Timber Design - Concept Map
Allowable Stress Design (ASD) for Wood Members
Related concepts
Adjustment Factors System
Sub concepts
- Load Duration Factor (CD): 0.9–2.0 range
- Wet Service Factor (CM): Moisture reduction
- Temperature Factor (Ct): Environmental adjustment
- Size Factor (CF): Sawn lumber scaling
- Beam Stability Factor (CL): Lateral-torsional control
- Column Stability Factor (CP): Slenderness reduction
- Repetitive Member Factor (Cr): Multiple members sharing load
Relationship to central
Core method to convert reference design values to allowable stresses; unique to timber ASD
Bending Design (Flexural Members)
Sub concepts
- Bending Stress Formula: fb = M/S
- Section Modulus: S = bh²/6 (rectangular)
- Allowable Bending: Fb′ = Fb × CD × CM × Ct × CF × CL × Cr
- Lateral Torsional Buckling (CL): Critical for deep beams
- Repetitive Member Factor (Cr): Multi-joist optimization
- Check Condition: fb ≤ Fb′
Relationship to central
Primary serviceability check for beams and joists using allowable bending stress
Shear Design (Parallel to Grain)
Sub concepts
- Shear Stress Formula: fv = 3V/(2A)
- Rectangular Section Area: A = b × h
- Allowable Shear: Fv′ = Fv × CD × CM × Ct
- Notched Support Effect: Reduced dn, amplification factor d/dn
- Common CELE Trap: Notched beams lose significant shear capacity
- Check Condition: fv ≤ Fv′
Relationship to central
Secondary check often governing short, deep wood beams; wood weakness in horizontal shear
Compression Design (Axial Members)
Sub concepts
- Compression Stress Formula: fc = P/A
- Allowable Compression: Fc′ = Fc* × CP
- Intermediate Value: Fc* = Fc × CD × CM × Ct × CF (no CP yet)
- Euler-like Buckling: FcE = 0.822 E′min / (le/d)²
- Column Stability Factor CP: Nonlinear reduction with slenderness
- Sawn Lumber Constant: c = 0.8 (glulam c = 0.90)
- Effective Length: le accounts for end conditions
- Check Condition: fc ≤ Fc′
Relationship to central
Column and post design governed by slenderness via CP stability factor
Reference Design Values (Species/Grade Dependent)
Sub concepts
- Fb: Reference bending (MPa)
- Fv: Reference shear parallel to grain (MPa)
- Fc: Reference compression parallel (MPa)
- Fc⊥: Reference compression perpendicular (MPa)
- Ft: Reference tension parallel (MPa)
- E: Modulus of elasticity (MPa)
- Emin: Minimum E for buckling calcs (MPa)
- Wood Species/Grade Tables: NSCP 2015 Chapter 6
Relationship to central
Starting point before any adjustment factors applied; from code tables
Load Duration Factor (CD)
Sub concepts
- Permanent Load: CD = 0.9 (10+ years)
- Normal/Ten-Year Load: CD = 1.0
- Two-Month Load: CD = 1.15 (construction season)
- Seven-Day Load: CD = 1.25 (short-term event)
- Wind/Seismic Load: CD = 1.6
- Impact Load: CD = 2.0 (mechanical shock)
- Governs Most Stress Limits: Applied to Fb, Fv, Fc, Ft
Relationship to central
Unique to timber; rewards brief loads; key multiplier in all stress adjustments
Beam Stability & Lateral Torsional Buckling
Sub concepts
- Lateral Restraint: Degree of bracing perpendicular to loading
- Unbraced Length: lu measured along beam axis
- Depth-to-Width Ratio: d/b affects slenderness in weak direction
- Fully Braced: CL ≈ 1.0 (decking, floor systems)
- Partially Braced: CL < 1.0 (open beams, trusses)
- CL Interaction: Competing with CF; often apply smaller governs
- Critical for Long-Span, Tall Timber Beams
Relationship to central
CL factor prevents lateral-torsional failure of deep, slender beams under bending
Column Stability & Slenderness Effects
Sub concepts
- Slenderness Ratio: le/d (effective length to least dimension)
- Euler Critical Stress: FcE from global buckling theory
- Stability Index: β = FcE/Fc*
- CP Formula: Quadratic solution (sawn lumber c = 0.8)
- Short Column: CP → 1.0 (crushing governs)
- Long Column: CP → 0.0 (buckling governs)
- Intermediate Range: 11 < le/d < 26 (most design range)
- Effective Length Factor K: Accounts for end conditions
Relationship to central
CP factor (analogous to steel column buckling) reduces allowable compression in slender timber columns
Notched Beam Design
Sub concepts
- Reduced Effective Depth: dn = d − notch depth
- Amplification Factor: (d/dn) reduces shear capacity nonlinearly
- Shear Stress at Notch: fv = (3V/2A) × (d/dn)
- Combined Tension/Shear: Interaction near notch end
- Design Rule: Avoid deep notches; limit dn/d > 0.75
- Common in Timber Trusses: Gusset plate connections often require notching
Relationship to central
Special case of shear failure at support; classic CELE pitfall due to net section loss
NSCP 2015 & Philippine Design Standards
Sub concepts
- Chapter 6: Wood Design (ASD method)
- Reference Standard: NDS (National Design Specification, USA) adopted with local mods
- Allowable Stress Basis: ASD philosophy for wood
- Load Combinations: Per NSCP 2010 Ch. 2 (mostly working loads)
- Material Properties: Tables for Philippine-market timber species
- Professional Practice: RA 544 (Professional Regulation Act) requires PE stamp
Relationship to central
Regulatory framework governing all timber design in Philippine projects
