Seismic design

How earthquakes load a steel building

During an earthquake, the ground moves and the building's mass resists that motion, producing inertia forces. For a light pre-engineered building with metal cladding the mass is small, so seismic forces are often lower than wind forces and wind governs the frames. Seismic design becomes critical where mass is high – concrete or masonry walls, mezzanine floors, heavy roof equipment, cranes – or where the hazard is high, such as the western United States, Canada's west coast, New Zealand, Japan, Turkey, Chile or Indonesia. Even when wind governs strength, seismic detailing requirements may still apply.

Code approach by region

Choosing the seismic system for a PEB

Two strategies are possible. In low-ductility design the frames are designed for higher forces with modest detailing requirements – for example "steel systems not specifically detailed for seismic resistance" with R = 3 (permitted in Seismic Design Categories A–C under ASCE 7-22), ordinary moment frames, NBC "conventional construction", μ ≈ 1–2 in AS 1170.4, or DCL in EN 1998. This usually suits light PEBs because wind already requires substantial strength. In ductile design lower forces are used but connections, member slenderness and bracing must meet capacity-design rules (AISC 341, CSA S16 Clause 27, NZS 3404, EN 1998). Tapered, thin-web members typical of PEB have limitations in high-ductility systems, so in high-hazard zones the system choice must be confirmed early.

• Longitudinal direction: rod or angle bracing must meet code limits on slenderness and connection strength; tension-only rod bracing has restrictions in high seismic categories.
• Connections in seismic force paths are often designed for overstrength (Ω0) forces.
• Diaphragm action of the roof and anchorage of walls to the frame must be checked.
• Drift must be checked with amplified deflections (Cd in ASCE 7-22) and limited per code.
• Non-structural components (heavy services, mezzanine handrails, cladding) need seismic anchorage.

Seismic weight and where the forces act

Seismic force is proportional to the effective seismic weight – the mass that moves with the structure. For a PEB it is built up from the items below (ASCE 7-22 wording; other codes use similar rules with their own fractions):

In a plain PEB most mass sits at roof level, so the base shear is applied at the eave. Mezzanines and crane bridges are applied at their own levels, which raises column moments and bracing forces. For single-storey steel moment frames in high seismic design categories, ASCE 7-22 permits ordinary moment frames only within height and roof and wall weight limits, which is one reason why concrete wall panels can force a change of system.

Seismic data needed at quotation

1. Project location (coordinates) and governing code edition.
2. Risk / importance category of the building.
3. Geotechnical site class or soil class – this can change seismic forces several-fold.
4. Masses: mezzanines, cranes, wall materials, roof-mounted equipment and solar panels.
5. Preferred seismic system and any client or authority requirements.

World Steel's engineering team applies the destination code and states the seismic parameters and system on the drawings and calculations. Seismic combinations are described in design loads, and drift limits in serviceability and deflection. Connection detailing is covered in connection design.