In general, a design code is calibrated by: （1） designing a range of structures according to current code procedures;（2） identifying random variables and developing load and resis- tance models based on the statistical parameters of actual loads and resistances; （3） choosing an appropriate reliability technique and computing reliability indices for the code-designed structures using the load and resistance models developed;（4）identifying target reliability indices from the results, usually such that the most typical structures represent the target indices; and （5） suggesting adjustments to current code design procedures that would minimize variations in reliability index among structural components of a similar type.

The objective of this study is to complete the calibration process and determine appropriate design parameters for wood bridges. This research fills this gap and provides recommendations that result in a consistent level of reliability for wood bridges.

Professor, Dept. of Civil Engineering, Univ. of Nebraska, Lincoln,NE 68588-0531.

Assistant Professor, Dept. of Civil Engineering, Mississippi State Univ., MS 39762-9546.

Note. Discussion open until April 1, 2006. Separate discussions must be submitted for inpidual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possible publication on February 9, 2004; approved on January 31, 2005. This paper is part of the Journal of Bridge Engineering, Vol. 10, No. 6,November 1, 2005. ©ASCE, ISSN 1084-0702/2005/6-636–642/$25.00.

Structural Types Considered

The calibration work is performed for selected representative types of wood bridges. In particular, simple span, two-lane, nonskewed bridges with wooden components of short to medium spans, from 4 to 25 m （from 13 to 80 ft）, are considered. In general, there are two types of wood bridges: structures that span by beams （stringers or girders） or structures that span by a deck.

Stringer bridges made of sawn lumber are typically short,spanning to a maximum of about 8 m (25 ft). Readily available sawn lumber stringers are usually from 100 to 150 mm (from 4 to 6 in.) wide and from 300 to 400 mm (from 12 to 16 in.) deep, and these sizes often limit spacing to no more than 400–600 mm (16–24 in.) on center. However, the use of greater widths such as 20 mm (8 in.) and larger depths may allow stringer spacing to be increased, until ultimately limited by deck capacity. Stringers of glulam can be manufactured with much greater depths and widths, and can thus span much greater distances and allow wider beam spacing. Spans from 6 to 24 m (from 20 to 80 ft) are common.

The stringers support various wood deck types, which may be glued-laminated (glulam), nail-laminated (nail-lam),spike-laminated (spike-lam), plank (4 in.х6 in., 4 in.х8 in.,4 in.х10 in., and 4 in.х12 in.), stress-laminated (stress-lam), and reinforced concrete (noncomposite). Laminated decks are made of vertical laminations, typically 50 mm (2 in.) thick and l00–300 mm (4–12 in.) deep, which are joined together by nails, glue,spikes, or transversely prestressed. The latter method is typically used for deck rather than stringer bridges, however. Laminations are made into panels that are usually from 900 to 1,500 mm (from 3 to 5 ft) wide. The designer may specify that these panels either be interconnected or noninterconnected (in a direction parallel to the laminations). Interconnected panels may be secured together by spikes, metal dowels, or stiffener beams, to form a continuous deck surface, whereas noninterconnected panels are left independent of one another, although in some cases the Code requires that transverse stiffener beams be used to provide some continuity. As with stringers, various wood species and commercial grades of deck laminations are available. Attachment of the deck to stringers is made by nails, spikes, or special fasteners. The structures may have decks running either perpendicular or parallel to traffic.Stringer bridges with longitudinal decks require transverse floor beams to support the deck and distribute load to longitudinal stringers. Diagrams of these structures are presented in Figs. 1 and 2.