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Portuguese pine forests frequently have high density due to the large number of young trees, which are maintained in the stands due to the high costs of thinning operations and the lack of financial return provided by the markets available for small-diameter poles. With this problem in mind, a national project was launched to investigate the use of Maritime pine small-diameter poles in structural applications. It is believed that this utilization could promote stand thinning operations by providing extra income to forest owners. Poles for the study were collected in an interior central region of Portugal and visually graded according to European standard EN1310. Poles were then tested (bending and compression) following European standard EN 14251. The results obtained are presented and discussed along with the results obtained for round timber from other wood species and for Maritime pine strength graded structural timber with rectangular cross section. Considering future development of national strength grading standards (based on the results of this project), some correlations are presented between wood features and mechanical properties and between bending strength and stiffness. Good correlations were obtained between bending strength, density, and local modulus of elasticity (MOE). A significant difference in the bending strength between round and rectangular cross sections was observed but not for MOE.
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Portuguese forests have problems of high density due to lack of organization. Similar problems have been reported in United States, such as in the forestland of the Inland Northwest (Erikson et al. 2000), Appalachia, southern pine plantations in the south (Patterson et al. 2002), and in the forestland along the Colorado Front Range (Mackes et al. 2005). This situation results in a higher risk of wild fires, such as the catastrophic fires in 2003 in Portugal. It also promotes the spread of diseases.
Usually stands of Portuguese forest Maritime pine (Pinus pinaster Ait.) are the result of natural regeneration. Increasingly they are a consequence of forest fires or established plantation with no after-care operations (cleaning, thinning, etc.). Therefore, pine stands generally have a large number of young trees that if removed in due time could increase the quality of mature trees and decrease the risk of fire. The large number of young trees in the stands, which should have been removed by regular thinning operations, is generally due to the high cost of this operation and the lack of profitable end-uses for the small-diameter poles (Wagner et al. 1998, Fight et al. 2004).
Although applications for wood material removed from the stands by after-care operations exist (pulp, biomass, or poles for fences), these are applications with small added value. To help offset thinning costs, higher value products using small-diameter material should be identified and developed, such as structural applications. Nevertheless, the use of small-diameter poles in structural applications is only possible if poles are selected (graded) and the mechanical properties for each grade are known.
Several studies have been done with that purpose, de Vries (1998a, 1998b) determined modulus of elasticity (MOE) and bending strength of roundwood larch (Larix kaempferi) from the Netherlands. For the same specie, de Vries and Gard (1998) conducted studies for predicting bending strength of the piece with nondestructive tests. Ranta-Maunus (1999) presented results for MOE, compression strength, and bending strength for roundwood of various species of various countries: Scots pine (Pinus sylvestris), Norway spruce (Picea abis), sitka spruce (Picea sitchensis), larch, and Douglas-fir (Pseudotsuga menziesii). Within the same project, a draft of a standard for visual grading was presented. Similar studies were also undertaken in United States. Wolfe and Moseley (2000) determined MOE, bending strength, and compression strength in roundwood Douglas-fir, white fir (Abies concolor), and radiata pine (Pinus radiata) from southwest Oregon. Larson et al. (2004) and Mackes et al. (2005) have determined bending strength of ponderosa pine (Pinus ponderosa).
There is no research on this topic in Portugal. Furthermore, the tree species grown in Portugal were not considered in other European studies. In order to change this situation, a national research project was promoted by a local association in collaboration with the University of Coimbra and the National Laboratory of Civil Engineering. The objective was to provide the basis for the use of small-diameter Maritime pine poles in structural applications. It is believed that this utilization could promote stand thinning operations by providing extra income for forest owners.
The study had two different tasks. The first task comprised the determination of the mechanical properties, the establishment of a Portuguese visual strength grading standard, and a proposal for a certification scheme to support the grading process. The second task, which will be finished in 4 years, concerns the development of special connections and design rules to promote the use of roundwood in construction, increasing the added value of small-diameter Maritime pine poles obtained from forest thinning operations. This paper presents the results obtained within the first task.
