Abstract
Poplar laminated veneer lumber (LVL) orthogonal rib box floor is a new type of floor composed of orthogonal LVL rib beams and oriented strand board (OSB). To study the creep performance of the box floor, four 3600 mm × 4800 mm floor specimens were designed and manufactured. The creep tests of the box floor with local damage, repeated load, and different stress ratio loads were conducted. The creep of the floor increased with ambient temperature and humidity. Because of the local damage of the box floor, the creep increased. Repeated loading increased the creep deformation of the floor, and increasing the load accelerated the creep of the floor. Combined with the creep mechanism of wood materials, a creep theoretical calculation formula of the box floor with LVL orthogonal ribs was established. Comparing the creep model analysis with the test data, it was found that the modified Burger mode can well simulate the creep performance of LVL box floor. Therefore, the modified Burger model can be used to calculate the creep deformation of the box floor.
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Creep Behavior of Orthogonal Rib Box Floor of Poplar Laminated Veneer Lumber
Yan Liu,* Jijia Long, Jing Chen, Xingyu Song,* and Wenbo Jia
Poplar laminated veneer lumber (LVL) orthogonal rib box floor is a new type of floor composed of orthogonal LVL rib beams and oriented strand board (OSB). To study the creep performance of the box floor, four 3600 mm × 4800 mm floor specimens were designed and manufactured. The creep tests of the box floor with local damage, repeated load, and different stress ratio loads were conducted. The creep of the floor increased with ambient temperature and humidity. Because of the local damage of the box floor, the creep increased. Repeated loading increased the creep deformation of the floor, and increasing the load accelerated the creep of the floor. Combined with the creep mechanism of wood materials, a creep theoretical calculation formula of the box floor with LVL orthogonal ribs was established. Comparing the creep model analysis with the test data, it was found that the modified Burger mode can well simulate the creep performance of LVL box floor. Therefore, the modified Burger model can be used to calculate the creep deformation of the box floor.
DOI: 10.15376/biores.17.4.6158-6177
Keywords: Orthogonal ribbed beam box floor; Creep test; Creep model; Finite element simulation; Creep prediction
Contact information: College of Civil Science and Engineering, Yangzhou University, Yangzhou, Jiangsu 225127, China; China; *Corresponding authors: ppsxy@163.com;liuyan@yzu.edu.cn
INTRODUCTION
Poplar laminated veneer lumber (LVL) is a kind of engineering wood that is made from raw poplar by rotary cutting, drying, coating, veneer striation, and hot pressing. In addition to its excellent performance, it has better mechanical properties than natural poplar wood (Liu et al. 2017). There have been many studies on the mechanical properties of LVL members and structures, but few studies on their creep properties. A great deal of investigation has been carried out to understand the relationship between creep deformation and molecular structure. Mature creep theory seeks to simplify the wood polymer to reveal its rheological characteristics. Armstrong et al. (1960, 1961) proposed that changing moisture content affects wood creep properties. Schniewind (1967) found that periodic variations in relative humidity and temperature in the environment decreased the average time it took for small Douglas fir beams to fail. Aipo (2000) studied the bending creep of LVL, plywood, spruce I-beam, and other components after different anti-corrosion treatments for up to eight years under natural environment and low stress levels. When the stress level is low, the creep increases with the increase of stress, and the creep and time are in the form of power function. Yazdani et al. (2003) studied T-section LVL simply supported beams by full-scale model test under natural environment. The loading time was 895 days, and then the creep recovery stage was observed for 90 days. The Burger model can well fit the creep test results and verify the accuracy of the model. The parameters of the Burger model are obtained according to the creep test results. Dinwoodie et al. (1992) studied the creep properties of wood composites, finding that the relative creep increased slightly and linearly with respect to stress within the range of stress levels adopted, as well as increasing with severity of the environment. All materials showed greater sensitivity to alternating humidity than to alternating temperature. Pierce et al. (1979) studied the creep properties of wood composites, finding that the Burger model with an additional viscous damper element had higher prediction accuracy. Leichti and Tang (1989) carried out the creep test of I-shaped timber beams and conducted a comparative analysis of the creep of reinforced and unreinforced timber beams. Under long-term load, the stiffness of the reinforced timber beam changes little and has good creep performance.
