information sheet Structural Materials

Transcription

1 information sheet Structural Materials Sawn Timber Manufacture Conversion into sawn timber The information provided below has been taken from the New Zealand Timber Design Guide 2007, published by the Timber Industry Federation and edited by Professor A H Buchanan. To purchase a copy of the Timber Design Guide, visit When trees in a forest are large enough they are felled, the branches are cut off, the trunk is cut into logs which are transported to processing facilities. For conversion into sawn timber, logs are taken to sawmills. Despite the use of advanced technology, converting a round log into rectangular timber is rather inefficient, and only about half of the log volume becomes sawn timber. The remainder usually ends up as slabs, chips or sawdust, which become feedstock for panel products or energy production. Diameter and shape (sweep, taper, ovality) do not usually limit the kinds of processing systems which can be used. Sawing of logs is the most common processing method used. Peeling and slicing and the manufacture of a range of reconstituted wood products are increasing in importance. Excellent results have been obtained with bandsaws, circular saws, frame saws and chipper canters in all the common sawmill configurations. New Zealand pine is similar to other medium-density softwoods in that more saw tooth side clearance is required than for hardwoods. A good surface finish can be achieved with appropriate feed speeds and sharp saws. The full range of breakdown methods can be used with New Zealand pine and the conversion levels achieved are dependent on the log and product mix and the mill efficiency level. Cutting patterns are selected according to the machinery available, the log size and quality, and the products required. Conversion patterns Most traditional conversion patterns can be used with New Zealand pine, provided the quality zones are recognised Grade sawing commonly applied to high-value pruned logs. Boards are removed around the log to maximize the recovery of high-value clearwood. Cant sawing commonly used to segregate the wood quality zones in unpruned logs. The juvenile wood zone is isolated in the inner boards. Suitable for small and medium-size logs. Live sawing used where only basic equipment is available or when wide boards are needed. This pattern allows recovery of some quarter-sawn boards. Peeling standard method for plywood and LVL production, used on pruned and industrial peeler grades. Log quality The quality of the logs is assessed visually and sonically. Additional information on log quality may be obtained from silvicultural records of pruning, thinning, seedling quality etc. Logs for conversion into structural timber are assessed for quality with an estimate of the modulus of elasticity (MOE or E-value), made either in the forest or at the sawmill, with a device which measures the time taken for a sound wave to travel the length of the log, after one end is hit with an electronic hammer. Low stiffness logs are sawn into appearance grade timber or chipped for manufacturing into panel products. Modern sawmills are sophisticated industrial facilities, with much of the work done by computer-controlled machinery. Logs are often scanned and centred before sawing to ensure maximum yield of timber from each log. 1

2 Flat vs quarter sawn timber In the sawmill, the logs are sawn into timber. There are many different possible patterns for cutting a log. The resulting timber is usually described as quarter-sawn or flat-sawn. In quarter-sawn timber the annual rings form angles of 45 to 90 with the surface, whereas in flat sawn timber, they are at 0 to 45 with the surface. Quarter sawn timber suffers much less distortion than flat sawn timber when it is dried. It is also much less prone to surface checking if exposed to sunlight. Diagram 1: Quarter-sawn and flat-sawn wood Seasoning When logs are converted to sawn timber the moisture content of the wood is very high, maybe more than 100% of the weight of the dry wood. The timber needs to be seasoned (dried) to reduce the moisture content before use. This is done either by air drying or kiln drying. Changes in moisture content result in shrinkage and swelling of the wood. Changes in dimension after installation can be minimised by seasoning the timber to near the equilibrium moisture content before use. Radiata pine shrinks across the grain by 3% and along the grain by 0.02% when drying from green to 12% moisture content. Kiln-drying The key to a successful timber drying operation is to dry as rapidly as possible whilst keeping timber quality within specification. A range of kilns is now available to meet the varying needs of producers. High temperature kilns (i.e. in excess of 100oC dry bulb) are recommended for the drying of structural timber. Medium temperature kilns (typically operated in the range of oC) are proving ideal for companies drying for quality markets. Conventional dry bulb temperatures of 70oC. Final moisture content depends on the market, but will typically range between 9 and 15%. Boards dry at different rates and it is sometimes necessary to run a conditioning step at the end of the drying schedule. This will bring the boards to the required moisture levels and reduce variability between boards, and is achieved by conditioning at close to saturation using steam at atmospheric pressure generated in a water bath. Controls against: Warping: During drying the worst distortions can occur in pieces cut from near the centre of the log where the presence of spiral grain causes twist during drying of framing timber in high and medium temperature kilns with a fast heat-up. This problem is reduced provided the timber is well stacked and top weight of up to 1000kg/m2 is added (reduced to kg/m2 for remanufacturing grades). The plasticising effect of the elevated temperature assists the restraining effect of the top weight, and the weight of the stack itself, in restricting warp. These drying methods not only keep warping within acceptable limits but also help to improve wood stability 2

