Do structural screws have shear strength?

Structural screws are often used in construction projects as an alternative to nails, bolts, or welds. They provide strong connections between materials like wood, steel, and concrete. A key consideration when using structural screws is their ability to resist shear forces.

What is shear strength?

Shear strength refers to the ability of a fastener or material to resist forces that act perpendicular to the fastener’s axis. When two materials are joined with a fastener like a screw, shear forces try to make one material slide over the other. High shear strength allows the materials to stay firmly connected under load.

For structural screws, shear strength indicates how much sideways force the screw can withstand before breaking or being pulled out of the surrounding materials. This is an important factor in structural integrity and performance.

Do structural screws have good shear strength?

Yes, structural screws are designed to provide excellent shear strength along with tension and pullout strength. Here are some key reasons why:

• Thread design – Structural screws have specialized threads meant to maximize hold in the substrate materials. The threads resist lateral movement and slipping.
• Screw materials – Steel is commonly used for its high shear strength. Stainless steel provides corrosion resistance without sacrificing shear capacity.
• Head design – Large screw heads distribute force over more material for extra resistance.
• Screw coatings – Some coatings like double barrier zinc improve grip for higher shear strength.

How is shear strength tested and rated?

Shear strength values for structural screws are determined through standardized testing methods. Here are some details on shear testing procedures:

• Single shear – The screw joins two materials and force is applied perpendicular to the screw shank. This tests shear strength for one screw.
• Double shear – Force is applied to two screws joining three materials. This simulates real-world connections.
• Pushout tests – Force bears down on a screw driven into the end of a board, testing withdrawal and slip resistance.
• Block shear – Measures shear strength of the connected materials as well as the screw.

Based on lab test results, screws receive shear strength ratings in pounds or newtons. Manufacturers publish these ratings so engineers can choose appropriate screws for their projects.

How does screw size impact shear strength?

In general, larger screws have higher shear strength capacities. Some factors related to screw size and strength include:

• Screw diameter – Wider screws can better distribute shear forces.
• Thread area – More threads grip material more effectively.
• Material volume – Larger screws contain more shear-resistant steel.
• Embedment depth – Longer screws dig deeper into materials.

However, larger fasteners are not always better. Oversized screws can cause cracking or splitting in wood and other substrates. The optimal screw size depends on the materials being connected and other design factors.

Shear strength vs. screw diameter

Screw Diameter	Shear Strength (Single)
#8	1,060 lb
#10	1,700 lb
1/4″ Lag Screw	3,200 lb
5/16″ Lag Screw	4,300 lb

This table displays the general relationship between increasing screw diameter and higher shear strength.

How does thread design impact shear strength?

Specialized threads are a key factor in the superior shear strength of structural screws. Some common thread features that boost shear performance include:

• Type of thread – Certain thread shapes like square and tri-lobular grip better than V-threads.
• Thread pitch – Smaller distance between threads increases shear resistance.
• Thread depth – Deeper threads provide more gripping power.
• Number of thread starts – More threads spread shear forces over a larger area.
• Thread fit – A tight thread-to-hole fit creates friction against shear.

Manufacturers can optimize these thread parameters to design screws with exceptional shear strength across different materials and applications.

What are typical shear strength values for structural screws?

Shear strength values vary based on factors like screw size, materials, coatings, and design. But in general, structural screws exhibit high shear strengths as follows:

• Small wood screws – Around 1,000 lb shear strength
• Large timber screws – Over 5,000 lb shear strength
• Light gauge steel screws – Around 2,000 lb shear strength
• Concrete screws – 1,000 to 4,000 lb shear strength

These shear strength capacities often exceed the shear strength of the connected materials like wood and light gauge steel. This helps ensure the screw itself does not shear and fail before the surrounding materials.

Average Shear Strength by Screw Type

Screw Type	Average Shear Strength
Wood screw into lumber	1,500 lb
Lag screw into wood	3,500 lb
Self-drilling screw into steel	2,500 lb
Concrete screw	2,000 lb

This table displays typical shear strength values for common structural screw types.

How can you increase shear strength?

If an application requires higher shear strength than standard screws can deliver, here are some ways to increase performance:

• Use larger diameter, longer screws
• Choose screws with specialized high-strength threads
• Use additional screws spread over more area
• Pre-drill clearance holes to prevent wood splitting
• Use threaded inserts or metal sleeves in soft materials
• Select screws with shear-boosting coatings and treatments

Bolt-shear connections and structural adhesive can also supplement screws when extreme shear loads need to be resisted.

Conclusion

Structural screws are engineered to provide substantial shear strength for structural connections. Key factors that contribute to their excellent shear resistance include optimized thread patterns, high-strength steel alloys, and design features like oversized heads and protective coatings. With shear strength ratings typically ranging from 1000 to 5000 pounds depending on screw size and type, structural screws can withstand significant lateral loads. Understanding the shear performance of different screw types and sizes allows engineers and builders to confidently utilize them as shear connectors.