Inspection and Evaluation Methods for Aged Truss Bridges

As a steel structure engineer at ZHENJIANG GREAT WALL HEAVY INDUSTRY TECHNOLOGY CO.LTD., a professional integrated industrial and trade enterprise specializing in modular steel bridges and bailey bridges for global infrastructure, we have extensive experience in the inspection and evaluation of aged truss bridges-critical structures widely used in South America, Africa, Southeast Asia and other regions for their large-span and load-bearing advantages. Aged truss bridges face structural degradation due to long-term service, harsh environmental erosion (e.g., Africa's high temperature, salt spray, Southeast Asia's high humidity) and repeated load action, making scientific inspection and evaluation the core of ensuring their safe operation, guiding maintenance and reinforcement, or determining replacement schemes. Below is a systematic summary of the inspection content, core evaluation methods and key technical points for aged truss bridges, combining international standards (AASHTO, Eurocode EN 1993) and on-site engineering practice.

I. Pre-Inspection Preparation: Basic Information Collation & On-Site Preliminary Survey

Preparatory work is the foundation of accurate inspection and evaluation, which avoids blind detection and ensures the comprehensiveness of subsequent work.

Basic data collection

Collect the original design documents (structural drawings, material performance parameters, load design criteria), construction records (welding process, anti-corrosion treatment), operation data (traffic volume, load type, maintenance records) and historical accident records of the truss bridge. For aged bridges with missing original data (a common situation in developing countries), supplement the basic structural parameters through on-site mapping and material sampling testing.

On-site preliminary survey

Conduct a visual walkthrough of the bridge site to confirm the overall structural form (through truss, half-through truss), support system (pier, abutment, bearing), and the surrounding environmental conditions (e.g., coastal salt spray, industrial corrosive gas, river scouring of piers). Mark the obvious structural defects (e.g., paint peeling, steel plate rusting, bolt loosening) and record the traffic operation status to formulate a targeted inspection plan (including inspection scope, methods and safety protection measures).

Material baseline confirmation

For the main steel components of the truss bridge (chord, web member, connection plate), sample test the material mechanical properties (yield strength, tensile strength) and chemical composition through non-destructive sampling, to confirm whether the material has aging, embrittlement or performance degradation compared with the original design. This is especially important for aged truss bridges without clear material records.

II. Core Inspection Content and Methods for Aged Truss Bridges

Inspection of aged truss bridges focuses on component-level defects, connection reliability and overall structural deformation, combining visual inspection (VI), non-destructive testing (NDT) and on-site physical testing-the three core methods, and prioritizing non-destructive testing to avoid secondary damage to the already degraded structure.

1. Inspection of Main Truss Components (Chord, Web Member, Diagonal Member)

The main truss components bear the primary axial force (compression/tension) and bending moment of the bridge; their damage directly affects the overall structural safety, and the key inspection defects include corrosion, fatigue cracks, section loss and local buckling.

Corrosion inspection: ① Visual inspection to record the corrosion location (especially the lower chord, web member near the bearing and components in the water spray zone), corrosion grade (mild, moderate, severe) and paint peeling range; ② Use a ultrasonic thickness gauge to measure the actual thickness of the steel plate at the corroded part, calculate the section loss rate (the ratio of lost thickness to original thickness)-a key index for evaluating corrosion damage (section loss rate >20% is generally considered a serious defect requiring reinforcement); ③ For coastal or industrial corrosive environments, detect the corrosion depth of the steel surface with a corrosion depth gauge to evaluate the corrosion development trend.

Fatigue crack inspection: Fatigue cracks are the most dangerous defect of aged truss bridges, caused by repeated traffic load and structural vibration, and are prone to appear at steel plate fillets, weld joints and section transition parts. Adopt ultrasonic testing (UT) and magnetic particle testing (MT) as the main methods: MT is suitable for detecting surface and near-surface cracks of ferromagnetic steel components (high detection efficiency for truss bridge joints); UT is suitable for detecting internal cracks of thick steel plates and welds, and can accurately measure the length and depth of cracks. For critical force-bearing parts, phased array ultrasonic testing (PAUT) can be used to improve the detection accuracy of small cracks.

