Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres CAMPBELL ACT 2612 Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
MGP Grades Vs F Grades
Anyone who
has worked with timber will tell you that different species behave differently
under varying types of loads, fail in different ways and generally behave like
different materials. This is not
surprising as species have vastly different cellular structures. The F Grading system attempts to lump all
species (yes hardwoods and softwoods) together on the basis of flexural
strength, and takes the lowest of all the remaining properties for the other
parameters.
For the
most part this system is acceptable as flexural strength is more often than not
the most critical design parameter. On
the other hand Radiata, Slash and Caribbean Pine are similar in nature and
represent a large and growing sector of the market. It makes good commercial sense to group these
together to make the most of their properties so that other properties such as
stiffness and compressive strength are not effectively ‘dragged down’ by other
timber species. The MGP grading system is an attempt to do just this.
To give you
an idea of the benefits of the system take a look at the table below and
compare the properties of F5 softwood with MGP10 (its equivalent flexural
grade).
|
Grade |
Characteristic Strength (MPa) |
Modulus of Elasticity (MPa) |
Modulus of Rigidity (MPa) |
|||
|
Bending f’b |
Tension f’t |
Shear f’b |
Compression f’c |
|||
|
F5 |
16 |
8.2 |
1.8 |
12 |
6900 |
460 |
|
MGP10 |
16.2 |
8.9 |
5.0 |
23.6 |
10000 |
670 |
|
F8 |
25 |
13 |
2.5 |
20 |
9100 |
610 |
|
MGP12 |
28 |
15 |
6.5 |
28.9 |
12700 |
850 |
|
F14 |
40 |
21 |
3.7 |
30 |
12000 |
800 |
|
MGP15 |
41.3 |
22.7 |
9.1 |
35.4 |
15200 |
1010 |
What the
comparison shows is that if you are designing say a timber stud for compression
you have a strength gain of 96 percent by using MGP equivalent. If deflection is the governing criteria for a
lightly loaded beam you have a potential reduction in deflection of 44
percent. Increases in shear strength are
even more dramatic but the F Grade values are known to be conservative and you
may note that AS1684 (Residential Framing Code) allows much higher values to be
used and differentiates between hardwoods and softwoods (see appendix A).
We are not
implying that MGP graded timber is that much stronger or stiffer than its F
Grade equivalent. The difference is that
we are allowed to use the additional strength because certain manufacturers
have invested money in both testing and quality assurance systems to justify
the greater values.
You will
see advertisements in the magazines claiming MGP pine to be more stable and
have reduced shrinkage but we have seen no evidence to suggest that this is
anything more than “marketing hype”.
What’s
Happening Around Canberra
I rang
around today (January 2002) to see what percentage of suppliers carried MGP grades
and found about 80% were carrying them (most exclusively) although nobody
seemed to understand that there was any difference between the two grading
systems. The price variations between
the F5 and MGP10 were not consistent, with MGP10 even being cheaper in some
instances. Bunnings Warehouse only
carried MGP10 and MGP12 which were often sold at the same price as they were
bundled together on delivery. AAJA
carried mainly MGP10 and advised us that builders never wanted to pay the 10 to
12 percent premium on the MGP12 (note it is 72% stronger and 27% stiffer). This is probably because builders are too far
down the design process to take any advantage from the strength or stiffness
gain.
FAW on the
other hand had never heard of MGP and carried F5 90*45’s a little cheaper than
most of the others.
What we
do
Our current
policy is to use the MGP grading system in design but to give an equivalent F
grade where flexural strength proves to be the governing design factor. This approach gives the builder the option to
source cheaper timber where appropriate.
Conclusions
For all
builders there are opportunities to save money by having their designers use
the MGP grading system, especially heavily loaded wall framing (such as in
three storey walk-ups)where compressive capacity may govern or lightly loaded
beams where stiffness governs. Relative
pricing changes daily so we give options wherever we can.
With other
timber species and many innovative products on the market such as Hyspan,
Hybeam, Posi-STRUT, Pryda Longreach, LVL, Easybeam and various cold formed
steel solutions, the best answer is not always obvious and may depend on
manpower and cranage available durability or bulk discounting by suppliers.
If you need
some specific answers or just want a few options on floor, roof or wall
framing from a structural engineer in
Canberra ACT call Mal Wilson from Advanced Structural Designs on (02) 61612171a
call on Ph 02 61612171.