Bridge inspection relies on a variety of visual and mechanical measurement techniques that are used to determine bridge safety. Gusset plates are often assessed with a visual inspection. Corrosion of bolts is measured using calipers. Corrosion pits are typically measured using pit gauges. The variability in these techniques results in a large uncertainty in the assessment. While many inspectors and engineers recognize the inherent uncertainty associated with these visual and mechanical methods, the impact of measurement uncertainty is often overlooked when determining the safety of a bridge. This can lead to catastrophic results. For example, the investigation into the causes of the collapse of the I-35W bridge in Minneapolis, MN resulted in a recommendation by the Federal Highway Administration to better evaluate gusset plate connections. A variety of approaches have been investigated: all of which require accurate geometric measurements.
Figure 1: Performing bridge inspections with visual and mechanical methods
3D imaging can provide information on accurate, repeatable information on bridge damage and can be used to better assess the safety of bridges. Metal loss assessment of bolts, nuts, and plate can be performed using 3D data. Distortion of gusset plates can be determined using 3D data. Finite element analysis can be performed by incorporating the 3D measurements into models of the as-built structure, often in Abaqus or similar software, to assess the impact of the damage on the structures safety.
An important transportation bridge, located in Asia, needed to be assessed to determine the if the remaining strength of the bridge could continue to support the bridge traffic. To do this, NDIC used Seikowave 3D imaging tools to acquire the necessary data and Seikowave 3D analysis software to assist in the assessment of the bridge.
Figure 2: Using 3D imaging to assess bridge plates and bolts
The process starts with field collection of 3D data on critical parts of the infrastructure. The field ruggedized 3D imaging systems from Seikowave are ideal for this application. Battery operated, IP67 compliant, and constructed with a shock resistant housing, the 3DSL Rhino, and other Seikowave 3D imaging systems, are able to meet the demanding requirements associated with the assessment of bridges
Figure 3: A 3D model of a bridge plate with bolts
Once the 3D is acquired, Seikowave 3D image assessment tools can be used to determine volumetric metal loss and reduction in the width of critical portions of the nuts used to secure gusset plates. A reduction in the width of the nut or the overall volume of the nut can trigger a replacement.
Figure 4: 3D model of a bolt showing clear evidence of corrosion
The high-density mesh created using the 3DSL Rhino system produces and ideal data set for finite element analysis (FEA). FEA performed using Seikowave 3D data is an excellent method for assessing the impact of damage and determining the overall safety of bridges.