Transportation Research Record 1053 l
An Overview of Timber Bridges
FONG L. OU and CLYDE WELLER
ABSTRACT
This paper contains a review of literature on timber bridges. It presents recent developments and evolving concepts in timber bridge technology, including aspects concerning wood material, bridge design, construction, inspection, rat ing, and maintenance. This review indicates that timber bridge technology has advanced with the design and construction of a prototype of a prestressed, laminated timber bridge in Ontario, Canada. In the United States, the main ef fort of government and the timber industry is to promote timber bridge tech nology transfer.
Timber was probably the first type of material that bridge. Third, the fabrication of glued-laminated humans used to construct a bridge. Although concrete (glulam) timber members may take longer than the and steel replaced wood as the major materials for construction of steel beams or concrete sections (6); bridge construction in the 20th century, the use of however, a newly developed prefabricated modular sys wood in short-span bridges remains as great as ever. tem Of production may eliminate this time delay Today, much is known about wood, yet its propensities Wood Preservation are not fully exposed. Given its many advantageous character is tics, treated wood is a good source for Chemical preservation of wood is often used to main construction materials. Research on its cost-effec tain the material in serviceable condition. Two main tive use is important and is continuing (11,_!l, preservation methods consist of brushing, spraying, 15-18). or dipping and vacuum-pressure processes CW • Brushing, spraying, or dipping provide shallow pene tration, l ow chemical absorption, and superficial DESIGN treatment. of the wood surface. Vacuum-pressure pro cesses penetrate the wood more deeply. They consist U. s. timber bridge design f ollows the requirements of the Bethel, Lowry, and Rueping processes, each of of Section 13, Timber Structures, Division I, Design, which uses timed pressure and vacuum treatments. of the thirteenth edition of the Standard Specifica vacuum-pressure processes are referred to as full- or tions for Highway Bridges (19), published by AASHTO. empty-cell processes depending on the al11Qunt of pre The Al\SHTO design specifications are adjusted or servative solution or toxicant that remains in the amended by state designers or Forest Service bridge wood after treatment (13). The Bethel process is a engineers to fit local conditions based on new ex full-ce.11 process and fii°-applied to products such as perience or research results (20). The AASHTO speci marine piling where maximum protection is required; fications include a list of allowable stresses for both the Lowry and Rueping methods are empty-cel.l stress-qrade lumber and glulam timber under normal processes and provide good distribution with a loading; permanent loading; and wind, earthquake , or limited amount of preservativ.e. The Rueping process short-term loading conditions. The specifications is more flexible and can be used to achieve a wider provide formulas for the computation of stresses , as range of results than the Lowry process. However , well as detailed design procedures for simple col the Lowry process is simpler to pe.r form, particularly umns, spliced columns , pile and fcamed bents, and when the wood is readily treatable. trusses. The Forest Products Laboratory also has Three groups of preservatives are used for wood documented in detail a design procedure fo:: glulam Ou and Weller 3 bridge decks (21-23) that was adopted by AASHTO and research on the design of prestressed wood decks included in the-bridge specifications. (30,34), which resulted in a comprehensive set of The AASHTO design code does not address the design desi9n specifications (35 ,36). These new specifica of log bridges for trails and roads. The Forest Ser tions were developed using-Probability-based methods vice often uses untreated logs for temporary struc of timber stress analysis and have been included in tures when short-term (5 to 10 years) needs justify the 1983 edition of the Ontario Highway Bridge Design use. However, some of these "temporary" structures Code (36). are up to 25 years old. Glulam bridges also have However, the analytical methods used in the been designed and constructed for temporary struc specifications for AASHTO and for the Ontario design tures in cases in which the load duration charac code differ. AASHTO uses allowable stress to design teristics of wood (time dependency; that is, change and evaluate timber bridges, while the Ontario design in strength over time of use) were a consideration code uses probability-based methods as an analytical (.