Bridge Collapse Investigation

The collapse of a bridge in DeKalb County, Illinois on August 19th 2008 was investigated. The bridge consisted of three 42 feet precast prestressed concrete deck beams simply-supported by concrete pile caps with a timber pile foundation. Assessment based on site investigation, in situ readings, and analysis indicated that a geotechnical or superstructure failure may be ruled out.

Six specimens retrieved from the site and tested under pure compression exhibited capacity greatly exceeding the design capacity of the piles. Two specimens, tested with a modest eccentricity, showed a 60% reduction in the capacity compared to that of concentrically loaded specimens. These tests illustrated that the bent capacity is sensitive to eccentric loading, which could occur when only a single span of the bridge is loaded.

An analytical model of a single pile was created to determine the capacity of the pile under concentric and eccentric loading. The material properties used in the model were calibrated based on the response of the experimental testing. The structure-soil interaction was represented by nonlinear springs developed based on in situ soil conditions. Flexural buckling was accounted for with a geometrically nonlinear formulation. The analysis revealed that the live load capacity of a single pile is greatly reduced by eccentric loading compared to concentric loading. The single pile model was utilized to develop a full model for the bent. The skew of the bridge resulted in an uneven loading of the bent. The analysis results showed that the bent is capable of withstanding an Illinois legal load if all eight piles are resisting the load. However, a moderately overloaded vehicle was found to be sufficient to initiate collapse of the structure.

Based on the experimental and analytical work conducted it is recommended that the effect of eccentrically applied load should be considered when determining the capacity of bridges supported on timber piles, especially when the superstructure is simply supported. The National Design Specification for wood construction provide provisions for eccentrically loaded timber piles.