Section Downloads. Lumber Design Values. Lumber Standard. Western Lumber Product Use Manual. Section 05: Truss Materials.

Transcription

1 Section Downloads Download & Print TTT I Sec 05 Slides TTT I Sec 05 Problem Handout TTT I Sec 05 Design Values Section 05: Truss Materials 1 PS Non-Printable Downloads Version Lumber Design Values Lumber Standard Design Value Download NDS Supplement Southern Pine Council Northeastern Lumber Manufacturers Association Western Wood Products Association Canadian Wood Council PS 10 U.S. Department of Commerce Voluntary Southern Pine Council Southern Pine Use Guide Western Wood Products Association Western Lumber Product Use Manual www2.wwpa.org Effective June

2 Canadian Wood Council The U.S. Span Book for Major Lumber Species TPI Selections 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 7 Selections from ANSI/TPI Standard Sizes Truss Materials Outline Lumber Metal Connector Plates 50% 5% 9 10 Table 3 Page 8 Lumber Outline Wood Structure Factors Affecting Strength Lumber Production Sizes & Grades Design Values Influences on Final Grade Lumber versus Wood Lumber & wood are not the same. Once the shape of wood is changed, the wood is called lumber

3 Wood Structure Wood Cells Chemical Composition Cellulose Lignin Hemicellulose Extraneous Materials Resin & Gum Growth Rings Tree Parts A = Pith B = Wood Rays Latewood Earlywood 15 Courtesy of Wood Engineering and Construction Handbook 16 Factors Affecting Wood Strength: Natural Characteristics Direction of Wood Fibers Specific Gravity Moisture Content Rate of Growth Direction of Wood Fibers (Grain)

4 Base Values for Western Dimension Lumber Specific Gravity (G) 19 Density = Mass Per Unit Volume 20 Moisture Content (MC) Moisture Content Southern Pine Design Values Rate of Growth Section 06 Design Principles

5 Rate of Growth Factors Affecting Wood Strength: Environmental Characteristics Temperature Load Duration Chemicals Decay Fungi Insects Greater # of Rings per Inch = Stronger Temperature Inverse relationship between strength & temperature Temperature Initial Char Rate = 1 / 30 per minute After 8 minutes: Char Rate = 1 / 40 per minute Load Duration Wood is subject to fatigue Need to determine Load Duration Factors 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 Section 07 Load Development

6 Decay Fungi Exposure to chronic moisture Attack beyond the wood surface Stain Fungi Black Yeast Mold Strength unaffected Needs food, temperature, water, & air to survive Eliminate one of these & fungi will cease Other Fungi Facts Regarding Mold on Wood Structural Building Components TTB Builder Advisory on Mold TTB TTBMOLD-D TTBMOLDBA-D Insects Many different types of insects attack & destroy wood Insects Subterranean Termites Carpenter Ants Ground Separation Detailing Treated Wood

7 Lumber Production Lumber needs to be sized & graded Lumber Production Kiln-Dried Lumber 7% MC 19% Nominal vs. Dressed Example: a nominal sized 2x4 is actually 1.5 x 3.5 Lumber groups: Boards Dimension Lumber Timbers Lumber Sizes Dimension Lumber Grade Categories Framing Lumber Grades intended for structural applications Appearance Lumber Non-structural grades Industrial Lumber Structural & non-structural intended for specific applications Framing Lumber

8 Appearance Lumber Industrial Lumber Dimension Lumber Grades 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 Influences on Final Grade Naturally Occurring Characteristics Knots Shake Distorted Grain Reaction Wood Juvenile Wood Knots Section 02 Terminology Review

9 Shake Distorted Grain Pitch Pockets Bark Pockets Pitch Pockets & Bark Pockets Sometimes the result of high winds Reaction Wood Juvenile Wood Lower density Lower strength Compression Wood 51 CROSS SECTION OF 14-YR-OLD HIGH YIELD FOREST TREE CROSS SECTION OF 24-YR-OLD TREE IN UNMANAGED FOREST 52 Influences on Final Grade Manufacturing Characteristics Checks Wane Warp Dimensional Variability Moisture Content Checks Section 02 Terminology Review

