|Small Diameter Timber|
to our work at the University of Maine (some of this is very Preliminary):
In 2000, over 7 million acres were lost to forest fires across the United States. This value is almost twice the 10-year average for acreage lost as reported by the National Interagency Fire Center in Boise, Idaho. A number of factors are cited for the increase in the acreage lost to fire. Persistent dry, hot and windy weather in the affected regions of the country has coincided with years of large losses to fires. Perhaps most importantly though, a large amount of fuel has accumulated in many of these areas due to forest management practices over the last century. The fuels includes small diameter timber as well as overgrown brush that would normally be eliminated through periodic fast moving small brush fires that would have periodically burned in unmanaged forests. While changes in global weather patterns may help the situation, a need remains to alter forest management practices in order to reduce the risk to people and property posed by these fires. Depending on the terrain and the condition of the forest, some combination of prescribed burning and mechanical thinning will be required to restore the health of the forests while reducing the fire risk.
Prescribed burning attempts to recreate smaller, more frequent fires that naturally reduce the amount of fuel in the forests. However, in many areas prescribed burns can be technically difficult because of terrain, and as may not be suitable due to proximity of human habitation. In other cases, removal of trees by selective thinning can reduce the technical difficulties of a particular burn and reduce the risk of run-away fires. Finally, in some forests, the existence of intermediate age trees and a large accumulation of fuel can change the nature of the fire. Many periodic burns that occur in a more natural forest include fast moving low burns that retain the forest canopy and remove fuel from the ground. The existing fuel load that includes a large inventory of small diameter timber can cause the fire to burn too hot, which will burn established trees and scorch the soil. Again, as in some of the more technical burns, the forest is likely to require mechanical thinning or thinning and a limited prescribed burn to restore the forest to a healthy condition. Thus, mechanical thinning is rapidly becoming a critical tool for forest management.
Mechanical thinning involves physically entering the forest and removing the small diameter timber. Mechanical thinning is similar to logging, but is focussed on removing trees that have limited commercial due to the small size of the trees removed. In addition, mechanical thinning is very labor intensive due to the selective cutting that is employed. The labor costs incurred increase the investment required to remove the trees while currently providing limited payback. One approach to reducing the cost of mechanical thinning is to increase the commercial value of the trees that are removed. However, significant technical barriers currently exist to the use of these materials in high value applications
Current markets for small diameter timber range from firewood and chip material to fence posts, lumber, and small poles. Costs for small diameter timber vary according to end use and range from $26 to $60 per green ton. At this price, mechanical thinning is not currently commercially viable. What further erodes the value of the material is the variability of the thinned wood. The wood must be thinned based on forest management priorities rather than selection of the wood for its commercial value. As a result, new markets need to be developed that can support the costs of mechanical thinning. However, it is also important that information on the quality of the material be obtained early in the selection, cutting and sorting process in order for the materials to be directed to the highest and best use.
One suggested new market for small diameter timber is as a structural material. Wolfe  presents the inherent advantages of using a full dimension round member instead of cutting a sawn lumber piece out of the same size of log. These advantages include distribution of defects, increased stiffness, reduced processing, and, in some cases, increased strength. However, disadvantages are also associated with the use of round structural members. These include unfamiliarity in construction, fit and workability due to the current design methodology, and lack of accurate and usable design values for round timbers.
Several approaches are currently being investigated to explore the utility and efficiency of small diameter timber as structural members. Small diameter timber is currently being used in demonstration structures that make use of novel connection details while methods are also being developed to evaluate the use strength and stiffness of the resulting members early in the thinning process. The demonstration structures have been planned in order to show how small diameter timbers can be used to build modern structures. The demonstration structures employ round timber in a variety of structural elements instead of traditional sawn lumber. These elements include beams, columns and tension members. Currently, small diameter timbers are most efficient if used in beams and beam/column elements due to their round cross-section.
Due to the shape and predominance of spiral grain in many of the relevant softwood species, the design of connection details is critical role to the design of structures built using small diameter timber. The strength of small diameter timber elements in a structure depends on the amount of load transferred through the connection. The round cross-section precludes the use of many existing connection designs. Further, spiral grain in the wood also requires that the structural element be allowed to rotate in the connection detail as the log dries. Many of the most obvious connections are axially symmetric and would be attached to the center of the structural member. This type of design also produces additional challenges since the connection detail is likely to be partially or fully anchored in juvenile wood. As a result, the small diameter timber creates a unique set of parameters that must be addressed in order to obtain an efficient connection. These variables include reduction of section, moisture shifting of the material, and geometry of the section. These connection details are the focus of a significant amount of attention and probably present the biggest challenge to researchers in this area .
The other important research need for SDT is the need to rate the strength and stiffness of he material early in the process of mechanical thinning. The rating of the material will allow the wood to be directed to the appropriate usage early in the handling. Early direction of the material to particular uses is important since much of the cost of this material is associated with transportation and handling. The materials that cannot be used for structural applications can then be directed to more traditional low value applications such as firewood and chip material. Material that is more difficult to classify can be directed for use as fence posts and small poles that represents an intermediate value product.
In addition to examining to the structural uses for small diameter timber, the material properties of small diameter timber need to be examined. Due to the naturally occurring growth cycle, a single tree contains different types of wood within the cross section. The strength requirements in a young, small diameter tree are much different than that of an older, large diameter tree. The young tree does not require as much strength and benefits from a larger amount of flexibility since the weight bearing capacity is low but lateral forces are high. This is the familiar flexible small tree that bends rather than breaks on impact. In order for the young tree to compete for sunlight the emphasis is on fast growth and flexibility in the wood created in the early years. An older tree requires more strength since the increase in volume of the tress, and therefore the weight, is faster than the increase in the cross-sectional area. The higher bearing stress is accompanied by increased wind loading as the tree begins to reach above the surrounding plants. Flexibility is sacrificed to obtain increased strength. The resulting woody material of the tree is in direct correlation to its growth cycle. At the center of the growth ring cross-section, low strength, low modulus juvenile wood is encountered while higher strength mature wood is encountered towards the outside of the growth ring cross-section. Typically, milling of a large diameter tree eliminates the low strength juvenile wood from the resulting sawn lumber by cutting around the interior cant of the tree. The removal of juvenile wood is not possible in small diameter timbers due to the small cross section. As a result, full section small diameter timber members are typically used.
Due to the high percentage of juvenile wood found in small diameter timber, the expected strength of the member is lower than comparable sawn lumber products. However, a spiral grain does not impact the bending strength and stiffness of small diameter timber since unlike sawn lumber small diameter rounds are not being loaded across grain. Similarly, the impact of knots and other material variability tends to be reduced for small diameter timber. Thus, it is hypothesized that the quality and final end use of the small diameter timber removed from the forest can be determined based on the amount of mature wood in the small diameter timber. This effort is an initial test of this idea with the effects of several factors on the bending strength of small diameter timbers considered. The other factors are considered to determine if the relation between the amount of mature and juvenile wood in the member cross-section is the primary determinant of bending strength.
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