Abstract
Laminated veneer lumber (LVL) is an engineered wood product that is commonly used for joists in wooden buildings. Holes in joists are often necessary to allow piping systems to pass through. Introducing a hole into an LVL joist remarkably changes the distribution of stresses in the vicinity of the hole. Tensile stresses occur perpendicular to the grain, and the capacity of the joist can be decreased accordingly. This study presents the experimental results of LVL joists with holes and reinforcement methods around the holes. The results showed that cutting a large enough hole contributed substantially to the strength reduction of LVL joists. Holes with a diameter-to-joist depth ratio of 0.4, 0.5, and 0.6 reduced the load-capacity 50.1%, 59.6%, and 68.8%, respectively. Glued plywood and glued-in threaded rods were both effective methods for reinforcing LVL joists with holes having a diameter-to-joist depth ratio of less than 0.5. The reinforcement effect of nailed plywood was relatively poor, increasing the load-capacity less than 30%. The reinforcement effect of all of the methods depended on the effective joint with the LVL. The thickness of the plywood, the number of nails, and the withdraw strength were also important factors.
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Experimental Behavior of Laminated Veneer Lumber with Round Holes, with and without Reinforcement
Xiaolan Zhang,a,c Yilin Que,b Xinmeng Wang,a Zherui Li,a,c Liuliu Zhang,a Cong Han,a Zeli Que,a,* and Kohei Komatsu a,c
Laminated veneer lumber (LVL) is an engineered wood product that is commonly used for joists in wooden buildings. Holes in joists are often necessary to allow piping systems to pass through. Introducing a hole into an LVL joist remarkably changes the distribution of stresses in the vicinity of the hole. Tensile stresses occur perpendicular to the grain, and the capacity of the joist can be decreased accordingly. This study presents the experimental results of LVL joists with holes and reinforcement methods around the holes. The results showed that cutting a large enough hole contributed substantially to the strength reduction of LVL joists. Holes with a diameter-to-joist depth ratio of 0.4, 0.5, and 0.6 reduced the load-capacity 50.1%, 59.6%, and 68.8%, respectively. Glued plywood and glued-in threaded rods were both effective methods for reinforcing LVL joists with holes having a diameter-to-joist depth ratio of less than 0.5. The reinforcement effect of nailed plywood was relatively poor, increasing the load-capacity less than 30%. The reinforcement effect of all of the methods depended on the effective joint with the LVL. The thickness of the plywood, the number of nails, and the withdraw strength were also important factors.
Keywords: Laminates; Wood, Fracture; Hole; Strength
Contact information: a: College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, 210037, China; b: High School Affiliated to Nanjing Normal University, Nanjing, 210037, China; c: Research Institute for Sustainable Humanosphere, Kyoto University, Uji, 6110011, Japan;
* Corresponding author: zelique@njfu.edu.cn
INTRODUCTION
Laminated veneer lumber (LVL) is an internationally used generic descriptor for an assembly of veneers laminated with adhesive, in which the grain direction of the outer veneers and most of the other veneers is in the longitudinal direction, as described in AS/NZS 4357.2 (2006). Laminating distributes the natural defects of the wood and provides uniform structure, dimensional stability, and strength. As a result of these characteristics, LVL has become one of the most important engineering wood products used today. As a structural material, LVL is commonly used as a carrying component, such as in joists, and the structural strength is the focus.
Holes in joists represent frequent constructive requirements in buildings. Economical and architectural reasons often prohibit any constraints to room height, e.g., caused by pipes for ventilation, water, and sewage (Aicher and Hofflin 2004). The pipes have to penetrate through the joists via holes located generally at mid-depth of the cross-section. The holes can be of considerable size, acting as stress concentrators and giving rise to greater tensile stresses perpendicular to the grain. Wood is very weak when exposed to tensile stress perpendicular to the grain. A large enough hole in the shear dominant region of the joist can change the failure mechanism to crack initiation and propagation around the hole (Ardalany et al. 2013b). Global strength is most often limited by perpendicular-to-the-grain fractures with crack initiation at the hole periphery and crack propagation along the grain direction. Hence, additional reinforcement should be given to joists with holes.
Studies of beams with holes and corresponding reinforcement methods have focused on glulam and I-beams (Johannesson 1983; Petersson 1995; Riipola 1995; Afzal et al. 2006; Pirzada et al. 2008). The reinforcement methods, described in the standard DIN 1052 (2008), use plywood and screws. Other reinforcement materials, such as steel bars, fiberglass, and self-tapping screws, were proposed and investigated (Hallström 1996; Aicher and Hofflin 2009). Formulas and design models for reinforced beams with holes are less.