£!) • Logs used for road bridges range in diameter tool (37). The AASHTO approach does not provide for at the tip from about 24 to 40 in. (61 to 102 cm). the determination of the actual safety reserve in The bridge spans reach up to 80 ft (24.4 m). Although the structure (38). However, developing a comprehen the loading varies in road designs, the minimum sive probability-based design code would require loading for the Forest Service is the AASHTO HS 20- considerable research because of the current lack of 44 truck. Markedly higher loads are used for logging a good data base on the mechanical properties of roads. In Forest Service engineering practice, load wood (~, iQ.l • ings for trail bridges include hikers, livestock, motorcycles, snowmobiles, and sometimes, four-wheel dr ive vehicles. The usable width of trail bridges BRIDGE LOAD-CARRYING CAPACITY varies from 3 to 8 ft (0.92 to 2.44 m) (~). An important aspect of bridge design is analyzing the adequacy of the bridge structure to carry the Timber Bridge Design expected traffic load. Nine independent elastic con stants are used in the analysis: three moduli of Traditionally, the deck design has involved a nail elasticity, three shear moduli, and three Poisson's laminated assembly of nominal, 2-in. (5-cm) dimension ratios. However, because of large variations in the lumber placed transverse to the supporting stringers. longitudinal modulus of elasticity of wood, actual Connections include through-nailing of laminations values of dead and live load moment and shear and toe-nailing to the stringers. The major short stresses in timber bridges are difficult to deter coming of this system is that nail connectors grad mine. Many methods have been used to estimate the ually loosen as a result of the deck deflections and longitudinal modulus of elasticity, such as the the shrinking and swelling caused by repeated wetting orthotropic plate theory (21,30,Q,42), the Grillage and drying cycles. Because of this problem, the deck analogy (_il,_!!), the AASHTO simplified approach cannot serve as a roof over the entire bridge struc (19,45), the statistical approach (46), and prob ture, protecting the supporting members and the deck ability-based methods A pilot test using low-power, high-resolution, Visual Inspection ground-penetrating radar (GPR) to detect deter iora tion in concrete bridge decks was performed in 1977 Visual inspection of timber bridges is based on two (84). After this pilot test, additional work was groups of visual indicators (79). The first group done to improve the accuracy of the technique (~- includes three visual indicators of the presence of 87). decay: (a) characteristic fungus fruiting structures, - The GPR system consists of a monostatic antenna, (b) abnormal surface shrinkage or sunken faces, and a control console that contains a transmitter and (c) insect activity. The second group of visual receiver, and an oscilloscope. This system transmits indicators is used to identify six conditions con impulses of microwave frequency into an overlaid ducive to decay. The indicators are concrete bridge deck and detects the extent of deterioration based on the reflection from the sur 1. Excessive wetting (evidenced by water marks faces of the bituminous concrete, the portland cement or stains) : concrete, and the reinforced concrete slab. When the 2. Rust stains on wood surfaces: concrete slab is delaminated, there is an additional 3. Growth of vegetation on bridge members: reflection from the deteriorated area. The more 4. Accumulation of soil on any wood surfaces, severe the delamination, the more pronounced is the which can trap water and increase decay hazard: resulting reflection. 5, Joint ~nterfaces, mechanical fasteners, field Re.cent studies indicate that radar can be used to fabrication, and wood adjacent to other water-trap survey the condition of overlay decks and decks that ping areas, which are potential sites of decay fungi have their original surfaces (78,83). Although the growth: and speed and accuracy of the technique~eed improvement, 6. water-catching seasoning checks in exposed the experiment of concrete bridges indicates that wood faces. Of these, the first condition is probably radar can be a potential tool for inspecting timber the most common and noticeable indication of the bridges. It should be noted, however, that like other development of decay. nondestructive techniques, radar is not adequately capable of identifying the areas of debonding. Also, The advantage of visual inspection is its sim the radar technique requires further development to plicity and quickness. However, it depends a great automate data collection and analysis and to improve deal on the inspecting engineer's judgment. This the interpretation of signals. method has two major limitations. First, it is less accurate for inspecting internal decay: for example, a timber pile may be completely decayed even though Ultrasonic Technique the external material appears sound. Second, members immersed in water or covered by asphalt or concrete Ultrasonic technique has been used to detect defects are difficult to inspect. However, a newly developed and to measure the strength of concrete for many photographic technique could overcome the difficulty years (88-93). The technique involves the use of of inspecting underwater bridge components (~_Q). The propagated hlgh-frequency sound waves to test mate equipment allows divers to obtain clear, underwater rials based on the relationship between the wave photographs under typical inspection conditions. velocity in a material and its properties. For exam ple, undamaged wood is an excellent transmitter