ARCH 542 Week 11 Lecture 1 An introduction to wood as a construction material.

1. WOOD Andrea Wheeler
ARCH 542
Sci-Tech 2
Week 11

2. Week 11 WOOD
Week 12 MASONARY
Week 13 STEEL
Week 14 CONCRETE

3. THIS WEEK’S CONTENT (Lectures Tues and Thurs)
1. Physical properties of wood – introduction to
understanding wood as a structural and constructional
material.
2. Lumber (timber) and the production and supply of
timber for the construction industry
3. Wood composites, laminated timber and new
materials (e.g. OSB).
4. Wood and moisture: movement, rot and insects
5. Construction uses for wood and timber framing (US).
6. Fastenings, and connections
7. Sustainable use
8. Case studies of timber in structure and construction

4. Labs: 4 groups
GROUP RESEARCH THEMES
1. Is wood a sustainable construction
material? The case for new innovative
wood based construction materials.
2. Why have we seen a development in
structural connections and fastenings in
timber: From the Old English Barn, log
structures to Shiguru Ban?
3. The structural limits of timber
structures: How high can we go?
4. Designing with wood – new materials
and new methods of engineering,
design and construction.

5. Attendance question ?
Name four
advantages and
four disadvantages
of wood in
construction … ARCH 542

6. Advantages Disadvantages
Strong Natural material, not precise, not perfectly straight
and subject to flaws (growth defects)
Light Affected by moisture – will size, and shape (will
change dimensionally through seasons).
Easily worked and shaped Can split and warp. Grading prevents this.
Fastened quickly and economically Burns easily (Oak is more resistant in a fire than
exposed steel as it produces a charred layer and
burns slowly)
Recyclable Decays (hardwoods are more naturally durable than
softwoods). Some species very durable –
Greenheart.
Biodegradable and nontoxic
(preservatives added to timber are
toxic)
Subject to insect damage (keeping timber dry and
detailing well will prevent this)
Renewable (it can regenerate and it
can be properly managed and
harvested)
Some rare timbers should not be used.

7. Approximately 90% of buildings constructed in the
US each year are framed with wood, whilst that
percentage is only 15 – 20 percent in the UK.
The use of green oak is becoming more common in the
UK as a structural material than any other country. Green
oak means freshly cut oak, drying (which will normally
take place before construction) takes place in service
(over 20 – 30 years). Green oak frames are subject to
movement.

8. Freshly cut timber has a high moisture content, medieval
carpenters used “green oak” but also developed a vocabulary of
structural connections that allowed movement to happen.

9. Strategies to allow for movement in green oak structures

10. Six Story Office Building
Design by Shigeru Ban,
Switzerland
http://www.designboom.com/architecture/shigeru-ban-tamedia-office-
building-in-zurich-completed/

14. Physical properties of wood
• Strong in tension, weak in compression
and shear
• Steel and concrete are isomorphic
materials, wood is not – orthotropic
/directional (evident where parallel to the
grain 100% stronger than perpendicular to
the grain).
• Movement – sensitive to humidity,
absorbs water from the air.
• Durable. Above 13% moisture content,
subject to rot, insect infestation.

15. Because wood is a natural
material and the tree is
subject to many constantly
changing influences (such
as moisture, soil
conditions, and growing
space), wood properties
vary considerably, even in
clear material.
The variation of strength between different
directions can be attributed to the fine structure
of the wood cells.

17. Directions of Wood
• Longitudinal direction: parallel to the
long axis of the stem.
• Radial direction: perpendicular to both
the growth rings and the long axis of the
stem.
• Tangential direction: tangent to the
growth rings.

18. Wood composition
The anatomy of wood

19. Electron microscope images of porous wood cell
structure.
It is the long cells (20m in pine) with tough cellulose bound by lignin walls that provide the
strength of the woods.
Spring wood and summer wood – spring wood cells and denser/ summer wood cells larger and less
dense.
WOOD – material structure = “A bundle of straws”

21. SOFTWOODS – Evergreen trees. Douglas Fir, Southern
Pine, fir. Typically less expensive than hardwoods.
Structural and constructional use. Limited uses in
finishes or as veneers.

