What Is a Truss Bridge?

Truss bridge refers to the bridge with truss as the main load-bearing component of the superstructure. A truss bridge is generally composed of a main bridge, vertical and horizontal linkages up and down, a bridge gantry and an intermediate cross brace, and a deck system. In a truss, a chord is a member that forms the periphery of the truss, including an upper chord and a lower chord. The member connecting the upper and lower chords is called a web. According to the direction of the web, the chord is divided into a diagonal bar and a vertical bar. The plane where the chord and the web are located is called the main truss plane. The bridge height of the long-span bridge is changed along the span direction to form a curved truss; the middle and small spans adopt the same truss height, which is the so-called flat string truss or straight string truss.

With the continuous improvement of China's road infrastructure, the detection and evaluation of bridges in use will gradually become the focus of work of relevant departments. However, the relevant domestic and foreign codes have not provided a reliable method for evaluating steel bridges. Taking "Technical Specifications for Urban Bridge Maintenance" (CJJ 99-2003) as an example, Table D-2 of its Appendix stipulates that the degree of corrosion of steel members is evaluated by the rust scale based on the empirical scoring method, but it does not explain the rust scope. The impact of bearing capacity cannot meet the needs of structural bearing capacity and remaining life assessment. The methods for checking the bearing capacity of bridges and the methods for assessing the remaining service life in other related literatures also need to be improved. In view of this, this paper combined with Shanghai Waibaidu Bridge to study a detection and evaluation method suitable for aged steel truss bridges.

Waibaidu Bridge was built in 1907 and consists of a simple supported steel truss with a span of 52 m + 52 m. It belongs to the old steel truss bridge and is a cultural heritage protection building in Shanghai. In 2008, in order to cooperate with the construction of the Bund Channel, the bridge was moved to land for testing, evaluation and reinforcement. In order to avoid large-area replacement of components and to ensure that the bridge can be used for another 50 years, a scientific detection and evaluation method is used. This article focuses on the traffic load analysis, the assessment of the bearing capacity of rusted members, and the calculation of the remaining life of the structure using the fracture mechanics method.

The original design of the Waibaidu Bridge was a 2-lane tram, and it has undergone many transformations since then. Prior to this reinforcement, the bridge allowed 3 lanes of vehicle loads, but the actual load level was unknown. In order to ensure the safe use of the bridge in the next 50 years, its traffic load must be analyzed.

2.1 Survey of traffic load

The traffic load survey items mainly include: load weight, load quantity, and space loading position of the load. These three items are obtained by vehicle weight survey, traffic volume survey, and headway spacing and headway detection. Before the implementation of the project, the traffic of the Baidu Bridge had been interrupted, and the vehicle weight could not be measured using the BWIM method. Therefore, the statistical data of nearby bridges were used as the vehicle weight information of the bridge.

The traffic volume data and the headway spacing information are obtained by the management unit's vehicle induction coils embedded in the bridgehead of Waibaidu Bridge. The detector classifies vehicles into five types of cars, passenger cars, small trucks, large trucks, and trailers based on the electromagnetic induction of the vehicle chassis, and records the traffic flow of various types of vehicles in each lane in units of 0.5 h. According to the relevant records of previous years.

2.2 Analysis of traffic load

The Monte-Carlo method is used to establish a traffic load model and analyze the effects of traffic loads.The steps are as follows:

(1) Generate a random number sequence of vehicle type, vehicle weight, and head distance according to uniform distribution, empirical distribution, and log-normal distribution. A 3-lane convoy is formed by combining lane driving directions. The simulated fleet was stored in units of days, and a total of 60 days of data was simulated.

(2) The simulated fleet is moved and loaded on the bending moment influence line and axial force influence line of the component to be analyzed, and the corresponding bending moment history and axial force history can be obtained. It is assumed that the superposition according to the flat section is the stress history.

(3) The maximum stress in the stress history on day i is recorded as i, max. Assuming that i, max obey the normal distribution, the probability distribution parameters are determined based on the 60 maximum stress samples. According to the "Uniform Standard for Reliability Design of Highway Engineering Structures" (GBT 50283-1999), the safety of the bridge structure is Grade II, and the reliability index is 4.2. Based on this, the maximum stress of the structure is calculated and used as the traffic load when checking the bearing capacity of the component.

(4) The rain flow method was used to count the stress history of 60 d to obtain the fatigue stress spectrum of the component, which was used to evaluate the remaining life of the tensile member. After analysis, it is determined that the most unfavorable component of the bridge is the mid-span inclined web. The traffic load corresponding to its 4.2 reliability index is 45.17M Pa and the maximum fatigue stress amplitude is 47 MPa.