of these waves, whereas damaged and decayed wood delays Sounding transmission. The technique, therefore, requires accurate measurements of the velocity of the propa The sonic technique lias been used to locate the gated stress wave. relatively severe delaminations of concrete bridges A recent study (76) indicated that the velocity by monitoring the audible sounds that result from of propagation of the waves parallel to the grain striking the deck surface of a bridge. Sounding VL and in the radial direction VN (normal to the relies on subjective judgments of hollow sound that grain) in an orthotropic material with a Poisson's are produced when concrete is struck with a hammer ratio for transverse strain in the longitudinal or when a chain is dragged across the deck surface. direction, when stress is applied in the radial Interference from traffic noises may result in an direction, in the range of 0.01 to 0.04, is ap inaccurate judgment The principle of using infrared (IR) thermography as RATING a means of defining delaminated areas caused by cor rosion of the reinforcing steel in concrete bridge decks is based on the detection of differences in As discussed previously, the field inspection estab the surface temperature between the sound concrete lishes the extent of deterioration in load-carrying members of bridges. After inspection, the informa and the delaminated concrete. The concrete emits IR radiation generated by the vibration and rotation of tion gathered must be rated so that the bridge engi neers can make safety decisions about the repair, its atoms and molecules. From an IR scanner, the rehabilitation, posting, closing, or replacement of temperature can be determined indirectly by measur an existing bridge. The rating of bridge strength ing the intensity of the emitted IR radiation. generally follows procedures similar to the AASHTO IR thermography has proven to be a faster and design code, including checks for specified design more effective method than conventional sounding loads, girder distribution and impact factors, and methods for surveying the extent of delaminations in allowable stresses. Therefore, bridge rating is an concrete decks that have not been overlaid with important step in producing acceptable safety. bituminous concrete (96,97). For overlaid concrete Most bridge rating methods were developed for bridge decks, the application of an IR scanner is steel and concrete bridges. Some of these methods still in the experimental stage. However, two recent use. load spectra (101), and others apply the prag studies have shown· enc o ura ging r esults. one study indicated that the scanner dete c ted more than 90 matic approach for rating (102). The pragmatic ap p roa ch uses a procedure to rate highwa y b ridges for percent of the known delaminations, some of which regulation loads without causing yie lding of the were less than 6 in. (150 mm) in diameter (78). The bridge materials. Federal and state governments have results of the other study were not conclusive for developed several computer systems that inventory quantity estimates, but the applicability of IR the actual conditions of bridges (103,104). These equipment to general rehabilitation programming is systems may provide a data base for --rating and warranted (1.!!_,~) • selecting a preferred bridge maintenance program A number of commercial IR systems have been tested (105-107) • using various configurations of the equipment, in There is not however, a well-developed rating cluding aerial and van-mounted apparatus. Between method for timber bridges. The Forest service has the two, the van-mounted equipment, which provides developed a system for bridges and major culverts simultaneous recordings of the infrared scan and a (BMC) to be used in conducting inventories of forest video of the actual surface appearance, is more road structures (108) • Implemented several years practical. The results of recent tests have con ago, this nationai--5ystem is progressively being cluded that the optimum height above the deck for enhanced. Because most Forest service bridges are the scanner is in the range of 13 to 20 ft (4 to 6 timber bridges, the BMC is considered to be a timber m) (78,97,98). bridge inventory system. A major , e1ement of this AlthougtlIR thermography is a promising tool for system is composite rating, in which a numerical bridge inspection, it has limitations. First, de rating, ranging from 0 to 9, is assigned to the con tecting the point of debonding or the lateral extent dition of the bridge as a whole. This rating reflects of debonding with any degree of accuracy is diffi the inspector's evaluation of the more critical fea cult. Second, there are difficulties associated with tures of the bridge affecting safety and cost. isolating and identifying hot spots, such as bitu The rating scores rely partly on the mathematical minous patches, crack sealers, and tire marks. The ratings for load capacity and on the inspecting presence of these affects the quality of interpreta engineer's judgment. Scientific approaches for timber tion and requires appropriate visual records to com bridge rating have yet to be developed. plete the interpretation. Third, a