Info

Parana pine

GS (C16)

SS (C24)

Caribbean pitch pine

GS (C18)

SS (C27)

Redwood

GS (C16)

SS (C24)

Whitewood

GS (C16)

SS (C24)

Western red cedar

GS (C14)

SS (C18)

Douglas fir-larch (Canada and USA)

GS (C16)

SS (C24)

Hem-fir (Canada and USA)

GS (C16)

SS (C24)

Spruce-pine-fir (Canada and USA)

GS (C16)

SS (C24)

Sitka spruce (Canada)

GS (C14)

SS (C18)

Western white woods (USA)

GS (C14)

SS (C18)

Southern pine (USA)

GS (C18)

SS (C24)

♦Timber graded in accordance with BS 4978:1996; based on Table 1.2 , BS 5268-2:2002.

♦Timber graded in accordance with BS 4978:1996; based on Table 1.2 , BS 5268-2:2002.

the load to induce a known deflection) is then automatically measured and compared with pre-programmed criteria, which leads to the direct grading of the timber section and marking with the appropriate strength class. An example of the grading marking , based on the requirements of BS EN 14081-1:2005, is shown in Figure 1.7.

In general less material is rejected if machine graded; however, timber is also visually inspected during machine grading to ensure that major, strength-reducing , defects do not exist.

1.5.3 Strength classes

The concept of grouping timber into strength classes was introduced into the United Kingdom with BS 5268-2 in 1984. Strength classes offer a number of advantages both to the designer and the supplier of timber. The designer can undertake the design without the need to check on the availability and price of a large number of species and grades that might be used. Suppliers can supply any of the species/grade combinations

Key:

PRODUCT: producer identification CODE: Code number of documentation DRY GRADED: used if appropriate NBODY: identification of notified body M: machine graded

C24: strength class or grade and grading

Fig. 1.7. Example of grading marking.

that meet the strength class called for in a specification. The concept also allows new species to be introduced to the market without affecting existing specifications for timber.

BS EN 338:2003 [11] defines a total of 18 strength classes: 12 for softwoods - C14, C16, C18, C20, C22, C24, C27, C30, C35, C40, C45 and C50; and six for hardwoods -D30, D35, D40, D50, D60 and D70. The letters C and D refer to coniferous species (C classes) or deciduous species (D classes), and the number in each strength class refers to its 'characteristic bending strength' in N/mm2 units; for example, C40 timber has a characteristic bending strength of 40 N/mm2. It ranges from the weakest grade of softwood, C14, to the highest grade of hardwood, D70, often used in Europe.

1.5.3.1 Material properties

Section 3 of BS EN 1995-1-1:2004 (referred to in the text as EC5) [12] deals with the material properties and defines the strength and stiffness parameters, stress-strain relations and gives values for modification factors for strength and deformation under various service classes and/or load duration classes. EC5, in common with other Eurocodes, does not contain the material property values and this information is given in a supporting standard, i.e. in Table 1 of BS EN 338:2003, reproduced here as Table 1.3.

The characteristic values are defined as the population 5th-percentile values obtained from the results of tests with a duration of approximately 5 min at the equilibrium moisture content of the test pieces relating to a temperature of 20° C and a relative humidity of 65%.

In addition to providing characteristic strength and stiffness properties and density values for each strength class (and the rules for allocation of timber populations, i.e. combinations of species, source and grade, to the classes), BS EN 338:2003 lists the equations that form the relations between some of the characteristic values given in Table 1.3 for properties other than bending strength, mean modulus of elasticity in bending and density.

The relationships between the characteristic strength and stiffness properties are given as follows:

• Tensile strength parallel (0) to grain, /t,0,k = 0.6/m,k

• Compression strength parallel (0) to grain, /c,0,k = 5(fm,k)0'45

• Shear strength, fv k = minimum of {3.8 and 0.2(fm,k)0'8}

• Tensile strength perpendicular (90) to grain, /t,90,k = minimum of {0.6 and 0.0015^ }

• Compression strength perpendicular (90) to grain,

/c,90,k = 0.007 pk for softwoods /c,90,k = 0.015pk for hardwoods

• Modulus of elasticity parallel (0) to grain,

E005 = 0.67E0,mean for softwoods E0.05 = 0.84E0,mean for hardwoods

• Mean modulus of elasticity perpendicular (90) to grain,

E90,mean = #0,mean/30 for softwoods E»,mean = #0,mean/ 15 for hardwoods

Table 1.3 Strength and stiffness properties and density values for structural timber strength classes, (in accordance with Table 1, of BS EN 338: 2003
0 0

Post a comment