Zhu and Zhou (2009) studied the influence of LVL creep on structural stability, and obtained the tensile-compression creep law of LVL material under normal use environment in China. They established the LVL creep constitutive model and the creep buckling finite element model of wood structure under long-term deformation. Chen (2017) found that the loading level, cross-section form, and reinforcement ratio prestress value can affect the long-term bending performance of wood beams. With the help of power law model, the value of creep deformation coefficient was obtained. Cao (2017) conducted three groups of creep tests of plywood beams with different stresses and selected the modified Burgers model as the constitutive model of plywood. The creep characteristic constant of larch plywood beam was obtained by fitting the test data with the modified Burgers model. This model has good fitting accuracy and it can be used to predict the short-term creep of plywood beams. Zuo et al. (2021) compared the long-term flexural performance of laminated timber beams strengthened with different replacement proportions and loads by experiments. It was found that when the replacement rate is constant, with the increase of loading value, the initial deflection and creep value of laminated timber beams increases, and with the increase of load, the deformation rate increases. When the loading ratio is fixed, the deflection deformation can be reduced by increasing the replacement proportion of the reconstituted bamboo in the plywood beam. Yang et al. (2017) studied the long-term bending performance of prestressed plywood beams under the same prestress, different number of prestressing tendons, the same number of prestressing tendons, and different prestress. It was found that prestress regulation can effectively reduce the creep of wood beams. Zhou et al. (2020) conducted creep tests on steel-wood composite floors under long-term load. The combination of the two materials can give full play to the advantages of materials, so that the floor has good integrity and bearing capacity.
Wang et al. (2020) designed and made three wood beams strengthened with CFRP bars to study their mechanical properties under long-term load. The initial defects of wood beams affect the long-term performance, and the deflection of wood beams without initial defects is smaller than that of wood beams without reinforcement. The deflection of reinforced beam with obvious initial defects in midspan was higher than that of unreinforced beam, and obvious cracks appeared around the midspan joint. He et al. (2016) studied the number of prestressed tendons, prestress, load, and other factors on the long-term bending performance of prestressed plywood bamboo beam. Increasing the number of prestressed tendons can reduce the creep, and when the number of prestressed tendons increases to a certain number, the effect on reducing creep is no longer obvious. Increasing prestress can reduce the creep of bamboo beam; the greater the load, the greater the creep. According to the test results, the long-term stiffness calculation formula of bamboo beam is deduced. Sheng (2015) conducted creep tests on three poplar LVL columns with the same size, and applied stress ratios of 0.1, 0.3, and 0.5 to study the effects of different load sizes on the strain and deflection of poplar LVL columns. The specimen basically conforms to the creep law of wood. The larger the stress ratio, the greater the creep and the greater the deflection in the column. According to the wood creep mechanism, the creep model of poplar LVL column was established and the creep equation was obtained. Liu (2018) applied 0.3 and 0.5 failure loads to four LVL beams of poplar with 75 mm width, 125 mm height, and 2400 mm length for 110 days of creep test. Under the same load, the creep of wood beams parallel to the adhesive layer direction was smaller, where the greater the stress ratio, the greater the creep.
Previous research on the creep characteristics of timber and engineering wood structures provide a valuable reference for this paper. Through the creep test of poplar LVL orthogonal ribbed box floor and the series research on the mechanical performance of the box floor, the results may provide technical support for the application of poplar LVL orthogonal ribbed box floor in building structures.
EXPERIMENTAL
The size of the floor specimen was 3600 mm × 4800 mm. The parameters of the floor specimens are shown in Table 1 and in Figs. 1 and 2. The internal rib beam joints of each sample adopted the 3 mm thick Q235 steel L-shaped connector, which was connected by M4 × 20 mm cross-countersunk self-tapping screws. The size of connectors and the number of screws were increased at the end nodes of the rib beam, which were connected to the edge-sealing rib beam by M4 × 40 mm cross-countersunk self-tapping screws. OSB plates were used as the upper and lower floor slabs of the box floor, which were connected to the rib beam by 2.8 × 50 mm round nails. The spacing between the round nails was 150 mm and 300 mm at the edge-sealing rib beam and the internal rib beam, respectively (Su et al. 2022).
