Earthquake demand

The first step is to determine the earthquake demand on all of the components that make up the engineering systems within the building. This is the procedure to determine earthquake demand:

1. Classify the building importance level and component categories.
2. Determine the earthquake load demand (static forces).

Find out about:

• Component categories
• Calculating earthquake load demand
• Determining relative displacement
• Other design considerations

Component categories

NZS 4219:2009 Table 2 sets out general objectives and performance requirements for each component in an engineering system. This requires the designer to classify all components on the basis of the consequences of their expected performance under earthquake actions.

(From NZS 4219:2009, provided by Standards New Zealand under licence 001149)

Note that category P5 only applies to importance level 4 buildings as this is a requirement of NZS 1170.5:2004. Design loads derived for components in lower importance level buildings may be increased unnecessarily if category P5 is applied instead of P6 or P7 to other components in importance level 1, 2 and 3 buildings.

Calculating earthquake load demand

The earthquake load demand (F) on a component for each design criterion should be calculated by multiplying the lateral force coefficient (C) by the operating weight of the component (W) in the equation:

F = C x W

The lateral force coefficient can be found using the component’s height (C_H), the building’s earthquake zone factor (Z), the component performance factor (C_p) and the component risk factor (R_C) in the equation:

C = 2.7 x C_H x Z x C_p x R_C

where C_H is 3.0 for components that are above the ground floor or 1.0 for components that are on or below the ground floor. C need not be greater than 3.6.

Earthquake zones

The earthquake zone factor (Z) represents the relative level of seismicity for the building’s location in New Zealand. It can be determined from NZS 4219:2009 or NZS 1170.5:2004 or derived from the following map. These must all be modified in accordance with Amendment 10 of the Building Code citation of NZS 1170.5:2004. This states that, for buildings in the Canterbury Earthquake Region with a building structure period less than 1.5 seconds, the zone factor should not be less than 0.3.

Relative seismicity for the design of seismic restraints for engineering systems. (From NZS 4219:2009 Figure 2, provided by Standards New Zealand under licence 001149) 

Performance factor

NZS 4219:2009 Table 4 provides for a performance factor (C_p), which can be used to determine the load on each part of the restraint system.

(From NZS 4219:2009, provided by Standards New Zealand under licence 001149)

NZS 4219:2009 Appendix C shows normative performance factors for specific components and their specific type of installation.

(From NZS 4219:2009, provided by Standards New Zealand under licence 001149)

Risk factor

The component risk factor (R_C) can be determined from NZS 4219:2009 Table 5 using a known importance level and component category. Note that, following the 2010–11 earthquakes, the component risk factor for buildings in the Canterbury Earthquake Region should not be less than 0.33. 

(From NZS 4219:2009, provided by Standards New Zealand under licence 001149)

Determining relative displacement

Components connected to the building structure at more than one level must be designed to sustain the relative seismic displacement (D) between the levels for the appropriate design criterion.

The displacement can be determined from the building’s calculated design displacement, where known. If the displacement is not known, it may be calculated using the component risk factor (R_C), the height between fixing points (H_Z) and the following equation:

D = 0.025 x R_C x H_Z

Note that, when using this equation, R_C should be no greater than 1.0.

The non-specific design pathway allows relative displacement to be accommodated using flexible joints. The joints must be able to accommodate the relative displacements between fixing points. Where a component is attached to the structure using rigid joints that are not designed to allow for the relative displacement, the component must follow the specific design pathway.

Other design considerations

In building design, the role of engineering systems in protecting life and property and providing safe egress from the building should be considered along with seismic resistance of services and their components. The basic concepts of a building layout should be examined against the likely effect of earthquakes on the particular building.

The effect of heavy equipment on the structure and other building components should be considered. Heavy equipment breaking loose is a potential source of injury and building damage. Locating heavy plant at a high level within the building not only adds to the earthquake loading, it may require pipes and electric power to run the full height of the building.

The seismic resilience of connections to public utility services should be assessed. The design should aim to reduce the probability of damage and provide for connection to temporary alternative services when required. Vehicle access, mains pipe connections, the position of shut-off valves, meters, risers and main reticulation are all important.

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