Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
73 Cards in this Set
- Front
- Back
|
What are the ideal properties of wood and their corresponding molecular structure? |
Polymer Chains - Cellulose Alignment to form fibres - Micro-fibrils of cellulose Maximize strength/weight ratio by forming tubes in matrix of glue - Tube "cells" made of micro-fibrils embedded in resin matrix (lignin) |
|
|
What are the major advantages of wood? |
- Reusable, recyclable, biodegradable - CO2 Storage - Aesthetics - Cost effective - Renewable resource |
|
|
What are the major disadvantages of wood? |
- Food source for fungi, insects, and bacteria - Many imperfections that tend to control structural behavior |
|
|
What orientation type does wood fall under? What does this orientation type imply? |
Orthotropic - properties vary between parallel, tangential and radial axis. ** Mechanical properties for each axis may vary by a factor of 20 |
|
|
What is the difference between wood, timber, and lumber? |
Wood - defect free material, small clear pieces Timber - natural material which includes defects Lumber - timber sawn into structural members |
|
|
What molecular structure is wood composed of? |
49% Carbon Composed of the following organic polymers - Cellulose 40-44% - Hemicellulose 15-35% - Lignin 18-35% - Extratives 1-5% |
|
|
What is cellulose and their main contributions to the mechanical properties of wood? |
- Linear polymer chain of glucose units (covalently bonding leads to high strength and stiffness) - Laterally bonded into linear bundles - Provides stength and framework of cells |
|
|
Hydroxyl units (OH-) provide bonding for what part of wood? |
- It attracts cellulose molecules making microfibrils - Attracts water which is largely responsible for swelling and shrinking |
|
|
What are lignins and their main contributions to the mechanical properties of wood? |
- Complex high molecular weight polymer of phenolpropane units - Permeate cellulose microfibrils in cell walls - Lignins and hemicellulose surrond cellulose units, bonding them together (similar to a surrounding matrix) - Imparts rigidity and compressibe strength to cell walls - Adds to woods toxicity (resistant to decay and insect attack) Bonus ** Lignins are washed out during the paper making process; black liquor is used as an admixture for water reduction in concrete. - |
|
|
What are Extratives and their main contributions to mechanical properties of wood? |
- Variety of chemical substances - Provides properties such as colour, odour, taste, strength, flammability, hyrgroscopicity Not part of basic wood structure |
|
|
What are Microfibrils and their main contributions to mechanical properties of wood? |
- Threadlike bundles of cellulose molecules that are arranged parallel - Consist of cellulose, hemi-cellulose, and lignin |
|
|
Describe the orientation of the different layers of microfibrils. |
From Outside to Inside: Middle Lamella - Lignin rich area which joins neighbouring cells Primary wall - thin, oriented randomly Outer Layer (S1) - left and right hand helix 50-70% inclination Middle Layer** (S2) - Very important layer for elastic modulus and stiffness of wood; Steeper angled S2 increased stiffness and wood from older trees tend to be steeper Inner Layer (S3) - left and right hand helix 50-70% inclination |
|
|
Page 1-14 of notes- Schematic Illustration of parts of a tree |
|
|
|
What is the purpose of outer bark? |
Protection |
|
|
What is the purpose of inner bark? |
Transport sap from leaves to the growth parts |
|
|
What is Cambium? |
Layer of tissue 1 cell thick; between bark and wood - cell division forms new wood |
|
|
What is sapwood? |
Roughly 1/3 of a trees's wood; responsible for moisture conduction and food storage |
|
|
What is heartwood? |
Inner, non-living core. More resistant to decay than sapwood and is drier and harder |
|
|
What is the difference between Spring wood (early) and Summer wood (late)? |
Spring wood- rapid growth of large cells with thin walls Summer wood- thicker cell walls; stronger mechanically |
|
|
What is a pith? |
Center of wood |
|
|
What is the difference between softwood and hardwood? |
Softwood- Needle or scale-like leaves (SPF, D-F, Hemlock, Cedar, Cypress, Redwood, Larch) Hardwood- Broadleaf trees (lose leaves in winter), Alder, Poplar, Birch, Oak... etc. Used more for furniture, rather than structural purposes |
