Tension wood structure

Hemicelluloses in reaction woods are quite different from those in the normal woods, namely, galactan and P-(l-3)-gIucan in compression wood and galac-tan in tension wood. It is also well known that a remarkable amount of a water-soluble polysaccharide, arabinogalactan, is contained in the heartwood of larch. Since this polysaccharide occurs mainly in the lumen of tracheids and is not a cell wall component, it may not be included in hemicelluloses. Although structures and distributions of hemicelluloses have been comprehensively studied in the last 20 years, their physiologic meanings in a cell wall are not known yet. This must be the most important point for the future study of hemicelluloses. 

The other significant structural feature of tension wood fibers is the nature of the rest of the secondary wall, which may lack an S3 or S3 and S2 (36) (Figure 34). 

Sigglekow IL (1974) A comparision of tension wood in some New Zealand trees. MSc thesis. University of Canterbury, Christchurch, New Zealand Simperingham P (1997) A structural engineer s approach to forestry production. Wood Technology Research Centre, Workshop 1997. University of Canterbury, Christchurch, New Zealand, 5p... 

The exo-anomeric effect enforces a value of cp around 90° for anomeric axial linkages. This ensures a proclivity towards helical structures (as distinct from the ribbons of 1,4-diequatorially linked polysaccharides), as in starch and glycogen. p-(1 4)-Linked galactans, 1,4 axial-equatorial in the other sense, are known as hemicellulose components of compression and tension wood, the storage polysaccharides of lupins and as arabinogalactans attached to the rhamnogalacturonan I component of pectin, but have not been subject to conformational studies they appear to be biosynthesised from UDP-Gal. " ... 

T Akiyama, Y Matsumoto, T Okuyama, G Meshitsuka. Ratio of erythro and threo forms of beta-O-4 structures in tension wood lignin. Phytochemistry 64 1157-1162, 2003. 

Wood structure within a given tree species is not uniform but varies depending on the conditions under which the tree is growing. For example, trees compensate for exposure to wind or other types of bending pressure by the production of reaction wood. In softwood, the formation of reaction wood is induced on the compressed side of a bending trunk (compression wood), whereas in hardwood, reaction wood is formed on the elongated side of the trunk (tension wood). Reaction wood cells are morphologically similar to normal wood cells but differ in their cell wall structure and chemical composition. 

This technology finds its practical application in the strengthening and upgrading of existing wood structures subsequently transferred by tension-resistant bonding of carpentry connections, which in its original condition substantially takes only compressive loads, OFig. 49.1. 

It is important to note material such as those plastics or wood that are weak in either tension or compression will also be basically weak in shear. For example, concrete is weak in shear because of its lack of strength in tension. Reinforced bars in the concrete are incorporated to prevent diagonal tension cracking and strengthen concrete beams. Similar action occurs with RPs using fiber filament structures. 

If a force is applied to wood within the proportionality limits, the wood will bend and if the force is released, the wood returns to its original form with an elastic recovery. In contrast, if the wood is dried under stress, a substantial superposition of stresses occurs in conjunction with the drying and shrinking process. Since the ordering of macromolecules or larger structural elements under tension is different from those under compression, as the water molecules are removed, new hydrogen bonds can form between different subunits of the structure to support the distorted structure in its new form. 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

Membranes are thin flexible fibrous or foil materials which are stabilised solely by tension forces and have a constant stress rate over their entire thickness. In contrast to other materials like glass panes, wood, stone or concrete, membrane structures are extremely Ught because the ratio of material weight to tensile strength is excellent. The low weight of the membrane material also reduces the weight of the primary supporting structure. Because textile membranes are made of thin and flexible materials, usually a double curvature and biaxial pre-tension is needed to stabiUse the structure. 

Heat treatment also causes some unfavourable effects, such as lower bending and tension strengths, reduced toughness and, thus, increased brittleness of wood [2, 5, 8-13], Modified chemical, physical and structural properties of wood after heat treatment can affect the bonding process with adhesives. 

Joints are often the weakest part of the structure, normally susceptible to premature risk of splitting near the mechanical fasteners. By improving the tension perpendicular to the grain of the timber in this location, failure will not occur in the wood but... 

Composite structures are remarkably tough when tested in tension. However, when stressed in other ways the composite may be more brittle than expected. Rope is a good example. The tensile properties are excellent, but the rope is floppy when bent or compressed. Similarly, wood is difficult to chop across the grain, but cleaves easily along the grain. The problem is to define the composite properties under different loading circumstances. 

Figure 11.1 (a) An example of triangulated truss structure where the wood is in compression, the steel wire is in tension, with metal brackets as transition pieces, (b) An example of triangulated truss structure, where steel mbes are in compression, and steel rods are in tension, (a) Boeing (1916) structure photo by the author, (b) Boeing B-40 (1925) structure, photo by the author. 

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