Applications of Wood Materials for Innovative Bridge Systems

Transcription

1 In: Suprenant, Bruce S., ed. Serviceability and durability of construction materials: Proceedings of 1st materials engineering congress; 1990 August 13-15; Denver, CO. New York: American Society of Civil Engineers; 1990: Vol. 1. Applications of Wood Materials for Innovative Bridge Systems Abstract Russell C. Moody, Michael A. Ritter, 1 and Hota GangaRao 2 This paper describes completed research and the status of research underway on timber bridges that are part of the USDA Forest Service timber bridge initiative. The initiative includes a research and technology transfer program to develop new and improved bridge system and increase awareness of the attributes of timber bridges. Research on material properties, preservative treatments, and innovative system development is included, and programs are underway at several universities in the United States, many of which are cooperating with the Forest Products Laboratory. Introduction A significant opportunity has been identified in the United States to improve rural transportation networks by using wood for bridge construction. Over 40 percent of the approximately 575,000 highway bridges across the nation are either structurally deficient or functionally obsolete and need to be replaced. Many of these bridges are ideally suited for wood construction. Technological advances in bridge design and preservative treatments have made the wood bridge an economical, safe, and attractive alternative to conventional bridge systems. To address this opportunity, the United States Congress funded the Timber Bridge Initiative, beginning in This legislation emphasizes increased use of locally available material, which for most regions in the populus Northeast and Midwest, is hardwoods. Hardwood growth exceeds harvest in the eastern United States, which is where many of the deficient bridges are located. In addition to providing needed bridges, an objective of the program is to stimulate rural economies through harvest and processing of local materials and local fabrication and construction of bridge components. As a result of the Timber Bridge Initiative, research on material properties, preservative treatments, and system development has significantlyexpanded in the past 2 years. Most of this research has addressed the development of new bridge systems with an emphasis on hardwoods, which are currently underutilized for structural applications throughout the eastern United States. Several experimental bridges have been developed and constructed on National Forests using innovative wood materials and systems as part of the research effort. In 1 Research Engineers, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, Wisconsin The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. 2 Professor of Civil Engineering, West Virginia University, Morgantown, WV. 423

2 424 CONSTRUCTION MATERIALS SERVICEABILITY/DURABILITY addition, more than 100 timber bridges have been funded for construction as demonstration projects for the Forest Service technology transfer effort. These include more than 50 bridges in West Virginia and 18 bridges in Pennsylvania. Most of the bridges in these two states will use significant amounts of structural hardwood lumber and innovative designs developed through research programs at the Forest Products Laboratory (FPL) and West Virginia University (WVU). Timber Bridge Research Material Properties Until recently, design values for most species of hardwood lumber were not available to engineers. Because of low demand and negligible production of structural hardwood lumber, the development of applicable grades, grading rules, and design stresses, such as are available for many softwoods, could not be justified. However, in anticipation of increased use of hardwoods for applications such as bridges, this information has been developed for several species groups and is now published in the National Design Specification for Wood Construction (NFPA 1986, 1988). Marketing classes and actual hardwood species permitted in those classes are given in Table 1. Research in this area deals with developing improved grading systems for hardwood species. An evaluation of structural grades of red oak is underway, and a similar evaluation of red maple is planned at FPL. These studies are based on a visual grading system to classify material, as is commonly used for softwood species. Both the FPL and WVU are investigating alternative grading procedures to improve the yield of structural material from the resource. Acoustic emission (AE) techniques are being used by WVU to determine if the AE signals obtained during testing can be related to physical properties. The applicability of a stress-class grading system, which is similar to that used in several other countries, is being considered (Green & Kretschmann, in press). Preservative Treatment Systems Hardwoods have traditionally been pressure treated with wood preservatives for products such as railroad ties. Traditional oil-type preservatives, such as creosote, have greatly extended life and provided good field performance. However, little is known about the treatment of hardwoods with waterborne treatments. Limited information suggests that higher retentions of the waterborne treatments will be required to provide the same level of protection in hardwoods as in softwoods. Thus, research has been initiated by FPL in cooperation with Mississippi State University to determine appropriate treatment levels of waterborne preservatives for proper protection of hardwoods. Treatment additives that reduce the rate of moisture changes in the waterborne-treated wood under exterior exposure are also being evaluated. Both laboratory research and exterior exposure tests are proceeding for three hardwoods (red oak, red maple, and yellow poplar) and one softwood (Southern Pine). Another related study at WVU is aimed at determining if

