A General Introduction to Wood Construction

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A General Introduction to Wood Construction
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Journal of Architectural Education

• An Exploration of the Relationships between
  Houses and Forests in American History
• William Gould Sturges
• University of Oregon

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There is a Supply Demand Imbalance for Wood
Products

• In 1986 we consumed 57.2 Billion Board Feet (bbf)
  of Wood Products, yet only 70 was Produced
  Domestically
• This is 40 greater Consumption than in the 50's
  and 60's
• The Trend is Getting More Acute
• In 2040, the estimates are gt70bbf

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Forests

• Provide
• species habitat
• watershed protection
• biological gene banks
• medicine i.e.. taxol

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Primeval Forests 1620
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Primeval Forests 1850
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Primeval Forests 1989
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Population Density Maps 1776, 1876, 1976
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Annual Housing Unit Production
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Iroquois Longhouse
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Iroquois Longhouse

• Minimal Impact on the Forest
• High Density Configuration
• Same Quantity of Wood as Todays SF Detached
  BUT...
• they housed 50 - 60 people compared to today's
  average family of 2.5 persons

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Log Cabin
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Log Cabin

• First Response to Housing Need
• Forests were Viewed as an Impediment to
  Agriculture
• 9216 in3/lf

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Timber Framing
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Timber Framing

• Coincided with the Development of Sawmills
• Maine ca. 1620
• Water Powered
• Mortise and Tenon
• Labor Intensive
• 6210 in3/lf

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Balloon Framing
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Balloon Framing

• Coincided with the Development of
• Steam Powered Sawmills
• Machine Made Nail
• 4234 in3/lf

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Western Platform Framing
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Western Platform Framing

• Coincided with the Development of Veneer Panel
  Products
• early 1900's
• 1368 in3/lf

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Wood Use per Lineal Ft of 8' High Wall
Source Jerry Finrow, The Conflict between
Dwelling and the American Forest. A Revolution in
Basic Cultural Values
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Energy Consumption of Common Building Materials
Mining, Processing, and Transporting
Combined Concrete Contributes 3 of Greenhouse
Gases
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Strategies for the Future

• Sustainable Forests
• Create Housing with Less Demand

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Sustainable Forest Farming

• Forest Stewardship Council (FSC)
• Limit Timber Removal to Prevent Decline in
  Quality/Quantity
• Provide Variety of Ages
• Allow Trees to Mature Fully
• Maintain Plant and Animal Habitat
• Protect the Soil

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Create Housing w/ Less Demand

• Recycle
• Don't Over-design
• Engineered Wood Products
• Higher Density Configurations

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Wood Construction Topics

• Wood Botany
• Lumber
• Panels Products
• Engineered Wood Products

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Tree Structure

• Bark
• Cambium Layer
• Sapwood - conduction, storage and support
• Heartwood - support only
• Growth (Annual) Rings
• Late Wood - summer/fall, small, densely packed
  cells
• Early Wood - spring, large cells

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Cross Section through a Tree
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Wood Botany

• Two Major Groups
• Coniferous
• Deciduous

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Coniferous

• Softwoods (not a measure of hardness)
• Gymnosperms - naked seeds
• Evergreen e.g. Douglas fir, larch, pine
• Narrow leaf
• Coniferous trees lack vessels

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Deciduous

• Hardwoods (not a measure of hardness)
• Angiosperms - covered seeds
• Shed leaves annually
• Broadleaf e.g. oak, maple, cherry
• Deciduous trees have vessels for support and
  conduction of nutrients

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Wood is Anisotropic

• An anisotropic material has physical properties
  that are different with respect to their three
  major orthogonal planes

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Three Major Planes

• Longitudinal - along the length of the log
• Radial - with respect to the center of the log
• Tangential - with respect to the growth rings

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Wood is Hygroscopic

• An hygroscopic material has an affinity for water
  molecules which causes swelling and shrinking as
  the material reaches equilibrium with its
  environment

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Wood is Viscoelastic

• enough is enough!