The creep characteristics of the box floor under local damage loading, repeated loading, and different stress ratios were studied. During the short-term loading test of specimen L2-1, when the load reached 15 kN/m2, the OSB joint in the middle area of the floor bottom was separated by 3 to 4 mm, but the whole specimen was not seriously damaged, and still maintains good integrity. Poplar LVL box floor was placed on the steel base. A water tank for loading as placed on the upper part of the floor. To ensure the safety of the water tank, a reinforced steel frame was set around the water tank, and the loading device is shown in Fig. 2. To keep the load constant in the creep test, a waterproof film was coated on the water surface inside and on the top of the water tank to prevent water evaporation, and the water surface height was regularly checked during the creep test to ensure that the load value remains unchanged. The creep test load and the relationship between load and duration are shown in Table 2 and Fig. 3, respectively.
Table 1. Floor Specimen Number and Parameters
Fig. 1. Plane and detailed configuration of test specimen
(a) loading water tank and support base (b) Test overview
Fig. 2. Test setup
Fig. 3. Load-time relationship
Table 2. Floor Specimen Number and Parameters
Experimental Scheme and Measuring Arrangement
A TS3860 type static resistance strain gauge was used to record the floor deflection change during creep test, and a CX601 type industrial high-precision thermometer and hygrometer were used to record the environmental temperature and humidity change, as shown in Fig. 4. To measure the creep deformation of each box floor specimen under long-term load, the displacement meter was arranged along the longitudinal and transverse axis of the floor. Based on the symmetry of the box-type floor and the symmetry of the load, a quarter of the floor area was selected for the layout range of the displacement meter. The distribution of each measuring point is shown in Fig. 5(a), and the number of the displacement meter is W1 to W14. The layout of the displacement measuring point of the specimen is shown in Fig. 5(b).
(a) Static resistance strain gauge (b) Temperature and humidity meter
Fig. 4. Measuring equipment
(a) Plan of displacement measuring points (b) Installation of displacement meter
Fig. 5. Design and arrangement of displacement measuring points
RESULTS AND DISCUSSION
Temperature and Humidity Record
The creep test time of each specimen was 55 days, and the ambient temperature and humidity were recorded regularly during the test. In the first week of the experiment, the data were recorded every 2 hours, and then every 4 hours. The temperature fluctuation range in the environment during the test was 6.2 to 14.4 °C, and the humidity fluctuation range was mainly in the range 33 to 98%, as shown in Fig. 6.
Fig. 6. Change of temperature and humidity during the test
Floor Deflection
To determine the creep deformation of poplar LVL orthogonal rib box floor under load, the deflection-time relationship of each test point of specimens was drawn with time as the horizontal axis and the deflection of the test point as the vertical axis, as shown in Fig. 7.
Fig. 7. Creep deflection–time curve
Figure 7 reveals that the creep process of the test specimen could be divided into two stages: the instantaneous deformation stage, which produces elastic deformation when the load is just applied to the specimen; and the second stage that is the creep stability stage, in which the specimen has viscoelastic and viscous deformation. When the applied long-term load is small (Table 2), the deformation increases limited with time. The creep deformation of each measuring point of the floor gradually increases with time from about the first 15 days after the load is applied. When the specimen entered the creep stability period, the creep growth rate gradually decreased. Although the creep values at different measuring points were different, the curves are basically parallel, indicating that the specimen has good integrity and stiffness. The deflection development law of each measuring point of the floor was the same, and the maximum deflection appeared in the middle of the whole floor specimen.
Figure 7 shows that when the test specimen was unloaded, the specimen had an immediate recovery of the deformation; with the extension of time, the specimen had a small amount of delayed recovery of deformation, that is, elastic aftereffect; finally, there was a certain amount of residual deformation, as shown in Table 3. Generally, the creep of wood materials consists of elastic deformation, viscoelastic deformation, and viscous deformation. The immediate recovery deformation of specimen is elastic deformation, and the elastic aftereffect that gradually recovers after unloading belongs to viscoelastic deformation, while the residual deformation is viscous deformation, which cannot be recovered after unloading (Jozsef and Benjamin 1982).