Some research on LVL joists has been conducted recently. Ardalany et al. (2012) studied the effect of the location of a hole. The results indicated that the load-carrying capacity underwent a subtle change when the hole position was moved perpendicular to the grain, a slight difference when the hole position was moved to below the neutral axis, and a 10% positive difference when the hole position was moved to above the neutral axis in the compressed part of the joist. Additionally, in the three-point-bending test, the load-carrying capacity was nearly constant when the hole position was moved along the length of the joist, as long as there was enough distance that the hole is at least one beam height from the support and the loading location. Some reinforcement methods have also been tested by Ardalany et al. (2013a). The research showed a promising solution using glued plywood.
In the present study, LVL joists with holes of varying diameters (80 mm, 100 mm, and 120 mm) were tested for the purpose of evaluating the effect of opening diameter. Various options for reinforcement were investigated for the sake of proposing effective and convenient reinforcement methods. Nailed plywood, glued plywood, and glued-in threaded rods were used to reinforce LVL joists with holes and several variables not reported in the previous research were investigated.
EXPERIMENTAL
Materials
Laminated veneer lumber was produced from Douglas fir and manufactured by Weyerhaeuser (Seattle, WA, USA). There were 13 layers, averaging 45 mm in thickness, and all laminae were in the same direction. The mean density and moisture content, measured according to GB/T 20241 (2009), were 560 kg/m3 and 7.7%, respectively. Other mechanical properties of the LVL are presented in Table 1. Tension strength perpendicular to the grain was a critical variable in this experiment, and the data were measured according to BS EN 408 (2012). The LVL was cut into test pieces with dimensions of 1500 mm × 200 mm × 45 mm, and round holes with various diameters were cut into these pieces according to the experimental program in Table 2.
Table 1. Material Properties of LVL
Table 2. Experimental Program
The plywood was made of fast-growing poplar grown in Siyang County of Jiangsu Province, China. Three common thicknesses of plywood in the Chinese market were selected (5 mm, 9 mm, and 12 mm) to compare the strengthening effect of plywood of varying thickness. The plywood, with dimensions of 1220 mm × 2440 mm, was cut to specimens with dimensions of 200 mm × 200 mm, and round holes of various diameters were cut into the specimens according to the experimental program in Table 2. The mean density and moisture content, measured according to GB/T 9846.3 (2013), were 0.43 g/cm3 and 11.4%, respectively.
Plywood specimens were nailed or glued to both sides of the LVL joists with holes (Fig. 1). No adhesive was used when the plywood was nailed to the LVL to compare the reinforcement effects of nailed plywood and glued plywood. The adhesive was epoxy resin. Two kinds of nails were selected, round nails and screw nails, both with 40-mm length and 2.0-mm diameter. The main difference between them was the thread. Three configurations of nails (Fig. 1) were tested.
Fig. 1. Different joint modes between plywood and LVL
The threaded rods were made of carbon steel, with a strength grade of 4.8 (i.e., the tensile strength was 400 MPa, and the yield ratio was 0.8), according to GB/T 3098.1 (2010). The length of the threaded rods was 250 mm, and the nominal diameter was 6 mm. Two threaded rods were glued and inserted into pre-drills on both sides of the hole (Fig. 2). The adhesive was epoxy resin. Two nuts and two washers were used to limit the rod from moving. Other criteria, such as pre-drill diameter and distance from the hole edge, should be complied with to prevent splitting in the surrounding wood. In this experiment, to minimize strength weakness caused by pre-drills, a diameter of 7 mm was chosen. To prohibit cracks around the hole caused by the pre-drill, a distance of 30 mm from the hole edge was chosen (Ardalany et al. 2012).
Fig. 2. Schematic drawing of reinforcement with threaded rods
Methods
The three-point bending test, following AS/NZS 4063.2 (2010), was carried out on all of the LVL joists, with a constant rate of 2 mm/min. The LVL joist was put on the roller supports of the WDW-300E mechanical testing machine (Jinan Wanyuan Test Equipment Co., Ltd., Jinan, China). Three steel plates (200 mm × 90 mm × 40 mm) were used on the roller supports and load point during the three-point bending test to avoid crushing the timber and ensure uniform loading. Four displacement sensors (W1, W2, W3, and W4, shown in Fig. 3) with an accuracy of ± 0.1 mm were used to measure deformations at the supports and global deformation and local deformation at mid-span. A load cell with an accuracy of ± 0.5 kN was used to measure the applied load.
Fig. 3. Schematic drawing of experimental setup
The experimental program is presented in Table 2. Joists without holes, joists with non-reinforced holes, and joists reinforced with nailed plywood, glued plywood, or threaded rods were included. Three specimens were tested for each configuration.
RESULTS AND DISCUSSION
A typical failure scenario was observed for the joists with holes and without reinforcement that was characterized by two distinct major cracks, starting at the hole edge at the diagonally opposite location of the maximum tension stress, perpendicular to the grain, and extending parallel to the longitudinal direction (Figs. 4a, b). The ultimate load emerged when the crack appearing under the specimen arrived at the end of the joist, and shear fracture suddenly occurred (Fig. 4c).