Study guide
3.4 Wood Material Properties
NCEES PE Civil Construction Study Guide Exam
Track your progress as you read
Finished sections get a check mark.
1 Chapter 1: Soil Mechanics
1.1 Lateral Earth Pressure 1.2 Soil Consolidation and Settlement 1.3 Effective and Total Stresses 1.4 Bearing Capacity 1.5 Slope Stability
2 Chapter 2: Site Layout and Development
2.1 Layout and Control 2.2 Horizontal and Vertical Curves 2.3 Site Investigations and Utilities
3 Chapter 3: Material Properties
3.1 Soils, Rock, and Aggregates 3.2 Concrete Material Properties 3.3 Steel Material Properties 3.4 Wood Material Properties
4 Chapter 4: Estimating Quantities and Costs
4.1 Quantity Take-Off Methods 4.2 Cost Estimating 4.3 Engineering Economics 4.4 Work Measurement and Productivity (Earned Value)
5 Chapter 5: Project Planning and Scheduling
5.1 Construction Sequencing 5.2 Activity Time Analysis 5.3 Critical Path Method and Network Analysis 5.4 Resource Scheduling and Leveling 5.5 Time–Cost Trade-Off and Crashing
6 Chapter 6: Material Production, Execution, and Quality Control
6.1 Specification Conformance and Hold-Points 6.2 Field Testing: Concrete, Soils, and Aggregates 6.3 Compaction: Principles, Calculations, and Equipment 6.4 Concrete Placement, Consolidation, and Finishing 6.5 Bolting and Welding: Structural Steel Connections 6.6 Acceptance Sampling and Statistical Quality Control
7 Chapter 7: Structural Mechanics
7.1 Loads (Dead, Live, Construction, Wind, and Preloading) 7.2 Statics and Stresses (Bending, Shear, Deflection) 7.3 Beams, Columns, and Slabs 7.4 Foundation and Retaining Wall Mechanics
8 Chapter 8: Hydraulics and Hydrology
8.1 Stormwater Collection and Drainage (Open-Channel Flow) 8.2 Runoff Analysis 8.3 Detention and Retention Ponds
9 Chapter 9: Operations and Methods
9.1 Lifting and Rigging 9.2 Crane Capacity and Stability 9.3 Dewatering and Pumping 9.4 Equipment Selection and Production Rates 9.5 Deep Foundation Installation 9.6 Excavation and Embankment (Cut and Fill)
10 Chapter 10: Support of Construction Loads
10.1 Formwork Design 10.2 Falsework and Scaffolding 10.3 Shoring and Reshoring 10.4 Bracing and Anchorage 10.5 Support of Excavation (Trenching, Cofferdams) 10.6 Erection and Equipment Loads on Permanent Structures
11 Chapter 11: Health and Safety
11.1 OSHA Construction Regulations and Safety Management 11.2 Work Zone and Public Safety (Traffic Control)
3.4 Wood Material Properties
Key Takeaways Explain This Topic AI
Wood exhibits orthotropic behavior, bearing much higher loads parallel to the grain compared to perpendicular to the grain.
Engineered wood products like GLULAM and LVL provide higher strength and fewer natural defects than sawn lumber.
Wood dimensions used in structural calculations must be the actual (dressed) dimensions, not the nominal sizes.
Design values must be modified using NDS adjustment factors like the Load Duration Factor and Wet Service Factor.
Wood shrinks significantly when its moisture content drops below the fiber saturation point, primarily in the tangential direction.
Last updated: July 2026
3.4 Wood Material Properties
Quick Answer: Wood is an orthotropic material, meaning its properties vary dramatically depending on the direction of the grain. The exam tests your understanding of structural lumber versus engineered wood, moisture effects, and the critical adjustment factors (moisture, temperature, duration of load, size) used in timber design according to the NDS.
Wood is a highly versatile, renewable construction material. However, because it is a biological product, its properties are inherently variable and susceptible to environmental conditions. Understanding how to account for this variability is key for construction engineering.