better definition is needed of the weather conditions under which IR the rmo graphy is most effective . Fi nally , IR ther mog MAINTENANCE r aphy requires the development of software to produce a scaled hard copy from videotape. Based on the field inspection and the rating, a The application of IR scanners on timber bridges bridge maintenance program can be developed. Govern- Ou and Weller 7 ment agencies use numerous methods to develop bridge timber is subjected to frequent decay-causing wet maintenance programs. For example, the Minnesota ting. Proper bridge surface drainage as well as a Department of Transportation has employed a systems clear waterway will reduce decay or collision damage. approach as a planning tool (.!Q2_,110), while the Periodic inspections during periods of minimum mois Wisconsin Department of Transportation has used the ture to ensure that no glueline separation is devel linear programming method (111) to minimize mainte oping would be appropriate (121). nance costs. Timber bridge maintenance can be classified into three categories: replacement, repairs, and preven RESEARCH AND TECHNOLOGY TRANSFER tive maintenance. As indicated previously, the age of a structure is a major factor in the deter iora The outcome of the future wood utilization research tion of a bridge. However, a thorough inspection will have significant impact on the research and should determine the actual condition of the struc development of timber bridges. Three major institu ture before a decision is made on a maintenance plan. tional groups in the United States that are involved in research and development concerning the use of wood are the federal government, the forest industry, Replacement and academic institutions. The federal government funds and performs research and development, the When the load-carrying capacity of a member has forest industry develops products and improves manu decreased by 50 percent or more of its design capac facturing processes, and academic institutions con ity, the member must be replaced, repaired, or duct training and basic research. [The research and strengthened by adding reinforcements (100 ,ill) . development activities of the three groups are Replacement includes the replacement of timber reported elsewhere (18).J Two of the groups' major stringers, plank decks, defective piling, and so concerns with respect to the timber bridge technology forth. The construction procedure for replacement is development are research and technology transfer. documented elsewhere (114,115). In the case of a partial replacement, the replacement must extend 2 ft beyond the defective area of the member (ill) . Research Onsite preservative treatment is recommended for the area surrounding the defect and the newly exposed As stated in a 1983 report tural Engineering, Vol. 109, No. 9, ASCE, Sept. Division, Vol. 98, No. ST3, March 1972, pp. 1983, pp. 2175-2191. 729-740. 10. B.F. Hurlbut. Basic Evaluation of the Struc 30 . R.J. Taylor and P.F. Csagoly. Transverse Post tural Adequacy of Existing Timber Bridges. In Tensioning of Longitudinally Laminated Timber Transportation Research Record 647, TRB, Na Bridge Decks. Research Report 220. Ontario tional Research Council, Washington, D.C., Ministry of Transportation and Communications, 1977, pp. 6-9. Downsview, .Ontario, Canada, June 1979. 11. U.S. Forest Products Laboratory. wood: Its 31. J. Youngquist, D. Gromala, R. Moody, and J. Structure and Properties. Clark c. Heritage Tschernitz. Press-Lam Timbers for Exposed Memorial Series on Wood, Vol. 1, University of Structures. Journal of the Structural Divi Wisconsin, Madison, 1981. sion, Vol. 105, No. ST7, ASCE, July 1979. 12. R.C. Moody, R.L. Tuomi, W.E. Eslyn, and F.W. 32. Standard Specifications for Highway Bridges. Muchmore. Strength of Log Bridge Stringers AASHO, Washington, D.C., 1973. After Several Years' Use in Southeast Alaska. 33. Glulam Bridge Systems: Plans and Details. Research Paper FPL 346, Forest Products Labo American Institute of Timber Construction, ratory, u.s. Department of Agriculture, Madi Englewood, Colo., 1974. son, Wis., 1979. 34. R.J. Taylor, B.D. Batchelor, and K. Van Dalen. 13. u.s. Forest Products Laboratory. wood as a Prestressed Wood Bridges. Structural Research Structural Material. Clark c. Heritage Memorial Report 83-01. Ontario Ministry of Transporta Series on Wood, Vol. 2, University of Wiscon tion and Communications, Downsview, Ontario, sin, Madison, 1984. Canada, 1983. 14. B.A. Richardson. wood Preserving. The Con 35. R.J. Taylor. Design of Prestressed wood Bridges struction Press, Lancaster, England, 1978. Using the Ontario Highway Bridge Design Code. 15. J.R. Goodman and J. Bodig. Orthotropic Elastic Structural Research Report 83-03. Ontario Properties of Wood. Journal of the Structural Ministry of Transportation and Communications, Division, ASCE, Nov. 1970, pp. 2301-2319. Toronto, Ontario, Canada, 1983. 16. u.s. Forest Products Laboratory. Adhesive Bond 36. Ontario Highway Bridge Design Code, 2nd ed. ing of Wood and Other Structural Materials. Ontario Ministry of Transportation and Com Clark c. Heritage Memorial Series on Wood, munications, Downsview, Ontario, Canada, 1983. Vol. 3, University of Wisconsin, Madison, 1985. 