CHAPTER 10 Flashcards
2 types of growth
endogenous
exogenous
What is endogenous
intertwined growth.
very strong and lightweight
What is exogenous
outward growth
more predictable engineering properties
Difference between deciduous and confierous
Deciduous = hardwood Coniferous = softwood
Dark vs light ring
Early wood : light ring
Latewood : dark ring
What is early wood
rapid spring growth of hollow thin-walled cells
What is late wood
dense summer growth of thick-walled cells which are much harder and stronger
from center axis outwards
- pith
- heartwood (darker)
- sapwood (ligher)
- cambium (very thin layer)
- inner bark
- outerbark
Longitudinal properties
strongest and least shrinkage
Radial
perpendicula ro the growth rings
Tangential
weakest and most shrinkage
The direction influences
strength, modulus, thermal expansion, conductivity and shrinkage
Definition of anisotropic
properties change with direction
Chemical composition of wood
Cellulose Lignin Hemicellulose Extractives Ash-forming
What is cellulose
polymer that forms strands (fibrils) that make up cell walls.
high density indicates higher strength
What is the lignin?
The glue
What are extractives?
tannins, coloring matters, essential oils, fats, resins, waxes, starches
What are ash-forming?
calcium, potassium, phosphate, silica
What depends on the moisture content?
shrinkage
strength
weight
What is EMC?
equilibrium moisture content.
moisture content for average atmospheric conditions
What is the FSP?
fiber saturation point
moisture content when cells are completely saturated with bound water but no free water inside cell cavities.
what is bound water
water held tightly in cell cavities, wood shrinks on removal
what is free water
water inside cell cavities
doesnt affect shrinkage
above fsp?
changes affect only wet weigth
below fsp?
small changes strongly affect all physical and mechanical properties
largest shrinkage is in
tangential direction
smallest shrinkage is in
longitudinal direction
zero shrinkage @
above fsp, regardless of direction
for glulam, assume
1% shrinkage per 5% change in m/c
wood production steps (6)
- harvesting
- sawing
- seasoning (drying)
- surfacing (planing)
- grading
- preservative treating
5 types of wood products for construction
- dimensional lumber
- heavy timber
- round stock
- specialty items
- engineered wood products
harvesting : 3 things
- minimal sap wanted
- concerns of fire hazard
- other plant growth and underbrush is minimal
3 types of sawings
live (plain)
quarter
combinatio0n
3 types of board cuts
flat-sawn
rift-sawn
quarter-sawn
describe flat sawn
worst quality, most problems and defects
grain is
describe rift-sawn
45-80°
describe quarter-sawn
80-90°
best quality, least shrinkage problem
moisture content of green wood when leaving mill
approximately 15%
3 methods of seasoning
air drying (cheap and slow) kiln drying (fast and expensive) combination
s4s means
surfaced 4 sides = dressed
planing involves taking off
1/4 inch from each side
4 types of grades for canadian timbers
- select structural (ss)
- no 1
- no 2
- standard utility
causes of lumber defects (5)
- natural wood growth
- seasoning too fast
- wood diseases
- animal parasites
- faulty processing
what is the specific gravity of the cell wall?
1.5
specific gravity closer to 1.5 means
more cell walls which is denser and stronger
what is the R value?
1/conductivity
what is thermal diffusivity
rate at which material absorbs heat from surroundings
applying heat to wood
first expands the wood from thermal expansion
then shrinks from moisture loss (when below FSP)
electrical properties of wood
good electrical insulator which decreases with moisture content. more water is a better electrical conductor
modulus of elasticity of wood depends on
- species variation
- moisture content
- specific gravity and
- direction of grain
which is better : tensile or compressive?
tensile
which is stronger : parallel or perpendicular to grain for tensile strength?
parallel to grain 20x greater than perpendicular
loads over time for wood?
wood can support higher loads of short duration than sustained loads
load duration : what is the time assumed for design?
10 year loading and/or 90% full max load throughout life of structure
short-duration loads, how to design?
multiply design values by load duration factors fr short-duration loads
vibration damping and wood
vibration damping increases w/ moisture content up to FSP.
better than structural metals
what are the two main testing techniques?
- testing of timbers of structural sizes (ASTM D 198)
- testing of representative, small, clear specimens (ASTM D 143)
3 types of tests
- 2-point, 3rd-point or center-loading
design considerations for wood
- load duration
- temperature
- size
- flat use
- column stability
- repetitive member
- wet service
- beam stability
- volume (glulam only)
- curvature (glulam only)
- bearing area
5 organisms that degrade wood
- fungi caused dry rot
- bacteria damage black heartwood
- termite damage
- spruce ips beetle
- marine-borer damage to a buried pile
wood preservation, 2 types
- petroleum-based,
- waterborn preservatives (salts)
2 applications techniques for wood preservation
- superficial treatment (Generally not effective)
- liquid penetration (pressure treating at high temp, heat and moisture)
what are engineered wood products?
- made by bonding together wood strands, veneers, lumbers or other forms of wood fibers to produce large units.
2 advantages of engineered wood products
- produce specific and consistent mechanical properties that are better than natural large ppieces
- hard to find high quality large natural pieces
what is plywood
thin sheets (plies) flued together with the grain at right angles toeach other so it has the same propertiesin both direction
how is plywood made?
veneer is peeled from a soaked log on a giant lathe
what is particle board?
sawdust sized particles
what is chip board
randomly oriented wood chips
what is osb
wood chips and strand oriented in specific direction
how are floor joists made of wood?
two 2x4s or 2x6s as flanges and an OSB web
what is glulam?
lumber glued together with the parallel grain
advantages of glulam
- ease of manufacturing large pieces from standard commercial
- can vary cross section along length
- special architectural designs
- can use lower wood grade in less stressed areas
- minimizes shrinkage defects