The main disease that affects the bearing capacity of aging steel bridge members is rust. For this reason, in the test, a detailed rust test was performed on all members with visual cross-section loss. The inspection found that due to the difference in structure and location, the corrosion of auxiliary members such as steel plates showed uniform corrosion on the full section, and the corrosion of the main members was mostly localized pits or cross-section corrosion. As an evaluation index of the degree of rust.

According to the research results of relevant scholars, corrosion has little effect on the stable bearing capacity of steel structures, so only the effect of corrosion resistance on tensile strength is studied in this project. In order to determine the influence of the degree of corrosion on the tensile bearing capacity of the members, the members with different degrees of corrosion were cut from the bridge (samples were sampled from the severely damaged components that must be replaced) and standard tensile test pieces were made to test the mechanical properties. The triangle represents the measured data. It can be seen that as the degree of corrosion increases, the bearing capacity of steel decreases approximately linearly.

There are two main methods for assessing the fatigue life of steel bridges: traditional fatigue life analysis methods based on the Weller curve and Miner's linear product quotient theory, and damage tolerance calculation methods based on fracture mechanics theory. Compared with the former method, the latter method has two major advantages: it is not necessary to consider the loading history of the structure; the relevant material parameters in the calculation formula can be obtained through the standard CT test piece, instead of the costly full-scale test Pieces. Therefore, in this study, the fracture mechanics method is mainly used to study the remaining service life of the structure. The initial crack length and critical crack length of the member are different; K is the stress intensity factor amplitude, which can be calculated according to the stress intensity factor manual of the fatigue stress amplitude.

The analysis steps of the damage tolerance method are as follows:

(1) A fracture mechanics test is performed to determine the curve of crack growth rate-stress intensity factor amplitude, and C and m are determined according to the curve.

(2) According to the simplified method of fracture mechanics model introduced, the components with rivet hole edge defects are simplified into bilateral cracked members or central cracked members.

(3) Use ultrasonic non-destructive testing to inspect the tensile member to determine the initial crack (defect) length a0. If no defect is detected, it is assumed that there are cracks corresponding to the 90% detection probability at the edge of the rivet hole.

(4) Calculate the maximum tensile stress that may occur during the service life of the component. KIC is the fracture toughness, which is the material constant; max is the maximum tensile stress; Y (acr) is the shape function related to the crack length.

(5) Load the fatigue stress spectrum into equation (2), and use the iterative method to calculate the remaining service life N. Among them, step (3) is the difficulty of the implementation of this solution. The Chinese standard specifies the ultrasonic flaw detection test result as the reflected sound intensity value of a defect equivalent to a 3 mm round hole. After the test, the test result must be converted into the corresponding crack length. The corresponding relationship can be calculated according to the method provided in [5]: a = + ln (P) + (4) where a is the crack length; and are instrument-related parameters; is the error , Is a normal function with a mean value of 0; P is the detection result.

In order to determine the parameters and , the rivet-containing members removed from the solid bridge were subjected to fatigue loading. After the initial crack was obtained, the ultrasonic flaw detection was used for repeated inspection, and then the X-ray method was used to determine the crack length. Calculated = 0.218 6, = 0.340 9, and the variance of is 0.408 5.

Material parameters of fracture mechanics are measured through fracture mechanics tests. Standard CT test specimens are selected from the severely damaged components on the solid bridge. The fatigue fatigue propagation test method for metal materials under GB / T 6398-2000 is performed under constant amplitude fatigue loads. In the crack propagation test, the crack propagation amount and stress intensity factor amplitude were recorded simultaneously, and the results of this test were compared with the results of relevant foreign tests. The comparison results show that the foreign test data is conservative, and if the parameters recommended abroad are directly copied, it may lead to adverse results. The test data was fitted with a 90% guarantee rate, and C = 1.28 × 10 and m = 3 were taken as safe. After calculation, it was found that under the current load, the remaining life of 8 members of the Waibaidu Bridge could not meet the requirements for continued use, and it must be reinforced.

In this paper, a method suitable for the assessment of the bearing capacity and remaining life of an aged bolt-riveted steel truss bridge is introduced in conjunction with Waibaidu Bridge. Compared with existing maintenance codes and assessment methods, this method determines the effect of corrosion on the structural bearing capacity through experiments, and infers the remaining bearing capacity of the component based on the quantified corrosion degree test results; the actual bridge traffic load is used for the bearing capacity check instead of The design load is used to understand the actual safety level of the structure. The relevant parameters are obtained by testing and ultrasonic detection methods before the remaining life of the structure is evaluated, which improves the calculation accuracy of the fatigue life of the structure. According to the conclusions of this method research, Waibaidu Bridge completed the replacement and reinforcement of the members and gusset plates in December 2008, and was put back into use in early 2009. ^[1]