WOOD

Andrea Wheeler

ARCH 542

Sci-Tech 2

Week 11

Week 11 WOOD

Week 12 MASONARY

Week 13 STEEL

Week 14 CONCRETE

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

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.

Attendance question ?

Name four

advantages and

four disadvantages

of wood in

construction … ARCH 542

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.

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.

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.

Strategies to allow for movement in green oak structures

Six Story Office Building

Design by Shigeru Ban,

Switzerland

http://www.designboom.com/architecture/shigeru-ban-tamedia-office-

building-in-zurich-completed/

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.

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.

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.

Wood composition

The anatomy of wood

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”

SOFTWOODS – Evergreen trees. Douglas Fir, Southern

Pine, fir. Typically less expensive than hardwoods.

Structural and constructional use. Limited uses in

finishes or as veneers.

Softwoods and

hardwoods:

Differences in the

anatomy

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

Medullary Rays in

Oak giving figure /

interest to wood

Strong in tension, weak in compression and shear (but

isomorphic) Strength is also affected by moisture content.

Shear (splits)

Compression

(weak)

Tension

Mechanical behavior

Nails close to the end splits lumber easily

because of the weak bonds between the

fibers.

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.

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.

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.

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.

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%

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

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.

Lumber production - SAWING

• Maximum yield – cost efficiency

• Varying grain pattern (especially in

hardwoods) to maximize the figure of the

wood (pattern of the grain)

Through cut

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.

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

Seasoning flaws

• Warp – bow, crook, cup

• Surface checks and splits

• Stains (resins)

ESSENTIAL INFORMATION FOR ARCHITECT/ DESIGNERS

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.