Xiang Wei

(Sichuan College of Architectural Technology,Deyang 618000)

Abstract:With the continuous promotion of urbanization,more and more cities choose to set up highway bridges,which are followed by frequent overturning events of urban highway bridges,making the calculation of overturning resistance checking and reinforcement modification one of the important study project in the bridge construction field.Meanwhile,single-column bridges have become one of the major operation bridges in modern traffic engineering due to their good practicability and aesthetic appeal.However,due to some factors in design,construction and operation of such bridges,as well as the increase of vehicle overloading in recent years,bridge collapses occasionally occur.Hence,in order to prevent the re-occurrence of similar events,taking single column pier bridge as the study object,this article aims to investigate the overturning resistant stability of urban highway bridge,and propose a reinforcement scheme to provide the basis for the reinforcement of urban highway bridges.

Key words:Urban highway bridges;Overturning resistance checking calculation;Reinforcement and reconstruction;Design scheme.

1 Introduction

With the increasing urban population,the quantity of urban motor vehicles is also on the rise,leading to urban urban traffic congestion.In order to address the problem and improve the traffic situations,the local traffic management departments have constructed many urban viaducts and flyovers to alleviate the traffic pressure[1].Single column pier bridges are favored by urban highway bridge builders due to their lightweight and minimum impact on the traffic underneath the bridge.Single column pier bridge is characterized by single column supported continuous box girder bridge,which is well-suited for the construction of the curved bridges[2].Therefore,this bridge construction mode has been used during the past construction of expressway ramp and urban highway bridges.

However,with the wide application and construction of single column pier bridges,the problems have gradually exposed,mainly including overall lateral overturn of bridges,bridge overturning,pier cracking,and load bearing detachment[3].Among these problems,the overall overturning of bridges has attracted the attention of most researchers.In recent years,the overall lateral overturning of single column pier bridge in various places has caused significant economic loss and adverse social response.The typical events include the accidents of Minzu East Road Flyover in Baotou City of Inner Mongolia and Ramp of Tianjin-Taiyuan Expressway.The latter has resulted in the falling of five cargo carriers,six deaths and four injuries.Though the above accidents are somewhat related to overloading,the occasional bridge accidents have caused the designers to make reflection.In the past construction of highway bridges in China,emphasis was only laid on the bending resistance and shear bearing capacity of the bridges,ignoring the bridge stress calculation and the calculation of the horizontal overturning resistant stability of the bridge structure[4].Under this background,the study on the overturning resistance of single column pier bridge has been carried out in various places,and the bridges in need of maintenance and strengthening were reinforced and repaired.

Since single column pier bridges are favored by many bridge designers by virtue of their small footprint and lightweight and the ability to realize harmonious development of architectural aesthetics and environment aesthetics.As a result,in recent years,this bridge has been widely used.However,because of the lack of complete theoretical analysis on single column pier bridge during the initial construction,which made the design of bridge structure unsatisfactory,and the vehicle overloading occurs from time to time,thus causing frequent overturning of the superstructure of single column pier bridges in China.Therefore,the overturning resistance strengthening scheme of single column pier bridge has been emphasized by the related departments[5].In this context,this article attempts to study and analyze the overturning resistant stability and reinforcement scheme of single column pier bridge on urban highways,and expound theoretical basis for the construction and design of single column pier bridges,with the aim to provide the scientific guiding scheme for overturning resistance checking calculation and reinforcement scheme of single column pier bridges,thus providing the theoretical reference for improving the construction quality and extending the service life of the pier column bridges[6].

