1 superelevation design of curved cross slope of highway.
On the road curve (mainly on the overpass ramp), trucks often roll over or goods roll over, or trucks rush out of the guardrail. The essential reason is that the actual driving speed on the curve is too different from the design speed of the road section, and the design cross slope of the curve is seriously mismatched with the actual speed, which is also the contradiction between theory (specification) and practice. The design of cross slope of highway curve is related to design speed, curve radius and vehicle condition (height of center of gravity, width of tires on both sides). According to the technical standard, the maximum design superelevation cross slope of highway is 10%.
In reality, we must face up to the following facts: ① Most of the accidents on corners are super-high trucks, and the phenomenon of car rollover is rare. (2) On the main road, no matter how fast the design speed of this section of the road is, the speed of most ultra-high-load trucks is generally controlled at 60 ~ 100 km/h, and it is likely to drive more slowly, even stopping at corners (0 ~ 50 km/h). (3) There are two kinds of ramp curves of overpass. One is that when there are many vehicles, the driving speed is generally 0 ~ 30 km/h, and the rollover accident of trucks or goods mostly occurs in the low-speed driving or parking state of this section.
The other is that when there are few vehicles, the trucks coming from the main road will not slow down, speeding on the ramp, and will rush out of the ramp guardrail and roll over. Suggestions on the handling of vehicle rollover accident in curve: ① The design of the maximum superelevation cross slope in highway curve should mainly consider the actual speed of the superelevation truck, the matching relationship between the maximum rollover carrying capacity of the truck and the maximum cross slope, and it is safer and more reasonable to compromise the cross slope according to the maximum speed and parking of the superelevation truck. The transverse slope should not be completely designed according to the design speed of the highway (for example, the transverse slope in the ramp curve with a design speed of 50km is designed as 30km/h, and the transverse slope in the main curve with a design speed of 120km is designed as 100km/h); (2) The curve super-high cross slope designed according to the compromised speed may be slightly uncomfortable for the car driving at the designed speed, but it can avoid or greatly reduce the rollover accident of the truck; (3) For individual vehicles entering the interchange ramp from the main road, speeding out of the guardrail and overturning, comprehensive measures such as extending the gradual transition section, setting speed limit signs and speed bumps can be taken to solve the problem.
2. Requirements for refined construction of curved bridges
According to common sense, a flat curve bend can't be made into a segmented polyline, and it should meet the requirements of flat curve smoothness. When the radius of horizontal curve bend is small (such as interchange ramp bridge), the flange plates on both sides of the upper precast beam or cast-in-place beam must also be constructed approximately according to the concave and convex requirements of the horizontal curve (it cannot be made into a straight line within one span), otherwise it is very dangerous to make a curve only by the appearance of the concrete crash barrier, and the crash barrier reinforcement at the flange position of the beam slab cannot be welded (bent at the outer edge) or cut off (at the inner edge).
At present, the construction unit can't do the vertical curve of the bridge well, mainly including deck pavement and anti-collision guardrail. The elevation of the bridge deck at the top of pier is accurately located on the vertical curve, while the precast or cast-in-place beam in the middle of span is generally a straight space line, which has a gradual error with the vertical curve. Therefore, in the vertical concave curve part, the top plate of precast beam or cast-in-place beam should be constructed by reverse arch; In the part of vertical convex curve, precast beam or cast-in-place beam should be constructed by arching method.
3. Whether to set concrete leveling layer on the bridge deck of box girder with hanging basket.
Stripping and cracking of asphalt concrete pavement on long bridge deck is a common engineering disease, and whether the design of concrete pavement on bridge deck of continuous box girder with hanging basket is one of the important reasons for this disease has been controversial. In fact, the continuous box girder constructed by hanging basket is divided into several segments, and the factors affecting the deflection and deformation of cantilever box girder structure are complex, so it is difficult to control the cantilever deflection of each segment according to the monitoring design value.
Therefore, it is difficult for the top surface of cantilever bridge to meet the design elevation and smoothness requirements, and there are often outcrops and embedded holes of steel bars that are not handled well, which brings a series of problems to the construction of paving asphalt concrete surface directly on the bridge deck: ① If the bridge deck waterproof layer is cold sprayed (such as fyt- 1), it is easy to be punctured by steel bar heads or uneven concrete bumps; In the case of ready-mixed hot asphalt macadam waterproof bonding layer, uneven concrete surface will cause poor rolling and bonding, which will affect or even destroy the quality of waterproof layer to varying degrees; (2) The thickness of asphalt concrete pavement is seriously uneven (common height difference above 10cm), and the concave part is not compacted, which will directly affect the compactness and smoothness of asphalt concrete.
