Structural Wood Screws Entering the U.S. Market: Why AC233 Matters

Understanding the Difference Between Standard Wood Screws and Proprietary Structural Wood Screws

With the rapid development of wood-frame residential construction, Mass Timber, Cross-Laminated Timber (CLT), and prefabricated wood construction in the United States, wood connection technologies are continuously evolving.

Traditionally, wood structures mainly relied on nails, bolts, and conventional wood screws for connections. Today, due to their high load-carrying capacity, flexible installation methods, and excellent structural performance, Structural Wood Screws are increasingly used in modern timber construction projects.

At the same time, ICC and ICC Evaluation Service (ICC-ES) have continued to develop technical evaluation approaches for structural wood screws, highlighting the increasing importance of reliable engineering data and performance evaluation for structural fastening products entering the U.S. market.

1. Which Wood Screws Require AC233 Evaluation?

Not all wood screws used in the U.S. construction market follow the same evaluation pathway.

Depending on product geometry, dimensional systems, and intended structural applications, wood screws can generally be divided into two categories:

1. Traditional / Standard Wood Screws

2. Proprietary / Non-standard Structural Wood Screws

(Comparison Image: Standard Wood Screws vs Proprietary Structural Wood Screws)

The National Design Specification (NDS) for Wood Construction provides design provisions for traditional wood connection products, including nails, bolts, and wood screws.

For traditional wood screws that comply with established dimensional systems, their head configurations, dimensions, and basic characteristics have been standardized and have been incorporated into conventional wood connection design practices.

Typical examples include:

• Flat Head Wood Screws;

• Oval Head Wood Screws;

• Pan Head Wood Screws.

These products generally fall within traditional wood screw systems, such as those covered by ASME B18.6.1 Wood Screws, and may be evaluated using established design provisions.

However, with the development of Mass Timber, CLT, engineered wood products, and advanced timber construction systems, a large number of Proprietary Structural Wood Screws have been introduced.

These products often incorporate specialized designs, such as:

• Full-thread configurations;

• Large-diameter designs;

• Washer head configurations;

• Double-thread systems;

• Self-drilling or self-tapping features;

• Specialized thread geometries or cutting features.

Compared with traditional wood screws, these products may differ significantly in geometry, installation methods, load-transfer mechanisms, and structural applications.

For these products, traditional wood screw design methods may not fully represent their actual structural performance.

To address this need, ICC-ES developed:

AC233 — Acceptance Criteria for Dowel-Type Threaded Fasteners Used in Wood

AC233 provides a systematic evaluation pathway for dowel-type threaded fasteners that are outside the scope of traditional standard wood screws and are intended for structural applications.

It is commonly used to evaluate proprietary structural wood screws with special geometries, structural applications, or products requiring code-recognized design values.

AC233 is not simply a single test method. Instead, it provides a comprehensive evaluation framework supporting:

• Structural performance verification;

• Wood connection design;

• Establishment of allowable loads or design strengths;

• ICC-ES Evaluation Report (ESR) applications;

• Technical documentation and market acceptance.

Note: Lag Screws (hex-head wood screws) are generally treated as a separate fastener category under NDS and should not be considered the same as traditional wood screws or proprietary structural wood screws.

2. What Performance Properties Are Evaluated Under AC233?

The structural performance of wood screws depends not only on the strength of the fastener itself but also on factors including:

• Screw geometry;

• Wood species and density;

• Installation direction;

• Connection configuration;

• Edge distance, end distance, and spacing requirements.

Depending on product design and intended applications, AC233 evaluation typically includes the following key performance properties:

(Performance Image)

1. Fastener Shear Strength

This test evaluates the shear strength of the screw body under shear loading and provides fundamental data for fastener strength evaluation.

2. Fastener Tensile Strength

This test evaluates the axial tensile capacity of the screw and determines the ultimate tensile resistance of the fastener itself.

3. Bending Yield Strength

Structural wood screws typically experience bending deformation when subjected to lateral loads.

Bending yield strength is an important parameter used in NDS yield-mode analysis and directly influences connection ductility and failure behavior.

4. Withdrawal Capacity

Withdrawal testing evaluates the resistance of the threaded portion of the screw being pulled out from the wood member.

Withdrawal performance is affected by:

• Wood specific gravity;

• Thread diameter;

• Effective embedment length;

• Installation direction;

• Grain orientation.

It is one of the most important structural design properties for wood screws.

5. Head Pull-through Capacity

This test evaluates the resistance of the screw head penetrating through the wood-based side member under tensile loading.