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Moisture Content

• the weight of water as a percentage of the weight
  of an oven dry sample of wood
• freshly cut lumber can range from 30 to200

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Fiber Saturation Point

• the point at which the cell cavities are empty
  but the cell walls are full of water
• approx. 28 MC

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Moisture Content
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Wood Product Moisture Content
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Shrinkage Relative to the Major Planes
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Manufacture of Sawn Lumber
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Surfacing

• S4S, surfaced four sides
• S-Green, surfaced at gt19 MC
• S-Dry, surfaced at 19 or less

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Categories of Dimension / Stress Rated Framing
Lumber
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How Do We Determine the Strength of Lumber
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Visual Grading by Certified Graders

• based upon tested strength of clear sample
• reductions in strength (grade) made based upon
  defects, grain direction etc

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Grade Stamps
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Machine Testing

• every board is tested and stamped according to
  results

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Machine Stress Rated Lumber
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In-Grade Testing

• North American In-Grade Testing Program, 1977
• testing of 70,000 full size pieces (1 million
  board feet)
• created a database of empirical design values
• Bending values went up for narrows and high
  grades, down for wides and lower grades

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Engineered Wood Products
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Engineered Wood Products

• attempt to improve the basic properties of lumber
• strength
• appearance
• dimensional stability
• cost
• use fast-growing, low-density wood species in
  combination with plastic resins

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Engineered Wood Products

• potential disadvantages
• out-gassing from formaldehydes
• deterioration under certain conditions
• FRT plywood
• appearance

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Engineered Wood Products

• Sheathing and Decking
• Main Structural Members
• Stress Skin Foam Panels

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Sheathing and Decking

• Plywood
• Oriented Strand Board (OSB) and Waferboard

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Main Structural Members

• Light Frame Trusses
• Glue Laminated Lumber (Glulam)
• Laminated Veneer Lumber (LVL)
• Parallel Strand Lumber (PSL)
• Prefabricated Wood I-Joists

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Light Frame Trusses

• Flat Plate Connectors
• Stamped metal plates hydraulically pressed into
  wood to make connections
• Suitable for floors and roofs
• Proprietary Connectors
• Wood Top and Bottom Chord
• Fab. Steel Element for Web w/ Integral Fastener
• Split Ring Connector
• For Custom-Designed, Field-Fabricated Trusses
• Increase Effective Area of Joint

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Glue Laminated Lumber (Glulam)

• Old Technology
• Headers, Beams, Girders and Columns
• Strength Depends on Wood Species and Quality
• Limitless Variety of Straight and Curved
  Configurations

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Glulam Manufacture
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Laminated Veneer Lumber (LVL)

• Based on Plywood Technology
• Headers, Beams and Girders
• Sized to Flush Frame with Platform Framing
• i.e. 1 3/4" x 9 1/4"
• Fb 2800 psi, E 2,000,000 psi

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LVL Manufacture
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Parallel Strand Lumber (PSL)

• Based on Cross between Plywood OSB Technology
• Strands are up to 8' long and 1/2" thick
• Headers, Beams, Girders and Columns
• Sized to Flush Frame with Platform Framing
• i.e. 1 3/4" x 9 1/4"
• Potential Applications in Finish and Cabinet Work

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PSL Manufacture
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Prefabricated Wood I-Joists

• Profile Similar to Steel Beams
• Top and Bottom Flanges Resist Compressive and
  Tensile Stresses Respectively
• Solid Wood
• LVL
• Glulam
• Web Resists Shear Forces
• Plywood
• OSB

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Stress Skin Foam Panels

• Walls, Floors and Roofs
• Foam Core Bonded to Structural Skin
• Modular Design Principles
• Joints between Panel Are Key to Usability

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Joint Types

• Dual Spline e.g. R-Control
• Special Piece e.g. Ray-Core
• Stud
• Cam Lock

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