Plates or small squares with parallel wood grains are first terminated or edged in the length or width direction to form laminates, and then laminated and glued in the thickness direction. Also known as glue-laminated building blocks. Like artificial boards, it can improve materials and expand uses. Glulam has a wide range of uses, mainly used as roof trusses, beams, arches, columns, door and window frames in house construction; keels and masts of wooden ships; bottom beams and carriage frames of vehicles. It can also be used in furniture, sporting goods, sleepers, plow shafts, aircraft propellers, handicrafts, etc.
A brief history
The use of glulam for building structures began in 1893, when a concert hall built in Basel, Switzerland, used glulam as an arch frame. In 1905, German Otto Hetzer obtained a patent for the "glulam construction method" in Switzerland. This technology was used in Europe and introduced to the United States around 1900. Since the 1930s, the use of glulam in house construction has developed. In 1948, China built worker dormitories at the Huainan Coal Mine, all of which used glulam arched roof trusses manufactured by the Shanghai Yangzi Timber Factory. In 1963, Beijing Guanghua Timber Factory successfully trial-produced a metal-free glulam roof truss with a span of 26 meters. In 1989, the Beijing Anti-corrosion Plant of the Ministry of Railways, the Wood Industry Research Institute of the Chinese Academy of Forestry Sciences, and the China Construction Technology Development Corporation cooperated to make glulam beams with a span of 30 meters for the structure of the Kangle Palace Water Park for the Asian Games project. Casein glue was initially used for glulam, and later urea-formaldehyde resin glue, phenolic resin glue and resorcinol-phenol-formaldehyde resin glue were used, which greatly improved the gluing performance and expanded its uses.
Types of glulam
It can be divided into 3 categories according to usage conditions, shape and purpose: ① Glulam for outdoor use and glulam for indoor use. The former is glued with highly water-resistant and weather-resistant adhesives such as resorcinol-phenol-formaldehyde resin glue and phenolic resin glue; the latter is glued with moisture-resistant or water-resistant adhesives such as urea-formaldehyde resin glue or melamine-modified urea-formaldehyde glue. ②Straight glulam and curved glulam. The former are mostly used as beams and columns of houses, while the latter are often used as arches, keels, ribs, and bow and stern frame materials for shipbuilding. According to the cross-sectional shapes of the above products, they can be divided into rectangular, I-shaped, T-shaped, box-shaped, etc. ③ Structural glulam and non-structural glulam. The former is used as a load-bearing component, and the latter is used as a non-load-bearing component.
Main features of glulam
①High strength and performance. The static bending strength of structural glulam is about 23% higher than that of wood, and the static bending elastic modulus is about 22% higher. This is because during the manufacturing process, some natural wood defects such as knots and decay are removed or concentrated material defects are reasonably dispersed, thereby improving the uneven strength of the wood. ②Good dimensional stability. Because the laminated boards are dried and glued into large-sized lumber, the internal moisture content is uniform, which avoids problems such as difficulty in drying large-sized wood and easy cracking and warping. ③ Small-sized lumber can be made into large-sized lumber, which provides a way for the utilization of small-diameter wood, board skin, and short materials. ④ Variable cross-section components and curved and special-shaped components can be designed and manufactured according to strength requirements. Compared with sawn timber, the disadvantages of glulam are that manufacturing materials of the same specification requires high energy consumption, many processing steps, and requires the use of adhesives, so the cost is high. In addition, the requirements for processing technology, processing equipment and quality management are also strict.
glulam manufacturing
The manufacturing process is shown in Figure 1.
Figure 1 Laminate material
Select the tree species, grade, thickness, etc. of the laminate according to the purpose of the glulam. The tree species should be coniferous trees with good glueing properties and not easy to crack and warp, or broad-leaved trees with the above properties. China's national standard "Code for Design of Timber Structures (GBJ 5-88)" stipulates the wood grade of glued components, the selection of materials for load-bearing glued wood structures, and the selection of tree species. The thickness and tree species of each layer of glulam that make up a piece of glulam should be the same. Commonly used laminate thicknesses are 20 to 50 mm. When China uses coniferous trees and soft broad-leaved trees, the thickness of the laminate should not be greater than 40 mm. When using hardwood pine or hard broad-leaved trees, it should not be greater than 30 mm. For curved glulam, the thickness of the laminate should not be greater than 1/300 of its radius of curvature, and its thickness should not be greater than 30 mm.
The moisture content of the laminates should be within the range of 8 to 15, to minimize the difference in moisture content between the laminates, or to control the moisture content within the specified range of ±2.
