Strength and stiffness performance of FRP reinforced White Oak

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1 Strength and stiffness performance of FRP reinforced White Oak Martin, Zeno, A. 1, Stith, Joe K. 2, Tingley, Dan A. 3 ABSTRACT s have been conducted to determine the performance characteristics of vertically laminated White Oak, as used in truck trailer decks, reinforced with high strength fiber-reinforced plastic (FRP). Significant strength and stiffness increases have been shown for other such wood-frp composites, such as FRP reinforced glue laminated timber (glulam), although most previous research has focused on softwoods. results indicate that FRP reinforced White Oak decks provide many of the same benefits as FRP reinforced softwoods. INTRODUCTION Currently, truck trailer structural flooring (truck decking) is made of high quality laminated White Oak. White Oak is a relatively expensive and heavy wood, creating high initial cost and continuing energy cost associated with the transport of the heavy deck. In the past decade high-strength fiber reinforced polymer (FRP) laminates have been used to reinforce glue-laminated timber (glulam) members, and the resulting FRP-wood composite is significantly stronger, stiffer, and has less variation in structural properties than a conventional glulam (Tingley, 1994; 1996). The FRP laminates are composed of tensioned high-strength pultruded synthetic fibers embedded in a polymer matrix, resulting in a thin FRP panel designed for compatibility with wood using conventional adhesives. These panels are sold by FiRP Sales, Inc. and are marketed worldwide under the trade name FiRP Reinforcement. The performance enhancements realized with the FRP reinforced glulam have motivated research in other areas of potential application. The FRP reinforced truck deck, or other similar hardwood transport containers, show promise to decrease weight, cost, and variability in load response while increasing structural performance. Oak decking Conventional 35 mm (1 3/8-inch) thick truck decking consists of 25.4 mm (1-inch) wide by 35 mm (1 3/8-inch) thick oak strips laminated along their wide face to form a 305 mm (12-inch) wide plank that is 35 mm (1 3/8-inch) thick. These 305 mm (12-inch) planks are placed side by side in a truck trailer floor to span the width. The length of the deck is limited only by practical considerations, as the laminates are end jointed with a simple hook joint, as shown in Figure 1. This type of end joint is used predominately to facilitate manufacture and to resist moisture migration from external sources and cannot effectively resist tensile stresses that occur during heavy loading of the laminated truck deck planks. As a result, many failures of the unreinforced truck decking planks occur near these hook joints. To reduce this type of failure and to better utilize the compressive strength of the laminated oak truck decking, thin FRP laminate can be adhered to the bottom deck surface (tension side), as shown in Figure 1. TEST PROGRAM Two sets of tests, ultimate load and stiffness, were performed on both unreinforced and FRP reinforced laminated oak truck decks to determine the performance difference provided by the FRP reinforcement. specimens The Oak laminated specimens had depths varying between 19 mm (0.75-inches) and 35 mm (1.375-inches), the width of the individual laminates, making up the deck, remained 25.4 mm (1-inch). The overall deck width was 305 mm (12- inches) and the length was 406 mm (16-inches). Specimens were tested both with and without FRP reinforcement. Two different types of FRP reinforcement were used in this test program: glass-aramid reinforced polymer (GARP) and aramid 1 Structural Engineer, Wood Science and Technology Institute (N.S.), Ltd., Corvallis, OR Engineer, Wood Science and Technology Institute (N.S.), Ltd., Corvallis, OR Executive Director, Wood Science and Technology Institute (N.S.), Ltd., Corvallis, OR 97333

2 reinforced polymer (ARP), the properties of which are shown in Table 1. Extensive testing has been performed to determine FRP panel properties (Tingley, 1996; Tingley et al., 1997). The width of the FRP panels was approximately equal to that of the Oak deck. Table 1. Material Properties of FRP Reinforcement Panels Modulus of Elasticity Design Tensile Strength Thickness FRP Type (MPa) (ksi) (MPa) (ksi) (mm) (in) ARP 80,000 11, GARP 55,172 8, Bonding the high strength fiber reinforcement to the laminated oak truck deck can be done using different types of adhesives. A two-part epoxy was used in this study for its ease of application in a shop environment. In commercial manufacturing facilities, conventional wood adhesive (e.g. phenolics) can be used. Hook Joint High Strength Fiber Reinforcement Figure 1. A Hook Joint on a Truck Deck, Shown Reinforced with FRP (GARP) set-up The decks were tested on an apparatus that conforms to Truck Trailer Manufacturers Association (TTMA) report requirements. The decks were supported by steel plates that provide a 254 mm (10-inch) clear span, and a 305 mm (12- inch) center-to-center distance, to simulate actual 51 mm (2-inches) wide truck deck support flanges. A load is applied from two 89 mm (3.5-inch) long by 127 mm (5-inch) wide rubber pads, 25.4 mm (1-inch) apart, which simulates a single set of dual forklift tires. A diagram of the test set-up is shown in Figure 2. Oak deck supports 254 mm 305 mm Elevation View Plan View 89 x 127 mm rubber pad (simulated tire area) Figure 2. Diagram of Truck Deck Set-Up Ultimate load testing The ultimate load testing was performed for both unreinforced and GARP reinforced specimens. The ultimate load capacity of these specimens was determined by loading at a constant rate of N/s (40-60 pounds per second), to failure, which occurred within approximately 5-10 minutes.