3 INNOVATIVE BRIDGE SYSTEMS 425 Table I. Marketing classes and species for hardwood lumber Marketing class Aspen Black Cottonwood Cottonwood Mixed Hardwoods Mixed Maple Mixed Oak Northern Red Oak Red Maple Red Oak White Oak Species included Bigtooth aspen, quaking aspen Black cottonwood Cottonwood American beech, bitternut hickory, mockernut hickory, nutmeg hickory, pecan hickory, pignut hickory, shagbark hickory, shellbark hickory, sweet birch, water hickory, yellow birch Black maple, red maple, silver maple, sugar maple All oaks listed for other groups Black oak, northern red oak, pin oak, scarlet oak Red maple Black oak, cherrybark oak, laurel oak, northern red oak, pin oak, scarlet oak, southern red oak, water oak, willow oak Bur oak, chestnut oak, live oak, overcup oak, post oak, swamp chestnut oak, swamp white oak, white oak the temperatures used during pressure treatment and subsequent drying have a significant effect on strength properties. Guard Rail Systems Improved guard rail systems that meet proposed federal standards are needed for timber bridge systems. A requirement of these new standards is that rail system performance he proven through full-scale crash testing. The FPL and the University of Nebraska-Lincoln are cooperatively developing and verifying approved rail systems. The first phase of this work includes longitudinal deck systems at the lowest loading performance level (PL-l), which will be applicable for most secondary roads. However, for many applications, a higher performance level rail system may be needed. Thus, additional work is needed to develop and verify rail systems for both higher load levels and alternative deck systems.

4 426 CONSTRUCTION MATERIALS SERVICEABILITY/DURABILITY Glulam Bridge Systems Previous Forest Service research demonstrated the structural advantage of glued-laminated (glulam) decks over nail-laminated decks and presented analytical methods for predicting the performance of glulam panel decks with steel dowel connectors (McCutcheon & Tuomi 1973). Simplified methods were adopted in the design specifications of the American Association of State Highway and Transportation Officials (AASHTO 1989; McCutcheon & Tuomi 1974). Erection procedures were suggested that facilitate the construction of glulam decks with dowel connectors (Tuomi 1976) During the summer of 1983, Forest Service researchers investigated the long-term performance of timber bridges by inspecting 18 bridges across the northern United States (Gutkowski & McCutcheon 1987). A major feature of this study was the acquisition of extensive moisture content data. The researchers found that glulam decks with wearing surfaces protected the substructures from water penetration. Based on this finding, the American Institute of Timber Construction (AITC) allows dry-use stresses for designing stringers that are covered with glulam decks but requires wet-use stresses for decks and other exposed components. The study also found that glulam panel decks are performing well structurally after up to 20 years of service. Stress-Laminated Bridge Systems Stress-laminated Slab Decks The Ontario Ministry of Transportation (MTO) in Canada has developed a bridge construction concept, referred to here as stress laminating, for restoring longitudinal nail-laminated bridges (Taylor et al. 1983). In this system, steel stressing rods are installed on the bridge to compress the deck transversely and develop plate action in the timber deck (Fig. 1). The resulting system has desirable load distribution characterisitics. Although this system was conceived as a method for rehabilitating bridges, it has also been applied to new construction. It has been used successfully in at least nine rehabilitated bridges and four new bridges in Ontario. Based on research conducted by MTO in cooperation with Queen s University and favorable field experience, specifications for the design of these systems have been included in the Ontario Highway Bridge Design Code (OHBDC) (MTO 1983). Recent and current Forest Service research on timber bridges is directed toward developing methods for designing and constructing stress-laminated bridges. Design criteria for these bridges will be based on the results of laboratory research and information on field performance. Results of cooperative research conducted by the Forest Service and the University of Wisconsin provide information on analytical methods for predicting deflections, transverse stress distribution, and loss in transverse stress with time (Oliva et al. 1987, 1990). Research underway at WVU (described later) is extending this work to other configurations (Dickson and GangaRao 1988, 1989; GangaRao and Raju 1989).

5 INNOVATIVE BRIDGE SYSTEMS 427 Figure 1. Stress-laminated timber bridge concept. (ML ) Ongoing cooperative research with Georgia Southern University will evaluate the use of lumber treated with waterborne preservatives in stress-laminated deck systems. The idea of using waterborne preservatives in these types of decks has raised concerns about rapid changes in moisture content, the accompanying shrinking and swelling, and the effect on transverse load in the stressing rods. Various methods of minimizing moisture content changes will be investigated. Parallel-Chord Truss Systems A limiting factor in the performance and span capabilities of stresslaminated slab deck systems is deflection under vehicle loads. Span limitations are approximately the same as those of conventional decks constructed of glulam timber or nail-laminated sawn lumber. To increase the stiffness and span of stress-laminated bridges, a parallel-chord bridge has been developed cooperatively with the University of Wisconsin. Rased on the results of experimental and analytical evaluations, an experimental bridge was designed and constructed in 1987 (Fig. 2). This bridge has been monitored periodically and was load tested immediately after construction and 2 years later. Reports on this research are being prepared. Additional research is underway to develop more efficient truss systems. Metal plate truss connectors are compatible with creosote-treated wood under static loading conditions (Oliva et al. 1988). T- and Box Sections To further extend the span capabilities of stress-laminated timber, several alternative systems are now being researched at WVU. These include T-sections, bulb T-sections, and box sections (Fig. 3). In the research, hardwoods native to West Virginia have been used as the flange (or decking), and glulam, LVL,

