Applying Restraint Capacity Ratings (D4)
Chapter D4 of this manual provides a significant level information with regard
to the selection of appropriately sized, Seismically rated components for
particular applications. Differences between the ASD and LRFD rating systems
are addressed as well as how to read, understand and interpret the Seismic
rating charts which accompany each Seismic component submittal.
This Chapter includes information critical to anyone involved in making decisions
relating to sizing or locating restraints in the field, particularly with
regard to piping, duct or conduit installations. The installation of restraints
on these systems often requires that design decisions be made in the field.
This is the result of issues relating to access, modifications, inaccurate
drawing details or a myriad of other reasons that result in the initial drawings
not matching the installation. Understanding this information will allow the
user to safely size restraints if alternate restraint components or locations
are found to be necessary.
The section is recommended reading for Design Professionals or any group
responsible for sizing or evaluating the appropriateness of particular restraint
devices.
ASD
(Applied Stress Design) vs LRFD (Load Resistance Factor Design) (D4.1)- Older Codes have historically used ASD values when sizing components.
New Codes have switched to LRFD. When evaluating ratings, all load and capacity
data must be converted into the same units or the resulting mismatch can
invalidate the analysis. This paper discusses the differences between the
two and when a conversion is required to properly size components.
-
Horizontal/Vertical
Seismic Load Capacity Envelopes (Variable) (D4.3)
- In some combination Isolator/Restraint devices, the supported load affects
the seismic rating. Depending on the load or the device, it could increase
or decrease the restraint capacity. Charts used to evaluate these kinds
of restraints are slightly different than the charts mentioned above and
this section explains them and their application.
-
Force
Class (for Hanging Piping, Ductwork, Conduit and Equipment) (D4.4)
- Because of the significant number of variables involved, rating cable
and strut restraint systems are typically more complicated that rating conventional
stand alone restraints. In an effort to simplify sizing these components,
Kinetics Noise Control has developed the "Force Class" Rating System. In
it, A restraint location is assigned a "Force Class" load requirement (I-VI)
based on Seismic Zone, Code, Length of Run, Weight per foot of suspended
system and Location in the structure. Hardware is also assigned a "Force
Class" capacity based on Size, Anchorage and Worst case geometry. These
values are such that a "Force Class" I component can generate sufficient
capacity to with stand a "Force Class" I Load. It is then a simple matter
to select components appropriate to the load. This section describes in
detail and provides necessary data to use this system. It is of critical
interest to those involved in evaluating pipe, duct or conduit restraint
systems.
-
Force
Class Load Determination Table (Sample) (D4.5)
- A Sample "Force Class" load rating Table is presented in this document.
As these are customized for each installation, this example cannot be used
for design without being tailored to the application in question, but if
offers a typical example of what might be encountered in practice.
-
Maximum
Restraint Spacing, Run Offset and Drop Length (D4.6)
- This is a collection of Tables that allow a user to quickly select appropriate
maximum spacing for both lateral and axial restraints as well as determine
allowable unrestrained drop lengths maximum allowable offsets. It is appropriate
for piping, ductwork and conduit.
-
Hanger
Rod, Strut and Stiffener Tables (D4.7)
- When subjected to Seismic Loads (either in Strut of Cable restrained system)
Uplift forces are generated in hanger rods. Depending on the magnitude of
the force and the length and diameter of the hanger rod, a rod stiffener
is often required. This section provides guidance as to how big a stiffener
to use and when it is needed.