37. D.E. Allen. Limit States Design--A Probabilis 17. u.s. Forest Products Laboratory. Wood: Engi tic Study. Canadian Journal of Civil Engineer neering Design Concepts. Clark c. Heritage ing, Vol. 3, 1975, pp. 36-49. Memorial Series on Wood, Vol. 4, University of 38. A.S. Nowak and M.K. Boutros. Probabilistic Wisconsin, Madison, 1985 . Analysis of Timber Bridge Decks. Journal of 18. u.s. Congress. wood Use: U.S. Competitiveness Structural Engineering, Vol. 110, No. 12, ASCE, and Technology, Vol. 2, Tech. Report OTA-M-224, Dec. 1984, pp. 2939-2953. Office of Technology Assessment, 1984. 39. J. Zhan. Reliability-Based Design Procedures 19. Standard Specifications for Highway Bridges, for Wood Structures. Forest Products Journal, 13th ed. AASHTO, Washington, D.C., 1983. Vol. 27, No. 3, March 1977, pp. 31-38. 20. Manual of Bridge Design Practice, 3rd ed . 40. E.B. Galambos, J.G. MacGregor, and C.A. Cor California Department of Transportation, Los nell. Development of a Probability-Based Load Angeles, 1971. Criterion for American National Standards A58. 21. w.J. Mccutcheon and R.L. Tuomi. Procedure for Special Publication 577. National Bureau of Design of Glued-Laminated Orthotropic Bridge Standards, u.s. Department of Commerce, 1980. Decks. Research Paper FPL 210, Forest Products 41. A.R. Cuesens and R.P . Pama. Bridge Deck Analy Laboratory, U.S. Department of Agriculture, sis. John Wiley and Sons, Inc., New York, 1975. Madison, Wis., 1973. 42. w.w. Sanders. Load Distribution in Glulam Tim 22. W.J. Mccutcheon and R.L. Tuomi. Simplified ber Highway Bridges, Final Report. Engineering Design Procedure for Glued-Laminated Bridge Research Institute, Iowa State University, Decks. Research .Paper FPL 233, Forest Products Ames, Feb. 1980. Laboratory, U.S. Department of Agriculture, 43. E. Lightfoot. A Grid Framework Analogy for Madison, Wis., 1974. Laterally Loaqed Plates. International Journal 23. R.L. Tuomi and W.J. Mccutcheon. Design Proce of Mechanical Science, Vol. 6, 1964, pp. 201- dure for Glued-Laminated Bridge Decks. Forest 208. Products Journal, No. 23(b), 1973, pp. 36-42. 44. F. Sawko and B.K. Wilcock. Computer Analysis 24. L.I. Knab and R.C. Moody. Glulam Design Crite of Bridges Having Varying Section Properties. ria for Temporary Structures. Journal of the The Structural Engineer, Vol. 45, No. 11, 1967. Structural Division, Vol. 104, No. ST9, ASCE, 45. B. Bakht, M.S. Cheung, and T.S. Aziz. Applica 1978, pp. 1485-1494. tion of a Simplified Method of Calculating 25. L.D. Bruesch. Forest Service Timber Bridge Longitudinal Moments to the Proposed Ontario Specifications. Journal of the Structural Di Highway Bridge Design Code. Canadian Journal vision, Vol. 108, No. ST12, ASCE, Dec. 1982, of Civil Engineering, Vol. 6, No. 1, 1979, pp. pp. 2737-2746. 36-50. 26. A.D. Freas and M.L. Selbo. Fabrication and 4 6. B. Bakht. Statistical Analysis of Timber Design of Glued-Laminated Wood Structural Mem Bridges. Journal of Structural Engineering, bers. Tech. Bull. 1069, Forest Products Labo Vol. 109, No. 8, ASCE, 1983, pp. 1761-1778. ratory, U. S. Department of Agriculture, Madi 47. R. G. Sexsmith and S.P. Fox. Limit States Design son, Wis., 1954. Concepts for Timber Engineering. Forest Prod 27. B. Bohannan. Prestressing Wood Members. Forest ucts Journal, Vol. 28, No. 5, 1978. Products Journal, No. 12, 1962, pp. 596-603. 48. M.D. Vanderbilt, J.R, Goodman, and M.E, Cris 28. B. Bohannan. Prestressed Laminated Wood Beams. well. Service and Overload Behavior of Wood Research Paper FPL 8. Forest Products Labora Joist Floor Systems. Journal of the Structural tory, u.s. Department of Agriculture, Madison, Division, Vol. 100, No, STl, ASCE, Jan. 1974, Wis., 1964. pp. 11-30. 29. B. Bohannan. Forest Products Laboratory Timber 49. E. Gower. Trouble-Shooting Bridges With Lami Bridge Deck Research. Journal of the Structural nated Girder Construction. Journal of Logging 10 Transportation Research Record 1053 and Sawmilling, Vol. 16, No. 2, Feb. 1985, pp. ERI-85441. Engineering Research Institute, 15-17. Iowa State University, Ames, 1985. 50. G.P. Boomsliter, C.H. Cather, and D.T. Worrell. 68. Forest Service. Forest Service Standard Speci Distribution of Wheel Loads on a Timber Bridge fications for Construction of Roads and Floor. Bull. 24. West Virginia University, Bridges. Report EM-7720-100. U.S. Department Morgantown, May 1951. of Agriculture, 1977. 