Reliability Models for Plank Decks

Abstract

In 1993 AASHTO adopted a new load and resistance factor design (LRFD) code for highway bridges. The new code provides a rational basis for the design of steel and concrete structures. Although wood bridge design was also included in LRFD format, the calibration was not carried out for these structures (Nowak 1995; 1999). Therefore, there was a concern about the consistency of the reliability level for wood structures. Previous studies have shown that the reliability index for wood bridge components can be significantly different from those for steel or concrete structures (Nowak 1991). The degree of variation for wood properties is a function of dimensions, load duration, moisture content and other parameters. There is particular concern for plank decks, for which section aspect ratio also significantly affects results. The objective of this study is to summarize the load and resistance criteria for highway bridge plank decks, and to estimate the reliability of plank decks designed by AASHTO Code (2004 LRFD and 2002 Standard). A typical plank deck consists of planks placed on stringers that run either transversely to or parallel to the direction of traffic. The span length of stringers is usually 16-20 ft (5-6m), while stringers are typically spaced from 12-24 in (300-600mm). Stringers are commonly made of Southern Pine or Douglas-Fir sawn lumber, often nominally 6 in x 18 in (150x450mm), or larger. The planks are typically 4 in x 10 in (100x250mm) or 4 in x 12 in (100x300 mm).