Table 3 shows that the residual deformation of W10 of specimen L2-3 increased by 38.9% compared with that of control specimen L1-1, indicating that the load had the greatest impact on the residual deformation of specimens. The W10 of the specimen with damage and repeated loading was also increased by 3% and 19.3%, respectively, compared with L1-1. The residual deformation of L2-2 with repeated loading includes the residual deformation after the first unloading, so the residual deformation after the second unloading is also large. When the load was small, the creep deformation of specimen L2-1 with damage increased to a limited extent compared with L1-1. The variation law of residual deformation of measuring points W7 and W12 was the same as that of W10, and the closer to the end of the specimen, the smaller the residual deformation of the specimen.
Table 3. Residual Deformation of 3 Measuring Points of Each Specimen
The change of creep deformation with temperature and humidity of W10 in specimen L1-1 to analyze the influence of ambient temperature and humidity on the creep performance of the specimen, is shown in Fig. 8. Figure 8(a) shows the relationship between creep deformation and humidity of specimen L1-1 when the temperature changes little. Figure 8(b) shows the relationship between creep deformation and temperature of specimen L1-1 when the humidity is basically equal. Figure 8 (c) shows the relationship between creep deformation and temperature and humidity of specimen L1-1 when the temperature and humidity both increase.
Fig. 8. The relationship between specimen creep deformation and temperature and humidity
The change of temperature and humidity will affect the creep deformation of the specimen. When the temperature was maintained at 12.6 ± 0.4 °C and the humidity increased by 22%, the deflection of measuring point W10 of specimen L1-1 increased by 0.08 mm, as shown in Fig. 8(a). When the humidity was maintained at 58 ± 2% and the temperature increased by 2.9 °C, the deflection of measuring point W10 increased by 0.05 mm as shown in Fig. 8(b). The creep deformation of box floor increased with the increase of environmental humidity, which indicates that environmental humidity affects the moisture content of wood and plays a role in increasing plasticity in the floor. With the increase of ambient temperature, the mechanical strength and stiffness of box floor may decrease, while the creep deformation of wood materials, especially viscoelastic deformation and viscous deformation, will increase to some extent (Jozsef et al. 1982).
Theoretical Analysis and Discussion
The creep deformation of measuring point W10 of specimen
The creep deformation-time relationship of the displacement measurement point W10 in the middle of the floor is shown in Fig. 9. Due to the presence of local damage, the initial deformation of specimen L2-1 was 0.140 mm larger than that of specimen L1-1, and with time, the creep deformation of specimen L2-1 was greater than that of control specimen L1-1. The local damage of specimen L2-1 reduced the stiffness of the floor and increased the creep.
Fig. 9. Deflection-time curve of measuring point W10 of each specimen
Under the same load of 4.5 kN/m2, the creep deformation of specimen L2-2 and L1-1 within 55 days was compared to explore the effect of repeated load on creep performance. It can be seen from Fig. 8 that the creep deformation of specimen L2-2 at the first loading was almost the same as that of specimen L1-1. In the first creep test, the residual creep deformation of specimen L2-2 after unloading was 3.73 mm. Due to continuous measurement, the residual deformation will accumulate into the creep deformation of the second load. This resulted in the creep deformation of specimen L2-2 at the second load being 14.95 mm, which is slightly larger than that of specimen L1-1 (13.38 mm) at the first load. However, under repeated loading, the creep rate of specimen L2-2 was 1.42 mm/d lower than that of specimen L1-1 (2.89 mm/d).
In Fig. 9, also comparing the creep time relationship between specimen L2-3 (load of 7.5 kN/m2) and specimen L1-1 (4.5 kN/m2), it can be seen that the greater the load applied by the specimen, the greater the creep deformation, and the increase of load will obviously affect the creep deformation of the specimen.
Relative Creep Deformation
To eliminate the influence of elastic deformation on creep analysis and compare the creep viscous deformation and viscoelastic deformation of each specimen, the relative creep deformation was defined as follows,
(1)
where δ0 is initial elastic deformation, and δ1 is creep deformation in t1 time.
Under the same load of 4.5 kN / m2, the relative creep deformation of each specimen within 55 days was compared, and the relative creep deformation-time relationship of the displacement measuring point W10 in the middle of the floor is shown in Fig. 10.