Section 05: Truss Materials - SBCA Truss Knowledge Online · 1 Level 1: Section 05 – Version 3 ©...
Transcript of Section 05: Truss Materials - SBCA Truss Knowledge Online · 1 Level 1: Section 05 – Version 3 ©...
1Level 1: Section 05 – Version 3© 2014 SBCA
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Section 05: Truss Materials
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Section Downloads
Download & Print TTT I Sec 05 Slides
TTT I Sec 05 Problem Handout
TTT I Sec 05 Design Values
PS 20-2010
Non-Printable Downloads
Version 2.1
Lumber Design Values
Design Value Download
NDS Supplement
Southern Pine Council
Northeastern Lumber Manufacturers Association
Western Wood Products Association
Canadian Wood Council
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Lumber Standard
PS 10
U.S. Department of Commerce
Voluntary
2010
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Southern Pine Council
Southern Pine Use Guide
www.southernpine.com
Effective June 2012
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Western Wood Products Association
Western Lumber Product Use Manual
www2.wwpa.org
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Canadian Wood Council
The U.S. Span Book for Major Lumber Species
Major Species Combinations
Canadian U.S.
Spruce-Pine-Fir S-P-F S-P-F [S]
Douglas Fir-Larch D.Fir-L [N] D.Fir-L
Hem-Fir Hem-Fir [N] Hem-Fir
Southern Pine - SYP
www.cwc.ca/index.php/en/
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TPI 1-2007 Selections
Selections from ANSI/TPI 1-2007
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Standard Sizes
Table 3 – Page 8 10
Truss Materials OutlineLumber Metal Connector Plates
50% 5%
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Lumber Outline
Wood Structure
Factors Affecting Strength
Lumber Production
Sizes & Grades
Design Values
Influences on Final Grade
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Lumber versus Wood
Lumber & wood are not the same.
Once the shape of wood is changed, the wood is called lumber.
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Wood Structure
Chemical Composition Cellulose
Lignin
Hemicellulose
Extraneous Materials Resin & Gum
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Wood Cells
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Growth Rings
Latewood
Earlywood16
Courtesy of Wood Engineering and Construction Handbook
Tree Parts
A = Pith
B = Wood Rays
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Factors Affecting Wood Strength:Natural Characteristics
Direction of Wood Fibers
Specific Gravity
Moisture Content
Rate of Growth
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Direction of Wood Fibers (Grain)
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Base Values for Western Dimension Lumber
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Specific Gravity (G)
Density = Mass Per Unit Volume
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Moisture Content (MC)
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Moisture Content
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Southern Pine Design Values
Section 06 – Design Principles24
Rate of Growth
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Rate of Growth
Greater # of Rings per Inch = Stronger26
Temperature
Load Duration
Chemicals
Decay Fungi
Insects
Factors Affecting Wood Strength:Environmental Characteristics
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Temperature
Inverse relationship between strength & temperature
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Temperature
Initial Char Rate = 1/30″ per
minute
After 8 minutes: Char Rate =
1/40″ per minute
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Load Duration
Wood is subject to fatigue
Need to determine Load Duration Factors
Section 07 – Load Development30
Chemicals
Wood is susceptible to attack Chemicals can also help prevent attacks
Chemicals can help preserve wood & are used in fire-retardant treatments
Chemical exposure reduces strength properties Treatment manufacturers know
necessary reductions
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Decay Fungi
Exposure to chronic moisture
Attack beyond the wood surface
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Other Fungi Stain Fungi
Black Yeast
Mold Strength unaffected
Needs food, temperature, water, & air to survive Eliminate one of
these & fungi will cease
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Facts Regarding Mold on Wood Structural Building Components TTB
TTBMOLD-D
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Builder Advisory on Mold TTB
TTBMOLDBA-D
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Insects
Many different types of insects attack & destroy wood
Subterranean Termites
Carpenter Ants
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Insects
Ground SeparationDetailing
Treated Wood
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Lumber Production
Lumber needs to be sized & graded
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Kiln-Dried Lumber7% ≤ MC ≤ 19%
Lumber Production
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Lumber Sizes
Nominal vs. Dressed Example: a nominal
sized 2x4 is actually 1.5″ x 3.5″
Lumber groups: Boards
Dimension Lumber
Timbers
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Dimension Lumber
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Grade Categories
Framing Lumber Grades intended for structural applications
Appearance Lumber Non-structural grades
Industrial Lumber Structural & non-structural intended for specific
applications
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Framing Lumber
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Appearance Lumber
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Industrial Lumber