51. W.C. Huntington, W.A. Oliver, M.W. Jackson, 69. J.L. Koger. Factors Affecting the Construction and W.T. Cox. The Distribution of Concentrated and Costs of Logging Roads. Tech. Note B27. Di Loads by Laminated Timber Slabs. Bull. 424. vision of Forestry, Fisheries, and Wildlife University of Illinois, Urbana, April 1954. Development, Tennessee valley Authority, Nor 52. P. Zia, W.T. Wilson, and W.H. Rowan, A Study r is, Tenn., 1978. of Load Distribution Characteristics of Single 70. R.L. Tuomi. Erection Procedure for Glued-Lami an~ Double Layer 'l'imber Bridge Decks Supported nated Timber Bridge Decks With Dowel Connec by Multiple Stringers. North Carolina State tors. Research Paper FPL 263. Forest Products University, Raleigh, 1964. Laboratory, u.s. Department of Agriculture, 53. E.C.o. Erickson and K.M. Romstad. Distribution Washington, D.C., 1979. of Wheel Loads on Timber Bridges. Research 71. G.D. Bell and K.A. Olson. Bridge Structure Paper FPL 44. Forest Products Laboratory, u.s. Construction System that Uses Treated Lumber. Department of Agriculture, Madison, Wis., 1965. In Transportation Research Record 871, TRB, 54. T.L. Wilkinson. Strength Evaluation of Round National Research Council, Washington, D.C., Timber Piles. Research Paper FPL 101. Forest 1982, pp. 40-46. Products Laboratory, u.s. Department of Agri 7 2. S. Leliavsky. Arches and Short Span Bridges. culture, Madison, Wis., 1968. Monograph, Chapman and Hall Limited, New York, 55. T.R. Agg and c.s. Nichols. Load Concentrations 1982. on Steel Floor Joists of wood Floor Highway 7 3. Manual for Maintenance Inspection of Bridges. Bridges. Bull. 53. Iowa Engineering Experiment AASHTO, Washington, D.c., 1978. Station, Ames, April 1919. 74. Recording and Coding Guide for the Structure, 56. M.H. Hilton, L.L. Ichter, and D.W. Taylor. Inventory, and Appraisal of the Nation's Load Distribution on a Timber Deck and Steel Bridges. FHWA, u.s. Department of Transporta Girder Bridge. In Transportation Research Rec tion, 1979. ord 645, TRB, National Research Council, Wash 75. B.F. Hurlbut. Timber Bridge Inspection Guide ington, D.C., 1977, pp. 20-22. lines. Reprint 80-013. Presented at the ASCE 5 7. B .A. Bendtsen. Bending Strength and Stiffness National Convention, Philadelphia, Pa., 1983. of Bridge Piles After 85 Years in the Milwau 76. M.S. Aggour, A.M. Ragab, and E.J. White, Jr. kee River. Research Paper FPL 0229. Forest Determination of In-Place Timber Piling Products Laboratory, U.S. Department of Agri Strength. In Transportation Research Record culture, Madison, Wis., 1974. 962, TRB, National Research Council, Washing 58. w.w. Sanders, F, w. Muchmore, and R. L. Tuomi. ton, D.C., 1984, pp. 69-77. Behavior of Alaskan Native Log Stringer 77. J.M. Phelps and T.R. Cantor. Detection of con Bridges. In Transportation Research Record crete Deterioration Under Asphalt Overlaps by 665, TRB, National Research Council, Washing Microseismic Refraction. In Highway Research ton, D.C., 1978, pp. 228-235. Record 146, HRB, Nationa~ Research Council, 59. R.L. Tuomi, R.W. Wolfe, R.C. Moody, and F.W. Washington, D.C., 1966, pp. 34-49. Muchmore. Bending Strength of Large Alaska 78. D.G. Manning and F.B. Holt. Detecting Deterio Sitka Spruce and Western Hemlock Log Bridge ration in Asphalt-Covered Bridge Decks. In Stringers. Research Paper FPL 341. Forest Transportation Research Record 899, TRB, Nii= Products Laboratory, u.s. Department of Agri tional Research Council, Washington, D.C., culture, Madison, Wis., 1978. 1983, pp. 11-20. 60. F.W. Muchmore and R.L. Tuomi. Alaska's Native 79. F.W. Muchmore. Techniques to Bring New Life to Log Bridges. Journal of Civil Engineering, Timber Bridges. Journal of Structural Engi Vol. 50, No. 2, ASCE, 1980, pp. 54-55. neering, Vol. 110, No. 8, ASCE, Aug. 1984, pp. 61. Teng and Associates, Inc. Allowable Stresses 1832-1846. in Piles. FHWA, U.S. Department of Transporta 80. D.D. McGeehan. Underwater Photography for tion, Dec. 1983. Bridge Inspections. Report FHWA/VA-84/3. Vir 62. J.R. Goodman and E.P. Popov. Layered Beam Sys g ini.a Highway and Transportation Research tems With Interlayer Slip. Journal of the Council, FHWA, U.S. Department of Transporta Structural Division, Vol. 94, No. STll, ASCE, tion, 1983. Nov. 1968. 81. J.R. Van Daveer. Techniques for Evaluating 63. J.R. Goodman. Layered Wood Systems With Inter Reinforced Concrete Bridge Decks. Journal of layer Slip. Wood Science, Vol. 1, No. 3, 1969. the American Concrete Institute, Vol. 72, No. 64. E.G. Thompson, J.R. Goodman, and M.C. Vander 12, 1975, pp. 697-703. bilt. Finite-Element Analysis of Layered Wood 82. J.J. Agi. Nondestructive Testing of Marine Systems. Journal of the structural Division, Piling. Proc., 4th Symposium on Nondestructive Vol. 101, No. ST12, ASCE, Dec. 1975, pp. 2659- Testing of Wood, Vancouver, wash,, 1978, p. 2672. 187. 65. R.M. Gutkowski, J.R. Goodman, and J.D. Pault. 83. G.G. Clemena. Nondestructive Inspection of Tests and Analysis for Composite Action in Overlaid Bridge Decks With Ground-Penetrating Glulam Bridges. In Transportation Research Radar. In Transportation Research Record 899, Record 676, TRB, National Research Council, TRB, National Research Council, Washington, Washington, D,C., 1978, pp. 1-7. o.c., 1983, pp. 21-32. 66. G. Kechter and R.M. Gutkowski. Doubled-Tapered 84. T.R. Cantor and C.P. Kneeter. Radar and Acous Glulam Beams--Finite Element Analysis. Journal tic Emission Applied to the Study of Bridge of the Structural Division, ASCE, forthcoming. Decks, Suspension Cables, and a Masonry Tunnel. 67. w.w. Sanders, Jr., F.w. Klaiber, and T.J. Wipf. Port Authority of New York and New Jersey, Load Distribution in Glued Laminated Longi Report 77-113, New York, 1977. tudinal Timber Deck Highway Bridges. Report 85. J.V. Rosetta, Jr. Feasibility Study of the Ou and Weller 11 Measurement of Bridge Deck Bituminous Overlay Record 950, TRB, National Research Council, ment Thickness by Pulse Radar. Geographical Washington, D.C., 1984, pp. 53-59. Survey Systems, Inc., Hudson, N.H., Sept. 1980. 