Materials
Pole selection
Test specimens came from a small forest area called Pinhal Interior Sul which comprises five municipalities (Fig. 1) with a total area of 1906 [km.sup.2] of which approximately 51 percent is forest areas. In each municipality five to eight stands (approximately 0.5 ha) were selected, and from each stand 20 poles were collected for a total of at least 100 poles per municipality. Specimens were selected from the available material removed in normal forest thinnings and showed no biological degradation or deep fissures. Specimen length was equal to 25 times the diameter. The poles were debarked but not machine rounded.
Seasoning and conditioning
The 500 poles selected were piled outdoors, but protected from direct exposure to sunlight or rain, to dry until a moisture content (MC) below 18 percent was reached. This MC goal was achieved in 6 months, from March to August of 2007. The first row of the pile was placed 200 mm aboveground, and the poles in each row were spaced 50 to 100 mm from each other.
After drying to below 18 percent MC, a new selection was made based on the diameter of the poles, knot diameter, width of the growth rings, shape of the pole, and signs of biological degradation. This selection resulted in 200 final poles which were transported to the climatic chamber. There they were subjected to a relative humidity (RH) of 65 percent and a temperature of 20[degrees]C during a 4-month period until a constant mass (weight) was achieved.
Although a uniform distribution of poles along the range of diameters under analysis (70 mm to 190 mm) was intended, a large portion of the poles (81%) had a diameter of less than 120 mm.
Visual characterization of poles
During the final period of drying, several visual characteristics of the poles were measured and recorded, including spiral grain, ovality, taper, rate of growth, sweep, fissures, and knots. Measurement rules for features given in the European standard EN1310 (CEN 1997) were followed, in order to allow the future establishment of visual strength grading rules. The results obtained are presented in Table 1, and the limits admitted for structural application, suggested by Ranta-Maunus (1999), are presented in Table 2.
Spiral grain was assessed by measuring the inclination of the fissures relative to the longitudinal axis of the pole. A significant number of poles (98%) showed a spiral grain equal or inferior to 167 mm/m, the limit value established for lower visual strength grade of Maritime pine sawn timber according to NP 4305 (IPQ 1995).
Ovality, when significant, can compromise the application of the pole (both because of the shape and of the amount of compression wood present). This feature was assessed by measuring the maximum and minimum diameters in a section located at least 1 m away from the larger end. Ovality is given by a ratio of the difference between the two diameters and the maximum diameter. All of the specimens presented an ovality equal or inferior to 20 percent usually considered as the limit value for upper round timber grades (Ranta-Maunus 1999).
[FIGURE 1 OMITTED]
Taper was assessed based on the mean diameter (mean of maximum and minimum diameters) measured in sections at least 50 mm from the ends, and the distance between those sections. Taper was then considered as the difference between the two mean diameters divided by the distance between the measuring points. The limit for structural application suggested by Ranta-Maunus (1999) is 10 mm/m; this requirement was fulfilled by 86.5 percent of the specimens.
The rate of growth is a very important parameter for most European species of softwoods, since it reflects, in an indirect way, the density of the wood, which has a significant influence in the strength and stiffness of timber members. In practice this property was determined for each end as the ratio between the largest diameter and the number of growth rings, expressed in millimeters. In the lower visual strength grade of Maritime pine sawn timber, the limit for rate of growth is 10 mm (IPQ 1995). The results obtained in this project for roundwood show that all of the specimens fulfill this requirement.
Sweep was assessed by measuring the maximum distance between the rounded concave longitudinal surface and a straight line joining the innermost points of the surfaces. For multiple sweep, the highest value was recorded. The sweep value was obtained by the ratio D/L (Fig. 2). The results obtained are higher than the limits indicated in other studies for other species (Ranta-Maunus 1999). But, the fact that the specimens were extracted from natural regeneration forests or stands where an initial thinning should have taken place but was not carried out could explain these weak results.
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