brief history
Glued wood was used as a building structure from 65438 to 0893, and a concert hall built in Basel, Switzerland used glued wood as an arch. 1905, OttoHetzer, a German, obtained the patent of "Glued Wood Structure Construction Method" in Switzerland. This technology was applied in Europe around 1900 and introduced to the United States. Since 1930s, the application of glulam in building construction has been developed. During the period of 1948, all the dormitories built by China in Huainan Coal Mine adopted the glued wood arch roof truss produced by Shanghai Yangzi Timber Factory. 1963 Beijing Guanghua Wood Factory successfully trial-produced a metal-free glued wood frame with a span of 26 meters. From 65438 to 0989, the Beijing Anti-corrosion Factory of the Ministry of Railways cooperated with the Wood Industry Research Institute of China Academy of Forestry and China building technique development Company to build a glued wooden beam with a span of 30 meters for the water park structure of the Amusement Palace in the Asian Games project. At first, casein glue was used to glue wood, and then urea-formaldehyde resin glue, phenolic resin glue and resorcinol-phenol-formaldehyde resin glue were used, which greatly improved the gluing performance and expanded the application scope.
Glued tree species
According to the use conditions, shapes and uses, it can be divided into three categories: ① outdoor glued wood and indoor glued wood. The former is made of resorcinol-phenol-formaldehyde resin glue and phenolic resin glue with high water resistance and weather resistance; The latter is glued with urea-formaldehyde resin glue or melamine modified urea-formaldehyde glue, which is moisture-proof or waterproof. ② Straight glued wood and bent glued wood. The former is mostly used as beams and columns of buildings, while the latter is often used as keel, rib, bow and stern skeleton materials for arches and shipbuilding; According to the cross-sectional shape of the above products, they can be divided into rectangular, I-shaped, T-shaped and box-shaped. (3) structural glulam and non-structural glulam. The former is used as a load-bearing member and the latter as a non-load-bearing member.
Main characteristics of glued wood
① High strength performance. The static bending strength of structural glued wood is about 23% higher than that of wood, and the static bending elastic modulus is about 22% higher. This is because some natural defects of wood, such as knots and decay, are reasonably dispersed during the manufacturing process, which improves the strength unevenness of wood. ② Good dimensional stability. Because the laminated board is glued into large-size wood after drying, the internal moisture content is uniform, which avoids the problems of difficult drying, easy cracking and warping of large-size wood. (3) Small diameter timber can be made into large diameter timber, which provides a way for the utilization of small diameter timber, veneer and short diameter timber. ④ Variable cross-section members, flexural members and special-shaped members can be designed and manufactured according to the strength requirements. Compared with sawn timber, glued wood has the disadvantages of high energy consumption, many processing procedures and high cost. In addition, the requirements in processing technology, processing equipment and quality management are also very strict.
Glued wood manufacturing
The manufacturing process is shown in figure 1.
Figure 1 Laminates
According to the purpose of glued wood, choose tree species, grade and thickness. The tree species should be coniferous or broad-leaved trees with the above properties, with good bonding performance and not easy to crack and warp. China's national standard "Code for Design of Timber Structures (GBJ 5-88)" has provisions on the wood grade of glued components, the material selection of load-bearing glued wood structures and the selection of tree species. The thickness of each layer and the species of the glued wood should be the same. The commonly used laminate thickness is 20 ~ 50 mm In China, when conifers and soft broad-leaved trees are used, the laminate thickness should not be greater than 40mm, and when hardwood pine or hard broad-leaved trees are used, it should not be greater than 30 mm.. When bending glued wood, the thickness of laminate should not be greater than 1/300 of its radius of curvature, and the thickness should not be greater than 30 mm. The moisture content of laminates should be within 8 ~ 15%, and the difference of moisture content between laminates should be minimized or controlled within 2% of the specified moisture content.
welding technology
The extension of the laminate should be finger joint (see finger joint wood). Without finger joint technology, diagonal connection can be adopted, that is, the ends of two plates to be extended are cut into the same inclined plane and then glued together (Figure 2a). The strength of miter joint is good, but the wood loss is large. The bond strength depends on the inclination ratio t/ 1, which is generally t/l =1/8 ~112. In order to save wood and reduce the workload of gluing, butt joint can also be used in the parts with less stress on glued wood (Figure 2b). When butting, the thickness difference between two butting plates should not be greater than 0. 1 mm. Generally, the width of laminated plates is trimmed. The extended or widened laminates should be planed, and the planed quality should meet the following requirements: ① The upper and lower bonding surfaces should be closely bonded without local light transmission. The convex marks caused by knife-edge defects in some parts are not higher than 0.5 mm on the plate. (2) In wood shavings, the roughness length near the joint should not be greater than 10 mm ... After wood shavings, the laminated board should be glued within 12 hours, not more than 24 hours at most, and the dust and oil stains on the surface should be removed before gluing.