2 Build the overturning resistance checking calculation model for urban highway bridge

2.1 Rules for the model building

To study the overturning resistant stability,this article has chosen the most typical type of urban road bridges for modeling and analysis,established the overturning resistance checking calculation model by selecting different bridge horizontal curves and radiuses of curvature,as well as different single-bearing pre-eccentricity treatments and the distances between different double bearings.In this model,the determination of the study parameters follows the relevant provisions in“Code for Design of Highway Reinforced Concrete and Pre-stressed Concrete Bridges and Culverts”(JTG D62-2004) and“General Code for the Design of Highway Bridges and Culverts”(JTG D60-2004)[7].According to the relevant provisions of the above two documents,the vehicle centrifugal force,nonuniform settlement of the bearing,constant load,and temperature load are selected as the most unfavorable vehicle load conditions for the load combination,thus the overall overturning resistant stability is verified according to the specification requirements[8].In addition,the bridge structure finite element calculation program Midas Civil 2012 is used to calculate the finite element model of the constructed bridge.For purpose of the checking calculation of overturning resistance,the safety of the engineering bridge under the most unfavorable conditions was verified,and the detailed results are shown in Table 1 below.

As can be seen from Table 1,in the test of the bridge section bearing capacity,the resistance safety coefficient for the control section of main bridge is greater than 1.0,indicating that the monitored bridge can meet the load-bearing capacity requirements under the combination effect.In addition,for the urban road bridge under the combination effect,the maximum vertical external forces on the top of the bridge piers are shown in Table 2.

Table 1 Checking calculation of the bridge section bearing capacity

Table 2 Vertical external forces on the top of the piers

From Table 2,it can be seen that the external force on the top of pier 1 is the largest.From the field survey,it can be found that the diameter of pier 1 is 1.4cm,and the main construction material is concrete,coefficient of structure importance is 1.0,and axial compression stability coefficient is 1.0 as well.A conclusion can be drawn that the materials used in construction can determine the bearing capacity of the pier.

2.2 Basic hypotheses

To analyze the overturning resistant stability of the urban highway bridge,the following hypotheses are made in this thesis:

Hypothesis 1:urban highway pier has sufficient load-bearing capacity to prevent the main bridge and pier from damage before the structural overturn;

Hypothesis 2:main bridge and pier in urban highway have sufficient strength to prevent the main bridge and pier from the unbalanced destruction before the structural overturn;

Hypothesis 3:the bridge deformation factors are not considered when calculating the overturning resistance of urban highway bridges;

Hypothesis 4:single stress is applied to the bearing of urban highway bridge in the vertical direction;

Hypothesis 5:the bearing is used as the supporting point in the analysis of the overturning resistant stability of urban road bridges.

2.3 Checking calculation of overturning resistance of urban highway bridge

In this article,the overturning resistance of urban highway bridges was verified under the the horizontal curves and radiuses of curvature of single column pier single-bearing eccentric distance and double-bearing bridge.And the minimum supporting reaction force of the bearing has been calculated with 2.8m,3.1m,3.4m,3.7m as the spacing of double-bearing bridge,so as to calculate the overturning resistant stability of urban highway bridges,and the detailed load conditions are shown in Table 3.

From Table 3,it can be seen that the urban road bridges will suffer bearing disengagement and overturning destruction of the superstructure in severe overload condition regardless of the bearing spacing.Therefore,for the urban highway bridge,it is required to enlarge the spacing of two bearings at the initial stage of designing and reduce the disengagement of the bridge bearings.

Table 3 Overturning resistance checking calculation of urban highway bridge (unit:KN)

3 Design scheme for the strengthening of urban highway bridge

3.1 Bridge strengthening rules

After combing a large amount of the research literature onbridge strengthening,it was found that,in overseas developed countries,the emphasis of bridge construction has gradually shifted from new construction to the maintenance and strengthening of the existing bridges,and many disciplinary institutions have been established to be responsible for the bridge maintenance and strengthening,as well as the inspection,and check,verification,maintenance and strengthening of the road bridge by testing the reliability of the urban road bridges[9].Besides,in China,urban road bridge strengthening is realized by the close combination of the industry,university and research institutions,with fruitful results and a complete bridge strengthening evaluation system,including“Technical guide for strengthening highway concrete bridge”,and“Complete-set technology and engineering example for strengthening old bridges on highways”,to guide and standardize the bridge strengthening.According to the relevant provisions in the above study results,this article has summarized the following principles for bridge strengthening of urban highway:first,during the urban bridge strengthening,it is recommended to take the construction drawing of the original bridge and the related assessment report by the inspection institution as the main basis,and on this basis,formulate the new strengthening scheme;second,it is required to carry out the strengthening according to the design load at the time of bridge construction and materials used in the construction years;third,calculating the strength,rigidity and stability of the buildings during the construction stages to ensure safety;fourth,recovering the service life of the overall bridge structure to the service life of the original design bridge;fifth,reducing the pedestrian and vehicle interference and adopt environmental protection measures.

