Seismic Design Category

The Seismic Design Category is of great importance to everyone involved with MEP systems because it will determine whether seismic restraints are required or not, and if they qualify for exemption. Generally the structural engineer is responsible for determining the seismic design category for a project.

Seismic Design Category depends on:

*Occupancy Category (above)

*Long and Short Period Mapped Spectral Accelerations Parameters (S_D1 and S_DS)

S_DS & S_D1 are computed using the mapped long and short period spectral accelerations S₁and S_S respectively, and the project site class (below). S₁ and S_S values are mapped and can be obtained with the use of zip codes across the country and select international cities by the United States Geological Survey (USGS). The USGS also includes known fault lines into its calculated values. The worst case will be assigned to the project.

*Site Class

The type of soil and rock strata that directly underlies the building site. It ranges from A to F, progressing from the stiffest to the softest strata.

Long period values are generally applied to the buildings and other structures since they react more strongly to the long period excitation due to their relatively high mass and low stiffness. The code specifies the use of short period values when evaluating nonstructural components, such as pipe and duct, as they respond more strongly to the short period excitation due to their relatively low mass and high stiffness.

There are six seismic design categories that vary from A, the least stringent seismic restraint requirement to F, the most stringent,level of restraint required.*

Component Importance Factor

All MEP components must be assigned a Component Importance Factor that is designated as IP in the body of the code. This is done by the design professional who has responsibility for the MEP system in question. The design professional is also responsible for assigning the component importance factor to that system.

There are just two I_P values for MEP components, 1.0 and 1.5, neither of which are directly linked to the importance factor for the building structure.

Value I_P is 1.5 if the following life safety components are required to function after an earthquake:

1. Fire suppression systems in all occupancy categories where specifics are addressed by the National Fire Protection Association (NFPA).
2. Smoke evacuation systems in all occupancy categories.
3. Every component in occupancy category IV buildings.

Value I_P is 1.5 if components listed below contain hazardous (bio-hazardous) materials:

1. Bio-hazardous lab exhaust fans and ducts.
2. Process piping and equipment carrying flammable explosive or caustic material.
3. Duct and attached equipment carrying flammable explosive or caustic material.

Value I_P is 1.5 if components' failure would impair the continued operation or close an Occupancy Category IV facility. Even the failure of domestic water, sanitary and roof drain lines can flood a building and render it uninhabitable. So, all of the items listed above would typically apply to facilities in Occupancy Category IV, plus:

1. Plumbing systems including drain, waste and vent lines.
2. Air handling and conditioning systems.
3. Electrical supply and control systems.
4. Communication systems including servers and networks.

All other MEP systems that are not covered under items listed above may be assigned an I_P component importance factor of 1.0.

The component importance factor is very important to the designer responsible for selecting and certifying the seismic restraints for an MEP system or component. It is a key indicator as to whether a particular component will qualify for an exemption or not. If a component importance factor has not been assigned to an MEP system, the designer must assume that it is equal to 1.5. This could result in an unnecessarily large increase in the size and number of restraints required along with a corresponding increase in the cost for the system if, in fact, the component importance factor could otherwise be assigned a value of 1.0.

Seismic Restraints: A Summary

Mechanical and electrical equipment knocked off of its supporting structure due to earthquake-related building movement can threaten both life and property. The cost of properly restraining this equipment is insignificant compared to the associated costs of replacing or repairing the equipment, the cost of system downtime as a result of seismic damage to the building services, and damage to the building contents. A thorough analysis of seismic restraint hardware and seismic rated vibration isolators requires the consideration of four aspects of the system: Attachment of the Equipment to the Restraint ° The equipment must be securely attached to the restraint, and this attachment must demonstrate sufficient strength to withstand the imposed forces and to allow for transfer of seismic forces into the restraint. Restraint Design ° The strength of the seismic restraint must be sufficient to withstand the equipment-imposed forces. Manufacturers offer a wide variety of restraints suitable for many different applications. Attachment of Restraint to the Building Structure ° This attachment is typically via bolts, welds, or concrete anchors. In addition, the building attachment interface must be reviewed to ensure that it is capable of withstanding the imposed seismic forces. Typically this attachment is the 'weakest link' of the overall design, especially when post-installed concrete anchors are used. Equipment Fragility ° The ability of the equipment to continue to operate after being subjected to seismic force. Fragility information must be obtained from the equipment manufacturer.