2005 National Design Specification for Wood Construction (NDS)

Purpose and Scope

The NDS establishes a uniform system of rules for the design of structural wood members and connections. It is intended for use by architects, engineers, manufacturers, and building officials. The specification covers:

• Solidsawn lumber, gluedlaminated timber (glulam), laminated veneer lumber (LVL), parallelstrand lumber (PSL), and other engineeredwood products.
• All loading conditions defined by the International Building Code (IBC) and the American Wood Councils Design Guide series.
• Both allowablestress and LRFD (loadandresistancefactor) design approaches.
• Design of members for bending, axial compression and tension, shear, combined stresses, and compression parallel to grain.
• Guidelines for connections, including fasteners, metal plates, and adhesives.
• Special provisions for fire resistance, longterm performance, and moisture effects.

Key Design Values

Reference Design Values (Rvalues)

Reference design values are derived from material test data and represent the baseline properties of a wood product before any adjustment factors are applied. For each product class the NDS provides:

• F_b reference bending stress.
• F_c reference compressive stress parallel to grain.
• F_t reference tensile stress.
• F_v reference shear stress.
• E reference modulus of elasticity.
• G reference shear modulus.

Adjustment Factors

To adapt the reference values to specific conditions, the specification introduces a series of multiplicative factors (kfactors). The major groups are:

1. Duration of Load (C_D) reflects the reduced strength of wood under sustained loads.
2. Size Factor (C_F) accounts for the size effect on strength for members larger than the standard test dimensions.
3. Temperature Factor (C_T) modifies values for temperatures above 40C (104F).
4. Repetitive Load Factor (C_R) applies when a member is subjected to repeated loading cycles.
5. Incising Factor (C_I) for glulam members that have been incised to improve glue line performance.
6. Wet Service Factor (C_M) reduces strength when the member will be in a wet environment for extended periods.

The allowable stress for a given condition is computed as:

F_allow = (F_ref C_D C_F C_T C_R C_I C_M)/

where is the safety factor (typically 0.9 for allowablestress design).

Load Combinations

The 2005 NDS aligns its load combinations with those of the IBC. The main categories are:

• Dead load (D) permanent loads such as selfweight and fixed equipment.
• Live load (L) variable loads from occupancy, furniture, and equipment.
• Snow load (S) regionspecific uniform or varying snow pressures.
• Wind load (W) pressure and suction stresses on exterior surfaces.
• Seismic load (E) applicable for certain highrisk locations.

Typical allowablestress load combinations (with = 0.9) include:

1. D + L
2. D + S
3. D + L + 0.5W
4. D + 0.5W + 0.5E

For LRFD design the same combinations are used, but each load is multiplied by an appropriate load factor (e.g., 1.2D + 1.6L).

Design of Specific Member Types

SolidSawn Lumber

Solidsawn lumber is the most common material for residential framing. The NDS provides separate design tables for clearspans, joists, and studs. Key points include:

• Use of the design value for the worst case from the speciesgrade table.
• Application of the size factor only when the depth exceeds 8in. for bending and compression.
• Consideration of deflection limits: L/360 for floors, L/240 for roofs, or as dictated by architectural requirements.

Engineered Wood (GLULAM, LVL, PSL)

Engineered products are designed for higher strength and stability. The NDS requires:

• Verification of the design values supplied by the manufacturer (often already factored for size and duration).
• Use of the effective span method for glulam beams where a portion of the beam is considered supported by internal glue lines.
• Consideration of lateral-torsional buckling for long, slender beams, using the effective length factor (K) from the tables.

Panel Products (OSB, Plywood)

Panelized sheathing is covered in Chapter13 of the NDS. Design parameters include the panels effective span, the number of layers, and the direction of the grain relative to the load path. Shear strength is governed by the panels shear modulus and allowable shear stress, which is reduced by the C_D factor for longduration loads.

Connections

Connections are a critical component of wood design because failure often initiates at the joint. The NDS provides design equations for:

• Singlescrew, nail, and bolt fasteners, including withdrawal resistance and shear capacity.
• Metal plate connectors (e.g., joist hangers, beamtopost plates) with design values taken from the manufacturers tested data.
• Adhesive connections for glulam splices, where the design shear stress is limited by the adhesives allowable stress and the effective bonded area.

For each connection type the designer must check:

1. Direct shear of the fastener.
2. Withdrawal of nails or screws from the wood.
3. Combined shearwithdrawal (when applicable).
4. Embedding strength for screws set into concrete or masonry.

Connection design also incorporates the load path factor (C_c) to reflect the distribution of forces within a multimember assembly.

Special Provisions

Fire Resistance

The NDS outlines two methods for fire design:

• Prescriptive firerating of members (e.g., 1hour firerated 2by8 studs) with reduction factors for strength loss at elevated temperatures.
• Performancebased design using fireexposure tests and the temperaturestrength curves supplied by the manufacturer.

Moisture and Decay

When wood will be exposed to high humidity or potential decay, the specification requires the use of the moisture adjustment factor (C_M) and, if applicable, a decay factor (C_D). Preservativetreated lumber and naturally durable species have separate factor tables.

LongTerm Deflection

To control longterm sag, the NDS recommends applying a creep factor (K_c) to the calculated instantaneous deflection. Creep may be limited to 0.3% of span for floors and 0.5% for roof decks.

Implementation and Compliance

Compliance with the 2005 NDS is typically demonstrated through one of three pathways:

1. Standard Design using the tables, formulas, and charts in the NDS directly.
2. Custom Design performing a detailed analysis (e.g., finiteelement modeling) and documenting that the analysis satisfies the NDS criteria for each condition.
3. Reference Designs employing manufacturerprovided preengineered systems that have been approved as conforming to the NDS.

For buildingofficial approvals, the NDS must be referenced in the construction documents, and any deviations require a formal variance backed by engineering judgment.

Significance for the Industry

The 2005 edition represented a major update over the 1995 version. It introduced LRFD provisions, clarified the handling of engineeredwood products, and harmonized many of the adjustment factors with the International Building Code. Its influence can be seen in several areas:

• Greater Design Flexibility designers can choose either allowablestress or LRFD methods while maintaining consistent safety levels.
• Improved Material Utilization refined size and duration factors allow for optimized member sizing, reducing material waste.
• Enhanced Safety more rigorous connection equations and firedesign guidance help prevent catastrophic failures.
• Better Compatibility the NDS aligns with other design guides (e.g., the Design Guide for Wood Frame Construction) and with the National Building Code of Canada, facilitating crossborder projects.

Resources and Further Reading

Key resources for professionals seeking deeper insight into the 2005 NDS include:

• American Wood Council Official NDS PDF
• Design Guides Series (e.g., LG1, PG2)
• International Building Code (IBC) for load combination compatibility.
• Local building department handbooks to verify jurisdictional adoption and any amendments.

Professional societies such as the Timber Design Council and university woodscience departments often host webinars and workshops that discuss practical applications of the NDS.