10 Defects in Lumber: Wane Defects in Lumber: Warp Characteristics 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 Design Values Design Values Design Values

11 Design Values Visual Grading Section Dense, Regular, 06 Dense Design & NonDense Principles Select Structural highest visual grade Look for The Mark slope Ripped fewer May the of mark characteristics grain, lumber grade piece wane, at needs to the be knots, end the to end be of higher & trimmed re-graded each distances the piece grade between Visual Grade Stamp Visually Graded Lumber Select-Structural No. 1 No. 2 No. 3 Watch as lumber characteristics become more pronounced as grades decrease Species? Visually Graded Lumber: Select Structural Visually Graded Lumber: No

12 Visually Graded Lumber: No. 2 Visually Graded Lumber: No Machine Graded Lumber Machine Stress Rated (MSR) Grades Designated by F b and E Machine Graded Lumber Bending stress = 1650 psi Modulus of Elasticity = 1,500,000 psi Machine Evaluated Lumber (MEL) Grades Designated by Numerical Designation M-xx 69 Shear parallel to grain (F v ) and Compression Perpendicular to Grain (Fc ) depend on species & grade 70 Machine Grading Technologies Continuous Lumber Tester (CLT) Continuous Lumber Tester (CLT) Directly measures the stiffness of lumber X-ray Lumber Gauge (XLG) Directly measures the density of lumber

13 X-Ray Lumber Gauge (XLG) Technologies and Types Both CLT & XLG can be used in the production of MSR & MEL Machine Graded Lumber Lumber Grades TTB TTBGRADES-D F t = Tensile Stress Parallel-to-Grain SG = Specific Gravity F v = Horizontal Shear Fc = Compression Stress Perpendicular-to-Grain Section 06 Design Principles Metal Connector Plates Outline Quiz 1 How Plates Work Plate Testing Lateral Resistance Strength Shear Net Section Tension Net Section Full Scale Testing Early Truss Testing Example Problem

14 Metal Connector Plates 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 Structurally Safe Efficient Easy to Install How Connector Plates Work 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 81 Section 02 Terminology 82 Testing Standard Work began in st Standard developed by TPI in 1960 TPI-60 Design Specification for Light Metal Plate Connected Timber Trusses Ten editions: 1962 to 2007 Strength Properties Tested

15 Strength Properties Tested Lateral Resistance Strength Tooth Withdrawal Shear Net Section Tension Net Section Lateral Strength Tests TPI 1 requires the evaluation of truss plates in four orientations. Connector plate length parallel-to-load, parallel-tograin (LRAA) Lateral Strength Tests Connector plate length perpendicular-toload, parallel-to-grain (LREA). Lateral Strength Tests Connector plate length parallel-to-load, perpendicular-to-grain (LRAE) Lateral Strength Tests Connector plate length perpendicular-toload, perpendicular-to-grain (LREE). 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

16 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. Shear Strength Tests Shear Net Section Tensile Strength Tests Shear Failure Full Scale Testing

17 Full Scale Testing Full Scale Testing Full Scale System Testing Early Truss Tests Early Truss Tests Quiz

18 TTT 1 Sec 05 Handout TTT 1 Sec 05 Handout Given Situation You = Truss Manufacture Structure in Madison, WI Builder calls about truss uplift problem Constructed during rain last summer It is now February 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 Given Situation 1 gap between top of 3 rd floor wall & truss BC Gap increased over time Trusses arch Must be the problem Questions Answers (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? (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

19 Lumber Shrinkage Partition Separation Prevention & Solutions TTB TTBPARTSEP-D 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). Calculate Lumber Shrinkage 2x10 s 2 x 9.25" x x (25-8) = 0.629" Actual width of dry 2x10 Plates 8 x 1.5" x x (25-8) = 0.408" Studs 3 x 92" = 276" 276" x x (25-8) = " Vertical Length Shrinkage Factor Longitudinal Shrinkage Factor % change in Moisture Content % change in Moisture Content " " " = 1.35" 112 Solutions Wrong Solutions: Cut the truss bottom chord Pull the truss bottom chord down to the wall plate and fasten securely Quiz 3 Right Solutions (see TTB): Balanced attic ventilation Floating gypsum corners

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