lecture 3 timber construction

8/18/2019 Lecture 3 Timber Construction

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Lecture 3:Timber Construction

Building Construction and Technology 2


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Learning Outcomes

• Types of timber for construction

• Timber construction techniques

•Jointing systems

• Wood protection & treatment


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Types of timber for

Construction


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Timber for Marine & Heavy

structures

• close grained, dense wood,

• natural resistance to:

– impact (deliberate and accidental), close grained,high density

– heavy loading

– Chemical resistance – infestation (insects, sea creatures, borers)

– fungal attack (wet rot, other fungi)

– salt or wave erosion and temperature variations.

• Oak, Larch, and Teak

– natural oils for preservatives

Medium/Light Construction

•Roof trusses, partition, floors, walls

– resistance to insects and fungal attack,

– minimum dimension changes

– flame resistance

– market availability

• Douglas Fir, Southern Pine, Hemlock, Redwood, (other

softwoods)

Falsework

• Plywood, board, formwork, glue lam supports

– inexpensive

– available

– weight

– ease of handling and storage

– resistance to impact and abrasion (reuse and stripping

ease)

– any graded wood, hemlock, pine, softwood


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Types of Malaysian Timber for

Construction


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TimberConstruction

Techniques


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Timber Construction Techniques

• Depends on the building design

• Construction sequence:

- Sub-structure

- Superstructure/Frame

- Floors

- Roof

- Suspended Floors

- Staircase

- Walls


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Construction & Jointing System:

Sub-Structure Construction

Moisture- Moisture Barrier- Raised from ground

Foundation

- Masonry materials

(brick and/or concrete)- Stability: Rest on pad,

strip or raft foundation

Ground Floor- Raised Floor

- Sleeper wall

- Under floor ventilation

Elements:- Fad/strip or raft

foundation

- Column stump/load-

bearing wall

- Bean/bearer


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Sub-Structure Construction


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Ground Floor Construction


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Moisture- Moisture Barrier- Raised from ground

- Under floor insulation


Ground Floor Construction


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Frame/Primary

Structural Element- Post/column

- Beams/bearer

- Bracing

Frame/Secondary

Structural Element- Framed walls (see wall)

Jointing/connections


Superstructure Frame Construction


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Superstructure Frame Construction


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Roof Truss- Rafters- Tie

- Struts

- Bracing

- Hip

- Ridge

- Eaves

- Battens

- Purlins- Hip

- Gable

- Fascia

- Dormers

Roof Coverings- Insulation- Battens

- Tiles

- Sheet roofing

Rainwater Components- Gutters- Downpipes

- Flashings


Roof Construction


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Roof Construction - Connectors


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Roof Construction - Connectors


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Roof Truss


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Suspended Floors

Down

Joist

Bearer

Beam

Column

Column

Open

web joist


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Suspended Floors


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Staircase


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Staircase


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Walls


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Walls


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Walls


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Timber JointingSystems


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Joints


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Dowel Joint

• Dowels come in different sizes ¼”, 3/8”, ½” , 5/8” etc.

• These joints are hidden and add strength to the joint.

• Dowel holes are drill and glued and clamped

together.


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Dado

• Dado's are typically used in making book shelves,they support the shelf without the benefit of any

additional fasteners, any glue or hardware simply

holds the shelf in place.

• Dados may be made with a dado blade on a tablesaw


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Rebate/Rabbet Joint•

This type of joint is made by using thedado blade.

• Each side of wood is cut to a specific

length, then glued or brad nailed to

make a stronger joint.


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Lap Joint

• A lap joint is when two piece are cut ona dado and glued or nail together to

create a stronger joint.


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Dovetail Joints

Most commonly used toattach drawer sides to

drawer fronts, dovetails

joints almost always

indicate quality furniture.Typically cut using a

manufactured jig to cut

these. Can be done by

hand.


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Mortise and Tenon

• The mortise and tenon joint is one of thestrongest wood joints.

• Mortise and tenon joint is normally formed by cutting a

square tongue (the tenon) on the end of one piece of

wood and an equal size square hole or slot (the

mortise) in another.


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Mitre with Wood Spline


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Tongue and Groove• Tongue and groove or T&G is a method of fitting

similar objects together, edge to edge, used mainlywith wood: flooring, parquetry, panelling, and similar

constructions. Tongue and groove joints allow two flat

pieces to be joined strongly together to make a single

flat surface.

• The effect of wood shrinkage is concealed when the

joint is beaded or otherwise moulded


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Metal Jointing


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Timber

Defects


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Diseases and Defects• Trees are prone to diseases

and defects.

• Diseases and defects affectthe strength and quality of thewood. However, it can make

the timber more valuablebecause it looks well.