103. R.R. Johnston, R.H. Day, and D.A. Glandt. 86. A.V. Alongi, T.R. Cantor, C.P. Kneeter, and A. Bridge Rating and Analysis Structural System Alongi, Jr. Concrete Evaluation by Radar: (BRASS). Report FHWA-RD-73-502. FHWA, U.S. Theoretical Analysis. In Transportation Re Department of Transportation, 1973. search Record 853, TRB, National Research 104. J.A. Puckett, L.R. Reash, and C.H. Wilson. Council, Washington, D.C., 1982, pp. 31-37. Microcomputer Version of Bridge Ratings and 87. T.R. Cantor and C.P. Kneeter. Radar as Applied Analysis of Structural Systems (BRASS-PC) • to the Evaluation of Bridge Decks. !!!. Trans Paper presented at the 2nd Annual International portation Research Record 853, TRB, National Bridge Conference and Exhibition, Pittsburgh, Research Council, Washington, o.c., 1982, pp. Pa. , June 1985. 37-42. 105. B.L. Richards. Development of a Computerized 88. E.A. Whitehurst. Pulse-Velocity Techniques and Bridge Inventory for a State Ro.ad Authority. Equipment for Testing Concrete. Proc., Vol. In Transportation Research Record 664, TRB, 33, HRB, National Research Council, Washing National Research Council, Washington, D.C., ton, D.C., 1954, pp. 226-242. 1978, pp. 1-6. 89. A. Scanlon and L. Mikhailovsky. Nondestructive 106. Bridge Inspector's Training Manual. FHWA, u.s. Test Methods for Evaluation of Existing Con Department of Transportation, 1979. crete Bridges. Paper presented at the 2nd An 107. H.P. Koretzky, K.R. Patel, and G. Wass. Penn nual International Bridge Conference and Exhi sylvania's Structure Inventory Record System: bition, Pittsburgh, Pa., June 1985. SIRS. In Transportation Research Record 899, 90. V.M. Malhotra. Testing Hardened Concrete: Non TRB, National Research Council, Washington, destructive Methods. Monograph 9. American D.C., 1983, pp. 43-52. Concrete Institute, Detroit, Mich., 1976. 108. Transportation Information System User's Man 91. G. Swift and W.M. Moore. Investigation of Ap ual. Forest Service, U.S. Department of Agri plicability of Acoustic Pulse Velocity Mea culture, forthcoming. surements to Evaluation of Quality of Concrete 109. A .M. Shirole and J .J. Hill. Systems Approach in Bridge Decks. In Highway Research Record to Bridge Structure Rehabilitation or Replace 3 78, HRB, National Research Council, Washing ment: Decision-Making. In Transportation Re ton, o.c., 1972, pp. 29-39. search Record 664, TRB, National Research 92. W.M. Moore. Detection of Bridge Deck Deterio Council, Washington, D.C., 1978, pp. 22-31. ration. In Highway Research Record 451, HRB, 110. A.M. Shirole and J.J. Hill. Systems Approach National Research Council, Washington, D.C., to Bridge Structure Replacement: Priority 1973, pp. 53-61. Planning. In Transportation Research Record 93. W.M. Moore, G. Swift, and L.J. Milberger. An 664, TRB, National Research Council, Washing Instrument for Detecting Delamination in Con ton, D.C., 1978, pp. 32-36. crete Bridge Decks. In Highway Research Record 111. W.A. Hyman and D.J. Hughes. Computer Model for 451, HRB, National Research Council, Washing Life-Cycle Cost Analysis of statewide Bridge ton, o.c., 1973, pp. 44-52. Repair and Replacement Needs. In Transporta 94. K.A. McDonald and E.H. Bulgrin. Locating Lumber tion Research Record 899, TRB,-National Re Defects by Ultrasonics. Research Paper FPL search Council, Washington, D.C., 1983, pp. 120. Forest Products Laboratory, u.s. Depart 52-61. ment of Agriculture, Madison, Wis., 1969. 112. s.H. Park. Bridge Rehabilitation and Replace 95. K.A. McDonald. Lumber Defect Detection by ment (Bridge Repair Practice). S.H. Park, Inc., Ultrasonics. Research Paper FPL 311. Forest Trenton, N.J., 1984. Products Laboratory, U.S. Department of Agri 113. Bridges on Secondary Highways and Local Roads- culture, Madison, Wis., 1978. Rehabilitation and Replacement. NCHRP Report 96. F.B. Holt and D.G. Manning. Detecting Concrete 222. TRB, National Research Council, Washing Bridge Deck Delaminations With Infrared Ther ton, D.C., 1980, 132 pp. mography. Public Works Magazine, March 1979. 114. Rehabilitation and Replacement of Bridges on 97. G.G. Clemena and W.T. McKee!, Jr. Detection of Delamination in Bridge Decks With Infrared Secondary Highways and Local Roads. NCHRP Re Thermography. In Transportation Research Rec port 243. TRB, National Research Council, ord 664, TRB, National Research Council, Wash Washington, D.C., 1981, 46 pp. ington, D.C., 1978, pp. 180-182. 115. B. Bakht. Timber Bridges: State-of-the-Art 98. R.E. Knorr, J.M. Buba, and G.P. Kogut. Bridge (Discussion by R.M. Gutkowski and T,G. Wil Rehabilitation Programming by Using Infrared liamson). Journal of Structural Division, Vol. Techniques. In Transportation Research Record 110, No. 8, ASCE, Aug. 1984, p. 1929. 899, TRB, National Research Council, Washing 116. R.R. Avent, L.Z. Emkin, R.H. Howard, and C.L. ton, D.C., 1983, pp. 32-34. Chapman. Epoxy-Repaired Bolted Timber Connec 99. J.T. Kunz and J.W. Fales. Evaluation of Bridge tions. Journal of the Structural Division, Deck Condition by the Use of Thermal Infrared Vol. 102, No. ST4, ASCE, April 1976, pp. 821- in Ground-Penetrating Techniques. Paper pre 838. sented at the 2nd Annual International Bridge 117. R.R. Avent. Decay, Weathering, and Epoxy Repair of Timber. Journal of Structural Engineering, Conference and Exhibition, Pittsburgh, Pa., Vol. 111, No. 2, ASCE, Feb. 1985, pp. 328-342. June 1985. 