wood B

Question 1

why cant we rely on counting growth rings to determing age of tree

Answer 1

• abnormal environmental conditions can cause tree to produce multiple-growth increments or prevent growth entirely for a period

Question 2

why do we use soft wood more than hard wood

Answer 2

• soft wood is long and straight
• allows you to make a big member out of it
• cheaper

Question 3

why do we use wood specimens to study relationships between microstructure and properties

Answer 3

• small
• clear specimens
• clear of macroscopic defects

Question 4

why are knots at the top better than knots at the bottom

Answer 4

• knot is defect so its weak zone
• the top experiences compression: stronger bc whole solid material is being tested
• bottom experiences tension: pulling apart the knot so its weaker

Question 5

the moisture content changes in wood, can they be protected or prevetented

Answer 5

• retarded, not prevented
• with protective coatings like varnish, lacquer or pant

Question 6

why is curing important

Answer 6

• improves properties
• protect from interaction with external elements
• prevents shrinkinge bc part of moisture is removed

Question 7

why is it important to dry the wood

Answer 7

• avoid shrinkage after construction

Question 8

what is the most common way of failure with wood

Answer 8

• shear is most common
• then compression
• then tension bc wood is strongest in tension

Question 9

what is the 5% exclusion limit

Answer 9

• 5%: only 5% of specimens tested would be expected to fall below the specific value

Question 10

why not average, why 5%

Answer 10

• if we take the average, half above; half below: half of structural members would fail
• 5% is a lower probability of member at critical location causing failure

Question 11

what chemicals are the biggest threat to wood

Answer 11

• strong acids: attack cellulose or hemicellulose and weaken wood
• alkalis react with cellulose and lignin to weaken wood
• iron salts (corrosion): stain or soften wood

Question 12

how does nuclear radiation affect wood

Answer 12

degrade the strength of wood by decreasing degree of polymerization of the cellulose molecules

Question 13

how does pressure impregnateion work and what is it for

Answer 13

• protection against fires
• 1. put wood in vacuum chamber: pores are vacuums
     
     2. immerse in fire retardant
     3. release vacuum

Question 14

what are the natural limitations of structural timber

Answer 14

• size of timber is limited by tree size
• imperfections limit allowable stress levels