This report outlines the forensic investigation of the collapse of Bridge No. SN 019-5010 in DeKalb County,
Illinois on August 19, 2008. 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. Samples of
the oak timber piles were retrieved and tested under pure compression as well as combined compression and
flexure. Laboratory testing results of these specimens were used to calibrate an analytical model of a full-scale
single timber pile. The analytical model considered both symmetrical and unsymmetrical loading of the bridge.
The results of the analytical model were used to determine the required live load to initiate collapse. The results
of the investigation indicate that structural overload, leading to fracturing of the timber piles in combined
compression and flexure, is the likely reason for the bridge failure.ICT-R27-SP12published or submitted for publicationis peer reviewe