• Timber we get from trees canbe subject to any of thefollowing:

a. Natural Defects

b. Artificial Defects

c. Fungal Attack

d. Insect Attack


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Natural defects

• Natural defects are oftenpleasing to the eye while

others are unwanted

defects.

• Heart Rot- caused by a

fungus that rots the pith or

the heartwood

•

Spike Rot- reduces thestrength of the wood

• Spiral Grain- tree may

twist when growing


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Knots •

Knots form where branches of a tree are cut off or stopgrowing

• Loose knots are called Dead Knots – branch has

stopped growing before tree is felled

•Other knots are called Live Knots

• All knots reduce the strength of the timber.


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Resin Pockets•

Most common in conifers• Forms in internal cracks/cavities in the wood

• Cracks caused by high wind or extremes in

temperature

• Reduce the strength of the timber


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Natural defects•

Shakes- these are splits in the endgrain. They are either Radial,

Tangential or Cross(thunder) shakes

• Radial- along ray lines include heart

or star shake

• Tangential- in annual rings at old

age, seasoning or strong winds

• Cross- split across the grain when

tree is subject to shock in felling


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Artificial Defects

• Artificial defects occur asa result of stresses

caused by poor

stacking or season ing .

• They include:

Cupping

Bowing

Twisting/Warping End splitting

Case hardening

Honeycomb checks


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Artificial Defects

• Cupping-this form of shrinkageforms a curve if you look at theplank from either end. Caused byincorrect stacking duringseasoning.

• Bowing-where the timber istaking the form of a bend alongthe length of the piece. Likecupping. It is caused by incorrect

stacking during seasoning, oftenthe incorrect positioning of thestickers.


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Artificial Defects

• Twisting or warping-the two endsof the piece have twisted inopposite directions while the sidesof the piece have remained straight.

• End splitting-this is caused by theend of the piece drying out tooquickly. Most common in naturalseasoning where there has been

excessive exposure to the heat orsun. Can be prevented by coveringor treating the ends duringseasoning.


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Artificial Defects

•Case Hardening-this occurs whenthe piece is seasoned too quickly andmoisture is still trapped within thewood. The surface and the centrehave different drying rates creatingcompression at the centre andtension at the surface.

• Honeycomb checks-the inside ofthe wood splits and like casehardening the timber is seasoned tooquickly for the centre to be dried out.When it does there is shrinkage

causing splits within the piece.


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Fungal Attack• Fungi attack causes damage by feeding

on the cells of the wood . The woodbecomes soft and eventually decays.

• A fungus is made up of cells calledhyphae. A large number of hyphaetogether will form a mycelium.

• These hyphae penetrate the wood tofeed and get moisture. They will then

produce food bodies called sporophores.

• The cycle will now repeat itself when thespores produced are blown on to newwood.

• The following conditions are required:

Moisture

Food supply

Oxygen

Warmth


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Fungal Attack• Wet Rot (white rot)-Usually occursoutdoors where it will rot fence posts,window frames, logs, doors, etc.

• The affected wood will become very moistand slimy and will produce a whiteresidue.

• Dry rot (brown rot) -This type is usuallyfound indoors, eg under wooden floors.

• This fungi attacks wood and eats thecellulose found in the wood. Wood will

appear dry and contain rectangular cracksin it.

• Treatment of this type of rot must beimmediate and severe as the rot cancause extensive damage.


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Fungal Attack

Treatment process

1. All infected wood needs to be removed as well as

uninfected wood 500mm around the area.

2. All surrounding brickwork and materials must be

scorched or treated to kill remaining fungicide

3. The cause of the dry rot must be remedied before newpreserved timber is put in place.


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Insect AttackSome insects will attack and feed ontimber much like fungal attack.

1.The adult lays the egg in an oldinsect hole

2. The egg hatches and thelarva/grub will feed on the starch inthe wood – this creates the tunnels

3.The larva makes a cocoon-like shellaround itself before it settles for aperiod of time to change into an adultbettle.

4. The adult then emerges andbreaks out to the surface to begin thecycle again.


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Insect Attack

Furniture Beetle (woodworm)

• This common beetle, more

often known as ‘woodworm’ is

probably the most common in

Ireland.

• Woodworm will attack both

softwoods and hardwoods,

mostly just the sapwood ofthese timbers.


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Insect Attack

Treat ing t imber w i th woodworm

1. Apply coats of insecticide following the instructions on

the container.

2. Ensure that all surfaces and cracks are coated with

the chemical

3. Use an applicator to insert insecticide to the exit holes

of the woodworm.

C t ti D t ili t O


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Construction Detailing to Overcome

Decay and Insect Attack

lecture 3 timber construction

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