橋梁畢業(yè)設(shè)計(jì)外文翻譯--對(duì)木橋的負(fù)載和阻力系數(shù)的校準(zhǔn) 英文

1、Load and Resistance Factor Calibration For Wood BridgesAndrzej S. Nowak, F.ASCE,1 and Christopher D. Eamon, M.ASCE2Abstract: The paper presents the calibration procedure and background data for the development of design

2、code provisions for wood bridges. The structural types considered include sawn lumber stringers, glued-laminated girders, and various wood deck types. Load and resistance parameters are treated as random variables, and t

3、herefore, the structural performance is measured in terms of the reliability index. The statistical parameters of dead load and live ?traffic? load, are based on the results of previous studies. Material resistance is ta

4、ken from the available test data, which includes consideration of the post-elastic response. The resistance of components and structural systems is based on the available experimental data and finite element analysis res

5、ults. Statistical parameters of resistance are computed for deck and girder subsystems as well as individual components. The reliability analysis was performed for wood bridges designed according to the AASHTO Standard S

6、pecifications and a significant variation in reliability indices was observed. The recommended load and resistance factors are provided that result in consistent levels of reliability at the target levels.DOI: 10.1061/?A

7、SCE?1084-0702?2005?10:6?636?CE Database subject headings: Bridges, wooden; Calibration; Load and Resistance Factor; Design; Bridge decks.IntroductionIn 1993 AASHTO adopted a new load and resistance factor design ?LRFD? c

8、ode 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

9、 structures ?Nowak 1995, 1999?. Therefore, there was a concern about the consistency of the reliability level for wood structures. Previous studies showed that the reliability index for wood bridge components can be sign

10、ificantly different than for steel or concrete structures ?Nowak 1991?. The degree of variation for wood properties varies depending on dimensions, load duration, moisture content, and other parameters. In case of wood b

11、ridges, it is important to consider the structural system or subsystem as well as individual elements/components. In general, a design code is calibrated by: ?1? designing a range of structures according to current code

12、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 computin

13、g 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

14、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 com

15、plete the calibration process and determine appropriate design parameters for wood bridges. This research fills this gap and provides recommend- ations that result in a consistent level of reliability for wood bridges.St

16、ructural Types ConsideredThe calibration work is performed for selected representative types of wood bridges. In particular, simple span, two-lane, non- skewed bridges with wooden components of short to medium spans, fro

17、m 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 lumbe

18、r 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

19、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

20、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. Th

21、e stringers support various wood deck types, which may be glued-laminated ?glulam?, nail-laminated ?nail-lam?, spike-laminated ?spike-lam?, plank ?4 in.?6 in., 4 in.?8 in., 4 in.?10 in., and 4 in.?12 in.?, stress-laminat

22、ed ?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, o

23、r 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 m

24、ay specify that these panels either1Professor, Dept. of Civil Engineering, Univ. of Nebraska, Lincoln, NE 68588-0531. 2Assistant Professor, Dept. of Civil Engineering, Mississippi State Univ., MS 39762-9546. Note. Discus

25、sion open until April 1, 2006. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this

26、 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

27、-0702/2005/6-636–642/$25.00.636 / JOURNAL OF BRIDGE ENGINEERING © ASCE / NOVEMBER/DECEMBER 2005J. Bridge Eng. 2005.10:636-642.Downloaded from ascelibrary.org by University of Michigan on 02/26/15. Copyright ASCE. Fo

28、r personal use only; all rights reserved.heavy trucks? for 75 year period and for the three considered traffic volumes is 1. Low ADTT??100 trucks??0.5 s??365 days??75 years? ?15 days; 2. Medium ADTT??200 trucks??0.5 s??3

29、65 days??75 years? ?30 days; and 3. High ADTT??600 trucks??0.5 s??365 days??75 years? ?90 days. Although wood bridges are typically located on low volume roads, in the reliability analysis it is conservatively assumed th