Local buckling inspection: Check whether the chord and web member have local buckling (bulging, deformation) due to excessive compression or section loss, use a straightness meter and profile gauge to measure the deformation value, and judge whether it exceeds the limit specified in international standards (e.g., Eurocode EN 1993 specifies that the local buckling deformation of steel truss components should not exceed 1/1000 of the component length).

2. Inspection of Connection Nodes: The Most Critical Weak Link of Truss Bridges

Truss bridge components are connected by welds, high-strength bolts, rivets (riveted connections are common in early aged bridges), and the connection nodes are the stress concentration zone, with defect failure risk far higher than the main components.

Weld connection inspection

Focus on detecting weld cracks, incomplete penetration, slag inclusion and weld undercut. Use MT (surface cracks) and UT (internal defects) for comprehensive testing; for important load-bearing welds (e.g., chord-web member connection welds), conduct radiographic testing (RT) if necessary to confirm the internal quality of the welds. For welds with serious corrosion or paint covering, remove the surface coating first to ensure detection accuracy.

Bolt/rivet connection inspection

① Loosening detection: Use a torque wrench to check the pre-tightening torque of high-strength bolts, and judge the loosening degree (the pre-tightening torque loss rate >20% needs to be re-tightened); for riveted connections, check whether there is rivet shearing, loosening or base metal cracking around the rivet hole by visual inspection and tap test (metal hammer tapping to judge the tightness by sound). ② Corrosion and shearing inspection: Check the corrosion of bolt/rivet rods and connection plates, measure the section loss of bolt rods with a caliper; check whether there is shearing deformation at the bolt/rivet connection, and whether the rivet hole has oval deformation due to long-term load. ③ Missing fasteners: Record the number and location of missing bolts/rivets, especially for the nodes of the main force-bearing truss.

3. Inspection of Support System and Substructure

The support system (bridge bearing) and substructure (pier, abutment, foundation) are the basis for transferring the truss bridge load to the ground; their damage will cause uneven stress of the upper truss structure and induce secondary defects.

Bridge bearing inspection: Check whether there is corrosion, deformation, displacement or failure of the bearing (e.g., rubber bearing aging, steel bearing pin wear), measure the bearing displacement with a total station, and confirm whether the load transfer is normal.

Pier and abutment inspection: Check the concrete spalling, steel bar exposure and corrosion of reinforced concrete piers/abutments; for steel piers, conduct the same corrosion and crack inspection as the truss components. Detect the vertical and horizontal displacement of piers/abutments with a total station, and check whether there is settlement or inclination.

Foundation inspection: Focus on checking whether the foundation is scoured by rivers (for water-crossing truss bridges) or has uneven settlement; use a ground-penetrating radar to detect the integrity of the foundation soil, and combine the settlement observation data to evaluate the foundation stability.

4. Overall Structural Deformation and Displacement Detection

Overall structural deformation reflects the service performance of the aged truss bridge; excessive deformation will lead to uneven load distribution and even structural instability.

Deflection detection: Use a level gauge or total station to measure the mid-span deflection and segmental deflection of the truss bridge under dead load and live load (e.g., standard test vehicle load), and compare it with the original design deflection and the limit value specified in international standards (AASHTO LRFD specifies that the long-term deflection of steel truss bridges should not exceed 1/800 of the span).

Truss plane out-of-plumb detection: Measure the out-of-plumb value of the truss frame with a plumb line and a laser theodolite-excessive out-of-plumb (generally >1/500 of the truss height) will reduce the lateral stability of the truss bridge and increase the risk of lateral buckling.

Span and alignment detection: Use a total station to measure the actual span and horizontal/vertical alignment of the bridge, check whether there is overall displacement or alignment deviation, and analyze the impact on structural stress.

