Section 05: Truss Materials - SBCA Truss Knowledge Online · 1 Level 1: Section 05 – Version 3 © 2014 SBCA 1 Section 05: Truss Materials 2 Section Downloads Download & Print TTT

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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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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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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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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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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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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Connector plate length perpendicular-to-load, parallel-to-grain (LREA).

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Connector plate length parallel-to-load, perpendicular-to-grain (LRAE).

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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


115

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Documents

Section 05: Truss Materials - SBCA Truss Knowledge Online · 1 Level 1: Section 05 – Version 3 © 2014 SBCA 1 Section 05: Truss Materials 2 Section Downloads Download & Print TTT