30、at the live load duration is 2 months ?between medium and high traffic volumes?. For short spans, live load is caused by axle loads or even wheel loads. Therefore, the live load model is determined by variations in wheel

31、 load rather than the entire truck or axle. Statistical parameters for wheel load are derived from existing survey data ?Nowak et al. l994?. Based on axle load taken from field measurements on bridges located in Michigan

32、, as well as state police citation files for overload vehicles, the maximum observed axle load for a 1 year interval is close to 200 kN ?40 kips?, which produces 50 kN ?10 kips? per wheel ?two tires per wheel?. Therefore

33、, in this calibration, the mean maximum one year value for a wheel load is taken as 50 kN ?10 kips?. The coefficient of variation is taken as 0.15 ?Nowak et al. 1994?. Tire contact area is an important consideration for

34、live load distribution to short span components. Based on the measure- ments reported by Pezo et al. ?1989? and Sebaaly ?1992?, the transverse dimension ?width? of the contact area is 185 mm ?7.5 in.? for each tire, with

35、 a 125 mm ?5 in.? gap between tires for a dual tire wheel. A nearly linear relationship exists between the wheel load and length of the contact area. For a 50 kN ?10 kips? wheel load, tire length is approximately 250 mm

36、?10 in.?. There- fore, in this study, the contact area for a single tire is considered as a rectangle of 180 mm?250 mm ?7.5 in.?10 in.?, and for a dual tire, a rectangle of 250 mm?500 mm ?10 in.?20 in.? ?the gap is ignor

37、ed?. In the AASHTO Standard ?1996?, dynamic load is not consid- ered for wood bridges. In AASHTO LRFD ?1998?, dynamic loadis specified at 50% of the corresponding value specified for concrete and steel girders. Field mea

38、surements reviewed for the development of the AASHTO LRFD Code indicated the presence of a dynamic load effect in timber bridges ?Nowak and Eamon 2001?. It was also observed that the load effect was lower than that for o

39、ther materials. Dynamic load is associated with a very short duration, much shorter than the static portion of live load. However, the strength of wood can be considerably larger for shorter time periods. Because of thes

40、e observations, and of a lack of more detailed test data, the increase in component strength is not considered in the calibration process, but the dynamic load is taken as zero.Material Resistance ModelsThe deterministic

41、 models of resistance are summarized by Ritter ?1990?. The major mechanical properties of wood are modulus of rupture ?MOR?, modulus of elasticity ?MOE?, and shear strength. These properties are subject of a considerable

42、 variation, and the statistical parameters depend on dimensions, species, grade, moisture content, and load duration. For various grades and sizes of sawn lumber, a considerable data base was developed by Madsen and Niel

43、sen ?1978a,b?. For Douglas Fir, bias factors, with respect to tabulated strength values listed in the 1996 LRFD Manual for Engineered Wood Construction ?EWA 1996? vary from 1.41 to 1.98 for select grade and from 1.76 to

44、2.88 for Grades 1 and 2, whereas coefficient of variation ranges from 0.17 to 0.27 for select and from 0.23 to 0.30 for Grades l and 2. The higher variations correspond to sections with largest depth/width ratios. Resist

45、ance is taken as a lognormal random variable. For glulam girders, the statistical parameters for strength are taken from the report by Ellingwood et al. ?1980?, based on the test results obtained by the USDA Forest Produ

46、cts Laboratory on beams with Douglas Fir and Southern Pine with horizontally ori- ented laminating. The resulting bias factor is from approximately 2–3, with an average of 2.5, and coefficient of variation is fromFig. 4.

47、 Bias factor for live load Fig. 5. Coefficient of variation for live load638 / JOURNAL OF BRIDGE ENGINEERING © ASCE / NOVEMBER/DECEMBER 2005J. Bridge Eng. 2005.10:636-642.Downloaded from ascelibrary.org by Universit