5. Anti-Corrosion System Inspection

For aged truss bridges in harsh environments (e.g., Africa's coastal salt spray, South America's tropical rainforest), the failure of the anti-corrosion system is the main cause of component corrosion. Check the integrity of the anti-corrosion coating (paint, galvanizing) : record the peeling, bubbling, chalking range of the coating; test the adhesion of the coating with a cross-hatch adhesion tester (the adhesion grade should not be lower than Grade 2 according to ISO standards); for hot-dip galvanized components, check whether there is zinc layer peeling, pitting corrosion and calculate the zinc layer thickness with a zinc layer thickness gauge.

III. Comprehensive Evaluation Methods for Aged Truss Bridges

Inspection only obtains defect data of the bridge; comprehensive evaluation is to analyze the impact of defects on the structural performance, evaluate the current safety level and residual service life of the aged truss bridge, and provide a basis for subsequent maintenance, reinforcement or replacement. The core evaluation methods include qualitative evaluation, quantitative performance evaluation and structural finite element simulation analysis, combined with international bridge evaluation standards (AASHTO Bridge Inspection Manual, Eurocode EN 1993-1-10).

1. Qualitative Evaluation: Defect Level Classification and Risk Ranking

Based on the inspection results, classify the defects of each component and connection node into four levels (refer to AASHTO standards) to conduct a preliminary qualitative risk evaluation:

Level 1 (Minor defect): Slight corrosion, paint peeling, minor bolt loosening without affecting structural stress and safety; only regular maintenance is required.

Level 2 (Moderate defect): Local section loss <10%, small surface cracks (length <5mm), partial bolt pre-tightening torque loss; need to conduct regular tracking inspection and minor repair.

Level 3 (Serious defect): Section loss 10%-20%, obvious fatigue cracks (length 5-20mm), multiple bolt loosening/missing, local buckling of components; need to stop partial heavy load traffic and conduct urgent reinforcement.

Level 4 (Critical defect): Section loss >20%, large internal cracks (length >20mm), weld fracture, bolt shearing, serious overall deformation; need to immediately close the bridge for reinforcement or replace the components.

On the basis of defect level classification, rank the risk of each part according to its force-bearing importance (e.g., main chord > web member > secondary support member), and determine the priority of maintenance and reinforcement.

2. Quantitative Performance Evaluation: Structural Bearing Capacity Calculation

Quantitative evaluation is the core of truss bridge performance assessment, which calculates the actual bearing capacity of the aged truss bridge by considering the influence of various defects (corrosion section loss, fatigue cracks, connection reliability) and compares it with the design bearing capacity to obtain the bearing capacity ratio (actual capacity/design capacity)-the key index for evaluating the structural safety.

Component-level bearing capacity calculation: For each truss component, correct the original design bearing capacity according to the actual section loss rate (corrosion), crack reduction coefficient (fatigue cracks) and material performance degradation; for compression components, consider the impact of local buckling on the stability bearing capacity.

Node connection bearing capacity calculation: For weld connections, reduce the bearing capacity according to the crack length and weld defect grade; for bolt/rivet connections, calculate the shearing and tension bearing capacity according to the actual number of effective fasteners and section loss of bolt rods.

Overall structural bearing capacity check: According to the component and node bearing capacity calculation results, check the overall truss bridge under the standard load combination (dead load + live load + wind load + temperature load) specified in AASHTO/Eurocode standards, to confirm whether the overall structure meets the safety requirements.

3. Finite Element Simulation Analysis: Accurate Evaluation of Structural Stress and Deformation

For aged truss bridges with complex defects or large-span structures, finite element (FE) simulation analysis (using software such as ANSYS, MIDAS/Civil) is adopted to establish a three-dimensional finite element model of the truss bridge, and input the actual structural parameters (defect size, material performance, support condition) obtained from inspection to simulate the actual stress and deformation state of the bridge under various load conditions.

Simulate the stress concentration at the crack and corroded parts, and predict the crack propagation trend and component failure risk.

Analyze the influence of uneven settlement of piers, bearing failure and truss out-of-plumb on the overall structural stress distribution.

Conduct load test simulation (e.g., over-limit load, extreme wind load) to evaluate the structural safety reserve and anti-risk ability.