3 Stiffness testing The stiffness was determined from non-destructive load testing, and each individual specimen was loaded twice, first without FRP reinforcement, and then with ARP reinforcement. The decks were loaded to approximately 25-50% of their ultimate load and then unloaded. It is assumed that this load level applied for a relatively short duration does not cause any fiber damage or permanent deformation to the specimens, so that they could be re-tested a second time with ARP reinforcement. Load deflection data was acquired from a digital load meter and an analog dial gage. A constant load was applied at a rate of N/s (18-20 pounds per second), to 22,250 N (5,000 pounds), and deflection readings were recorded from an analog dial gage every 1,110 N (250 pounds). STIFFNESS ANALYSIS To determine stiffness, the deck was treated as a beam with a point load in the center. The deflection for a simply supported beam, of rectangular cross section, with a point load in the center is: Which can be rearranged to solve for the stiffness, EI: 3 PL 12(1.2)EI = EI GAL δ 2 (P / δ)l EI = L ((P / δ) 4GA The slope of the load-deflection graph for each deck is input as (P/δ). The modulus of elasticity, E, can be found by initially assuming a shear modulus, G, and dividing EI by I, the moment of inertia, which is based on the gross cross section. The shear modulus is then changed, or iterated, until E/G is 12, which is an approximate value published in the Wood Handbook (Forest Products Laboratory, 1999) for White Oak. To predict the stiffness of the ARP reinforced deck, the E is first calculated for each unreinforced plank, as described above. The moment of inertia is then calculated by transformed section theory using the E of the wood and the E of the reinforcement, 80,000 MPa (11.6 million psi) in this case. These two numbers (E and I) are multiplied to provide a predicted stiffness of the reinforced truck decks. TEST RESULTS Stiffness Stiffness test results for the 25.4-mm (1-inch) and the 35 mm (1 3/8-inch) unreinforced and ARP reinforced Oak truck decks are presented in Table 2 and Table 3, respectively. Both tables also show the percent difference between the two values for both the measured and calculated test results. The average modulus of elasticity of the oak was calculated to be 6.45 KN-m 2 (2,248,00 lb-in 2 ) for the 25.4 mm (1-inch) decks and 6.27 KN-m 2 (2,183,000 lb-in 2 ) for the 35 mm (1 3/8- inch) decks. Ultimate load A summary of the ultimate load test results is presented in Table 4, along with the percent increase in strength with the addition of the GARP reinforcement. Various failures were observed during ultimate load testing. For the unreinforced planks, failure was observed in the wood in the bottom (tension zone) of the deck, as well as failure of the hook joints. A failure in the tension zone of the wood can occur instantly with little or no warning.

4 The addition of the high strength fiber reinforcement shifted the failure mode from being tension based to compression based, as seen in Figure 3. A compression failure is a very slow failure, preceded by excessive deformation before a catastrophic failure. Table 2. Stiffness Results for the 25.4 mm (1-inch) thick Unreinforced and Reinforced Oak Truck Decks Specimen Unreinforced EI (KN-m 2 ) Measured Data Reinforced EI (KN-m 2 ) Percent Increase in EI from FRP Calculated Data Percent Difference Predicted EI (KN-m 2 Between Measured and ) Predicted A % % A % % A % % A % % A % % Average % % Std. Dev % % COV 5% 10% 26% 4% 89% Table 3. Stiffness Results for the 35 mm (1.375-inch) Unreinforced and Reinforced Oak Truck Decks Specimen Unreinforced EI (KN-m 2 ) Measured Data Reinforced EI (KN-m 2 ) Percent Increase in EI from FRP Calculated Data Percent Difference Predicted EI (KN-m 2 Between Measured and ) Predicted B % % B % % B % % B % % B % % Average % % Std. Dev % % COV 4% 16% 61% 2% 97% Table 4. Comparison between Unreinforced and FRP Reinforced Laminated Oak Truck Decks Specimen Thickness Unreinforced GARP Reinforced Thickness (mm) Thickness (in) Number of Specimens Average (N) Number of Specimens Average (N) Percent Increase in 19 3/4 1 38, , % 22 7/ , , , % / , /8 2 87, ,293 85% Note: 1 N = lbf

5 Figure 3. Compression Wrinkles on the Top Surface are Typically Seen in a Failure of FRP Reinforced Laminated Oak Truck Decks SUMMARY A test program has been conducted to evaluate the bending strength and stiffness performance of FRP reinforced White Oak vertically laminated decks, as used in truck trailer flooring. Results indicate that significant strength and stiffness increases can be achieved using thin FRP laminates as tensile reinforcement for these hardwood specimens. For equal White Oak truck deck cross sections, a thin FRP laminate adhered to the tension face is shown to: Increase stiffness by 40-70%. Increase ultimate load strength by %. Conventional elastic theory, using transformed sections, is shown to predict stiffness to within 15-25% of actual test data. The following assumptions, and aspects may account for this difference in predicted to actual result: E/G=12 may or may not be valid for this particular White Oak. With the span to depth ratios considered in this study, 7-13, shear deformation contributes a significant portion to the total deflection. Thus, the measured values are sensitive to the shear modulus assumption. The hook joints constitute a gross discontinuity in the wood material, so stress transfer in the wood is not as assumed using elastic theory. The true experimental load applied was partially distributed, but for analysis purposes it was treated as a point load. REFERENCES Forest Products Laboratory Wood Handbook: Wood as an Engineering Material. U.S. Government Printing Office. Agricultural Handbook 72. Washington DC: U.S. Department of Agriculture; rev Tingley, D. A Aligned Fiber Reinforcement Panel for Structural Wood Members. U.S. Patent Document No. 5,362,545. Tingley, D. A The stress-strain relationships in wood and fiber-reinforced plastic laminae of glued-laminated wood beams, Ph.D. Thesis, Oregon State University, Corvallis, Oregon. Tingley, D. A., Gai, C., Giltner, E. E ing Methods to Determine Properties of Fiber Reinforced Plastic Panels Used for Reinforcing Glulams. ASCE Journal of Composites for Construction, Vol. 1, No. 4, November, pp