Practical Applications in Philippines
Sub concepts
- Formwork & Scaffolding: Temporary, high CD (1.25–1.6)
- Residential Floor Joists: Normal loading, CD ≈ 1.0
- Roof Trusses: Wind load, CD = 1.6; snow minimal in PH
- Bridge Decking: Waterfront structures (wet service CM < 1.0)
- Agricultural/Rural Buildings: Lower cost, timber preferred
- Connections: Bolts, nails, screws (design per NDS Chapter 11)
Relationship to central
Common uses of timber design in Philippine construction practice
Concept connections
- Allowable Stress Design ASDAdjustment Factors SystemASD is the governing method; adjustment factors are the mechanism to convert reference to allowable
- Load Duration Factor CDAll Stress Limits Fb Fv Fc FtCD is multiplied into every stress allowable; unique to wood design
- Bending DesignBeam Stability Factor CLCL controls lateral-torsional buckling of slender beams; critical for deep unrebraced members
- Compression DesignColumn Stability Factor CPCP reduces allowable compression stress as slenderness ratio increases; analogous to steel column buckling
- Shear DesignNotched Beam DesignNotches reduce effective depth dn and trigger amplification factor d/dn; classic CELE failure mode
- Reference Design ValuesAdjustment Factors SystemReference values are the starting point; all C factors are multiplied to arrive at allowable
- Adjustment Factors SystemBending DesignBending allowable Fb' is Fb times six adjustment factors CD CM Ct CF CL Cr
- Adjustment Factors SystemShear DesignShear allowable Fv' is Fv times three adjustment factors CD CM Ct
- Adjustment Factors SystemCompression DesignCompression allowable Fc' is Fc* (with CD CM Ct CF) times CP (stability); two-stage process
- Wet Service Factor CMPractical Applications in PhilippinesWet service common in PH bridge decking and waterfront structures; CM < 1.0 reduces allowables
- Size Factor CFBending DesignCF applies to sawn lumber; smaller sections receive higher CF; affects beam sizing economics
- Slenderness Ratio le/dColumn Stability Factor CPCP is computed from le/d via Euler critical stress and quadratic formula; governs column behavior
- Effective Length leCompression DesignEffective length accounts for end conditions; critical input to slenderness and buckling calcs
- NSCP 2015 Chapter 6Allowable Stress Design ASDNSCP 2015 adopts NDS ASD approach for timber design in Philippines
- RA 544 Professional Regulation ActNSCP 2015 Chapter 6PE requirement mandates compliance with NSCP; professional responsibility for timber design
- Bending Stress Formula fb = M/SSection Modulus SSection modulus is the geometric property used to compute bending stress
- Shear Stress Formula fv = 3V/2ARectangular Section1.5 factor arises from parabolic shear distribution in rectangular cross-sections
- Compression Stress Formula fc = P/AColumn Stability Factor CPAllowable fc is adjusted by CP to account for buckling; check fc = P/A ≤ Fc' = Fc* × CP
- Notched Beam DesignShear DesignNotches trigger net section loss and shear stress amplification; integral to shear check
- Practical Applications in PhilippinesLoad Duration Factor CDFormwork and scaffolding use high CD (1.25–1.6); residential joists use CD = 1.0
Diagrams
Timber Design - Complete Concept Hierarchy
Comprehensive mind map showing the hierarchical structure of timber design from central ASD concept through adjustment factors, design checks, reference values, and Philippine standards. Displays CD factor values, stress formulas, and design component relationships.
Timber Beam Design Process Flow
Step-by-step flowchart for timber beam design showing load duration factor selection, reference value gathering, environmental adjustment, allowable stress computation, and stress checks. Includes feedback loop for section resizing and notch handling.
Timber Column Design & Stability Factor CP
Flowchart for timber column design emphasizing the critical slenderness assessment and CP stability factor calculation. Shows differentiation between short, intermediate, and long columns, with CP computation from Euler buckling concept.
Allowable Stress Adjustment Factor Application
Systematic flowchart showing how reference design values are progressively adjusted by all applicable C factors, then compared against actual stresses. Emphasizes the difference between bending (direct CF and CL product) and compression (CF into Fc* before CP stability calculation).
Column Behavior Under Axial Load: Short to Long Transition
State diagram illustrating how timber column behavior transitions from crushing-controlled (short) through nonlinear stability zone (intermediate) to buckling-controlled (long) as the slenderness ratio le/d increases. Shows the role of CP factor in governing behavior.
Typical Load Duration Factor Frequency in Philippine Construction
Pie chart showing typical distribution of load duration factors in Philippine timber construction projects. Normal loads (CD=1.0) and construction-phase loads (CD=1.15–1.25) dominate, with wind/seismic less frequent due to PH climate and mainly design wind (not snow load).