【正文】 wood that may result in little change of section properties when members are loaded ?atwise, whereas for an edgewise loaded member, the same size ?aw occupies a higher proportion of section width and thus may weaken it substantially. Coef?cient of variation ranges from to , where wider sections have the lowest variation. The variability of MOE is described by Nowak (1983).It is considered as a lognormal distribution, with a coef?cient of variation of . MOE is partially correlated with MOR. The correlation can be described with MOE as a linear function of MOR,as shown in MOE = [(MOR) + ]1,000 (1) From the standpoint of reliability, this relationship is important as in a system of wood ponents (such as a deck with multiple laminations), where the weakest (lessstiff) members absorb less force, increasing the reliability of the system. Variation of dimensions is negligible. Form the measurements performed by Madsen and Nielsen (1978a,b), the coef?cient of variation is about . The bias factor varies from to . Structural Resistance Models The current AASHTO LRFD Code (1998) girder distribution factor (GDF) formulas for wood bridges are given as a function of girder spacing only. The accuracy provided by this method is insuf?cient for developing a suitable resistance model. The GDF formulas for steel or concrete girders supporting a concrete deck predict load distribution well for a certain range of idealized structures, regardless of material. However, these formulas lose accuracy when girder spacing less than m or spans greater than 6 m are considered. Many wood bridges have beam spacing and spans less than these values. Therefore, in this study, load distribution to stringers is based 河北聯(lián)合大學(xué)輕工學(xué)院畢業(yè)翻譯部分 26 on ?nite element analysis. Based on results of analysis, for closely spaced sawn lumber stringers [400–600 mm](16–24 in.), a subsystem of three stringers tends to relatively equally share load when two trucks are side by side. For wider girder spacing, however, such as that for glulam girder bridges [5–8 ft][– m], only one girder substantially resists a wheel load. Based on model simulations, coef?cient of variation V of the threestringer subsystem is taken to be typical ponent V= , whereas for spacings much greater than 600 mm (24 in.)(glulam girder bridges), coef?cient of variation is not reduced from ponent V. A similar subsystem [with the width of 900 mm (36 in.)] is considered for stressed and glulam decks. The girder distribution factor is –. The statistical parameters of resistance of a stressed subsystem are based on the test data obtained by Sexsmith et al. (1979). The mean moment carrying capacity (resistance) of the subsystem [500 mm (20 in.) wide] is equal to the sum of mean capacities of individual elements (boards). The mean MOR of a system is the same as that for an individual element. However, the coef?cient of variation is (for typical single lamination V=). For glulam deck systems, no speci?c data on coef?cient of variation are available. Studies have shown, however, that glulam decks display a similar, slightly more stiff transverse behavior as pared to stresslam decks (Batchelor et al. 1979, 1981。 Bridge decks. Structural Types Considered The calibration work is performed for selected representative types of wood bridges. In particular, simple span, twolane, nonskewed bridges with wooden ponents 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 河北聯(lián)合大學(xué)輕工學(xué)院畢業(yè)翻譯部分 17 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 mon. The stringers support various wood deck types, which may be gluedlaminated (glulam), naillaminated (naillam),spikelaminated (spikelam), plank (4 6 in., 4 8 in.,4 10 in., and 4 12 in.), stresslaminated (stresslam), and reinforced concrete (nonposite). 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 mercial 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 bridges with longitudinal decks require transverse ?oor beams to support the deck and distribute