wood A Flashcards
what is the positive of wood bc it has thousands of distinct species with enormous variation?
variation in mechanical properties between species
structure of a tree
- bark
- cambium
- sapwood
- rays
- heartwood
- pith
bark description
outer protective surface of tree, protects the growth layer from negative environmental effects
cambium description
area where new growth occurs, subsequently differentiates into microstructure
sapwood description
area of active cells and food storage. Movements of nutrients from roots to leaves occur through this area
rays description
transportation of food and nutrients in the radial direction
heartwood description
area of inactive wood, maintains the cell structure of sapwood
pith description
core of the tree
harwood description
generally a denser wood with more complicated microstructure. Corresponds to Deciduous trees
softwoods description
generally a simpler microstructure that is lighter, corresponds to coniferous trees
what is the microstructure of wood
- primarily composed of hollow, elongated, spindle-shaped cells that arranged parallel to each other along the trunk of a tree
- anisotropy due to directionality of these long narrow cells
what are the main components in the cells
- cellulose (50%)
- hemi-cellulose (25%)
- ligning (25%)
- extractives (rest)
cellulose description
- long, strong, thin fibers held in parallel orientation via molecular attraction (linear polymer made of thousands of the covalently bonded glucose units)
- fibers attract water molecules and are responsible for swelling and shrinking)
hemi-cellulose description
- provides fiber to fiber bonding
ligning function
binds cellulose fibers together and adjacent cells together
extractives function
gives flavour, colour, not structurally active
what is the difference between wood and timber
wood: small samples that are generally clear from defects
timber: large samples found in lumberyard which probably contain flaws
what does anisotropic mean for the wood
different material resistance depending on direction of loading
what are the different types of material resistance depending on direction of loading
- tension along cell axis (resisted by cellulose and hemi-cellulose)
- compression along cell axis: causes localized buckling of cells
- tension perpendicular to cell axis: pulls apart, only resistance is lignin
- compression perpendicular to cell axis: crushed cells
macrostructural parallels: which are stronger/weaker
- tension along grain (longitudinal direction) strongest
- compression along grain weaker, still strong in compression
- tension across grain weak
- compression across grain weak
how is moisture content created
cavities within the wood cells channel water up from roots via capillary suction
what are the two different forms water can exist in our structural lumber
- free water within the cell cavity
- bound water within the cellular walls
the moisture content differs …. bc of what
different structural properties
what is the fiber saturation point (FSP)
unique point when all the free water has evaporated from cell cavity, but cell walls still fully saturated
what happens when water goes below the saturation point
- there is removal of water from cell walls which causes compaction of molecular structure
- wood will shrink and become stronger
- mechanical properties of wood are related to the amount of moisture available
what happens to the mechanical properties of wood when moisture goes above FSP
little to no effect on mechanical properties
how does shrinkage occur and where does it occur
- loss of water from cell walls induces attractive forces between microfibrils, causing them to bunch together
- since microfibrils are parallel to the grain, there is large reduction in volume perpendicular to the axis of the cells
- there is small amount of shrinkage longitudinally
- shrinkage: tangentially> radially> longitudinally
what are the three axis of wood
- radial
- longitudinal
- tangential
when does wood’s dimensions change? what is result of that
- gains or losses moisutre below FSP
- shrinks when losing moisutre from cell walls, swells when gaining moisture in the cell walls
- shrinkage and swelling can result in warping, checking and splitting
what are the types of deffects
- knots
- shake
- check
- warping
knots description
- area where branches begin off of main trunk
- may get absorbed into the trunk during additional growth phases if branch falls off
- point of weakness and additional stress concentration in timber
shake description
seperation between annular growth rings
check description
weak point through grain caused by drying process
warping description
anisotropic contraction caused by geometry during drying
specific gravity (SG) of air, wood and water? what does this mean?
air SG = 0
wood SG = 1.5
water SG = 1
dry: if average air and wood < 1 - wood floats
dry: if average air and wood > 1 - it sinks
saturated: average of 1+1.5 > 1 sinks ALWAYS!
what do termites, ants bacteria and fungus and worms and lice create in wood
durability concerns
what do termites eat?
cellulose that makes up most of the wood
where do termites live?
in large colonies that can do lots of damage to wood if exposed
different types of termites
- subterranean: hot areas, live underground
- dampwood: live in wood with high water content, may infest dry areas nearby
- drywood: live in dry wood, slower growing than others
carpenter ants concerns
- dont eat the wood, but make their homes out of the wood
- built their tunnels through structural elements - severe weakening of wood through mass loss
- need to live close to a food source to survive
bacteria concerns
- like to live in wet wood, are brought in through water contact
- larger threat to humans living around, may also destroy wood over long periods of time
marine borers concerns
- worms: bore into wood and eat it, make their homes there
- lice: make the wood its home, particularly just below tidal zone
what is the lifecycle of fungus
- spores land on wood and germinate in the presence of wate r
- spores send threads out that secretes enzyme which breaks down cellulose, hemicellulose and lignin in wood cells. Threads continue to spread through wood, eating components of cell walls and compromising more of the member strength
- Fungus releases spores to land in new spot
different types of fungi
- brown rot
- white rot
- soft rot
- dry rot
- moulds and stains
brown rot, what does it attack?
