A General Introduction to Wood Construction

Journal of Architectural Education

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

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 >70bbf

Forests

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

Primeval Forests: 1620

Primeval Forests: 1850

Primeval Forests: 1989

Population Density Maps: 1776, 1876, 1976

<1940

1950

1973

1992

Year Housing Units

800,000

2,000,000

3,000,000

1,100,000

Annual Housing Unit Production

Iroquois Longhouse

Iroquois Longhouse

… Minimal Impact on the Forest… High Density Configuration… Same Quantity of Wood as Today’s

SF Detached BUT...… they housed 50 - 60 people compared to

today's average family of 2.5 persons

Log Cabin

Log Cabin

… First Response to Housing Need… Forests were Viewed as an Impediment

to Agriculture… 9216 in3/lf

Timber Framing

Timber Framing

… Coincided with the Development of Sawmills…Maine ca. 1620…Water Powered

… Mortise and Tenon… Labor Intensive… 6210 in3/lf

Balloon Framing

Balloon Framing

… Coincided with the Development of:…Steam Powered Sawmills…Machine Made Nail

… 4234 in3/lf

Western Platform Framing

Western Platform Framing

… Coincided with the Development of Veneer Panel Products…early 1900's

… 1368 in3/lf

Log Cabin

Timber Frame

Balloon Frame

Platform Frame

Housing Type Cubic Inch / LF Material Savings

9216

6210

4234

1368

0%

33%

46%

85%

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

Concrete

Lumber

Plywood

Steel

MJ/kg

2**

4~7

18

25~35

Energy Consumption* of Common Building Materials

* Mining, Processing, and Transporting Combined** Concrete Contributes 3% of Greenhouse Gases

Strategies for the Future

… Sustainable Forests… Create Housing with Less Demand

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

Create Housing w/ Less Demand

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

Wood Construction Topics

… Wood Botany… Lumber… Panels Products… Engineered Wood Products

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

Cross Section through a Tree

Wood Botany

… Two Major Groups…Coniferous…Deciduous

Coniferous

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

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

Wood is Anisotropic

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

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

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

Wood is Viscoelastic

… enough is enough!

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%

Fiber Saturation Point

… the point at which the cell cavities are empty but the cell walls are full of water

… approx. 28% MC

Moisture Content

Wood Product Moisture Content

Shrinkage Relative to the Major Planes

Manufacture of Sawn Lumber

Surfacing

… S4S, surfaced four sides… S-Green, surfaced at >19% MC… S-Dry, surfaced at 19% or less

Categories of Dimension / Stress Rated Framing Lumber

How Do We Determine the Strength of Lumber

Visual Grading by Certified Graders

… based upon tested strength of clear sample

… reductions in strength (grade) made based upon defects, grain direction etc

Grade Stamps

Machine Testing

… every board is tested and stamped according to results

Machine Stress Rated Lumber

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

Engineered Wood Products

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

Engineered Wood Products

… potential disadvantages…out-gassing from formaldehydes…deterioration under certain conditions

…FRT plywood

…appearance

Engineered Wood Products

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

Sheathing and Decking

… Plywood… Oriented Strand Board (OSB) and

Waferboard

Main Structural Members

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

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

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

Glulam Manufacture

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

LVL Manufacture

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

PSL Manufacture

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

Stress Skin Foam Panels

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

Usability

Joint Types

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