100. J. Raloff. IR Can Spy Plant Stress Before Eyes 118. T.R. Truax and M.L. Selbo. Results of Acceler Do. Science News, Vol. 127, No. 5, 1985, p. 70. ated Tests and Long-Term Exposures of Glue 101. F. Moses, M. Ghosn, and R.E. Snyder. Applica Joints and Laminated Beams. Transactions of tion of Load Spectra to Bridge Rating. !!!. the American Society of Mechanical Engineers, Transportation Research Record 950, Vol. 1, Vol. 70, No. 5, 1982, pp. 393-400. TRB, National Research Council, Washington, 119. R.R. Avent. Design Criteria for Epoxy Repair D.C., 1984, pp. 45-53. of Timber Structures. Journal of the Struc 102. s.c. Peng. A Pragmatic Approach to Rating tural Division, ASCE, forthcoming. Highway Bridges. .!!! Transportation Research 120. E.E. Hedgecock. In-Place Preservative Treat- 12 Transportation Research Record 1053 ment of Deteriorating Wood Bridge. Engineering 124. Timber Bridge Technology Transfer Plan. Forest Field Notes, Vol. 15, Forest Service, U.S. Products Laboratory, u.s. Department of Agri Department of Agriculture, July-Sept. 1983, culture, Madison, Wis., 1984. pp. 33-43. 125. Glulam Wood Bridge System--Technical Manual . 121. B. Roberts, M.B. Scott, and C.F. Scholer. Minor Weyerhauser Company, Takoma, Wash., 1980. Maintenance Manual for County Bridges . Publi cation H-84-10. Purdue University, West Lafa yette, Ind., Aug. 1984. 122. R.O. Foschi. Reliability of Wood Structural The information contained in this paper reflects the Systems. Journal of Structural Engineering, views, opinions, and conclusions of the authors, and Vol. 110, No, 12, ASCE, Dec. 1984, pp. 2995- does not necessarily represent those of the Forest 3013. Service, U.S. Department of Agriculture. This mate 123. Important Research Needs in Wood as a Struc rial was developed, written, and prepared by em tural Material. Journal of the Structural Di ployees of the u.s. Governmenti therefore, it is in vision, Vol. 105, No. STlO, ASCE, Oct, 1979, the public domain, and private parties or interests pp . 2069-2089. may not hold copyright for this material. Designing Timber Bridges for Long Life FRANK W. MUCHMORE ABSTRACT Wood is a marvelously adaptable structural building material. When treated with a compatible preservative to prevent early decay deterioration, it is an eco nomical and practical structural material for many short-span bridges (spans in the range of 15 to 60 ft). Timber's inertness to deicing chemicals, as well as some new design developments, such as glued-laminated deck panels and pre stressed laminated decks, make it more attractive than ever for use in highway structures, Important factors in assuring long, useful lives for timber bridges include designing to avoid water-trapping details, use of effective and compat ible preservative treatment, and following a systematic inspection and mainte nance program. Attention to these factors will provide a lifespan that is com petitive with other structural materials, such as steel and concrete, and will, in most cases, dramatically increase the useful life of timber bridges. Timber bridges are remarkable structures--when prop • Is simple to fabricate, erly designed, built, and maintained, they can (a) • Is lightweight and easy to install, carry heavy loads without showing material fatigue, • Has a high strength-to-weight ratio, (b) resist the deteriorating action of deicing • Has excellent sound and thermal insulation chemicals, (c) be constructed by unskilled labor, properties, and (d) last for long periods of time. Timber is • Has good shock resistance, particularly adaptable to short-span bridges (spans • Is immune to deicing chemicals, up to 60 ft) and can be economically designed to • Has unique aesthetic qualities, carry heavy highway loads. For example, some timber • Is a renewable resource, bridges in western Canada routinely carry logging • Is long-lasting (when properly protected) , and trucks and machinery weighing in excess of 100 t. •Has good fire resistance Cll· Wood was one of man's first building materials. However, as modern technology has yielded its wonders Note that the unusual claim of good fire resis in mass-produced steel, aluminum, concrete, and tance of structural timber is well founded. When a plastic construction materials, wood has been largely large structural timber is exposed to fire, there is overlooked as a primary structural material for some delay as it chars and eventually flames. As bridges. Wood is still a marvelously adaptable burning continues, the charred layer has an insula structural material. For example, structural wood tive effect, and the burning slows to an average rate of about 1/40 in. (0. 6 mm) per min [or 1-1/2 in. (38 mm) per hr], for average structural timber species. This slow rate of fire penetration means Northern Region, P.O. Box 7669, Forest Service, u.s. that timber structural members subjected to fire Department of Agriculture, Missoula, Mont. 59807. maintain a high percentage of their original