Fig. 2 Adhesive
Adhesives should be selected according to the purpose, environment and working conditions of glued wood. Resorcinol-phenol-formaldehyde resin is often used as a structural material, which can be cured at room temperature and has excellent adhesion and aging resistance. Phenolic resin can also be used. Urea-formaldehyde resin adhesive or melamine modified urea-formaldehyde adhesive is often used in unstructured glued wood. Commonly used gluing equipment includes four-roller gluing machine, glue spraying machine and glue spraying machine. The coating amount of small materials is generally 250g/m2, and the coating amount of large structural materials is generally 350 ~ 400g/m2.
Blank component
When stacking blanks, we should pay attention to the following points: (1) The textures of the two bonding surfaces should be similar (because chord cutting materials should not be bonded with radial cutting materials); Distribute seams and flat seams to avoid concentration or stratification; The finger joint spacing between the two layers should not be less than 10t, the miter joint spacing between the two layers should not be less than 20t (Figure 3), and the flat-fell seam spacing between the two layers should be greater than the plate thickness (Figure 4). The outer layer of glued wood should use high-grade sawn wood, and the core layer can use low-grade sawn wood. When bending glulam, the outer layer should be a whole plate, and laminated plates with longitudinal or edge splicing should not be used.
Figure 3
Fig. 4 Blank pressurization
It is required that all parts along the length of the material be uniformly compressed. The pressure is determined by the density of wood, the processing accuracy of board surface and the viscosity of glue solution. Unit pressure for pressurization: softwood is 0.5 ~1MPa; Hardwood is 1 ~ 1.5 MPa. In the process of pressing, pressing and curing can be completed in the same equipment or in two equipment respectively. Pressurization equipment should be selected according to heating mode, product specifications and output (Table 1).
Table 1 Spiral pressurization and cylinder pressurization device is suitable for long-term pressurization of long and curved wood at room temperature and medium temperature. Pressing, continuous pressing, high-frequency heating and hot plate heating are suitable for short-time pressing of materials with high output and specifications. After planing, sanding, cutting and surface decoration, the pressed glued wood blank is made into glued wood products.
Physical and mechanical properties of glued wood
The properties that have an important influence on the use are adhesive strength, peeling rate of adhesive layer, static bending strength, static bending elastic modulus and combustion safety.
bonding strength
One of the important indexes reflecting the bonding quality of glued wood. It is the stress per unit bonding area measured when the adhesive layer is destroyed under the action of external force. When the specimen is subjected to parallel shear test, it is calculated according to the following formula:
According to the Chinese national standard GBJ 5-88 Code for Design of Timber Structures, the bonding strength of glued wood should meet the specified values listed in Table 2.
Table 2 Wood damage rate
Another index to measure the bonding quality. It is the ratio of the damaged area of wood on the bonding surface to the whole bonding area when the specimen is damaged by shear when measuring the bonding strength. Usually, the visual method is used, and the accuracy is 5 ~ 10%. The wood breakage rate is calculated by the following formula:
Stripping rate of adhesive layer
Reflect the durability of glued wood. In the process of using glued wood, it is affected by atmospheric temperature, especially humidity change, ultraviolet radiation and harmful substances in the atmosphere, which makes the wood expand and contract, causes the dimensional change between the plates, and acts on the adhesive layer to form internal stress, which leads to the cracking and peeling of the adhesive layer and affects the service life of glued wood. Generally, the artificial accelerated aging test is used to promote the peeling of the end-face adhesive layer of glued wood, 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 as follows:
Static bending strength and static bending elastic modulus
They are two main indexes to measure the strength characteristics of glued wood, especially in structural application (Table 3). The main factors affecting the static bending strength of glued wood are: the strength of wood itself; Lamination configuration of each layer of board; Bonding quality, etc. In glued wood with 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 cross section of glued wood used for building components is large, which shrinks when it meets fire and burns slowly, thus prolonging the collapse time and improving the fire safety. The safe burning time of glued wood 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 combustion (cm); D is the beam height (cm) after combustion; β is the carbonation speed, and the average carbonation speed of bonded beams is 0.05 ~ 0.06 cm/min. In order to improve the safety, the surface of glued wood is generally coated with fireproof asbestos layer or fireproof paint to prolong the ignition time.