Seismic Restraint of Suspended Equipment, Piping and Ductwork
Failures in piping systems that have led to the release of water or other fluids have been found to be the cause of most of the monetary damage to buildings and contents during seismic events. The most frequent occurrence of these failures has been in systems that were not restrained to the standards set forth in the building codes or the guidelines issued by SMACNA (Sheet Metal and Air Conditioning Contractors National Association).
The code bodies have revised their focus in these areas, ensuring that the current Code and Guideline information is followed.
Broadly stated, in applications where significant motion can occur, the restraint requirements for piping and ductwork systems are to be adequately sized in both the lateral and axial directions. These restraints must be used with spacings short enough to prevent local failures in the pipe/duct runs between restraints.
In the field, seismic restraint systems must attach and interface with numerous piping, ductwork, and electrical systems. It is difficult if not impossible to specify locations for these restraints prior to completion of the runs since the routes of these systems frequently changes over the course of construction. It is recommended that the seismic restraint be installed after the installation of the mechanical and electrical systems. The SMACNA Seismic Restraint Manual offers general guidance for field installation of these restraints. It includes tables with maximum spacing and restraint component sizes for various sizes of piping and ductwork in the various seismic zones. The SMACNA manual is easily understood and can be effectively used by installation contractors on systems already in place. The value listed as “S” in the drawings below comes from tabulated data from SMACNA and computation sheets provided by Kinetics Noise Control for case specific applications.
Typically piping/ductwork systems are restrained either with cable restraints or rigid braces that run upward at an angle from the pipe/duct to the ceiling. Because these links run at an angle, the application of a horizontal load generates a vertical load component on the hanger rod which supports the pipe or duct. This vertical component can frequently be as large as double the horizontal force. This vertical force needs to be taken into account when designing the anchorage.
When using cable restraint systems, the secondary vertical force component generated by the horizontal load is always directed upward, loading the support hanger rod in compression. With rigid braces, the vertical force component can be either in compression or tension, depending on the direction of the seismic load. To resist the compressive load, a stiffener is required on the hanger rod when the critical buckling length of the hanger rod is exceeded. This dimension is tabulated in the SMACNA guidelines for various piping configurations. Where a rigid brace is used, not only does the long hanger rod require a stiffener, the anchor itself must also be capable of taking a downward load comprised of both the weight load of the pipe and the downward force generated by the seismic event.
It has been found that piping or ductwork that is hung on rods such that the dimension from the top of the pipe/duct to the underside of the supporting surface is 12" or less will not be excessively excited by a seismic event. It has also been found that pipes under 2-1/2" in diameter are sufficiently small and ductile such that they will flex and not be damaged by an earthquake. The same holds true for ducts that are under 6 ft in cross-sectional area. Most of the codes exclude such systems from seismic restraint requirements.