Timber: Advantages, Disadvantages, and Processing, Lecture notes of Architecture

Download Timber: Advantages, Disadvantages, and Processing and more Lecture notes Architecture in PDF only on Docsity!

Lecture 12

Timber

Advantages

^

Natural products and renewable resources.
^

Less energy required than alternative materials.
^

No disposal problem after use.
^

High strength weight ratio.
^

Warm

‘ feel

^

Easily cut with hand toolsand joined with nails or screws.
^

Strong joints formedwith adhesives.
^

Good fire resistance forstructural timber.

Wood Structure

^

Softwood and Hardwood
^

Not correspond to thehardness of the wood. ^

Softwood ^

Any species such as pines that have needlelikeleaves and that are generally evergreen. ^

Grow relatively quickly, the resultantwood generally being soft, of lowdensity and easily worked. ^

More economical thanhardwoods but less durable.

Wood Structure

^

Hardwood ^

Trees such as oak

橡樹

,^ 橡

木^

, maple

楓木

& teak

柚木

with broad leaves that areshed in winter. ^

Usually

grow more slowly

than softwood but

harder,

denser, stronger and moredurable. ^

Used in

window, door,

high quality joinery

and

veneers

薄板

,^ 膠合板

,^ 單板

for

doors and furniture

.

Wood Structure

^

Sapwood & Heartwood ^

Sapwood (

The newly

formed

outer wood located

just inside the vascularcambium of a tree trunk andactive in the conduction ofwater. Sapwood is usually lighter in color thanheartwood.

Wood Structure

^

Heartwood ^

The

inner layer of the tree

^

No longer storesfood

but

supporting

the tree. ^

Cells is

acidic

for

preservation of the tree

^

Darker in color

than

sapwood. ^

More heartwood in oldtrees

and

better durability

Effects of Moisture

Moisture content

is the

mass of water

contained in the wood

, expressed as

percentage of the mass of oven-drywood

Moisture content

=^

mass of water present

in as sample x 100%

mass of that sample when oven dry

Seasoning

乾燥

(Wood drying)

^

Seasoning is the

controlled reduction of

the moisture content of timber to a levelappropriate to its end use

^

2 methods of seasoning

^

Air seasoning

^

Kiln seasoning

,^ 爐

Kiln Seasoning

^

Involves

heating the timber

in

sealed

chamber

, initially

using steam

to

maintain

saturation

, the

humidity then being

progressively reduced to produce drying

^

Takes about

one week per 25mm

thickness for softwoods

and about

two

weeks per 25mm thickness forhardwoods.

^

Has

advantage

that it

sterilizes the wood

and

can reduce moisture contents to

lower levels than air seasoning (about12%).

Air Seasoning

^

The timber stacked in

open-sided sheds

to

promote

drying

without artificial assistance.

^

Stacked about

450 mm clear of the ground

^

This process

minimizes damage

but this

process very slow often taking several yearsto complete

^

This process

cheap

compare to drying kiln

but will

not reduce the moisture content

below about 16%.

Air Seasoning

Fungal Attack

Growth of all fungi in timber requires amoisture content of at least 20%.

Fungi attack

can be

classified

as

dry

rot and wet rot.

Dry rots

occurs when there is

no

satisfactory ventilation.

Wet rots

occurs when there is

higher

moisture content usually 50%

in the

timber.

Insect Attack

Insects have a characteristic

life cycle

egg, larva, pupa, and finally adultinsect.

Insect attack

does not require wet

timber,

though

higher moisture

contents are

preferred

Sapwood is much more susceptible toinsect attack than heartwood.

Attach is less likely in wood productssuch as chipboard, containingsynthetic adhesives.