|
|
What is a Tracheid? |
- Longitudinally Oriented cells - 2 to 5mm long with aspect ratio of 100 - Make of 90% of volume of wood in softwoods ** Responsible for mechanical support of tree and vertical conduction of water |
|
|
What is a Parenchyma? |
- Transversely oriented cells - 30um wide by 200um long - located in rays extending radially within a tree ** Stores food and transports horizontally |
|
|
What is the density of wood controlled by? |
Void spaces located within the wood |
|
|
Why is density important when taking about mechanical properties of wood? |
Void space (porosity) control density, which also results in large changes in mechanical properties |
|
|
Moisture Content exists in wood in what two states? |
Free water - which liquid is trapped in cell cavities (>28% M.C) Bound water - physically absorb water by the cell walls (<28% M.C) **Bigger holes dry more quickly |
|
|
What does green wood entail? |
Green wood has high moisture content due because it contains sap and the cell cavities are full or partially full |
|
|
What is fibre saturation point? |
When all free water is evaporated, but the cell walls are still fully saturated |
|
|
As drying occurs below Fiber Saturation Point, what wood properties change? |
Wood becomes stronger mechanically, but starts to shrink below the FSP the volumetric shrinkage is approximately proportional to the volume of water lost |
|
|
Drying wood too quickly leads to what? |
Cracking and warping |
|
|
What are the codes and relative M.C of Dry and Green Lumber |
S-DRY - < 19% S-GRN - > 19% |
|
|
How do shrinkage minimized in wood |
- Slower rate of drying - Coatings - Protective wraps - Protect from rain - Use wood near EMC such as Kiln Dried Lumber |
|
|
What are the tensile properties of wood? |
70 to 150 MPa parallel to grain - Requires rupture of primary bonds - Small failure strain (1%) 2 to 9 MPa perpendicular to grain - Secondary bonds between tubs are weaker - large failure strain due to distortion of tubes |
|
|
What are the compressive properties of wood? |
25 to 60 MPa parallel to grain - develop small kinks within microfibril at failure and localized buckling of cell walls - kinks can be seen on surface 3 to 10 MPa perpendicular to grain - cells collapse or flatten at low stress |
|
|
What key properties of wood are important in regards to flexural strength? |
Failure will occur first in compression, then neutral axis will lower and eventually tensile failure will occur 40 to 100 MPa |
|
|
What are the three types of failure in shear wood exhibits? |
Shear perpendicular to grain, parallel to grain (5 to 15 MPa), and rolling shear (25% of previous) |
|
|
What does temperature do to properties of wood? |
- Prolonged exposure at high temperature degrades wood irreversibly - Made worse by higher M.C ** Rule of Thumb 1 Degree changes the mechanical property by 0.6 to 1 % |
|
|
What effect does the rate of loading have on wood? |
increase in apparent strength as the rate of load increases (aprrox. 8% for every 10 times increase) |
|
|
What effect does duration of loading have on wood? |
Increased load duration results in reduced strength |
|
|
What effect does creep have on wood? |
- initial deformation is elastic - if load is maintained for long periods of time, creep will occur - occurs at low stress levels **higher temp and moisture condition increases creep by approx 10 times **Wet/dry cycles further increases creep |
|
|
What is fatigue? |
Progressive damage that occurs in a material that is subjected to cyclical loads |
|
|
What factors effect fatigue? |
- Number of cycles - Frequency of cycles - Stress level relative to failure - Stress differential (between max and min values) - Stress reversal (tension and compression) - Temperature, moisture, size |
|
|
What is a knot in regards to defects? |
portion of a limb surrounded by subsequent wood growth **shape and type of knot (encased(dead) knot and intergrown knot) influence mechanical properties |
|
|
What is a check in regards to defects? |
Lengthwise separation of wood which usually extends across the growth rings Caused by differential in temperature of the surface and inner core |
|
|
What is a wane? |
lack of wood on the face of a piece for any reason |
|
|
What is a shake? |
