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Strength and stiffness of reinforced yellow-poplar glued ... Strength and Stiffness of Reinforced Yellow-Poplar Glued-Laminated Beams Roland Hernandez, Research General Engineer Forest

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  • United States Department of Agriculture

    Forest Service

    Forest Products Laboratory

    Research Paper FPL–RP–554

    Roland Hernandez Julio F. Davalos Somnath S. Sonti Youngchan Kim Russell C. Moody

    Strength and Stiffness of Reinforced Yellow-Poplar Glued-Laminated Beams

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  • Abstract In bridge applications, it is often necessary to minimize the depth of the bridge structure to provide for the required hydraulic opening or reduce the volume of approach fill. For bridges that utilize structural glued-laminated (glulam) tim- ber beams as stringers, reinforcement using thin strips of pultruded E-glass-fiber-reinforced plastic (GFRP) composites may permit reduced depth, because the reinforcement has the potential to increase stiffness and strength. This study is part of an overall effort aimed at evaluating the potential for com- mercial production of glulam-GFRP beams in current wood- laminating plants and a wood adhesive compatible with existing equipment. Twelve Yellow-Poplar glulam GFRP beams were commercially manufactured, and their perform- ance was evaluated. The GFRP panels were bonded to the wood with a resorcinol formaldehyde adhesive to provide the reinforcement. The simplicity of the process used to manufac- ture the test beams indicates that the commercial production of glulam-GFRP beams is feasible. Increases of 18 percent in stiffness and 26 percent in strength were achieved by adding 3 percent of GFRP by volume. The bending strength values of the beams predicted by the ASTM D3737 procedure correlate well with the experimental values. However, the observed delamination of the reinforcement indicates that improved bonding strength of wood–GFRP interfaces is needed. Results of this study will be useful to manufacturers interested in improving the performance of glulam timber beams.

    Keywords: Glulam, fiber-reinforced plastic, Yellow-Poplar, composites

    July 1997

    Hernandez, Roland; Davalos, Julio F.; Sonti, Somnath S.; Kim, Youngchan; Moody, Russell C. 1997. Strength and stiffness of reinforced Yellow- Poplar glued-laminated beams. Res. Pap. FPL–RP–554. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 28 p.

    A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53705–2398. Laboratory publications are sent to more than 1,000 libraries in the United States and elsewhere.

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    Acknowledgments We thank Creative Pultrusions, Inc., for providing the fiber- reinforced plastic laminates, Indspec Chemical Corporation for providing the adhesives, and Unit Structures for manufac- turing the glued-laminated beams.

    Contents Page

    Introduction................................................................1

    Background................................................................1

    Objective and Scope.....................................................2

    Research Methodology .................................................2

    Beam Design...........................................................2

    Conventional Beam...............................................2

    Reinforced Beam...................................................2

    Material Selection and Characterization ........................3

    Material Selection.................................................3

    Material Characterization........................................4

    Finger Joint and Beam Manufacture.............................4

    Evaluation Procedures...............................................5

    Finger-Jointed Lumber...........................................5

    Glulam Beam.......................................................5

    Results and Discussion.................................................7

    GFRP Evaluation.....................................................7

    Lumber Evaluation...................................................7

    Finger-Jointed Lumber Evaluation...............................7

    Glulam Beam Evaluation...........................................8

    Failure Response ..................................................9

    Stiffness Performance .............................................9

    Strength Performance.............................................9

    Strain Response..................................................10

    Discussion of Alternative Design Approaches..................11

    Conclusions .............................................................12

    Literature Cited.........................................................12

    Appendix A—Finger-Joint Evaluation...........................14

    Appendix B—Beam Failure Descriptions.......................17

    Appendix C—Beam Failure Maps................................19

    Appendix D—Load–Displacement and Strain–Load........23

  • Strength and Stiffness of Reinforced Yellow-Poplar Glued-Laminated Beams Roland Hernandez, Research General Engineer Forest Products Laboratory, Madison, Wisconsin

    Julio F. Davalos, Associate Professor Somnath S. Sonti, Research Engineer Youngchan Kim, Graduate Research Assistant Department of Civil and Environmental Engineering West Virginia University, Morgantown, West Virginia

    Russell C. Moody, Supervisory Research General Engineer Forest Products Laboratory, Madison, Wisconsin

    Introduction The abundance and underutilization of Yellow-Poplar lumber in the regions of Virginia and West Virginia provide a poten- tial resource for a variety of structural applications. Long-span, glued-laminated (glulam) beams are currently needed for timber bridges in these regions. Although struc- tural glulam timbers are produced primarily from Southern Pine and Douglas Fir lumber, Moody and others (1993) recently showed that Yellow-Poplar beams can be manufac- tured with comparable engineering properties to those of softwoods. As a result, standards for Yellow-Poplar glulam timber beam combinations with stiffness and strength proper- ties comparable to softwoods are being incorporated by the American Institute of Timber Construction (AITC) into the hardwood glulam standard (AITC 1996). For some bridge applications, it is important to minimize the depth of the superstructure to provide for the needed hydraulic opening or minimize the volume of fill in the approaches. By reinforcing glulam timber with a material that has a high level of stiff- ness and strength, the required depth of the bridge stringers can be reduced and an increase in bending stiffness and strength can be achieved.

    Background Previous researchers have studied the feasibility of reinforcing glulam beams with metals and fiber-reinforced plastics. Spaun (1981) presented reviews of previous work on the reinforcement of wood with other materials. Metal reinforce- ments included aluminum and steel; various adhesive sys- tems and mechanical fasteners were used to attach the metal reinforcements. Mark (1961) used a formaldehyde adhesive to bond aluminum strips, coated with casein-latex primer, to wood cores. Failure of the beams occurred mainly by separa- tion and buckling of the aluminum facings. Similarly, Peter- son (1965) reinforced wood beams with pretensioned steel

    strips. Lantos (1970) used round steel bars to reinforce wood beams. Curtis (1972) used steel-plate reinforcement that extended through the entire depth of wood beams, a method called the flitch plate beam design, and obtained a 12- to 31-percent increase in stiffness but no increase in strength. Krueger and Sandberg (1974) reported on ultimate strength design of Southern Pine reinforced on the tension side with a bronze-coated woven steel wire, which was bonded with epoxy. The beams exhibited compression failures.

    Fibrous composite reinforcements were also used by several researchers. Theakston (1965) used resin-preimpregnated fiberglass reinforcements bonded with epoxy. The beams showed a 39-percent increase in bending strength. Spaun (1981) selected E-glass fiber rovings for reinforcement be- cause of its low cost and phenol-resorcinol adhesive for all gluelines. He observed significant increases in tensile strength in proportion to the fiber-volume fraction of the E-glass reinforcement. Tingley (1994) described a method of reinforcing glulam timber using aramid fiber and glass- reinforced laminates. Plevris and Triantofillou (1992) studied the response of wood beams reinforced with thin layers of carbon-fiber-reinforced plastic composites. Triantofillou and Deskovic (1992) used prestressed carbon-fiber-reinforced plastic sheets as external reinforcements of wood