In a word, it is found through on-site fact analysis that the asphalt concrete on the bridge deck and its waterproof layer will be more damaged if the deck concrete leveling layer is not set on the hanging basket suspension bridge. Therefore, it is suggested to reserve 6 ~ 8 cm thick reinforced concrete leveling layer on the top surface of box girder of segmental continuous bridge, and reserve steel dowel. As long as the top surface of box girder is washed and chiseled, fine stone concrete is used to reduce the slump of concrete, the strength grade of screed-coat concrete is reduced to C35 (which can meet the requirements of structural strength), and the surface layer is brushed, the quality of screed-coat concrete can be guaranteed. In a word, the quality control of concrete leveling layer is much easier than the deflection control of hanging basket box girder. Therefore, it is easier to ensure the quality of asphalt concrete pavement by designing concrete leveling layer on the hanging basket suspension bridge surface.
4 on the design and construction of bridge prestress
In the actual construction of bridge prestress, there are often great contradictions in the tension sequence and spatial layout at present. The names of main beam and secondary beam are not used in bridge structures like industrial and civil structures, but the structural stress mechanism of main beam and secondary beam also exists. For example, in the structural form of cast-in-place continuous box girder with no cover beam at the lower pier, relative to the longitudinal box girder web beam of the bridge span, the hidden beam at the top of pier support is the main beam, and the longitudinal web beam is the secondary beam; The transverse cantilever flange plate on the top surface of box girder is the secondary beam relative to the longitudinal span box girder web beam, and the longitudinal span web beam is the main beam.
Therefore, for the continuous box girder structure with obvious or implied concepts of main beam and secondary beam, the principle of determining the prestressed tensioning construction sequence of bridge structure should be symmetrical and balanced, and the main beam should follow the secondary beam, or the prestressed tensioning construction should be carried out at the position where the main beam and secondary beam cross each other by half. When there is a contradiction in spatial layout, it is a basic principle of steel bar construction that "the secondary reinforcement is replaced by the main reinforcement and the main reinforcement is replaced by the prestressed reinforcement". However, in actual operation, it is often necessary to cut steel bars, which will cause some troubles such as weak structure, large amount of rework by workers, disagreement among construction parties, supervisors and owners. Therefore, first of all, the design institute must consider the vertical and horizontal spatial layout of steel bars together when designing drawings. Where there are spatial conflicts, the design drawings are basically "digested" in advance, leaving only some minor problems to be solved on the spot. "Fine design" must be ahead of "fine construction". Only by doing a good job in fine design and fine construction can high-quality products appear.
5. Problems related to temporary structures, such as bearing supports and concrete formwork.
The collapse of concrete bearing support and concrete formwork structure is a big worry for almost all construction practitioners. Many accidents happen not because of people's ignorance, but mainly because the field personnel are not clear about the stress mechanism of these temporary structures, do not know the key details and how to check and accept them, that is to say, they are mainly caused by insufficient professional and technical level and blind operation. In view of the frequent accidents of temporary structures in construction, the design calculation methods and key details treatment measures of each process in construction should be more detailed and clear in construction specifications. At present, many construction bidding documents stipulate that steel plates with a certain thickness are used as surface templates, which is actually an incomplete understanding of formwork engineering.
The concrete formwork structure should be a complete structural stress system, which consists of the panel and its back, horizontal and vertical sides, and the final opposite pulling screw or external top bracing. The lateral pressure (or vertical gravity) of concrete is transmitted to the following structural members layer by layer through the panel, and the stress and deformation mechanism of each member are different. The stress and deformation of the panel are controlled by the comprehensive factors such as its own material and thickness and the distance between the back surfaces, not just the panel itself. In fact, as long as the template structure is carefully designed, the panel is replaced, the combination template with smooth bamboo veneer as the panel and steel (or square wood) back support is also very effective, and the steel plate with a certain thickness should not be rigidly specified in the bidding documents.
6. Comparative analysis of design load standard and actual situation of highway bridges
In the current technical standards for highway engineering, the vehicle load standard is 550kN for five-axle trucks, with the vehicle length 15m, the distance between vehicles in each lane before and after driving about 45m, and the axle weight of vehicles about100kn; Bridge calculation is a combination model of concentrated load (180 ~ 360 kN) and uniformly distributed load (10.5 kN/m) in the middle of the span. Three-axle vehicles are used in the bridge completion acceptance test, with a front and rear track distance of 4.8m and a total weight of 35 ~ 40t, with three vehicles in front and three vehicles in the back of each side-by-side lane. At present, the heavy vehicles driving on the highway are generally six-axle vehicles with a length of 15 ~ 18m, with a total weight of 85 ~ 150~200kN and an axle weight of 150~200kN. Moreover, many trucks in a team often follow behind, and the driving distance is often less than 20 m. When parked in the right lane, they are concentrated in front.