3 Checking: Getting the timber into the kiln as soon as possible after sawing is strongly advised as checking can develop while the filleted timber is outside waiting to go into the kiln. Internal checking is of more concern. Stress Levels: Final steam conditioning at the end of drying, to relieve drying stresses (case hardening), is vital to the final quality of drying, especially for material that is to be resawn. Colour changes: It is important to guard against fungal staining by prompt drying after sawing and use of anti-sapstain chemicals. Chemical stains of various sorts can also cause discoloration. The surface of timber darkens increasingly as the drying temperature is raised, but after subsequent machining it is difficult to detect any difference in colour. Source: Windsor Kilns web site Features of sawn timber Several important features are apparent in sawn timber. These depend on how the tree grew, its species, and how it was sawn and dried. In order of importance, the features that affect strength and stiffness are: knots sloping grain corewood (or juvenile wood) compression wood splits, checks and shakes warp (or distortion) pitch pockets wane Limitations on these features are the basis of visual grading rules, as described below. Knots Knots appear in sawn timber as a result of the wood grain flowing into the branches in the living tree. The shape of a knot on a sawn surface depends upon the direction of the saw cut with respect to the axis of the branch. When a branch is sawn through at right angles to its length, a nearly circular knot results as shown in diagram 2 (a). When the branch is sawn through lengthwise a spike knot appears as shown in diagram 2(b). Sawing diagonally produces an oval knot. A dead knot (encased knot) results when the tree has grown around a dead branch; these knots sometimes fall out leaving a hole right through the board. Most often, live knots (inter-grown knots) result from sawing trees in which the branches were still alive. All types of knots will cause a reduction in strength and stiffness because: There is loss of load carrying cross section. The fibres in the area of the knot are distorted, resulting in perpendicular-to-grain stresses. Checking or splitting often occurs around knots when the wood dries. Because sloping grain causes a large reduction in tension strength most knots cause a greater reduction of tension strength than of compression strength. For a simply supported beam, a knot will have the greatest strengthreducing effect when it is situated in the centre of the span on the lower side, where the tension stress is highest. Since knots also affect stiffness, they can decrease the buckling strength of columns. 3

4 Diagram 2: Knots in sawn timber Sloping grain Sloping grain in sawn timber lowers its strength and stiffness. The reduction in tension strength is greater than the reduction in compression strength. Sloping grain may be local, or it may be over the whole cross section. Sloping grain may be due to several causes: The grain was disturbed locally in the growing tree due to a branch. The board was sawn parallel to the pith of the tree, but the log had pronounced taper (resulting in diagonal grain). The log had fibres growing in a spiral direction about the trunk of the tree instead of in a straight direction (spiral grain). Sloping grain is not always easy to detect visually, although it may have significant effects on strength. Corewood Corewood is the wood within about 10 growth rings from the centre of the tree, including the pith. This wood is less dense than other wood and usually contains knots. It has higher longitudinal shrinkage, and is weaker than wood from further out in the tree. For this reason, visual grading rules for some higher grades of timber exclude any boards containing the pith, or growth rings near the pith. Douglas fir exhibits more uniform properties within stems than radiata pine and therefore does not have a well defined, low quality, corewood zone. Compression wood Compression wood is weaker and has greater longitudinal shrinkage than normal wood, and is comparatively brittle. Unfortunately it is very difficult to identify compression wood in sawn timber, although a greater proportion of summerwood, and increased opacity may be present. Another identifying feature is that boards with compression wood over part of their cross section are likely to suffer greater distortion when the wood dries, in which case the boards will be downgraded for the distortion rather than for the compression wood itself. Splits, checks and shakes Splits, checks and shakes are cracks or fissures parallel to the main axis of the tree, but they each have slightly different definitions: A split is a separation along the grain, forming a crack or fissure that extends through the piece of wood from one surface to another. A check is similar, except that the fissure does not extend all the way through the piece. A shake is a separation occurring between annual growth rings. 4

5 A shake and a check are shown in diagram 3. Splits and checks are usually the result of differential shrinkage during drying which often occurs because the end grain of a piece of wood will dry more quickly than the rest of the piece, and thus will shrink faster. The differential strain can cause splits or checks. Coating the end grain or drying at a slow rate can reduce this occurrence. Checks and splits can cause significant loss of shear strength. They also create cosmetic and durability problems, because an unbroken paint film cannot be maintained over a split or check. A pitch pocket is an opening between growth rings (i.e. a shake), which contains (or has previously contained) resin, or bark, or both. Wane Wane is a lack of wood at the corner of a board, as shown in diagram 3. Wane is usually caused by the sawmiller cutting too close to the outside surface of the log. A small amount of wane is not a serious strength-reducing defect because the wood at the outside of the log is usually the strongest wood in the tree, so that boards with wane are often the strongest boards in a given population. Wane is considered to be a visual defect for timber exposed in its final location. Diagram 3: Shake, check and wane Warp If different parts of a cross section shrink at different rates, the result will be distortion of the board. This distortion is known as warp. Warp can be due to several factors, all related to shrinkage when the wood is dried: Different rates of shrinkage in the radial and tangential directions of the cross section. Higher longitudinal shrinkage on one side of the board, due to the local presence of corewood or compression wood. Uneven drying, with one side of the board drying more quickly than the other, and thus shrinking more. Spiral grain in the board, resulting in twisting when the board dries. Warping can cause the sides of the board to deviate from plane surfaces. There are four main types of distortion referred to as warp as shown in diagram 4. bow crook cup twist Warp can cause an apparent decrease in strength, because straightening forces will add extra unexpected stresses. Warp is also inconvenient for building for many practical reasons. Warp is generally prevented by careful seasoning practices, and restraining the timber in a straight position during drying. When dry, Douglas-fir has the reputation of retaining its shape and size better than radiata pine, without shrinking, swelling, cupping, warping, bowing or twisting, and it generally will not check or show a raised grain. 5

6 Diagram 4: Warp in sawn timber 6