Stress grading

Traditional forms of termination using the cast iron box, compound and suitably shaped bushings overcome these problems but, as already described, are cumbersome and difficult to install. Also, when it fails it often explodes causing much damage to surrounding parts of the installation. 

These three requirements are considered in turn below. 

In a high-voltage installation it is important to avoid or reduce the electric stress which can result from abrupt termination of the insulating sheath or screen of the cable. Such conditions of electrical stress can 

Experiments with heat-shrinkable stress cones led to the conclusion at Raychem that the best approach to stress grading was to design a heat-shrinkable tube with a combination of the correct resistive and capacitive properties. It emerged that the required properties were  

The volume resistivity is defined at the electrical resistance of a unit length of the material with uniform cross section, and is a measure of the degree to which a medium can resist the flow of electric current. It is measured in ohm metre. 

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National Design Speciricalion for Stress-Grade Lumber." National Forest Products Avsociation, Washington D.C.. 1968. 

National design specifications for stress-grade lumber and its fastenings." Natl. For. Prod. Assoc.,... 

Stress-Grade Lumber and Its Fastenings" NFPA Washington, D.C., 1982, 81 p. 

There are three main methods to stress-grade timber for structural use visual stress grading, mechanical stress grading, and proof grading. 

Some visual stress grading rules limit the growth rate and exclude all pith. There are two separate issues. The exclusion of with-pith material is understandable as corewood in its vicinity has a number of undesirable characteristics, including low density, low stiffness, and dimensional instability. However, a restriction on growth rate is less satisfactory unless it is coupled with a criterion that addresses ring curvature to ensure that such sorting only excludes material in proximity to the pith. 

Decay in most forms is prohibited or severely restricted in stress grades because the extent of decay is difficult to determine and its effect on strength may be greater than visual observation would indicate. Further without preservative treatment arrested decay in dry wood will resume if the material were to be rewetted. There are circumscribed exceptions. Limited decay of the pocket type, e.g. Fames pini, can be permitted in stress grades, as it occurs in knots but not in the surrounding wood. 

An orderly, voluntary, but circuitous system of responsibilities has evolved in most countries to handle visual stress grading. Most visually stress graded lumber is dimension lumber produced in sizes from 35 to 90 mm (1-1/2 to 3-1/2 in.) thick and 35 to 200 mm (1-1/2 to 8 in.) wide. Each country has its own series of grades that are applied to their products. Table 10.4 provides a partial list. 

Stress grade Bending Basic working stresses (MPa) Tension Shear Compression MOE (GPa)... 

The updated BS 4978 (BSI, 1986) specification for softwood grades for structural uses replaced the four grades by just two visual stress grades, general structural (GS) and select structural (SS), which have strength ratios of the order of 0.3-0.35 and 0.5-0.6, respeetively. However the strength ratio is no longer derived. 

In Canada and the United States, the relationships between the 5th percentile 10-year bending stress and those in tension and compression are based on limited timber testing of the three properties but supported by years of successful experience in construction with visual stress grades of timber (ASTM, 2005J). For tension, it is assumed that the ratio of design tensile stress Ft to design bending stress Fb is 0.45 unless other test data are available for compression the ratio is assumed to be... 

Archer KJ (1985) Bacterial modification of Douglas fir roundwood permeability. PhD thesis. University of Canterbury, Christchurch, New Zealand Archer RR (1986) Growth stresses and strains in trees. Springer-Verlag, Berlin AS/NZ (1997) Timber stress-graded product requirements for mechanical stress-graded timber. AS/NZS 1748. Australian/New Zealand Standard Ashworth JC (1977) The mathematical simulation of the batch-drying of softwood timber. 

SAA (1986a) Timber Softwood visually stress-graded for structural purposes. AS2858. 

Todd KL (1982) An evaluation of potential markets for New Zealand softwood plywood exports. MForSc, University of Canterbury, Christchurch, New Zealand Tokoh C, Takabe K, Fujita M and Sadd H (1998) Cellulose synthesized by Acetobacter xylinun in the presence of acetyl glucomannan. Cellulose, 5 249-61 TRADA (1974) Visual stress grading of timber. Timber Research and Development Association, High Wycombe, UK... 

Table 26.4 Typical Physical Properties of Thermofit SC Stress Grading Tubing...

HVTM), 330 impedance, 312 initial tracking voltage, 318 material selection, 319 non-tracking insulation, 317-34 outdoor environment, 324 permittivity, 312 pollution, 326 power cable types, 308 resistivity, 312 stress grading, 311 termination methods, 309... 

For wood railings Wood components shall be minimum 1500 Ib-ft/in2 fiber (stress grade) construction grade lumber the posts shall be at least 2-inch by 4-inch (5 cm X10 cm) lumber spaced not more than 8 feet (2.4 m) apart on centers the top rail shall be at least 2-inch by 4-inch (5 cm X 10 cm) lumber, the intermediate rail shall be at least 1-inch by 6-inch (2.5 cm x 15 cm) lumber. All lumber dimensions are nominal sizes as provided by the American Softwood Lumber Standards, dated January 1970. 

Mechanically stress-graded timber is graded based on stiffiiess, but as can be seen in Fig. 1, strength is not directly proportional to stiffness. 

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