When ever an item such as a beam (joist, lintel, purlin etc.) is used within a building, it will have to be capable of carrying or supporting a specific load.
We have seen how commercial grading rules apply when grading timber for appearance. In this case, (irrespective of species) grading rules will be based on the ability of timber to withstand internal stresses which could be brought about by external forces. Therefore, every piece of timber that may be put into a situation of risk with regard to it’s potential strength must be tested for its physical properties – then placed into a particular strength class and labelled accordingly.
This type of testing is called strength grading. Strength grading may be carried out at the saw Mill after conversion and drying (examples of possible strength graded timber sections are shown in table 1.8) or undertaken under factory conditions at the place of manufacture of struc- tural components, such as a roof trusses.
There are two types of strength grading:
1 visual grading, and 2 machine grading
a Visual strength grading of softwood – visual-grading is a skilled operation, the operative must be qualified and hold a current certificate from an approved certification body. Graders have to determine the strength of each piece of timber handled, by inspecting all its surfaces for any:
● Natural inherent defects – such as type, size and distribution and number of Knots, wane, growth rate (growth rings), growth pattern (sloping grain etc) worm holes and rot etc. – see section 1.6
● Drying defects – such as splits, checks, twist, bow etc. – see section 1.7.8
● Non-visible factors – such as that density of the wood (relative weight – see section 1.11b) and moisture content – see section 1.7.1
Before rejecting it or classifying it into one of two grades;
1 General structural coded as GS 2 Special structural coded as SS
NB. grading SS. if the higher of the two grades
Figure 1.67(a) shows how markings may have appeared on the face of a stress graded timber prior to 1997.
Figure 1.67(b) shows how a suitable grade may now appear when graded within the UK.
Both visual strength graded timber from Canada under National lumbar Grades Authority (NLGA) rules, and USA National
Table 1.8 A Guide to availability of sawn sizes of pre-stress graded softwood timber (extracted from ‘Nordic Timber Council’ fact file No. 5)
Thickness Width (mm) (mm) 75 100 125 150 175 200 225 38 ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ 47 ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ 50* ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ 63 ⫻ ⫻ ⫻ ⫻ 75 ⫻ ⫻ ⫻ ⫻
Note: * Check on availability strength graded before specifying
Grading Rules for Softwood Dimensional Lumbar (NGRDL), will display different grade codes, for example: Under structural ‘Joist’ and ‘Plank’ (J & P) will be labelled either Select (Sel) No 1, No2, or No3.
Examples of how these grades compared with other strength grades can be seen in table 1.9.
b Visual strength grading of hardwood – although outside the scope of this book details can be found within BS 5756 1997 c Machine strength grading – in this case
lengths of timber are passed quickly through a machine (fig 1.68) that comprises a series
of rollers, one of which presses the timber against a curved plate causing it to deflect – the pressure exerted enables the timber stiffness to be measured. Any defects would of course reduce the resistance of the wood enabling the machine to make comparisons with a similar yet defect free sample. It is this stiffness difference which, classifies the timber into a specific strength class. A grade mark is then automatically marked via the machine onto the surface of the timber face. As seen in the illustration with previous strength class (SC) grades, a colour stripe (splash of dye) maybe used every 150 mm to continuously colour code the specified strength grade – previously, timber was stress graded by similar means and marked with either:
● MGS (machine general structural) ● MSS. (machine special structural ) or, ● M 50 (50% strength value compared to
timber with straight grain and defect free)
● M 75 (75% strength value compared to
timber with straight grain and defect free – see figure 1.67c)
You’ll notice from figure 1.67(a & d) that a code SC (strength class) has been used. There used to be 9 strength classes – higher the number greater the strength. Softwood strength classes (SC) ranged from SC1 to SC5. Hardwood strength classes (SC) extended to SC9.
NB. Building regulations approved document ‘A’ (Part B) may still referred to SC3 and SC4. As far as European Redwood and Whitewood are concerned, these two grades could be com- pared, (although not truly compatible) with cur- rent strength class ‘C16’ or (GS) and ‘C 24’ or (SS).
Since the introduction of BS EN 338 1995, strength classes have been based on the charac- teristic values of timber strength and stiffness and placed into two groups:
1 softwood (pre-fixed ‘C’, for Conifer) 2 hardwood (pre-fixed ‘D’ for Deciduous) Softwoods shown in table 1.9 have 7 strength classes: C14, C16, C18, C22, C24, C27, and C30.
Hardwood (not shown) five strength classes: D30, D40, D50, D60, and D70.
There is also a special grade to meet end uses of trussed rafter material – this is TR26.
Grading timber 37
Sweden
Company licence no. Grader no. Mark or logo
of certification body Wood species group
Visually graded Grading company number British Standard number for stress grading British Standard Moisture content condition (alternatives KD or WET) Moisture content condition Grade Grading company name (logo) B.S. Institute Kite Mark
Machine grade
Grade and strength class
or splashed with coloured dye every 150 mm to indicate the grade at that
point. Colour examples: MGS - green, M50 - blue, MSS - purple, M75 - red Work species or
species group Company licence no.
Grader no. Mark or logo
of certification
Note: Methods (a) and (c) no longer used
Strength class
Standard number
Moisture content condition [alternatives 'KD' (kiln dried) or 'wet']
Strength class Grade
GS-General Structure Strength
class
Wood species group EW - European whitewood
ER - European redwood
Wood species group EW - European whitewood ER - European redwood Licence no. of stress grading machine Moisture content condition GS SC 3 EW/ER BS 4978 DRY S 001 V
C.A.T.G. A0001/000 EW/ER
BS 4978 DRY C16 GS BS 4978: 1988 0000 SC 5 EW/ER DRY 0000 EW/ER C22 BS EN 519 DRY
M75
M75 SC5 M7 VI SU AL S TR ENG TH GR ADES M A C HINE S TRE N G TH G R ADES (a) (b) (c) (d) (e)Softwood graded to North American Machine
Stress-Rated lumbar rules (NAMSR) are graded
with a number followed by ‘f ’. and a further number followed by ‘E’, for example: 1200f- 1.2E .
By using table 1.9 you can make a compari- son between all the common visual and machine softwood grades and their strength classes.
1.8.5 Moisture content of strength graded