Separation of along the grain between the annual growth rings (caused by wind damage) |
|
|
What is a pitch pocket? |
Opening between growth rings containing resin or bark |
|
|
What are the four types of warping observed in wood? |
Bow, Crook, Cup, Twist |
|
|
What are the visual grading criteria? |
- Density - Decay - Heartwood or sapwood - Slope of grain - Defects |
|
|
How does machine stress grading work? |
Machine measures the stiffness of a piece of timber by using the empirical relationship between stiffness and strength |
|
|
Why is it important to grade wood? |
Permits the designer to choose appropriate mechanical properties for structural purposes |
|
|
Mechanical properties of wood are factored or reduced to account for: |
- Variability of wood (using 5% limit) - Conversion from small wood to lumber or timber size - Conversion from perfect lumber or timer to specific grade category |
|
|
What stress modification factor does Kh account for? |
Load Sharing Factor - Accounts for redistribution of load from a highly stressed member to surrounding members |
|
|
What stress modification factor does Kd account for? |
Dependent of duration/rate of loading. Reduces or increases allowable stress depending on short or long term loading |
|
|
What stress modification factor does Kt account for? |
Treatment factor; chemicals for preserving or fire retardant may reduce strength properties |
|
|
What stress modification factor does Ks account for? |
Service Condition Factor; strength dependent on M.C, used to modify conditions other than dry |
|
|
What stress modification factor does Kz account for? |
Used to further account for effect of size on strength (Reduces) |
|
|
List 5 types of manufactured wood products. |
- Fingerjoined Lumber - Glulam - Structural Composity Lumber (SCL) ; Includes Laminated Veneer Lumber (LVL), Laminated Strand Lumber (LSL), Parallel Strand Lumber (PSL), CLT, Nail Laminated Timber - Light frame truss - Wood I-Joists - Plywood and OSB |
|
|
What is CLT? |
Cross Laminated Timber - Kiln dried lumber stacked 90* and glued in alternating layers - 3 to 7 layers - good dimensional stability - Use for more strictly controlled fire rated occupancies |
|
|
What is CLT used for? |
- Create large panels for walls, floors, and composite systems - rapid construction erection |
|
|
What is NLT and its pros and cons? |
Nail Laminated Timber - Similar to CLT, but boards only run in one direction (nailed together) - good for one way spans; very weak in second axis - less cost than CLT |
|
|
What is Glulam and its pros and cons? |
Wood pieces glued together with adhesive to make long curved structures - Strong for long spans |
|
|
What is LVL and its pros and cons? |
Laminated Veneer Lumber - Layered composite of wood veneers and adhesive - Good for headers, beams, and flanges - Each grain runes in the longitudinal direction, making it strong as a beam or plank |
|
|
What is LSL? |
Oriented strands (like OSB) made from aspen or poplar - For headers, beams, and columns |
|
|
What is PSL? |
Structural composite lumber product made by gluing long strands of wood together under pressure (high strength, less prone to shrinkage, not for exterior exposure) |
|
|
What are the advantages and disadvantages of Wood I-Joists? |
- Dimensionally stable - Known engineering properties - well suited for long span - Has serviceability issues |
|
|
What is sheathing? |
Flat plates of sheets for roofs, walls, and floors - Plywood - OSB - Wafer Board - Sawn Lumber |
|
|
What is Plywood? |
Built up sheets of softwood veneer glued together with adhesive - Direction of each grain is rotated by 90* |
|
|
What is the difference between OSB and waferboard? |
Oriented Strand Board - laminated thin chips of Poplar Waferboard is randomly oriented OSB is oriented in long direction |
|
|
What are the major factors that decrease the durability of wood? |
Decay, fire, termites, and marine borers |
|
|
What are the three zones of burnt wood? |
Char layer, Pyrolysis zone, Normal wood |
|
|
What do termites do to wood? How do we mitigate? |
Create shelters in soil and travel in tubes into the wood structure. Best protection is to block access for the soil to wood through appropriate construct ion techniques. |
|
|
To thrive fungi need: |
Source of food, warmth, moisture, and oxygen |
