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Published byAlison Covell Modified about 1 year ago

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Design of Timber Structures

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Timber as a structural material The oldest construction material and still one of the most versatile A natural material with inherent flaws and variability We need to recognize its strengths and weaknesses Timber design therefore as much an art as a science

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One of nature’s most efficient structures: an Arbutus tree facing the onslaught of West Coast storms

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Decay of wood Requirements: nutrition (wood) modest temperature (~ 20 C) moisture (the only one that can be readily controlled)

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Preservative treatment of wood in marine environment

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Decay in a poorly constructed building envelope

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Wet column bases

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Comparative material properties Strain, % Stress (MPa) -20 -100 -200 -400 300 302010-10 400 -300 200 100 mild steel wood (parallel to grain) concrete

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One of the biggest challenges in light timber construction Also an important benefit of heavy timber construction Fire resistance

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Reliability and Safety Load distributions Strength distributions Load, Resistance Probability of occurrence Probability of failure (overlap area)

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Safety Factors Load distribution Strength distribution Load, Resistance Probability of occurrence Probability of failure (overlap area) L avg R avg Global safety factor = R avg /L avg

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Safety factors Load distribution Load, Resistance Probability of occurrence Measure of safety Resistance distribution 95 th percentile5 th percentile Nominal safety factor = R 95 /L 05 L 95 R 05

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Safety Index Load distribution Strength distribution Resistance - Load Probability of occurrence Probability of failure (overlap area) Probability of occurrence (Resistance – Load) distribution β (SDEV) β = Safety Index Probability of failure

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Normal Distribution Load, Resistance Probability of occurrence Resistance distribution R 05 R avg 1.645 SDEV

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Design equation L ≤ R Load factor Load (L 95 )Calibration factor Resistance (R 05 ) Factored Action ≤ Factored Resistance From National Building Code (same for all materials) From material specific design code, e.g. O86.1

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Material properties of wood lignin cellulose fibres … imagine a bundle of straws held together with elastic bands tension parallel to grain compression parallel to grain tension perpendicular to grain compression perpendicular to grain shear

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Design properties (approximate values, D-fir No.1/2) Strength property Clear wood (MPa) Structural timber (MPa) Tension parallel to grain ( f t )206 Compression parallel to grain ( f c )1814 Tension perpendicular to grain ( f tp )11 Compression perpendicular to grain ( f cp )88 Bending ( f b )3010 Shear parallel to grain ( f v )11

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Consequences of different design values Avoid tension perpendicular and shear stresses at all cost Make use of compression strength of wood as much as possible Simplify connections and use compression load transfer when possible Avoid stress concentrations and complex stress patterns

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Brittle failure of wood Tension perpendicular to grain Tension parallel to grain Shear

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Factors that affect the strength of clear wood Decay Direction of load w.r.t. grain orientation Others ….. ?

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Effect of density 200 00.40.21.0 150 100 0 Strength (MPa) 0.6 Relative density 0.8 50 Compression perpendicular Compression parallel Modulus of rupture Modulus of elasticity Density values: Douglas fir0.49 Pine0.37-0.44 Hemlock 0.43 Spruce0.37-0.43

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Grading of timber Defects that affect the strength of timber

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Visual Grading of Lumber Lumber is sorted for a specific application, e.g. –For tension members all knots and defects have a significant effect –For beams and stringers, the grader focuses on edge knots –For posts and timbers sloped grain is more important The larger the members, the higher the probability of missing some important defects

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The sorting process Sorting by species –Species of similar strength characteristics are lumped together Visual grading –A certified grader sorts wood by hand according to visual appearance –Lumber gets sorted according to end use –Grading criteria: Knots (type, location, size, frequency), wane, checks, slope of grain, pitch pockets Mechanical grading

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Testing of lumber Tension test Bending test Full size members are tested (a)To failure (full distribution is obtained) (b)Up to a proof load (only lower tail end of distribution is obtained) Strength distribution Probability of occurrence Strength 5 th percentile value Proof load

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Use of dimension lumber in residential construction

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Platform construction

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Residential construction

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Design values for structural joists and planks (MPa) General purpose members

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Design values for beams and stringers (MPa)

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Beams and stringers on the flat Adjustment factors when using beams or stringers on the flat: f b E or E 05 Select Structural0.881.00 No.1 or No.20.770.90

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Variability of material properties Engineered wood products (less variability) Closely spaced members (load sharing) Bridging (load sharing) Selection of members for specific applications (grading)

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Large glulam beams in buildings and bridges Defects are distributed among many laminations

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Design values for Douglas fir glulam (MPa)

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Design concepts Load distribution Engineered wood product Sawn lumber Load, Resistance Probability of occurrence Probability of failure (overlap area)

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Engineered wood products - pick the best member for each application plywood finger-jointed studs oriented strandboard I-joists laminated veneer lumber laminated strand lumber

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Structural design To minimize the probability of a very high stress (extreme load case) occurring at a location of very low strength (extreme weakness) high stress area low strength area

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Wall construction These elements for shearwalls only

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Loads on walls Gravity loads (dead load, snow, occupancy) Shear loads (wind, earthquake) Lateral loads (wind)

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Shrinkage of wood moisture content of wood (%) shrinkage (%) 10 0 5 05 20251530 tangential shrinkage radial shrinkage lengthwise shrinkage

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Wood shrinkage in platform construction When using green wood (25% MC) Shrinkage @ 5% results in (0.05)(235+38+38) = 15.6mm 2x12 (38x235) joists 2x4 (38x89) top plates

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Post and beam constructi on

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Post and beam construction C.K. Choi building, UBC campus

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Design values for posts and timbers (MPa)

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Mechanical grading of lumber P Machine stress rating non-destructive continuous feed elastic modulus is measured over entire length and averaged E-values are correlated with strength values Probability of occurrence Strength5 th percentile values MSR Visual

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Design values for MSR lumber (MPa) Note: no species separation

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Use of MSR lumber in trusses

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Engineered wood products A way to reduce the variability of the material Use low quality material to produce a high-grade product Use high quality material in high stress zones No size limitations (almost) Can be made for special applications

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Shrinkage in woodframe construction

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Shrinkage in connections

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Wood properties and connection design Avoid connections as much as possible Work with the strong properties of wood (compression) Avoid weak properties (tension perpendicular and shear) Consider shrinkage Design for durability Strive for simplicity

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Efficient use of timber for a long span roof (minimal connections)

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Bearing connections

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Bearing connections

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Complex connections ??

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The connection palace

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Wood in bending and compression

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The ultimate tree ??

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