Bridge Load Testing
Bridge load test is a popular means of demonstrating a bridge’s capability to carry safely the design loads. Typical situations that arise include:
Uncertainties associated with current as-built conditions
Suspect performance such as excessive creep deflection
Structural strength unknown and analysis.
Impractical change of use involving an increase in loading
Bomb or fire damage
Materials defect or structural deterioration
Handing over criteria for newly constructed bridge
Quality Assurance Method used after completing the repair and strengthening works (used to verify the loading capacity of the structure and confirm its structural integrity)
The common method used for load application is loaded trucks (whether moving or static) which is sufficient to trigger the needed behavior of the bridge. Monitoring of deflections is usually undertaken for bridge load tests using sensors installed on the structure to be tested, other qualities may also be measured such as crack widths, strains and temperature.
Methodology
It is essential that before proceeding with any load testing a thorough and clear plan be put in place to ensure successful implementation. Listed below is a summary of the main steps usually taken in any bridge load testing activity:
Meeting with owner/engineer/designer to understand the objective behind load testing, develop common understanding of the testing procedure and agree on the acceptance criteria
Site visit to have the testig team familiarize themselves with structure to be tested as well as defining logistics and accessibility challenges
Development of instrumentation plan (sensor types and locations) and defining the acceptance criteria
Visual Inspection of accessible areas to note any concrete defect/distress (this is usually carried before, during and after load testing to compare results and identify if new cracks/defects had been noted due to the load teseting).
Perform various nondestructive testing and material sampling/testing to verify certain physical attributes of concrete or steel (where needed)
Conducting the load test and collecting field data (tests are usually reproduced to verify the accuracy of field findings)
Preparation & Submittal of report showing the findings of tests, analysis and general recommendations.
Different Bridge Load Tests
Static Load Test
The basic description of a proof test is to determine a safe load limit experimentally by incrementally applying load to a structure until a target load is achieved or distress is observed. The target load consists of a load that would induce responses equivalent to a desired rating load plus the necessary factor of safety. This means the target load would likely be the largest load ever applied to the structure. Structural stress is generally defined as excessive stress or deflection, visible cracks, and/or non-linear or inelastic behavior between load cycles. In general, proof load testing is conducted by researchers or government institutions due to the logistics and costs involved. Only structures that can be considered highly ductile and highly redundant should be considered candidates for proof testing.
Dynamic Load Test
This test similar to the static load test but instead of using and keeping a target load for more than 24 hours, a moving load is used at various speeds. This is usually used to assess the impact of dynamic effects on the bridge structural response and eventually complement the findings of the static load test.
Semi-Static load Test
Is a combination of a static and dynamic load testing through which a single truck rolls across structure at crawl speed along specific paths (usually lane lines). This method has become popular in the recent years due to the low cost and minimal closure and testing time involved in performing this test as well as its nondestructive nature. The goal of a diagnostic load test is also to determine a safe load limit. The load test portion of the process is done with loads less than or equal to the desired rating loads. The measurements are used to calibrate or validate a computer model. Actual load limits are determined by the resulting computer model and not directly from the test data. Due to significantly different loading requirements, while also conducted by government and research institutions, this method is also feasible for private consultants.