The finite element simulation results can provide a quantitative basis for the optimization of maintenance and reinforcement schemes (e.g., determining the reinforcement position, reinforcement material and cross-section size).

4. Residual Service Life Prediction

On the basis of the above evaluation methods, combine the environmental erosion rate (e.g., corrosion rate in salt spray environment), fatigue load action frequency (traffic volume) and structural defect development trend to predict the residual service life of the aged truss bridge:

For bridges with minor defects and good anti-corrosion performance, the residual service life can be predicted by the fatigue life calculation formula (Miner's linear cumulative damage rule) specified in Eurocode EN 1993-1-9.

For bridges with serious corrosion or fatigue cracks, the residual service life is determined by the defect development rate, and a clear maintenance and reinforcement cycle is proposed; if the reinforcement cost exceeds 60% of the new bridge construction cost, it is recommended to replace the truss bridge with a new modular steel bridge/bailey bridge (our company's core product, with the advantages of rapid installation, standard compliance and green recyclability).

IV. Key Considerations for Inspection and Evaluation in Harsh Global Environments

For aged truss bridges in South America, Africa, Southeast Asia and other regions where our company's products are mainly exported, the inspection and evaluation need to fully consider the influence of local harsh environmental conditions and actual operation characteristics:

Adapt to extreme environmental erosion: For African coastal salt spray areas and Southeast Asian high-humidity rainforests, increase the detection frequency of steel corrosion and anti-corrosion system failure; for South American high-temperature and strong UV areas, focus on checking the aging of the coating and the thermal expansion deformation of the truss structure.

Combine local traffic characteristics: For regions with high proportion of heavy-duty truck traffic (e.g., African industrial zones, South American mining areas), focus on the fatigue damage detection of truss components and connection nodes, and appropriately improve the safety factor of bearing capacity evaluation.

Simplify the operation of on-site inspection: For remote areas with insufficient testing equipment (e.g., rural areas in Africa), prioritize the combination of visual inspection and portable non-destructive testing (ultrasonic thickness gauge, magnetic particle flaw detector) to ensure the feasibility of on-site inspection.

Align with local bridge standards: When conducting evaluation, combine the local bridge design and inspection standards (e.g., Australian AS 5100 for Papua New Guinea, South African SABS standards) on the basis of international standards (AASHTO, Eurocode) to ensure the evaluation results are recognized by local engineering departments.

The inspection and evaluation of aged truss bridges is a systematic work integrating on-site detection, data analysis and structural performance evaluation, which is the key to ensuring the safe operation of this type of large-span steel structure bridge. Scientific inspection methods (visual inspection + non-destructive testing + physical testing) and comprehensive evaluation systems (qualitative + quantitative + finite element simulation) can accurately identify structural defects, evaluate the safety level and residual service life of the bridge, and provide a reliable basis for formulating targeted maintenance, reinforcement or replacement schemes.

As a professional steel bridge enterprise with a manufacturing base in Zhenjiang, Jiangsu and an R&D team in Shanghai, ZHENJIANG GREAT WALL HEAVY INDUSTRY TECHNOLOGY CO.LTD. not only provides high-quality modular steel bridges, bailey bridges and steel box girder bridges complying with AASHTO, Eurocode and Australian AS 5100 standards for global infrastructure construction, but also provides one-stop technical services including on-site inspection, structural evaluation and reinforcement scheme design for aged truss bridges in South America, Africa, Southeast Asia and other regions. Combining our rich experience in steel structure design and on-site engineering practice, we help global clients solve the safety problems of aged truss bridges, and provide efficient, economical and sustainable steel bridge solutions for infrastructure upgrading and reconstruction.

Key International Standards for Truss Bridge Inspection and Evaluation

AASHTO Bridge Inspection Manual (BIM)

Eurocode EN 1993-1-1: Design of steel structures - General rules

Eurocode EN 1993-1-9: Design of steel structures - Fatigue

Australian Standard AS 5100: Bridge design

ISO 12944: Paints and varnishes - Corrosion protection of steel structures by protective paint systems