Wood Screw Connection Design: Combined Shear and Withdrawal Loads
- nick8284
- Jul 27, 2025
- 2 min read
Wood screws are widely used in timber construction for their convenience and versatility. Unlike lag screws, which typically handle heavy loads, wood screws are often used for lighter-duty connections — but that doesn’t mean their design can be overlooked. When a wood screw is subject to both lateral (shear) and withdrawal (tension) forces, proper design is essential to avoid brittle failures.
In this post, we’ll walk through the essentials of wood screw design under combined loading, following the principles of the NDS (2018). We’ll outline what influences screw performance, the key considerations for combined loads, and where these connections are commonly used.
Why Combined Loading Matters
Many real-world connections don’t experience purely shear or purely withdrawal forces. For example:
Roof framing connections often see uplift (withdrawal) and lateral loads from wind or seismic forces.
Ledger-to-rim board attachments for decks can experience both vertical shear and tension.
Bracing and panel fastenings may introduce angled forces that combine the two effects.
In these cases, a wood screw must be sized and detailed for both failure modes acting together—not in isolation.
Key Factors in Wood Screw Design
1. Load Characteristics
Start by identifying the shear and withdrawal forces acting on the screw. These come from your structural analysis and load combinations.
2. Wood Properties
Species, grade, and specific gravity all affect screw performance. Denser wood provides higher withdrawal strength but also requires careful pre-drilling to avoid splitting.
3. Screw Size and Penetration
Diameter and thread penetration length matter. The longer the threaded portion embedded in the wood, the greater the withdrawal resistance. Minimum penetration depths are required to ensure full thread engagement.
4. Installation Quality
Proper pilot holes, correct tightening, and compliance with spacing and edge distance requirements are critical to achieving the calculated capacities.
How Combined Loading is Checked
Rather than adding shear and withdrawal demands together, the NDS requires an interaction check. This ensures the combined effect of both forces doesn’t exceed what the screw can handle. In practice, this means:
Calculate the allowable withdrawal capacity (based on wood density, screw size, and penetration).
Calculate the allowable lateral (shear) capacity (based on screw size, wood thickness, and connection geometry).
Check that the applied forces in withdrawal and shear together stay within the combined capacity limits.
Where It’s Used
Wood screws under combined loading appear in:
Roof-to-wall or wall-to-floor connections
Deck ledger attachments
Shear wall hold-down details
Panelized roof systems with angle brackets or inclined forces
Example Problem
(Solutions Provided Using CalcBook) Problem Statement:
Design Basis:
Design Inputs:
Demand Calculation:
Fastener Penetration Length:
Load Angle & Bearing Lengths:
Lateral Capacity Inputs:
Nominal Lateral Capacities:
Controlling Lateral & Withdrawal Capacities:
Adjustment Factors:
Design Strength:
Conclusion
Combined shear and withdrawal design for wood screws is more than a quick lookup—it requires considering multiple variables, detailing rules, and interaction checks. Skipping this can lead to brittle failures or unsafe designs.
CalcBook makes this simple by:
Handling both withdrawal and lateral design checks automatically
Applying all NDS adjustment factors in the background
Producing clear, professional reports for your records
👉 Try CalcBook today for accurate, transparent timber connection design.