridge is basically completed due to long-term use, and the buried depth of the newly widened pile should be deepened to minimize its settlement value, so that there is no obvious settlement problem between the new and old piers and abutments.
(2) Gravity pier
It can be widened by gravity or pile structure. Gravity pier widening is generally used in the case of dense foundation and high bearing capacity. It is necessary to connect the new and old piers into a whole, and the foundation is buried deeply, such as Miao Zhuang Bridge on highway. In the river bed with scour, the foundation should be buried to a certain depth below the scour line according to the requirements of the specification. When the river is deep or the soil foundation is weak, the pile foundation structure can be adopted, and the capping beam and the old pier cap are integrated, which is convenient for construction, economical and applicable.
(3) Arch bridge
The arch structure is widened. First of all, we should pay attention to the widening of arch bridge: first, the abutment adopts ribbed slab pile-column composite abutment to reduce the abutment backward movement caused by structural thrust, especially the arch bridge built on bad soil foundation; Secondly, lightweight concrete should be used to replace the original lime-soil filler in the vault, which can not only avoid the bridge deck subsidence, but also meet the temperature deformation of the bridge.
Fourth, about the details.
(1) Telescopic device and bracket
Large and medium-sized bridges should change cement concrete pavement into continuous deck, and install expansion devices with large deformation capacity, but the joint length should be controlled at 80 ~ 100 m, and sliding bearings should not be installed. GQF-MZL80, GQF-C60 and other similar devices can be used as expansion devices. For small bridges, expansion joints can be cancelled, that is, 2cm thick wooden boards are set in the concrete pavement to separate the holes from each other, and asphalt concrete surface layers are continuously laid on them. Because most of the old bridges are simply supported, the deck is discontinuous, the rubber bearings are thin and the horizontal deformation capacity is limited. Therefore, in the process of bridge widening, the bearing should be replaced by rubber bearing, and its thickness is determined according to the recalculated longitudinal expansion and contraction of the structure.
(2) Cross slope of bridge deck
The cross slope of bridge deck should be consistent with the cross slope of pavement as far as possible. If there is no asphalt concrete pavement on the bridge deck, asphalt concrete pavement should be added, and its thickness can be consistent with the thickness of the upper layer of the pavement, generally not less than 3cm. Due to the widening, no matter whether the original bridge is a one-way slope or a two-way slope, the cross slope of the original bridge deck will always be adjusted with the change of the centerline of the route. When the bridge deck is changed from a two-way cross slope to a two-way slope, or from a one-way slope to a one-way slope, the adjustment amount is generally small and can be adjusted in asphalt concrete pavement. When the bridge deck changes from two-way failure to one-way slope, cement concrete can be used to adjust the transverse slope while maintaining the minimum thickness of the lower side. When the curved bridge changes due to superelevation, it needs to be adjusted greatly to meet the requirements of cross slope.
(3) Bridge guardrail
See Technical Specification for Design and Construction of Expressway Traffic Safety Facilities (GTJ074-94) for the types of guardrails. Generally, reinforced concrete wall type F or combined guardrail should be selected for bridges, and metal guardrail can also be selected according to the situation when it is located in urban areas. For a small bridge, it is enough to set a tire. When the original bridge is an anti-collision guardrail, the guardrail type on the widened side should be the same as that of the original bridge. When the second-class highway is changed to the first-class highway, the inner side of small and medium-sized bridges can be set as separate curb cover plates. Anti-collision barriers should still be installed inside bridges and extra-large bridges, and anti-glare facilities should also be considered when the bridge is located in a horizontal curve.
In the process of highway reconstruction, there are many problems to be considered in the utilization and treatment of old bridges, but one principle can be followed: leaving no future troubles and minimizing investment. On this basis, use the existing bridges as much as possible.