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Abstract:The goal of this study was to develop a reliable load-rating methodology for timber piles based on the level of documented damage. Louisiana currently has over 4,000 timber bridges in its inventory of over 13,800 bridges. A quarter of these 4,000 timber bridges are structurally deficient since they cannot support their design loads. One of the most common forms of deterioration is core decay resulting in a hollow pile with an undecayed outer shell. This outer shell may be solid or broken-up by vertical splits along the longitudinal axis of the pile. Pile deterioration may extend from a few feet up, to the entire length of the pile.
Bridge maintenance personnel must make judgments on a regular basis as to the remaining capacity for these hollowed/decayed piles. Biennial inspections are routinely conducted for bridge substructures (every five years for underwater inspections). District bridge inspectors report visible defects and measure the thickness of the sound outer shell when decay is suspected. This data is then used to model the pile and perform a load rating analysis.
The Bridge Maintenance Section of DOTD supplied approximately 30 deteriorated timber piles up to ten feet (3 m) in length with a representative range of hollowness and splitting (checking). Small coupons were taken from most of the piles to determine the basic material properties. The degree of damage was quantified and each pile tested in axial compression. Mathematical models were developed to predict the axial load capacity and included all significant variables as typically reported by bridge inspectors. The theoretical and experimental results were compared to verify the model. Finally, recommended procedures were developed for load rating decayed timber piles.
The investigation has led to the following conclusions: (1) The strength of the sound wood portion of decayed piles is significantly lower than that of the new piles; (2) Piles having void areas less than 20% of the gross area tend to fail primarily by crushing; (3) Piles with void areas greater than 20% tend to fail primarily by buckling of the outer shell; (4) A good predictor of pile capacity is the energy required for a specific depth of radial penetration by a nail/probe into the pile; and (5) Based on this concept and a safety factor of three, equations were developed for predicting the pile allowable load for decayed timber piles.
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