3.2 Strengthening scheme

To ensure a better study of the strengthening of urban bridges,the strengthening scheme used in this article only involves the strengthening of the lower part of the bridge,and the main schemes include the following types:first,the strengthening method of adding capping beams.This method generally adopts steel capping beam or pre-stressed concrete capping beam with different steel capping means according to the original bridge cross-sectional shape[10].When the original piers of the bridge are rectangular,bolted connections are required;when the original piers of the bridge are round,the steel capping method with hooped connections can be used.Steel capping bridge strengthening method can reduce the additional constant load of the original bridge structure,which presents the characteristics of low cost and short cycle time,with little impact on the traffic on the bridge deck.Second,strengthening method of adding the pier columns.Through this method,vertical support can be increased on the basis of the bridge-span structure of the original pier column bridge,reducing the torque design value of the bridge,avoiding the possibility of negative reaction and bearing disengagement of the bridge,improving the overturning resistant stability of the bridge structure,and ensuring the operation safety of the bridge.

3.3 Strengthening steps

To verify the practical results that could be obtained with the above strengthening scheme,the demonstration with the actual example has been made.The steps for the strengthening method of adding the steel capping beams is shown below,first,measurement marks were made at the locations of the added steel capping beams,and the bearing backing slabs were added at the bottom of bridge girder.In addition,the distance among the reinforcement holes is determined by using roughening machinery on the original bridge at the connection with the steel cap,and then the drilling hole positions are determined.The adhesive was injected into the reinforcement holes.When the adhesive reaches 100%,the adhesive quality is checked and the steel capping beams are welded together,and structural adhesive is applied to the joints to ensure the quality of bridge after strengthening.Afterwards,the surface of the steel capping beam is dusted and coated to complete the bridge strengthening.After the steel capping beam is added to the pier,the bridge will not show any negative reaction and the bearing disengagement under the normal load,thus effectively improving the overall overturning resistance of the bridge.

Similarly,the steps for the strengthening method of adding the bridge pier column are shown below:first,measure and place the line on the bridge position with the construction to be added,and use the bolts to fix the beam bottom bearing backing slab after corrosion-proof treatment on the bridge girder bottom.Second,assemble the steel pipe structure.In the first step,the steel pipe needs to be de-rusted and reinforced with temporary reinforcement measures.In the second step,pump grouting is carried out from low too high to guarantee uninterrupted operation,thus completing the concrete casting of the steel pipe.In the third step,a steel channel needs to be set between the new and original columns and the thickness of the bearing backing slab is determined according to the actual height of the new steel pipe concrete column.In the fourth step,the original main bridge was jacked up and placed in the bearing position with proper adjustment to make it consistent with the bridge cross section.The bearing backing slab of main bridge was welded with steel pipe concrete to ensure its stability.In the fifth step,steel pipe surface was dusted,and the bridge pier columns were added for strengthening.From the practical operation of the bridge,it has been found that,under any load force,the bridge with additional piers will not experience negative reaction or the bearing disengagement,thus the overturning resistant stability of the bridge can be effectively enhanced.

4 Brief conclusion

In summary,with the increase of the urban highway bridges,the overturning resistance verification and strengthening scheme design of urban highway bridges have become the hot-spot in the architectural field.Thus,the current research model of the overturning resistance of highway bridges is analyzed,and the strengthening methods of adding the steel capping beam and adding single-column piers are proposed according to the study results,and detailed description of the steps of these two methods are illustrated.In the practical operation of the bridges,it is found that these two strengthening schemes can greatly boost the overturning resistance of the bridge.