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
Annual Housing Unit Production
Year
Housing Units
<1940
800,000
1950
2,000,000
1973
3,000,000
1992
1,100,000
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
… Steam
with the Development of:
Powered Sawmills
… Machine Made Nail
… 4234
in3/lf
Western Platform Framing
Western Platform Framing
… Coincided
with the Development of Veneer
Panel Products
… early
… 1368
1900's
in3/lf
Wood Use per Lineal Ft
of 8' High Wall
Housing Type
Cubic Inch / LF
Material Savings
Log Cabin
9216
0%
Timber Frame
6210
33%
Balloon Frame
4234
46%
Platform Frame
1368
85%
Source: Jerry Finrow, The Conflict between Dwelling and the American Forest. A Revolution in Basic Cultural Values
Energy Consumption* of Common
Building Materials
MJ/kg
Concrete
2**
Lumber
4~7
Plywood
18
Steel
25~35
* 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.
… Fb
1 3/4" x 9 1/4"
= 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
Work
in Finish and Cabinet
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