Week 5: Timber + section properties

3 Primary directions in wood

Longitudinal (grain): along the trunk and grain

Radial: from center of the tree outward

Tangential: around the grown rings

What’s anisotropic and application to timber

Material has different properties depending on the direction in which it is tested or loaded. When load is applied vertically, timber is strong and stiffer parallel to the grain (longitudinal) and weaker perpendicular (tangential) to the grain. This is because the loading across the timber’s width makes cellular structure easier to crush or deform

Hardwood

Hardwood comes from flowering trees such as oak, maple, or teak

• Strength, hardness, and density is generally higher than softwood (NOT ALWAYS, depends on classification)

• Trees with broad leaves and in darker colors

• Tends to be used in high-quality furniture, flooring, walls

• Slower growing

• More expensive

Softwood: Comes from non-flowering trees such as pine, spruce, cedar

• Lighter in weight

• Lower strength, density, and hardness that hardwood but not always

• Fast growing

• Easy to work with, commonly used for structural framing

What does it mean by hygroscopic material and link its properties to timber

Ability of a material to absorb or release moisture from the surrounding atmosphere in response to environmental humidity.

Timber is a hygroscopic material because it exchanges moisture with its environment

High humidity: Timber absorb moisture from air → may cause swelling because water is absorbed into cell wall

Low humidity: Timber release moisture to air → may cause shrinking because water leave cell wall

The moisture exchange continues until timber approaches equilibrium moisture content (EMC) with the atmosphere

Environment change → moisture changes → timber changes

Dimensional movement of timber, and which timber direction has the greatest movement

Since moisture movement cause timber to shrink and swell the dimensional movement is not equal in every direction

Tangential: greatest swelling and shrinkage along the growth ring

Radial: moderate movement across growth ring towards the center

Longitudinal (grain): Least movement along the tree trunk

→ These unequal movements lead to shrinkage, checking, warping, and cupping

What does unequal movement in timber lead to

Shrinkage: Timber becomes smaller as it loses moisture

Checking: Cracks develop due to uneven shrinkage and internal stresses

Warping: Timber bend, twist, or change shape

Cupping (type of warp): Timber bend across its width, creating a U shape

What is seasoning for timber and its properties

Seasoning is the controlled drying of timber to reduce its moisture content

• better for structural strength

• better nail holding capacity than wet timber → wood fiber lock together better when there’s not a lot of fluid between them

• better protection against decay

• lighter in weight

If wet timber is installed in building and later dries, it might shrink and cause gap or distortion because loss in moisture content

What is equilibrium moisture content (EMC)

The moisture content at which timber is in balance with the surrounding air, meaning that it no longer gaining or losing moisture overall

Sawn timber and its methods

Sawn timber is produced by cutting logs into structural members such as beams, joists, and rectangular sections

Plain sawn: board cut approximately tangential to growth ring (parallel through log)

• efficient cutting

• more likely to cup because timber shrinks more in tangential (along the growth ring) direction than radial (across growth ring) direction

• less timber waste → simple parallel cut

• cheapest

Quater sawn: logs are divided into quatres before boards cut approximately perpendicular to growth ring

• more dimensionally stable

• less cupping → growth ring in perpendicular to the board’s face

• generally, more expensive

Rift sawn: Boards are cut to produce consistent grain angle of around 30-60 degree to growth ring

• straight grain appearance → every grain orientates to center of log

• maximum strength

• most stable

• more waste

• higher cost

Cutting method matters because orientation of the growth rings influences dimensional stability, appearance, structural behaviors, and nail holding capacity

Engineered timber products

It’s a manufactured wood product made by bonding veneers, strips, and fiber of wood particle together using adhesive to improve structural performance.

LVL: Manufactured from multiple thin veneers bonded together

• grains are aligned in the same direction

• aligning grains help create a strong, consistent structural members → efficient for beams, lintels, and other similar structural elements

Plywood: Manufactured in thin veneers bonded in layers, unlike LVL each adjacent layer perpendicular to each other

• reduces directional weakness and improve stability

Glulam: Manufactured by bonding layers (individual timber board) of timber together, where the grains are aligned along the length of the members

• commonly used in long span beams, columns, ad curved structural elements

MDF: Make by breaking down hardwood or softwood residual into wood fibers and combining it with wax and resin binder. The panels are formed by applying high temperature and pressure

• commonly used for cabinetry and interior applications rather than primary load bearing structures

• unlike LVL, plywood, GLT, it is not made from intact veneers or boards

Timber structural framing: light timber framing, post-and-beam construction, mass timber construction

Light timber framing: loads are distributed through many smaller members → commonly used in residential buildings

Post-and-beam construction: All structural members are responsible for their tributary area → large beams transfer loads into widely spaced columns

Mass timber construction: Uses large, engineered timber elements, including structural panels → all is made from engineered timber

Fire behaviors in timber

When large timber member burns, a char layer forms at the exposed surface

1) Outer layer becomes charred and loses most of its structural capacity

2) The char layer acts as an insulation, slowing heat transfer into the remaining timber

3) Inner timber can continue carrying loads while enough effective cross-section remains

Structural grading of timber

Process of sorting timber into stress grades so design capacity can be assigned

Visual stress grading: person examines the timber for visible characteristic that may affect strength such as knots, cracks, grain slope, and distortion → then these identifications are classified according to grading rules

Machine stress grading: A machine applies relatively small load to the timber and measures how much it bends. Its purpose is to assess the stiffness without significantly damaging the timber → fast process around 1 sec per piece

Machine proof grading: A machine applies much higher load, closer to the timber intended design strength. The aim is to identify weak pieces through failure under proof load → slower process, around 15-20 sec per piece

Visual characteristics that affect strength of timber

Knots: They’re location of branches that grew from the trunk

• It interrupts the grain pattern, making timber weak

Slope of grain: When grain is inclined, the member cannot use its longitudinal fiber strength as effectively for load along its length

Gum pockets: Cavities containing gum or resin like material, interrupting wood structure and aesthetic

Wane and want: Wane is the missing wood along the edge of sawn timber (the defect itself). Want measures the amount/dimension of the missing wood caused by the wane

Others:

• check

• twist

• cup

Centroid and centroid axis

The geometric center of an area, the “average” position of all the points in the shape

→ basically, the point on the shape in which the object would balance when placed on a needle

Centroid axis: Line that passes through the centroid → cross section usually has two centroidal axes (y-y) (x-x)

Centroid of rectangle

The centroid lies at the intersection of (x-x) and (y-y) axis, halfway across its width and halfway up its height

→ So, coordinate of the centroid axis would be x=b/2, y=h/2

Centroid of rectangle

• The centroid axis lies at the intersection of 3 medians → median is a line connecting a vertex to the midpoint of the opposite side

• The centroid is located 1/3 of the distance from each side to the opposite vertex so y=h/3

• For right angle triangle, the height and base from right angle corner reference point is also (x=b/3, y=h/3)

Centroid for complex shapes

Break complicated shape into simpler shapes, calculate each one’s centroid and area, then combine the result

• use method of composite parts

  • Need to find area of each shape

  • Identify the X and Y coordinate based on the shape (e.g. y=h/s or h/3)

Area moment of inertia or second moment of area

Describe how the area of a cross-section is distributed relative to a particular axis. It reflects the efficiency of the shape in resisting rotation

• Represented by I

Large I = greater resistance to bending → less deflection under the same load

Smaller I = lower resistance to bending → more deflection under the same load

So, more distance away from axis of rotation will have higher I