Seismic structural attachment options for steel and wood framing

Seismic structural attachments for steel and wood framing are standardized hardware and anchorage details that transfer brace forces into the primary structure without field welding or ad‑hoc connections. The right choice depends on: substrate (steel beam, bar joist, wood joist, concrete), load level, and governing standard (typically NFPA 13, ASCE 7, and local amendments).​

Steel framing options

For structural steel beams, bar joists, and channels, common seismic structural attachments include:​

• Clamp‑on beam attachments with cone‑point or break‑off-head set screws (e.g., devices similar to TOLCO Fig. 800/825/828) that grip the flange without drilling or welding and provide a concentric bolt hole for the brace.​

• Bar‑joist and truss adapters that wrap the chord or seat and use non‑friction bearing clamp action to provide a listed connection for rigid bracing where welding is undesirable.​

• Structural inserts and weld‑on lugs/plates for heavy loads, sometimes combined with strut (Unistrut or similar) as an intermediate element to which the brace is bolted.​

Wood framing options

For wood joists, beams, and stud walls, seismic attachments usually rely on nails/screws into framing and, where needed, through‑bolts:​

• Listed wood‑joist seismic attachments (e.g., hardware analogous to TOLCO Fig. 131) that clamp around or bear against a wood joist and present a concentric bolt hole for rigid braces, avoiding splitting and eccentricity.​

• Straps, hold‑downs, and hurricane/seismic ties that connect braces, collectors, or diaphragm edges to studs, plates, and beams (similar to Simpson Strong‑Tie seismic and hurricane ties, stud tie‑downs, and strap anchors).​

• Plywood shear panels and blocking with framing anchors in residential work, where the “attachment” is the nailing schedule tying wall, floor diaphragm, and foundation together per local seismic strengthening details.​

Connection to concrete and mixed substrates

When bracing must attach to concrete (slab, beam, or foundation), the structural attachment is typically a steel bracket plus a rated anchor system:​

• Cast‑in or post‑installed anchors (wedge, screw, epoxy) sized and embedded per manufacturer ESR/report and seismic tables, with a welded or bolted bracket that provides a concentric hole for brace hardware.​

• Deck inserts or pan‑deck anchors for concrete‑filled metal deck, giving a threaded stud or nut below the slab to which strut or brace hardware connects.​

• Powder‑actuated or screw fasteners to steel decks or joists used with strut‑based bracing systems, provided the system is tested and detailed for seismic applications.​

Key selection and detailing points

When choosing and detailing seismic structural attachments for both steel and wood framing, typical best practices are:​

• Use listed, tested components from a single manufacturer where possible, and follow that manufacturer’s seismic design manual (brace load tables, allowable fasteners, edge distances).

• Ensure concentric loading: brace line of action should pass through the connection’s designed hole/seat, avoiding large eccentricities that NFPA 13 and ASCE 7 warn against.​

• Verify the base structure (steel flange thickness, wood member size/grade, concrete strength and thickness) can accept the required fasteners or anchors and that local code/plan review accepts the hardware type.​

Practical next steps

For a specific project, a practical workflow is:

• Determine brace loads from the seismic bracing design (or software) and identify each substrate: steel W‑beam, bar joist, wood joist, stud wall, or concrete.​

• For each condition, select a cataloged structural attachment rated for that substrate and load, then check the required fasteners/anchors and spacing against the project’s structural notes.​

• Coordinate with the structural engineer to confirm acceptability of clamp‑on vs. welded vs. anchored solutions, particularly for wood members and high‑demand braces in higher seismic design categories.​