Structural Defects and Moisture Content in Wood

Structural Defects

• A defect is any feature that alters the natural grain of the wood.

• Examples of defects include:

  • Knot

  • Slope of grain

  • Resin pocket

  • Check

  • Split

  • Compression wood

  • Wane

  • Pith

  • Bow

  • Cup

  • Crook

Knots

• Knots are remnants of branch connections on the trunk.

• They can be "tight" or "loose."

• Knots are very dense.

• Knots are undesirable because they change the grain direction.

• To increase wood quality and value, branches are cut regularly to minimize knots.

Impact of Knots on Strength

• Knots in the tension zone and splits/checks in the connection area can be critical defects.

• A higher knot area ratio leads to lower compressive and tensile strength.

• The relationship between knot area ratio and compressive strength is represented by the equation: $y = -21.173x + 27.185$ with $R^2 = 0.4151$

• The relationship between density and compressive strength is represented by the equation: $y = 0.0412x + 5.6865$ with $R^2 = 0.1614$

• The relationship between knot area ratio and tensile strength is represented by the equation: $y = -54.799x + 37.429$ with $R^2 = 0.3867$

• The relationship between density and tensile strength is represented by the equation: $y = 0.06x - 1.4625$ with $R^2 = 0.0928$

Slope of Grain

• A slope of grain of 1 in 12 (angle of 4.75°) results in a 16% strength reduction.

• A slope of grain of 1 in 10 results in a 22% strength reduction.

• Grading standards typically limit the slope of grain to 1:10 or 1:12.

• At knots, the angle can be 45°, reducing strength by 93%.

• If $f<em>{t,0} = 25$ MPa and $f</em>{t,90} = 0.85$ MPa.

Splits and Checks

• Splits and checks are examples of timber defects that can occur in structural members, especially at connections.

Wane

• The cross-sectional area is calculated as $A = b \times h$.

• $A_w$ represents the area of one wane.

Water and Wood

• 90% of all problems with wood involve moisture.

• Wood in trees is very wet and contains excessive water (sap).

• Under normal conditions, much of this water dries out, causing the wood to shrink.

• A fluctuating moisture balance is eventually reached between the wood and its environment.

• Atmospheric humidity determines the moisture content of the wood, and the moisture content determines the dimension of the wood.

Moisture Sensitivity of Wood

• Exposure to water and changes in moisture content affect the properties and performance of wood, including:

  • Strength

  • Creep

  • Thermal conductivity

  • Decay

  • Burning point

Moisture Content

• The water-free weight of wood is referred to as oven-dry weight.

• The moisture content (MC) is calculated as: $MC(\%) = \frac{W<em>w}{W</em>{odw}} \times 100$, where $W<em>w$ is the weight of water and $W</em>{odw}$ is the weight of oven-dry wood.

• Example: If a piece of wood has a mass of 15 kg and after drying, its mass is 12.5 kg, then the moisture content is 20%.

• Note that it is possible to measure MC in excess of 100%.

Relative Humidity

• Humidity refers to water or moisture in vapor form in the atmosphere.

• Relative humidity (RH) is the ratio of the amount of moisture in the air at a certain temperature to the amount it would be able to hold at that temperature.

Equilibrium Moisture Content

• Wood remains hygroscopic, responding to changes in atmospheric humidity.

• It loses bound water as RH drops and regains bound water as RH increases.

• Equilibrium Moisture Content (EMC) is the balance point at which the wood is no longer gaining or losing moisture.

Moisture Content Variations

• Moisture content of a tree can exceed 30%.

• Dry timber has an MC below 16%.

• Wet timber has an MC above 16%.

• Green wood refers to freshly-cut wood.

• Example: For freshly cut Douglas fir, the MC of the heartwood is approximately 37%, and the MC of the sapwood is about 115%.

Fiber Saturation Point

• The Fiber Saturation Point (FSP) is the point at which water is only within the cell walls, not in the cavities.

Wood Treatment

• Seasoning

• Kiln drying

• Thermal modification

• Air-dry wood is wood whose moisture content is in equilibrium with the ambient air relative humidity.

• Kiln-dried wood is wood whose moisture content has been reduced to about 15-19%.

Moisture Content and Shrinkage

• Bound Water: Water absorbed into cell walls.

• Free Water: Water contained in the voids of the cells.

• Fiber Saturation Point (approx.): 28%.

• Weight of water affects shipping costs.

• Change in width and thickness, little change in length.

• No change in dimensions below a certain moisture content level.

Strength Variation Relative to Moisture Content

• Impact strength

• Modulus of elasticity

• Modulus of rupture

• Crushing strength

Shrinkage Effects

• Bow

• Spring

• Twist

• Cup

Moisture Measurement

• Moisture content can be measured using resistive type instruments with pins inserted or contact-only instruments.

Moisture Content in Protected Areas

• In protected areas with no rain, the MC varies from 14-18% in unheated areas and from 8-12% in heated areas.

• If wood is to be exposed to water, it should be protected by stain, paint, or treatment and appropriate design.

Wood Movement

• Due to changes in surrounding relative humidity, the accompanying shrinkage and swelling of wood need to be determined and accounted for.

Shrinkage of Wood

• Tangential shrinkage

• Radial shrinkage

• Lengthwise shrinkage

Shrinkage Properties for NZ Species

• Species-specific shrinkage percentages when drying from green to 12% moisture content are:

  • Radiata: tangential 3.9%, radial 2.1%

• Douglas fir: tangential 4.9%, radial 2.8%

Calculating Dimensional Change Due to Moisture Content

• To estimate the change in dimension due to a change in moisture content, use the formula: $\Delta dim = \frac{\%SH}{100} \times \frac{\Delta MC}{FSP-12} \times ID$

  • Where: $\%SH$ is Shrinkage, $\Delta MC$ is Moisture Content change, $ID$ is Initial Dimension, and $FSP$ is Fibre Saturation Point.

Example: Bolted Connection

• A bolted connection has 2 rows spaced at 125 mm. Steel plates are used on each side of a Radiata pine member.

• Estimate the reduction in row spacing if the holes were drilled when the member was wet (MC = 29%) and the EMC will be 12%.

• Using the formula: $\Delta dim = \frac{3}{100} \times \frac{29-12}{29-12} \times 125mm = 3.8mm$

• Given that the hole tolerance is about 1.6 mm, this will result in splits in line with the bolt rows, leading to a reduction in connection strength.

Example: Two-Level Building

• In a two-level building, the wood at construction time had a MC of about 29%.

• The building will be enclosed and heated, bringing the timber to an equilibrium moisture content of 8%.

• This type of construction is called “platform framing,” where each level is built in succession and the floors are resting on the wall elements.

• Total vertical dimension of the construction is 1115 mm (calculated as the sum of ceiling joist, top plates, sole plates, sheathing, floor joists and sill plate).

• Using the formula: $\Delta dim = \frac{3}{100} \times \frac{29-8}{29-12} \times 1115 = 41.3mm$

• If the timber was kiln-dried (MC of 19%), $\Delta dim$ would be 21.6 mm.

• An alternative to “platform framing” is “balloon framing,” where wall elements are continuous as much as possible to minimize transverse shrinkage.

Hybrid Structures

• Taking into account moisture changes and dimensional changes is very important when hybrid structures are designed.

• Hybrid structures use different materials that may not have the same characteristics.

• Last student residence built for Victoria University in Wellington is a 6-level gravity timber construction coupled to a steel lateral-load resisting frame.

Connections

• Connections requirements between the steel elements and the wood elements.

• Slotted holes allow for timber shrinkage.