How Do Site Wind and Seismic Criteria Affect a Prefabricated Steel Frame? | ESS

How Lateral Load Reaches the Ground

A light steel frame resists lateral load through a chain that must be continuous.

• Roof and floor diaphragms collect the load and transfer it to the walls
• Braced or sheathed wall panels act as vertical shear elements
• Hold-downs and anchors transfer overturning tension into the foundation
• Any break in the chain, such as a large opening, needs a designed transfer

What Changes With Higher Criteria

The structure does not simply get heavier; the detailing changes.

• More braced bays, or sheathing upgraded to a structural grade
• Larger hold-downs, strap ties and anchor bolts at the load path ends
• Stiffer floor and roof diaphragms to limit drift
• Ductile detailing and capacity design where seismic criteria govern

Coordination Points That Are Often Missed

Lateral design fails in the interfaces between systems.

• Foundation design must match the uplift and shear the frame imposes
• Large glazed openings must not interrupt the only braced line
• Service penetrations must not cut bracing or diaphragm chords
• Site fixing of sheathing must follow the specified pattern and fixing schedule

Frequently Asked Questions

Does sheathing count as bracing?

It can, if it is a structural grade board fixed to a specified pattern. If the sheathing is not structural, bracing must be provided by other means.

Can wind design be copied from a nearby project?

Only if the wind speed, exposure, altitude and geometry match. Copying bracing between sites is one of the most common causes of under-design.

Why does seismic design change connections more than members?

Energy dissipation happens at connections and detailing. Capacity design protects the members by making the connections and hierarchy behave predictably.