Gluing technology
Finger jointing should be used to connect the laminates (see finger jointing into materials). If finger jointing technology is not available, miter jointing can be used, which is a longitudinal joining method in which the ends of the two boards to be joined are cut into the same inclined plane and then glued together (Figure 2a). Miter joints have good strength but suffer from high wood loss. The joint strength is determined by the tilt ratio t/1, and generally t/l=1/8~1/12 is used. In order to save wood and reduce the workload of gluing, butt joints can also be used to a limited extent in parts of the glulam that are not subject to large stress (Fig. 2b). When docking, the thickness difference between the two docked plates should not be greater than 0.1 mm. The width of the laminates is generally determined by edge stitching. The laminates that have been lengthened or widened must be planed, and the planing quality should meet the following requirements: ① The upper and lower gluing surfaces should be tightly connected without local light transmission. The convex mark caused by the knife edge defect in some parts shall not be higher than 0.5 mm above the board surface. ② In planed wood boards, the rough length near the wood knot should not be greater than 10 mm. The planed laminates should be glued within 12 hours, and no more than 24 hours at most. Dust, oil, etc. on the surface should be removed before gluing.
Figure 2 Adhesive
When laminating and gluing, the adhesive should be selected based on factors such as the purpose of the glulam, the use environment, and operating conditions. Resorcinol-phenol-formaldehyde resin is commonly used as structural materials. It can be cured at room temperature and has excellent bonding properties and aging resistance. Phenolic resin can also be used. For non-structural glulam, urea-formaldehyde resin glue or melamine-modified urea-formaldehyde glue is commonly used. Commonly used gluing equipment include four-roller gluing machines, glue spraying machines and glue spraying machines. The amount of glue applied is generally 250 g/m2 for small materials and 350-400 g/m2 for large structural materials.
Gluing); arrange joints and seams in a dispersed manner to avoid concentration or stacking together; the distance between finger joints between two layers should not be less than 10t, and the distance between miter joints between two layers should not be less than 20t (Figure 3), two-layer laminates The spacing between seams should be greater than the thickness of the board (Figure 4). The outer layer of glulam should be made of high-grade sawn timber, while the core layer can be made of lower-grade sawn timber. In curved glulam, the outer layer should be a whole board, and it is not suitable to use vertical or edge-joined laminates.
Figure 3
Figure 4 Blank Pressure
It is required that all parts along the length of the material are pressed evenly. The amount of pressure is determined by the density of the wood, the processing accuracy of the board surface, the viscosity of the glue and other conditions. The unit pressure used for pressurization: 0.5 to 1 MPa for coniferous trees; 1 to 1.5 MPa for broadleaf trees. During the pressing process, pressurization and curing can be completed on the same equipment, or in two devices separately. Pressurizing equipment should be selected based on heating method, product specifications, output size, etc. (Table 1).
Table 1 Spiral pressurization and oil cylinder pressurization devices are suitable for normal and medium-temperature air heating methods that pressurize long and bent materials for a long time. Press, continuous pressurization, high-frequency heating and hot plate heating are suitable for high output and short-time pressurization of dimensional materials. The pressed glulam blanks also undergo processes such as planing, sanding, cutting and surface decoration to make glulam finished products.
Physical and mechanical properties of glulam
Properties that have an important impact on use include gluing strength, adhesive layer peeling rate, static bending strength and static bending elastic modulus, and combustion safety.
Gluing strength
One of the important indicators reflecting the gluing quality of glulam. It is the stress measured on the unit bonding area when the adhesive layer reaches failure under the action of external force. When performing a along-grain shear test on the specimen, calculate it according to the following formula:
According to the Chinese national standard GBJ 5-88 "Code for Design of Wooden Structures", the bonding strength of glulam should meet the requirements listed in Table 2 specified value.
Table 2 Wood damage rate
Another indicator of gluing quality. It is the ratio of the wood damage area on the gluing surface to the entire gluing area when the specimen is sheared and damaged when measuring the gluing strength of the specimen. Usually visual inspection is used, and the accuracy is 5 to 10.
The wood damage rate is calculated according to the following formula:
Glue layer peeling rate
Reflects the durability of glulam. During the use of glulam, it is affected by changes in atmospheric temperature, especially humidity, ultraviolet radiation, and harmful substances in the atmosphere, causing the wood to expand and contract, causing dimensional changes between the layers of wood and acting on the glue layer to form internal stress. , leading to cracking and peeling of the glue layer, affecting the service life of the glulam. Generally, an artificial accelerated aging test is used to cause the end-face adhesive layer of glulam to peel off, which is measured by the ratio of the total length of the end-face adhesive layer peeling to the total length of the end-face adhesive layer of the specimen. The peeling rate is calculated according to the following formula:
Static flexural strength and static flexural elastic modulus
They are the two main indicators to measure the strength performance of glulam, especially in structural applications. (Table 3). The main factors that affect the static bending strength of glulam include: the strength of the wood that makes up the glulam; the stacking configuration of each layer of wood; the quality of gluing, etc. In glulam of the same thickness, increasing the number of laminated layers can also significantly improve its strength properties.
Table 3 Combustion Safety
Although wood is flammable, the glulam used for building components has a large cross-section, which shrinks when exposed to fire and burns slowly. The collapse time is extended and fire safety is improved. The safe burning time of glulam beams can be calculated according to the following formula:
where t is the safe burning time (minutes); D is the height of the beam before burning (cm); d is the beam height after burning ( cm); β is the carbonization speed, and the average carbonization speed of glulam beams is 0.05 to 0.06 cm/min. In order to improve safety, a fireproof asbestos layer or fireproof paint is generally applied to the surface of glulam to extend the fire time.