- Structural Lumber vs. Engineered Wood
- Sawn Structural Lumber
Traditional sawn lumber is cut directly from logs. It is graded visually or mechanically based on defects like knots, splits, and slope of grain. Common species include Douglas Fir-Larch, Southern Pine, and Spruce-Pine-Fir (SPF).
Nominal vs. Actual Dimensions: Sawn lumber is sold by nominal sizes (e.g., 2x4, 2x10). The actual dressed size is smaller due to surfacing and drying. A 2x4 is actually 1.5" x 3.5". A 2x10 is 1.5" x 9.25". Always use actual dimensions for structural calculations (area, moment of inertia).
Engineered Wood Products
To overcome the limitations of sawn lumber (size constraints and natural defects), engineered wood products are manufactured by bonding wood strands, veneers, or lumber together with structural adhesives.
Glued Laminated Timber (GLULAM): Made by gluing together layers of dimension lumber. Used for massive beams and striking architectural arches.
Laminated Veneer Lumber (LVL): Manufactured from thin veneers glued together with the grain aligned. Extremely strong and commonly used for headers and floor girders.
Oriented Strand Board (OSB): Made of wood strands aligned in specific directions and compressed with resin. Widely used for roof decking and wall sheathing, largely replacing plywood in many residential applications.
Directional Strength (Orthotropic Behavior)
Wood is composed of longitudinal cellulose fibers bound by lignin. Because of this cellular structure, wood is highly directional.
Parallel to Grain: Wood is extremely strong when loaded parallel to the grain (in compression or tension). Think of standing on a bundle of drinking straws.
Perpendicular to Grain: Wood is significantly weaker when loaded perpendicular to the grain. In compression perpendicular to the grain, the fibers crush easily (like stepping on the side of the drinking straws).
This means that a wooden post loaded vertically (parallel to grain) can hold a massive load, but a wooden beam bearing on a support may fail by crushing at the support point (perpendicular to grain).
Moisture Content and Shrinkage
Wood is hygroscopic; it absorbs and releases moisture to reach equilibrium with the surrounding air.
Fiber Saturation Point (FSP): Usually around 25-30% moisture content. At this point, the cell walls are saturated, but the cell cavities are empty.
Shrinkage: Wood does not shrink or swell significantly when the moisture content is above the FSP. However, as it dries below the FSP, the cell walls shrink. This shrinkage is unequal: it is greatest in the tangential direction (along growth rings), less in the radial direction, and negligible longitudinally (parallel to grain).
Design Adjustment Factors (NDS)
The National Design Specification for Wood Construction (NDS) requires baseline reference design values (like bending stress F b F_b F b or shear stress F v F_v F v ) to be multiplied by several adjustment factors to account for the specific service conditions of the structure.
Load Duration Factor ( C D C_D C D ): Wood can support much higher loads for short periods than for long periods. C D C_D C D increases allowable stress for short-term loads (like wind or impact) and decreases it for permanent dead loads (creep).
Wet Service Factor ( C M C_M C M ): If the wood will have a moisture content exceeding 19% in service (e.g., outdoor exposed structures, marine construction), its strength is significantly reduced. C M C_M C M reduces the allowable stress accordingly.
Temperature Factor ( C t C_t C t ): Prolonged exposure to temperatures above 100°F (38°C) degrades wood strength. C t C_t C t applies a reduction for high-temperature environments, such as industrial roofs or attics.
Size Factor ( C F C_F C F ): Larger sawn lumber members are statistically more likely to contain a critical defect (like a large knot) than smaller members. As the depth of a beam increases, the allowable bending stress decreases via the size factor.
Volume Factor ( C V C_V C V ): Similar to the size factor, but applied specifically to structural glued laminated timber (GLULAM).
By correctly applying these factors, engineers ensure that the naturally variable wood performs safely over the life of the structure under varying environmental stresses.