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Busway Seismic Bracing: When Is It Required and How Does It Work?

By DHYN Engineering Team  ·  October 7, 2026

Quick answer: A bus duct needs seismic bracing whenever the building code treats it as a nonstructural component that must stay put in an earthquake, which in practice means most projects in Seismic Design Category C and above under ASCE 7. Bracing means cable or rigid struts tying the duct to the building structure, both across the run and along it. Short hangers under 300 mm and low seismic zones are often exempt.

If your project sits anywhere with an earthquake map, the busway question comes up late and hurts. The duct is already drawn, then the structural engineer asks for bracing calcs. Here is how the requirement actually works, and what a compliant installation looks like.

When does the code actually require seismic bracing on a bus duct?

In the US, ASCE 7 Chapter 13 covers nonstructural components, and a busway falls squarely in it. Seismic Design Category A and B buildings are mostly exempt. From Category C upward, expect a designed bracing system, and for essential facilities like hospitals the importance factor Ip goes to 1.5, which increases the design force by half.

Two common exemptions are worth knowing. Components hung on short, flexible hangers, typically 300 mm (12 in) or less, can swing freely and usually escape bracing. Very light runs can also fall below the weight thresholds. Outside the US, codes like Eurocode 8 follow the same logic: the heavier and more critical the duct, the more restraint it needs.

What does a brace on a busway actually look like?

Two directions, always. Transverse braces stop the duct swinging sideways, longitudinal braces stop it sliding along its own axis. Both run from the duct's hanger assembly up to the building structure, either as tension cables or rigid struts.

Brace angle matters more than people think. A brace steeper than about 45 degrees barely restrains anything, so most guidelines keep the angle between 1:1 (45 degrees) and 2:1 (about 63 degrees from horizontal). The top end anchors into concrete or structural steel. Not the ceiling grid, not the roof deck, not the duct housing itself. We have seen braces clamped to the busway's own cover screws. That anchor will tear out in the first real shake.

How far apart should seismic braces go?

A common starting point is transverse braces every 9 to 12 m (30 to 40 ft) and longitudinal braces at roughly double that spacing, with a brace within about a meter of each end of the run and at every elbow or tee. The real spacing comes out of the project's force calculation, not a table.

The load math is simple in concept: take the duct weight per meter, multiply by the seismic coefficient from ASCE 7, and check that the braces and anchors carry it with margin. A 2500 A copper duct can weigh 60 kg per meter or more, so the forces add up fast. One thing people miss: at an expansion fitting, the duct is designed to move. Bracing hard across that joint defeats it. Brace each side independently.

What changes on vertical risers?

Vertical bus duct already hangs on spring supports that carry its weight while allowing thermal movement. Seismic design adds lateral guides at each floor so the riser cannot whip sideways, but those guides must not clamp the duct rigidly.

This is where normal busway hangers and supports and seismic hardware can fight each other. A riser clamped tight at every floor cannot grow thermally, and a riser with no lateral restraint becomes a pendulum. The fix is sliding guides, detailed on the drawings, not improvised on site.

How does a manufacturer prove a busway survives an earthquake?

By shake table testing. In North America the usual route is ICC-ES AC156, which puts the component through a required response spectrum on a triaxial table. Substation-grade bus duct is often qualified to IEEE 693 instead. Ask for the test report by name, with the tested configuration matching yours.

A report on a 1600 A aluminum duct does not automatically cover your 4000 A copper riser. Joints and tap-off outlets are the weak points, so check they were in the tested assembly. After any real earthquake, recommission: insulation resistance, joint torque spot checks, and a thermal scan under load.

The mistakes that fail inspection

Bracing to the ceiling grid is the classic. The grid is not structure. Next: only fitting transverse braces because they are easier, leaving the duct free to slide along its length until the end fitting tears off. Then over-restraining, clamping every support rigidly so thermal expansion buckles the housing or loosens the joints. And finally forgetting the elbows, which concentrate force exactly where the duct changes direction.

Get the seismic requirement onto the busway submittal drawing early. Retrofitting braces around an installed duct, in a live ceiling full of other services, is the expensive way to learn this.