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Dimension Lumber Grades
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Determining Design Values
Methods for Assigning Allowable Design Stresses to Framing Lumber Visually Graded Lumber Select Structural, No.1, No. 2, No. 3, Stud
Machine Graded Lumber MSR, MEL
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Influences on Final Grade
Naturally Occurring Characteristics Knots
Shake
Distorted Grain
Reaction Wood
Juvenile Wood
Section 02 – Terminology Review48
Knots
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Shake
Sometimes the result of high winds 50
Distorted Grain
Pitch Pockets & Bark Pockets
Pitch Pockets
Bark Pockets
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Reaction Wood
Compression Wood
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Juvenile Wood
CROSS SECTION OF 24-YR-OLD TREE IN
UNMANAGED FOREST
CROSS SECTION OF 14-YR-OLD HIGH YIELD
FOREST TREE
Lower densityLower strength
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Influences on Final Grade
Manufacturing Characteristics Checks
Wane
Warp
Dimensional Variability
Moisture Content
Section 02 – Terminology Review54
Checks
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Defects in Lumber:Wane
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Defects in Lumber:Warp Characteristics
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Lumber Quality Issues
Structural Performance Related High Moisture Content Reduces Connector Strength
Wane, Knots & Other Lumber Characteristics Reduces Plate Holding Strength
Lumber Thickness Variation Reduces Plate Holding Strength
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Design Values
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Design Values Design Values
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Design Values
Select Structural – highest visual gradeDenseSection 06 – Design PrinciplesDense, Regular, & NonDense 62
Visual Grading
Look for slope of grain, wane, knots, & distances betweenThe fewer characteristics the higher the gradeMark the grade at the end of each pieceMay mark piece to be end trimmedRipped lumber needs to be re-graded
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Visual Grade Stamp
Species?64
Visually Graded Lumber
Select-Structural
No. 1
No. 2
No. 3
Watch as lumber characteristics become more pronounced as grades decrease
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Visually Graded Lumber:Select Structural
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Visually Graded Lumber:No. 1
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Visually Graded Lumber:No. 2
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Visually Graded Lumber:No. 3
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Machine Graded Lumber
Machine Stress Rated (MSR) Grades Designated by Fb and E
Machine Evaluated Lumber (MEL) Grades Designated by Numerical Designation
“M-xx”
Bending stress = 1650 psiModulus of Elasticity = 1,500,000 psi
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Machine Graded Lumber
Shear parallel to grain (Fv) and Compression Perpendicular to Grain (Fc┴) depend on species & grade
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Machine Grading Technologies
Continuous Lumber Tester (CLT)
Directly measures the stiffness of lumber
X-ray Lumber Gauge (XLG)
Directly measures the density of lumber
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Continuous Lumber Tester (CLT)
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X-Ray Lumber Gauge (XLG)
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Technologies and Types
Both CLT & XLG can be used in the production of MSR & MEL
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Machine Graded Lumber
Section 06 – Design Principles
Ft = Tensile Stress Parallel-to-Grain SG = Specific Gravity
Fv = Horizontal Shear
Fc┴ = Compression Stress Perpendicular-to-Grain
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Lumber Grades TTB
TTBGRADES-D
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Quiz 1
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Metal Connector Plates Outline
How Plates Work
Plate Testing Lateral Resistance
Strength
Shear Net Section
Tension Net Section
Full Scale Testing
Early Truss Testing
Example Problem
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Metal Connector Plates
Structurally SafeEfficient
Easy to Install
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How Connector Plates Work
Sufficiently sized plates are placed on both sides of joint
Plates are pressed into wood
Plates resist forces at joints by transferring them from lumber through teeth & into plate
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How Connector Plates Work
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Metal Connector Plate Testing
Capacities measured in terms of withdrawal strength
Each plate manufacturer has its own truss plate design & associated strength values
Plate manufacturers test their plates & register results with evaluation agencies
Section 02 – Terminology
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Testing Standard
Work began in 1959
1st Standard developed by TPI in 1960 TPI-60 “Design Specification for Light Metal Plate
Connected Timber Trusses”
Ten editions: 1962 to 2007
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Strength Properties Tested
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Strength Properties Tested
Lateral Resistance Strength Tooth Withdrawal
Shear Net Section
Tension Net Section
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Lateral Strength Tests
TPI 1 requires the evaluation of truss plates in four orientations. Connector plate length parallel-to-load, parallel-to-
grain (LRAA).
