Timber Flashcards
Desirable characteristics of timber
- Versatile material
- High strength to weight ratio
- Good resilience and impact properties
- Good sound insulation
- Good fire resistance
2 types of timber
Hardwood and Softwood
hardwood
- Broad leaved, deciduous trees
- 53 quoted species
- Includes hardest and densest wood e.g. Oak and Balsa
- Less well defined rings as cells are closer together due to slower growth
Softwood
- Coniferous Trees
- 10 quoted species e.g. Pines, firs
- Fast growing and therefore cheaper
- Well defined rings as growth is fast in the spring giving larger cells and slower in the summer and autumn giving smaller cells
What does anisotropic mean
The mechanical and physical properties vary longitudinally, radially and tangentially
What factors affect the properties of timber
- Species of tree
- geography and climate
- Age of tree
- Speed of growth
- Knots
- Variations within the tree
Cultivation of softwood trees
- Seeds are grown in greenhouses and once they have sufficiently grown are then planted out
- Some are cut down after 10-15 years to be used as fencing material
- Softwood trees don’t reach maturity until 60 years
What is conversion of timber
Cutting logs and turning into planks
Defects caused by conversion
- Bow
- Spring
- twist
- Cup
4 main factors that effect strength of timber
- Moisture Content, given as a percentage of the dry weight. There is a 4-5% increase in strength for each 1% reduction below the fibre saturation point
- Nominal Specific gravity, Mass oven dried 103C/Volume/ Density of water. For many species NSG is constant
- Growth effect, rapid growth leads to larger cells and hence reduced density and strength
- Statistical Variation, normal distribution assumed
How is Timber graded
- Visual Grading, inspected and measured for the proportions of the sections occupied by knots, the slope of the grain and the width of the growth rings
- Proof Grading, Timber is tested and needs to meet the required stress to pass. However stressing timber to near failure point may weaken it
- Mechanical grading, planks machine graded for stiffness and strength by continuously feeding them into a machine that bends them. The resistance is measured and strength predicted
Knot area ratio
Exists if more than half the margin areas are occupied by knots. Regulations give allowable limits for knots
What does FSP stand for
Fibre saturation point
Shrinkage and moisture movement
Below the FSP, dimensions reduce radially and tangentially but there is little movement longitudinally. The shrinkage isn’t uniform causing distortion
What is Glulam
It is where large sections of timber are made by gluing small sections together. They are layered next to each other with the grains parallel
