FIGURE 21.-Highway bridge in Pacific Northwest utilizing wood deck and laminated beams and girders placed on bents of timber piles and caps. establishment of design criteria. Working stresses are based on sound principles and improvements have been made in manufacturing processes. Industry practices in both fabrication and design rest largely on the results of research at the Forest Products Laboratory. Future potential Research on improved adhesives and bonding and fabricating methods, as well as on engineering aspects, is continuing. New uses for this material are being developed. A proposed commercial standard for laminated construction has been prepared by the American Institute of Timber Construction and is now being evaluated by the Department of Commerce. An inde pendent inspection agency has been set up by the institute to check quality of laminated structural members in accordance with this standard, and the agency will certify laminated members that conform with the standard. References 1. Freas, A. D., and Selbo, M. L. Fabrication and Design of Glued Laminated Wood Structural Members. U.S. Department of Agriculture, Technical Bulletin No. 1069, 1954. Description and use TIMBER HIGHWAY BRIDGES A great number of timber highway bridges are in use and are being constructed on roads in the national forests and on other public roads where an economical and durable bridge is required. Increasing attention is being paid to details of their design and particularly to the way in which the decking and framing interact as a single loadcarrying structural unit. This has led to consideration of various types of decking and various ways of fastening the decking to the framing, including the use of epoxy resins or other modern adhesives. Structural studies are now underway at the Forest Products Laboratory in cooperation with the Division of Engineering, U.S. Forest Service. These studies include simulated truckloadings on prototype and full-scale bridges in the Laboratory and actual truckloadings on several bridges in highway service in the national forests. Future potential New design criteria are being developed that are expected to make full use of materials and methods now available to produce a strong and durable bridge at minimum cost. WOOD-PLYWOOD STRUCTURAL MEMBERS Description and use Plywood has many uses, both structural and nonstructural, as indicated in an earlier section. Special impact has been given recently to the use of plywood in structural members by the development of a service to assist fabricators in the design and fabrication of woodplywood members. Basic data on the properties of plywood have been developed through research at the Forest Products Laboratory and elsewhere. Such data, together with design procedures that include the rigorous ones needed for aircraft design, are available in a number of publications. Based on such information, the Plywood Fabricators Service, established by the Douglas Fir Plywood Association, has prepared detailed manuals for the design of various types of wood-plywood members. In addition, however, they have published information on fabrication procedures, stressing those aspects necessary to maintain high quality. To assist fabricators in quality control and to maintain quality standards, they have also established a quality control service which maintains surveillance over the fabrication procedures of the subscribing plants. Those who maintain high standards are entitled to use quality marks on their products. FIGURE 22.-Curved sandwich panels utilizing plywood faces and paper-honeycomb cores for canopy over walkway and offices. FIGURE 23. Use of stressed-skin radial folded plates made of %-inch-thick plywood faces glued to 2- by 4-inch wood framing members. A number of types of structural components are involved: Woodplywood box beams; curved and flat stressed-skin and sandwich panels; trussed rafters; and Trofdek, a patented construction consisting essentially of a series of inclined wood-plywood beams joined at their edges to form a series of troughs. All of the components are suitable for a considerable range of building sizes. The components find application in many types of buildings. Longspan box beams, for example, would be suitable for warehouse or industrial construction. The roof might be stressed-skin or sandwich panels. Residences can use trussed rafters. The whole residence might be made with stressed-skin panels for floors, walls, and roof. The list could be extended considerably, but in general, these types of construction units are suitable for residential, light construction, and industrial applications. Future potential Except for stressed-skin constructions, which have been used extensively in prefabricated housing, and for trussed rafters, which are finding increasing use in conventional housing and other light-frame construction, wood-plywood components have had relatively limited use. Their high strength, lightness, and rigidity, however, indicate that they have considerable potential for use in a wide range of structures. For example, sign bridges over highways carry relatively light loads but must be relatively rigid; box beams would be well suited to such a use. References 1. U.S. Forest Products Laboratory. Wood Handbook. U.S. Department of Agriculture, Agriculture Handbook No. 72, 1955. 2. Munitions Board Aircraft Committee. Design of Wood Aircraft Structures. Air Force-Navy-Commerce Bulletin 18, June 1951. 3. Freas, A. D. Guides to Improved Framed Walls for Houses. Engineering News Record, volume 137, No. 16, 1946. 4. Douglas Fir Plywood Association. Technical Data on Plywood, 1942. 5. Douglas Fir Plywood Association. Fir Plywood Roof Sheathing Manual. 6. Douglas Fir Plywood Association. Fir Plywood Box Beam Handbook, 1958. Description and use STRESSED-SKIN CONSTRUCTION The stressed-skin principle developed at the Forest Products Laboratory is the basis for a structural component that consists of two facings, one glued to one side and the other glued to the other side of an inner structural framework to form a box girder. The facing may be of plywood or other suitable material. The stressed-cover principle is based upon the engineering concept that all material should contribute directly to the strength of the structural element. Stressed-skin panel construction can be used for floors, walls, partitions, ceilings, and roof construction. These panels can be designed to be adapted to dwelling units, farm structures, schoolbuildings, doors, trailers, and trucks. Future potential The future of the stressed-skin principle is unlimited in the field of construction. This principle could be adapted to different types of portable structures, fallout shelters, lookout towers, form work, folder plates, curved panels, other types of warped roof surfaces, and temporary types of retaining walls. References 1. Luxford, R. F. Prefabricated House System Developed by the Forest Products Laboratory. Forest Products Laboratory Report No. 1165, 1958. 2. U.S. Forest Products Laboratory. Manual on Wood Construction for Prefabricated Houses. Prepared in collaboration with the Housing and Home Finance Agency, 1947. Description and use SANDWICH CONSTRUCTION Sandwich construction is a composite of thin, strong facings bonded to a thick, lighweight core. The facings carry the principal direct and flexural stresses, while the core carries the shear stresses and stabilizes the faces against buckling. Such a composite provides structural components of high strength and stiffness in relation to weight. A variety of materials can be combined to utilize each to its advantage. Cores, for example, may be honeycombs of various materials such as paper, aluminum foil, or foamed plastic. The facings may be any of a number of sheet materials such as plywood, hardboard, fiberboard, or metals. FIGURE 24.-Plywood box beam A frames being covered with plywood-covered stressed skin panels |