22. Softwoods and
hardwoods:
Differences in the
anatomy

23. Softwood (e.g. Pine) Hardwood (e.g. Oak)

24. Medullary Rays in
Oak giving figure /
interest to wood

25. Strong in tension, weak in compression and shear (but
isomorphic) Strength is also affected by moisture content.
Shear (splits)
Compression
(weak)
Tension

26. Mechanical behavior
Nails close to the end splits lumber easily
because of the weak bonds between the
fibers.

27. Water in wood
• Wood is a hygroscopic material, i.
e., it will absorb water vapor from
the atmosphere
• Moisture content in wood is
expressed as a percentage of
ovendry weight.

28. Water in wood - moisture and movement
Water may be held in wood in two ways:
• bound water
• free water
Bound water is held in the cell walls by adsoption
force.
• It is generally believed that bound water is not in the
crystalline regions of the cell wall, but is absorbed in
the amorphous regions
• This has important implications for the volume
changes associated with moisture changes.
Free water is not held by any forces and is situated in
the cell cavities known as lumin.

29. Moisture Content - Fiber Saturation Point (FSP)
• The moisture content at which the cell wall is saturated with
bound water and at which no free water is present is called
the fiber saturation point (FSP)
• The FSP varies from species to species, but it averages
about 28% moisture content
• Addition of removal of water below the FSP has a
pronounced effect on practically all wood properties.
• Addition or removal of water above the FSP has a almost no
effect on any wood properties.

30. Shrinkage and swelling of wood
• The variation of shrinkage between different directions can
be attributed to the microstructure of wood.
• The latewood cells dominate shrinkage since they absorb
much more water and in the tangential direction there is an
unbroken alignment of latewood.
• The greater shrinkage in the tangential direction causes
distortion in lumber with different orientations.

32. Class question
A piece of wood containing moisture weighs 205g,
and after oven drying to a constant weight, it weighs
110g. What is the percent moisture content? [slide 28]
(205 – 110)/110 = 86.4%

33. Class question
A piece of wood contains 18% moisture. What must
its weight have been before oven drying if it has a
constant weight of 140g after drying? [slide 28]
(x-140)/ 140 = 0.18 x = 165.2g

34. Lumber (timber) and the production and supply of
timber for the construction industry – from harvest to
commercial product
• Hardwoods and softwoods differ in the production
for construction.
• The production of wood for construction – from
log to board: Sourcing, sawing/cutting, seasoning,
planeing/ surfacing, preservative treatment,
grading, and distribution.

36. Lumber production - SAWING
• Maximum yield – cost efficiency
• Varying grain pattern (especially in
hardwoods) to maximize the figure of the
wood (pattern of the grain)

38. Through cut

39. Lumber seasoning/ drying
• Air seasoning: lumber dries naturally. Piles of lumber are
stacked in a way that air can circulate freely. The process
can take months.
• Kiln seasoning: Warm air circulates through the pieces of
lumber, controlled – defects can happen if too quick or too
hot.

42. Why season or dry wood?
• Equilibrium: when wood is used as a construction
material, whether as a structural support in a
building or in woodworking objects, it will absorb or
desorb moisture until it is in equilibrium with its
surroundings.
• Drying causes unequal shrinkage in the wood, and
can cause damage to the wood if equilibration occurs
too rapidly. The equilibration must be controlled to
prevent damage to the wood (i.e. seasoned)
• Wet wood is subject to rot and beetle infestation.
• Increases strength and stiffness
• Reduces weight

45. Seasoning flaws
• Warp – bow, crook, cup
• Surface checks and splits
• Stains (resins)

49. ESSENTIAL INFORMATION FOR ARCHITECT/ DESIGNERS

55. Labs: 4 groups
GROUP RESEARCH THEMES
1. Is wood a sustainable construction
material? The case for new innovative
wood based construction materials.
2. Why have we seen a development in
structural connections and fastenings in
timber: From the Old English Barn, log
structures to Shiguru Ban?
3. The structural limits of timber
structures: How high can we go?
4. Designing with wood – new materials
and new methods of engineering,
design and construction.