- most severe
- rapid strength loss and material loss
- fungi attacks hemi-cellulose in cell walls
white rot what does it attack
- mostly attacks hardwood
- causes discolouration
- loss of strength
soft rot what does it attack
- only attacks wood in saturated condition
- mostly surface attack
dry rot attack
- tendrils carry water to attack area - making preventing tougher
moulds and stains attack
- primary surface attack
- may cause health problems for humans or aesthetic problems
when can a fungus and bacterial attack occur
- only above fibre saturation point
how does a chemical attack weaken the durability of wood?
- cellulose and lignin are weakened by acid attack
- alkali salts can weaken the cellulose and lignin
- rust can also weaken the cellular matrix if exposed in solution
how does a heat attack weaken the durability of wood
- wood burns when exposed to a flame, but the rate fo burning is highly dependent on the type of wood
- forms a char layer that limits the volume combusting
- proceeds from outside inward
- cellulose becomes denatured at high temperatures and long periods of time
how to prevent durability loss
- waterborne preservatives
- oil borne preservatives
- weather resistant coating
- fire retarding
how do waterborne preservatives work
- salts dissolved in water
- may leach out
- toxicity to insects or fungus in the wood
how does oil borne preservatives work
staining oil compounds that are toxic to insects and fungus
how do weather resistant coating work
- protective barrier to keep moisture out, may require multiple coats
how do fire retarding work
- improve the burn resistance
- surface coatings: apply on surface for some absorption
- pressure impregnation: soaks further into member, could leach in moist conditions
why do we use composites?
- more uniform product free from timber defects: better mechanical properties
- wide variety of shapes and sizes not possible from timber section
- economic efficiency: use more of the wood more possible ways
what is plywood
thin layers of wood laminate in large sheets with the grain in orthotropic directions in subsequent layers
plywood caracteristics
- strong, split resistant
- can be made into wide sheets
- anisotropic shrinking is minimized due to orientation
- knots only through on ply
what is parallam
thin veneers of wood pressed together into beam shape with adhesive, the sawn to arbitrary dimensions
parallam caracteristics
- stronger than timber since free of defects
- higher quality control
- larger possible sections
why does wood exhibit anisotropic properties when it comes to resisting loads? in what principal direction and loading condition (compression or tension) deos wood perform the best
- wood’s anisotropic behavior is from the orientation of the wood cells
- the cellulose fibers within the cell walls are primarily aligned with the axis (longitudinal) of the tree, leading to better strength in that loading direction
- across the grain or radial to this growth the resistance in compression and tension is weaker, only ligning and cell buckling that is resisting the load
- wood is best in tension along axis of the cells because cellulose fibers are working to resist the load. In compression there can be cellular buckling.
what resists tensile loading at the microscopic scale both perpendicular and parallel to the grain of wood
- parallel to the grain: breaking of strong primary bonds of the microfibrils (covalent bonds between each cellulose molecules), the microfibrils are parallel to axis of the cell in the S2 wall
- Breaking of the weak secondary walls, S1 and S3 walls are perpendicular to the acis of the cell are very weak, weak secondary bonds of the microfibrils of the S2 walls, also get lignin
what are the 3 possible failure modes of wood in bending
- compressive
- tension
- shear
how does the most common affect the __ at the macro scale
- shear failure is most common: the grain is seprarated, the weak point between growth rings is separated and at macro scale
- micro scale: ligning between the cells will be sheared off
assuming that no other design adjustments are relevant, why is it abad idea to use the mean strength of a set of wood cubes when adhering to the design of a structure? what is another statistical method we use instead of the mean?
- half of the specimens that your testing is gonna be weaker than the average: means that half the members that we build will fail at the strength we designed for
- 5% exclusion limit: only 5% of the specimens that are loading will fail at the design stress
why do very thick wood members usually survive fires, explain how the mechanism works?
- forms a layer of char on the outside and there’s another zone called the pyrolysis zone (when wood becomes to decompose)
- char is not flammable so you will not get any more fire progressing through (but there will still be heat)
how do you apply fire retardants on wood on a new structure and an existing structure?
- new structure
- existing structure: members are not detached from anything so you can dip them into a liquid: you can use surface coating (absorb through capillary suction (like a sponge), vaccum impregnation )
what are the environmental and economic benefits of using wood for construction
- renewable resources
- cheap
why cant wood compete with concrete
- volume requirements are just not comparable
what are the two main limitations of using structural timber in construction
- size is limited
- defects in timber - limit allowable stresses that we have
how can one avoid these limitations
- composites bc make them whatever size you want them and to reduce the amount fo defects
why is it more practical to curve parallam vs gullam
- cross section of a regular piece of timber: very high stresses at the end (NA has 0 stress) the stress is increased linearly the higher you are the higher the stress
- if many small pieces of timbers … small stresse that build up: bend individually then glue together will make smaller stresses
- since stress is increased the further away the extremities of the specimen are from the NA, by having a member which is less deep (gullam), less stresses develop in that member
whats common between mold and bacteria
- harmful for humans
- don’t significantly damage wood
explain why and how dry rot fungi is a misnomer
- wood has to be saturated for mold to exist
- it can take water from somewhere else which is 2-3 m away.. but the site where the fungus is living can be dry, the fungus itself has to be saturated
what protects the structure from the outside environment
- building envelope: anything that blocks the outside from the inside
what are the two main practices to which this can be degraded
- holes for dryer duct (pierce hole in building envelope)
- balcony railings (added after everything else has been built, pierce a hole)
- making holes without sealing them properly is what makes them degrade
how does holes affect woods susceptibility to fungal and bacterial attack
- most moisture gets into the structure which increases fungi and bacteria attack