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Lateral Strength Tests
Connector plate length perpendicular-to-load, parallel-to-grain (LREA).
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Lateral Strength Tests
Connector plate length parallel-to-load, perpendicular-to-grain (LRAE).
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Lateral Strength Tests
Connector plate length perpendicular-to-load, perpendicular-to-grain (LREE).
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Lateral Strength Tests At Critical Slip, divide
the load by 1.3.
Average the test values for each plate orientation & multiply by a specific gravity average.
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Lateral Strength Tests At Ultimate Failure,
divide the load by 3.2.
Average the test values for each plate orientation & multiply by a specific gravity average.
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Shear Strength Tests
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Shear Net Section
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Tensile Strength Tests
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Shear Failure
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Full Scale Testing
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Full Scale Testing
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Full Scale Testing
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Full Scale System Testing
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Early Truss Tests
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Early Truss Tests
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Quiz 2
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TTT 1 Sec 05 Handout
TTT 1 Sec 05 Handout
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Given Situation
You = Truss Manufacture
Structure in Madison, WI
Builder calls about “truss uplift” problem
Constructed during rain last summer
It is now February
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Given Situation
Floor joist & interior wall lumber was installed with 25% MC
Lumber has dried to 8% MC
Floor joists Douglas Fir-Larch
Studs & plate material SPF (south)
SPF (south)
Doug Fir-
Larch
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Given Situation
≈ 1” gap between top of 3rd floor wall & truss BC
Gap increased over time
Trusses “arch” Must be the problem
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Questions
(a) Do you agree or disagree? Why or why not?
(b) Assuming that there has been no foundation settlement, what check should be made on the trusses to prove or disprove the builder’s claim?
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Answers
(a) Disagree! Truss arching is typically a temporary condition Open/close NOT progressively worse
Progressively worse gap indicates settlement
MC 25% to 8%
Wood shrinkage
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Lumber Shrinkage
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Partition Separation Prevention & Solutions TTB
TTBPARTSEP-D
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Answers
(b) Methods to check for truss arching: Compare exterior wall heights to interior partition
wall heights
Measure MC in TC & BC No difference = No truss arching
Calculate expected lumber shrinkage
Vertical members exhibit less dimensional change because wood’s longitudinal shrinkage is quite small (approximately .003% to .006% for every 1% change in MC).
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Calculate Lumber Shrinkage2x10′s
Studs
Plates
2 x 9.25" x 0.002 x (25-8) = 0.629"Actual width of dry 2x10
Shrinkage Factor
% change in Moisture Content
3 x 92" = 276"
276" x 0.000067 x (25-8) = 0.3144"
Vertical Length
Longitudinal Shrinkage
Factor
% change in Moisture Content
8 x 1.5" x 0.002 x (25-8) = 0.408"
0.629" + 0.3144" + 0.408" = 1.35"
Solutions
Wrong Solutions: Cut the truss bottom chord
Pull the truss bottom chord down to the wall plate and fasten securely
Right Solutions (see TTB): Balanced attic ventilation
Floating gypsum corners
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Quiz 3