It can be seen that the bridge acceptance load exceeds the design load by about 1.5 times, and the actual heavy vehicle load on the bridge is overloaded (more than 2 times) compared with the design load, which is more common. If the distance between the front and rear vehicles is 45m according to the design, the overrun value of the actual total vehicle load on each bridge is not too large (about 2 times of the design load), but the focus is on the traffic jam caused by unexpected events such as faulty vehicles or car accidents. The distance between the front and rear vehicles is only 1 ~ 2 m, and the vehicles occupy the bridge deck. Even a 55t normal truck fleet is parked side by side in the right lane.
In view of the current reality of serious vehicle overload, some bridge design institutes have improved their design safety factors on the basis of standards and specifications. Although this forced "illegal design beyond the rules" has played a role in preventing the bridge structure designed by them from collapsing, it has also contributed to a series of bad social styles. In this way, vehicle manufacturers, car owners, traffic control departments and construction units will not strictly implement the standards, which will inevitably lead to a greater vicious circle and more and more bridge collapse accidents.
Therefore, from the point of view of engineering builders, suggestions are as follows: ① I hope that vehicle manufacturing enterprises, highway transportation management and other industries, together with highway engineering construction industry, will strictly abide by the technical standards formulated by the state and check at all levels to prevent overloaded and overloaded vehicles from driving on the road; (2) Highway technical standards should not only pay attention to the load standard of bridge longitudinal span, but also pay attention to or add some provisions on cantilever of bridge lateral flange, span of box, overall balance and stability in the lateral direction, and the influence on the lateral direction of structure after applying longitudinal prestress. ③ The most unfavorable combined load standard of highway vehicles, the load model for bridge design calculation and the load test of bridge completion should be basically consistent in terms of vehicle form and mechanical conversion (total vehicle load and maximum uniaxial load of single-span bridge), and the design standards of urban bridges and highway bridges should be unified; (4) According to the most unfavorable distributed load generated by the standard truck fleet (at least 5 trucks) and the full load, the balance and stability of the whole and the most unfavorable parts of the highway bridge are calculated and checked, and the computer software is combined with the manual calculation of the structural engineer.
How to deal with the contradiction between strictly observing existing norms and breaking through existing norms
Any existing technical standards are not all-inclusive, and they have limitations in history and coverage. On the one hand, it is necessary to timely supplement the emerging innovative scientific and technological achievements; On the other hand, after practice, it is necessary to make up or correct the defects of the existing norms. "Four major innovations" and new inventions emerge one after another, and there are no ready-made norms and standards to follow. Therefore, the contradiction that often appears in engineering practice is that all new things are not in the specification, and whatever is not in the specification, whether it is correct or not, is not allowed to be done, and it is necessary to bear "responsibility" if it is done.
The government should have a perfect legal mechanism for innovation risk, or establish a venture capital fund for innovation and scientific and technological achievements to bear economic risks, and stipulate which level of experts are effective for scheme design or review, which can be pre-implemented before the final results review, and what responsibilities the review experts, users and regulators have. Often, after inspection, it is found that some accidents basically conform to the specifications and management procedures, but in fact, even accidents have occurred, that is, the existing specifications do not cover some accidental factors (incomplete) or some unknown details (not in-depth). In this case, the conclusion of illegal operation by the construction unit is often used to "fool people".
The correct way to abide by the existing norms and break through the contradictions of the existing norms should be to respect the objective facts, apply scientific principles, scientific calculation and scientific and technological methods to solve practical problems on the basis of referring to the existing norms and standards (quality, safety and environmental protection indicators remain unchanged or improved), guide practice with theory, and constantly innovate, summarize and improve the theoretical level of national norms and standards in practice. 9 Conclusion 1) Only by fully implementing refined design, construction and management can we produce refined products and gradually improve and update the requirements of national codes and standards, which is the premise of strengthening refined thinking. 2) National standards and norms can only play an active role if they keep pace with the times, constantly update and improve, and conform to the laws of social practice.
The above discussion on highway engineering construction is the content of Zhong Da's consultation and arrangement.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd