What
Are Seismic Loads?
Seismic
Loads are the forces exerted on a structure during an earthquake.
Every structure is designed for vertical, or gravity loads. In the
case of ducts or pipes, gravity loads include the weight of the ducts
or pipes and their contents, and the direction of the loading is downward.
The ordinary supports designed for gravity loads generally take care
of the vertical loads imposed during an earthquake. Therefore, the
primary emphasis in seismic design is on lateral, or horizontal forces.
However, since vertical loads contribute to any overturning, they
are included in seismic analysis.
What Happens During An Earthquake?
A fault is a fracture in the earth’s crust,
and an earthquake results from slippage along the fault plane. Any
structure straddling the fault line will probably suffer damage,
no matter how well it has been designed. However, most effects of
earthquakes are not directly on the fault line. This is because
the movement caused by the slippage creates waves in the earth that
travel away from the fault plane. These waves change throughout
the duration of the earthquake, add to one another, and result in
extremely complex wave motions and vibrations. The direction of
forces on structures can be horizontal, vertical, or rotational.
In terms of the way they may affect a given building, they are not
only unpredictable in direction, they are also unpredictable in
strength and duration. The structural load is proportional to the
intensity of shaking and to the weight of the supported elements.
How
To Resist Seismic Loads
The general principle in
resisting seismic loads is that we want equipment, ducts, and piping
to resist seismic forces by the strength of their attachment to the
building’s structure. Naturally, we must assume that the building
has been designed to perform safely in response to earthquake motions.
So that they remain intact and functioning, we want equipment, ducts
and pipes to move with the building during an earthquake and not break
away from their supports. Therefore, the restraints are sized to insure
the chances of keeping these systems attached to the structure.
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Kinds
of Bracing
Because we cannot predict
the directionality of seismic forces, it is important to restrain
equipment and brace piping and ductwork in several directions. Floor
mounted equipment is typically restrained by use of a seismic isolator
or restraint which keeps the equipment captive. If the equipment
does not require vibration isolators, properly sized anchor bolts
can be used to seismically restrain the unit. In order to restrain
ducts and pipes against seismic forces, longitudinal (in the direction
of their run) and transverse (perpendicular to their run) bracing
together with their vertical support will resist lateral loads from
any direction. All in-line equipment must be braced independently
of the ducts or pipes.
Angle
Bracing vs. Cable Restraints
When
suspended equipment, piping or duct is hung using spring or rubber
vibration isolators, cables are required for seismic restraint so
as not to short circuit or bypass the isolators. Angle bracing can
be used when piping and duct is hard mounted to the structure.
General
Requirements For Seismically Restraining Ducts
Rectangular
ducts with cross-sectional areas of 6 square feet and larger, and
round ducts with diameters of 28 inches or larger generally require
seismic restraint. No bracing is required if the duct is
suspended by hangers 12 inches or less in length. Bracing
of ductwork shall be at 30 foot intervals, at each turn and at each
end of a duct run.
General
Requirements For Seismically Restraining Pipe
All piping
of 2.5 inches nominal diameter and larger requires seismic restraint.
All piping located in boiler rooms, mechanical equipment rooms,
and refrigeration mechanical rooms that have a nominal diameter
of 1.25 inches and larger require restraints. Fuel oil piping and
gas piping (fuel gas, medical gas, compressed air) of 1 inch nominal
diameter and larger require seismic restraint. No bracing or restraint
is required for piping suspended by individual hangers 12 inches
or less.
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