Views: 21 Author: Frank Ding Publish Time: 2022-05-13 Origin: Site
The F grade system has been used to grade structural timber for many years. In general, all timbers other than certain plantation pines are still F graded. The grading process can be done either visually (that is, by eye), or mechanically, using a machine stress grader. The alternative grading system used for specific plantation pines is MGP grading. This is always done with a machine.
Set out below is a brief summary of the two grading systems.
F grading
An F grade is a measure of the bending strength of a piece of timber. 'F' stands for force in megapascals (MPa), and is the amount of force a piece of timber can withstand without bending beyond an acceptable limit. A piece graded to F11, for example, will have a safe working stress in bending of 11 MPa.
Note that the range of grades available for some softwoods has been updated in recent years as the result of continuing research into their performance in structural applications. Radiata pine, for instance, used to be visually graded to as high as F14, which was then revised down to F11, and now has now been modified again to F8 as the highest available grade.
MGP grading
MGP stands for machine graded pine, and is a system used for particular pines, especially radiata, slash and Caribbean. Instead of being based on the bending strength of the piece, MGP is a measure of its modulus of elasticity (MOE), which takes into account other properties, such as stiffness, compressive strength and shear strength.
The higher the MOE of a piece, the stiffer it is. When this is measured by an MGP machine stress grader, the board is given a grade which is simply an abbreviation of the MOE category that it falls into.
That is:
MGP 10 has an MOE of 10,000 megapascals (MPa)
MGP 12 has an MOE of 12,700 Mpa
MGP 15 has an MOE of 15,200 Mpa
Using MGP in place of F graded timber
It's important to note that although MGP material may be substituted for F graded material, as shown below, F graded material must not be used where MGP material has been specified. This is because the MGP grades have design tables for a range of properties, whereas F grades are based on bending strength only. So it is possible that the architect or builder who has specified an MGP grade for a structural member is relying on design characteristics such as compressive strength or shear strength, which would not be accounted for in an F graded piece.
But for LVL tumber, things were different.
LVL is made of thin veneers bonded together with weather-resistant phenolic adhesive. This means that the dimensions of the final LVL product are not limited by the dimensions of the raw material, and even small-diameter logs can be used to produce large LVL beams and panels. When veneer logs are peeled to produce veneer, any natural defects in the wood, such as knots, are dispersed as small fragments across the veneer. This, together with the lamination effect, eliminates the impact of defects and results in exceptionally homogeneous material properties.
According to AS/NZS 4357 LVL standard, the F grade is measured by MOE, stiffness, compressive strength and shear strength, similar as MGP grade. it is done with a machine. The actual properties associated with the F-grades can be obtained from the Timber Structures Code. For example F8 LVL timbers have the following properties (from AS 1720.1 Table H2.1):
Stress grade | Characteristic values, MPa | ||||||
Bending ( fb' ) | Tension parallel to grain | Shear in beam ( fs' ) | Compression parallel to grain ( fc' ) | Short duration average modulus of elasticity parallel to the gain, MPa (E) | Short duration average modulus of rigidity, MPa (G) | ||
Hardwood | Softwood | ||||||
( ft' ) | |||||||
F34 | 84 | 51 | 42 | 6.1 | 63 | 21500 | 1430 |
F27 | 67 | 42 | 34 | 5.1 | 51 | 18500 | 1230 |
F22 | 55 | 34 | 29 | 4.2 | 42 | 16000 | 1070 |
F17 | 42 | 25 | 22 | 3.6 | 34 | 14000 | 930 |
F14 | 36 | 22 | 19 | 3.3 | 27 | 12000 | 800 |
F11 | 31 | 18 | 15 | 2.8 | 22 | 10500 | 700 |
F8 | 22 | 13 | 12 | 2.2 | 18 | 9100 | 610 |
F7 | 18 | 11 | 8.9 | 1.9 | 13 | 7900 | 530 |
F5 | 14 | 9 | 7.3 | 1.6 | 11 | 6900 | 460 |
F4 | 12 | 7 | 5.8 | 1.3 | 8.6 | 6100 | 410 |
In certain cases, F grader LVL can be used to replace MGP grade according to the technical parameters in the table.
The characteristic strength properties for LVL products have been validated by in-grade testing and whilst they use an F-grade designation, they are different properties to F-graded sawn timber and have a high level of reliability. Australian produced plywood is linked to an audited, quality control and product certification system, when designated with an EWPAA (Engineered Wood Products Association of Australasia) Grade Mark.
Some third-part audited quality control programme like Bsi could supply credible certificate like Benchmark complied with AS/NZS 2269 standard.You can check your supplier if they have this certificate and stamp on the board.
Hongcheng company can supply high quality Structrual plywood, Bracing plywood, Formply and Sturctrual LVL to our customers. All products had passed the Bsi Benchmark certificate compliance with AS/NZS standard.
