6 Best Structural Washers for Shimmed Connections

Structural connections are only as strong as the components holding them together, and when shims enter the equation, the margin for error disappears. A compromised connection doesn’t just fail; it compromises the entire load path of a structure, leading to costly call-backs or catastrophic failure. Choosing the right load-rated washer isn’t about following the path of least resistance—it’s about ensuring the assembly maintains its tension and integrity under stress. Understanding these specific hardware components is the difference between a building that settles correctly and one that faces premature structural fatigue.

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Simpson Strong-Tie SSTB: For Anchor Bolts

The Simpson Strong-Tie SSTB (Square Tight Bolt) is engineered specifically for use with anchor bolts in concrete foundations. Its primary advantage lies in the high-strength steel composition designed to withstand the rigorous tension requirements of modern seismic and wind codes.

When bolting sill plates to foundation walls, these washers provide the necessary bearing surface to prevent the bolt from pulling through the lumber. They are a staple in high-seismic zones where the connection must resist significant lateral force without the plate crushing the wood fibers underneath.

Always verify the bolt diameter before selecting the SSTB version, as a mismatch results in an ineffective bearing surface. The bottom line is simple: if the project requires a code-compliant foundation connection, use the manufacturer-matched hardware to ensure the stamped load ratings remain valid.

ASTM F436 Hardened Flat Washer: The Go-To Standard

ASTM F436 is the industry benchmark for heat-treated, hardened steel washers intended for structural bolting. Unlike standard hardware store washers, these are specifically manufactured to withstand the high torque and clamping force typical of structural steel bolts.

These washers are essential when dealing with A325 or A490 bolts, as standard “soft” washers will deform or collapse under the necessary tightening load. Using an undersized or non-hardened washer in a structural connection is a recipe for premature bolt failure as the assembly loses its “pre-tension.”

When working on large-scale steel framing or heavy timber construction, verify the F436 marking on the washer face. It is the guarantee that the component is hardened enough to maintain the connection’s integrity throughout the building’s lifecycle.

Malleable Iron Beveled Washer: For I-Beam Flanges

When attaching hardware to the tapered flanges of an I-beam, the bolt head or nut cannot seat flush against the steel surface. Malleable iron beveled washers are the specialized solution designed to correct this slope, ensuring the load is applied squarely to the bolt axis.

These washers are tough enough to handle the immense pressure of ironwork without cracking, a common failure point for cheaper cast alternatives. The beveled design creates a flat plane for the nut, which is critical for achieving accurate torque readings during the bolting process.

If the assembly is not square, the bolt will experience bending stress in addition to tension, drastically reducing its load-carrying capacity. Always use beveled washers on tapered surfaces to prevent eccentric loading and potential fastener shear.

Simpson Strong-Tie BP Bevel Washer: Top Choice

The Simpson Strong-Tie BP series stands out for its high-quality construction and precise bevel angles. These are a contractor favorite for timber-to-steel connections where precise, heavy-duty shimming is required.

These washers excel in scenarios where a structural beam meets a column at an angle or when leveling a beam on a steel bracket. They provide a predictable, load-rated surface that keeps the bolt centered and the connection tight.

Avoid the temptation to use stacked flat washers to compensate for a bevel, as they are not designed to transmit load in that configuration. The BP series offers a cleaner, more reliable path to structural compliance.

MiTek Square Plate Washer: For Truss Connections

MiTek square plate washers are designed to provide a larger bearing surface area than standard round washers. In truss-to-wall connections, especially in high-wind zones, this extra surface area is vital for preventing the fastener from “pulling through” the chord.

These washers are particularly useful when wood members are slightly undersized or when dealing with over-bored holes that leave too much clearance. By distributing the load across a wider area of the lumber, they protect against fiber crushing under heavy uplift.

Inspect the washer edges for burrs before installation to ensure they sit flat against the wood. If a connection is under high stress, these plate washers are essential for maintaining the integrity of the fastener’s grip on the timber.

Fastenal Hardened Structural Washer: For Bulk Buys

Fastenal offers a consistent supply of F436 hardened washers that are ideal for larger jobs requiring volume and certification. Their hardware is reliable for contractors who need to maintain a strict inventory of code-compliant fasteners on the job site.

When bulk buying, verify that every batch meets the ASTM F436 standard, as mixing soft and hardened washers can lead to inconsistent connection performance. Keep them organized by diameter and grade to prevent installation errors during the final framing stages.

For high-volume structural work, rely on their documented specifications to satisfy building inspectors quickly. The bottom line: consistent hardware procurement is a key factor in keeping a project on schedule and up to code.

Why “Load Rated” Matters in Structural Work

Load-rated hardware is tested to withstand specific forces of tension, shear, and compression. Using non-rated hardware essentially means gambling with the building’s structural calculations, as there is no guarantee how the material will behave under extreme load.

When an engineer specs a connection, they assume the hardware will perform within the parameters set by the ASTM or manufacturer data. Deviating from these specs by using generic hardware can void engineering warranties and lead to failed inspections.

Structural work is governed by the principle that the connection must be as strong as the members it joins. Always prioritize hardware that carries a clear load rating to ensure the building remains stable during environmental stresses like high winds or seismic activity.

Flat vs. Beveled Washers: When to Use Each

Flat washers are intended for parallel, flat surfaces where the bolt head or nut sits flush against the structural member. They are the standard for 90-degree connections in wood framing or flat-plate steel assemblies.

Beveled washers are necessary whenever the surface is sloped, such as on the inner flange of an I-beam or when dealing with misaligned framing members. Using a flat washer on a beveled surface causes “side loading,” which can lead to bolt fatigue and eventual snapping.

Understanding the difference is fundamental to basic structural safety. Match the washer to the physical geometry of the connection to ensure that the clamping force is applied evenly along the length of the fastener.

How to Properly Shim a Structural Connection

Shimming should never involve stacking multiple thin washers, as this creates a “loose” connection that can shift under stress. If a gap needs to be filled, use a solid, structural-grade shim or a properly rated bevel washer designed for that specific angle and load.

When shimming, ensure the shim material is compatible with the surrounding lumber or steel to prevent galvanic corrosion. For example, avoid putting aluminum shims against pressure-treated wood, which will cause the metal to deteriorate rapidly.

The goal is a solid, metal-on-metal or wood-on-metal connection that does not move. Once the shim is in place, re-torque the bolt to the final specification to ensure the entire assembly is tight and uniform.

Reading the Specs: What ASTM F436 Means

ASTM F436 is a specification covering the chemical, mechanical, and dimensional requirements for hardened steel washers. It ensures that the washer is hard enough to withstand the forces exerted by high-strength structural bolts without deforming.

Look for the “F436” stamp on the washer, which provides verification that the product meets national standards. This is the gold standard for structural integrity, and it distinguishes professional-grade hardware from the softer, generic washers found in standard hardware bins.

Always keep documentation for the fasteners used on critical structural connections. If a building inspector asks about the grade of your washers, being able to point to the ASTM stamp or the box documentation provides instant credibility and ensures a smoother project sign-off.

Selecting the right hardware for structural connections is the mark of a professional. By understanding the mechanical requirements of these washers—from the hardness of F436 standards to the geometry of beveled plates—every connection gains the integrity needed to hold a building together for years to come.