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7 INNOVATIVE BRIDGE SYSTEMS 429 Figure 4. The first stress-laminated bridge constructed using a T-beam concept spans 75-ft near Charleston, West Virginia. (M ) The stressed timber T-beam bridge designed at WVU and built in Charleston, West Virginia, in May 1988 has performed very well (Dickson & GangaRao 1989, 1990). Load tests conducted on this 75-ft span structure soon after construction indicated a defection level of about L/850 for an AASHTO HS-20 loading. Thus, the geometry of the T-beam, which included an LVL web and red oak lumber deck, provided adequate stiffness. Subsequent designs using the T-beam approach will provide more economical bridge systems. Continued monitoring of the Charleston bridge and others to be installed will provide information on long-term performance and maintainability. Further laboratory research at WVU is addressing various aspects of system response including the composite action between flange components and stringers, load-deflection relationships, diaphragm effects, failure modes, and transverse load distribution. Systems have been evaluated in the laboratory under different load conditions. The experimental deflections and strains of the stringers were compared with those predicted using orthotropic plate equations and compared with the experimental results. Reports on the results of this work are being prepared. Modular Decking for Rehabilitation Research underway at WVU, in cooperation with FPL, will develop methods for using stress-laminated slab systems for rehabilitating or replacing deck systems on steel or concrete stringers. Initial work is focused on determining the long-term response of anchorage or connector devices for securing modular bridge deck components to steel members. Predictability of the clamping or hold-down force is essential for design of the stringers to which the timber deck is connected. Initially, cyclic loads were applied directly to the top of the con-

8 430 CONSTRUCTION MATERIALS SERVICEABILITY/DURABILITY nectors. After many cycles in both new and old wood and on several specimens, a predictable relationship between clamping force and number of load cycles is established. Additionally, wheel loads rolling across the connectors, as in a bridge deck, are simulated, and the response of connectors is studied. Utilizing these results, a series of bridge floors will be designed, built in the field on an experimental basis, and evaluated for performance. Additional work is underway to characterize the fatigue behavior of northern red oak for use in the bridge deck. In addition to tests on small samples of the material, fatigue tests of laminated timber decks are being conducted. A damage model will be developed to predict service life of stressed timber decks under fatigue loads. Finally, composite action of the connectors between the small stressed timber decks and steel stringers under cyclic loading will be investigated. Elastic Properties for Design Mechanics-based analysis and subsequent development of design procedures requires elastic constants for the component materials. Thus, representative values are needed. Work in Ontario and in the United States provides information on some softwoods. Research at WVU is aimed at determining the longitudinal and shear moduli of individual hardwood boards of two species (red oak and hickory) and two grades (No. 2 and No. 3). In addition, the elastic moduli in both directions and the in-plane and transverse shear moduli of stressed lumber will be determined. Design of Stressed Deck Systems Design procedures for stress-laminated decks have been given in the OHBDC since 1953; however, the OHBDC design methods vary significantly from design methods currently given by AASHTO. The load distribution assumed for stress-laminated bridges in the OHBDC is considerably higher than currently allowed in the AASHTO specification for wood deck bridges with nailed connections. The AASHTO specifications do not have separate distribution factors for stress-laminated bridges. Efforts are now underway to obtain AASHTO acceptance of design criteria for stress-laminated deck systems. Two types of efforts are proceeding simultaniously-one is the development of design procedures and the other is the verification of performance of bridges built using this approach. Design procedures based on adaptations of theory are being developed and considered by the appropriate AASHTO committees. Bridge performance is being verified through a comprehensive bridge monitoring program described in the following section. Bridge Monitoring Both experimental bridges built as part of active research programs and demonstration bridges built as part of the Forest Service Timber Bridge Initiative

9 INNOVATIVE BRIDGE SYSTEMS 431 are included in a structural monitoring program. FPL is currently participating in the monitoring of approximately 20 bridges to measure stressing rod force levels, deck moisture effects, and long-term creep. A nondestructive test approach to monitor the stressing-rod force levels is being investigated at WVU. This is based on the principle that the dominant frequency of the stressing rod has distinguishable variations for various tension levels. WVU and FPL will cooperatively monitor several of the 30 bridges planned to be constructed in West Virginia in Many bridges have also been load tested to determine field performance characteristics. This monitoring and load test information is being used to validate and further refine proposed design criteria for stress-laminated bridge systems. Concluding Remarks An expanded research program to develop and improve timber bridge systems, which resulted from the Timber Bridge Initiative, involves research in the areas of material properties, preservative treatments, and system development. The majority of the research has been directed toward the development of new bridge systems that allow for the use of wood species that are currently underutilized for structural applications. Results are being implemented through experimental and demonstration bridges being construction across the United States. A bridge-monitoring program has also been implemented to provide comparative results between field performance and laboratory work. Appendix-References

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