BASIC DESIGN GUIDELINE INSTRUCTIONS


It is the intent of this guide to assist you in proper design and estimation to keep all mechanical and electrical piping, conduit and ductwork building system components in place during a seismic event. 

This guide should be considered to be minimum requirements for seismic consideration and is not intended as a substitute for legislated, more stringent, national, state or local construction requirements (i.e. California Title 24, California OSHPD, Canadian Building Codes, or other requirements).

The requirements for seismic bracing can generally be categorized under two levels of earthquake safety. The first is when the failure of the component poses no hazards itself but the hazard exists if the supports or attachments of the component fail and the movement or failing of the component could pose a hazard to persons in the area. Piping located in ceiling or corridor spaces, in mechanical equipment rooms or in other areas are examples which may be designated as paths of egress.

In the second level, the failure of the component itself poses a hazard. This may include the failure of a piping or duct system which could result in a release of toxic or explosive substances or where a failure could be functional to a requirement that the system remain operational after a seismic event. Examples include fire protection piping, duct systems used in smoke management/control systems, uninterruptible power supplies, heating and cooling supply systems, and piping systems containing medical and life support gases in hospitals.

Mechanical and electrical systems are defined as "non-structural" components and the codes contain specific sections for these components. The technical requirements provide a method to calculate the expected forces to which components may be subject.

The form of the force equation varies among the various codes but the various factors reflect essentially the same information. The codes contain maps and tables where these factors are found and incorporate factors, including distances to known seismic faults, soil conditions and location of the braced components within the building.

The basic premise in the bracing of piping or duct systems is to secure those systems to the building structure such that any movement of the system is in concert with the structure. While piping and duct systems are generally rugged and perform well when subjected to shaking motions, high deflections and movements must be restrained in moderate to severe seismic events. Providing rigidity and secure attachments to the structure are effective in limiting damage to those systems.

A seismic restraint for a piping or duct system has two major considerations; the design of the restraint components and the location of the restraints in the system being braced.

Restraint System Design

A properly designed restraint consists of three components:

·          The attachment of the mechanical system to the restraint. The system must be positively attached to the restraint and it must transfer the imposed forces to the restraint.

·          The restraint itself must also be capable of carrying the imposed forces and transferring those forces to the structure.

·          The attachment of the restraint to the structure is the most critical and the most costly element of the seismic restraint system. A properly designed and installed attachment is essential to providing the rigidity and functionality of the seismic restraint system.

Restraint Locations

The placement of the seismic braces is as important as the design of the bracing components. The restraints are of two types: Transverse Braces - those designed and installed to restrain movement in the direction perpendicular to the piping or duct run and Longitudinal Braces - those designed and installed to restrain movement in the direction parallel to the pipe or duct run.

The spacing of the transverse and longitudinal braces is determined by structural analysis and is larger for smaller and lighter pipes and ducts. As systems become larger and heavier, the spacing decreases accordingly. Similarly, as the severity of the earthquake threat becomes lower in various locations of the country, the spacing of the restraints is correspondingly greater. Two general rules apply: Each piping or duct run must have a transverse brace at each end of the run and each run must have at least one longitudinal brace.