This property is particularly important for:

• Wood panels;

• OSB;

• Plywood;

• Roof assemblies;

• Wall systems;

• Mass Timber applications.

6. Lateral Load Capacity

Through wood-to-wood or steel-to-wood connection testing, lateral load capacity evaluates the load resistance and deformation behavior of individual screws under lateral loading.

Depending on product geometry and evaluation scope, test results may also be used to verify whether NDS calculation methods are applicable to the product series.

3. Additional Factors Affecting AC233 Evaluation

AC233 evaluation is not simply a process of installing screws into wood and loading them until failure.

To ensure reliable, comparable, and engineering-usable data, various test parameters must be carefully controlled, including:

• Wood species and grade;

• Wood specific gravity;

• Wood moisture content;

• Screw diameter, length, and thread geometry;

• Effective embedment length;

• Installation angle;

• Load direction relative to grain direction;

• Wood-to-wood or steel-to-wood connection configuration;

• Edge distance, end distance, and spacing;

• Predrilling requirements and hole dimensions;

• Compatibility between screw head and connected materials.

Although specific gravity and moisture content are not direct fastener strength properties, they are essential parameters for ensuring test reliability and data comparability.

For products with special thread geometries, non-standard thread root configurations, different head designs, or multiple size ranges, a properly designed test matrix should be developed based on the product family and intended evaluation scope.

4. Why Are Manufacturers Exporting Structural Wood Screws to the U.S. Market Paying More Attention to AC233?

With increasing adoption of Mass Timber, CLT, glulam, LVL, and prefabricated timber construction, structural wood screws are evolving from general hardware products into critical structural components.

For U.S. customers, product selection is no longer determined only by:

• Product dimensions;

• Pricing;

• Manufacturing capacity.

Increasingly, engineers, contractors, distributors, and project owners are evaluating:

• Whether structural performance has been verified through recognized testing;

• Whether engineering design data are available;

• Whether test results represent actual application conditions;

• Whether the product supports wood-to-wood or steel-to-wood connection design;

• Whether sufficient technical documentation exists for ICC-ES ESR evaluation;

• Whether different diameters, lengths, and head configurations can be covered within a product family.

For manufacturers, establishing a complete performance database allows products to move from general fasteners toward engineered structural connection systems.

5. Is AC233 a Fixed Testing Package?

AC233 does not require every structural wood screw product to undergo an identical testing program.

The evaluation scope and sample matrix should be determined based on:

• Screw diameter and length range;

• Thread and shank configuration;

• Head type;

• Material and heat treatment;

• Protective coating and service environment;

• Intended wood-to-wood or steel-to-wood applications;

• Applicable wood species and specific gravity range;

• Use in sawn lumber, glulam, LVL, or CLT;

• Installation direction and predrilling requirements;

• Target design values and ESR evaluation scope.

A proper technical review before testing helps manufacturers avoid insufficient product coverage and unnecessary repeated testing.

6. ICTT’s AC233 Testing and Technical Support

ICTT is a professional laboratory specializing in construction product testing, evaluation, and certification support.

ICTT continuously follows developments in ICC-ES Acceptance Criteria, U.S. building codes, and advanced wood connection technologies.

For structural wood screw manufacturers, ICTT provides AC233-related testing and technical support, including:

• Fastener shear strength testing;

• Fastener tensile strength testing;

• Bending yield strength testing;

• Withdrawal capacity testing;

• Head pull-through capacity testing;

• Wood-to-wood lateral load testing;

• Steel-to-wood lateral load testing;

• Wood specific gravity and moisture content testing;

• Product family evaluation;

• Test matrix development;

• AC233 test plan preparation;

• Technical support for ICC-ES ESR applications.

Based on product drawings, specifications, and intended applications, ICTT can assist manufacturers in selecting representative products, optimizing test programs, and establishing reliable engineering data for U.S. market entry.

7. From Manufacturing Screws to Providing Engineering Data

The structural wood screw industry is moving toward a more specialized, standardized, and engineering-driven future.

Future competition will depend not only on manufacturing capability but also on:

• Testing capability;

• Technical documentation;

• Engineering data;

• Product evaluation experience.

For manufacturers seeking to enter the U.S. timber construction market, understanding AC233 requirements and establishing complete, traceable performance data can significantly improve product competitiveness and provide a foundation for acceptance in professional construction projects.

ICTT will continue to monitor international building codes, ICC-ES acceptance criteria, and wood connection technologies, providing professional testing and technical services for structural wood screw manufacturers and construction fastening product suppliers worldwide.

For more information about AC233 evaluation requirements or structural wood screw testing programs, please contact ICTT.