This quote from Professor Cross, inventor of the
moment distribution method, emphasizes what the world expects from a
structural engineer. The public takes for granted that the structural
engineer will design building systems of adequate strength to protect
the health, safety and well-being of the occupants. As well they should,
for the public has the right to demand these basic tenets. But given
the expectation that all strength requirements will be met, excellent
structural engineering also must consider serviceability issues such as
vibration.
Steel framed laboratory building; University at
Albany Center for Excellence in Cancer Genomics.
Vibrations caused by mechanical equipment, by
people walking on a structured floor, or by airborne noise can be
annoying to the occupants of any type of building. Vibrations in
laboratory buildings can be more than just annoying - they can interfere
with sensitive equipment, disrupt experiments, and affect the behavior
of laboratory animals.
Higher strength steel and lighter weight
materials allow longer spans in modern buildings, making proper design
for vibration more important. With critical manufacturing processes,
vibration criteria are well defined, and extensive vibration controls
are employed to meet those specific criteria. However, general
laboratory buildings for most university and research clients do not
have specific vibration criteria other than "vibration is an issue."
For these types of buildings, engineers must take
special measures to ensure that the entire structure is more resistant
to vibration problems. Design considerations include using heavier
concrete floor slabs that provide more damping capacity, placing columns
on both sides of corridors to reduce the impact of foot-fall vibrations
propagating into adjacent lab spaces, and structurally isolating spaces
particularly sensitive to vibration disturbances.
Vibration table used to place vibration
sensitive equipment; Rochester Institute of Technology.
Noise Measurements
Understanding common measurements for sound
transmission, noise reduction, and impact insulation provides a useful
background for vibration design. Each measurement addresses a different
concept, and each concept involves various adjustments to construction
methods.
The Sound Transmission Class (STC) measures the
ability of a wall assembly to prevent the transmission of sound from one
side of a wall to the other. The higher the number, the less sound can
pass through a partition. A typical 4-inch metal stud wall with a layer
of 5/8-inch gypsum board on each side offers an STC of 38. Adding
fiberglass batt insulation to the wall increases the STC to 43, and
increasing the mass by adding a second layer of drywall to both sides
increases the STC to 45.
The real trick to improving the STC is to break
the pathway by which noise travels through a wall. The most common way
to accomplish this is to mount the layer of drywall on one side of the
wall on resilient metal clips. The resilient clips absorb airborne
vibration, preventing the sound waves from propagating through the
structure. This results in an STC of about 52.
Recommended sound isolation between adjacent
laboratory spaces is STC 42 to 48. Use caution if designing stud shear
walls with gypsum board sheathing, because gypsum board mounted on
resilient channels offers no structural capacity.
Lab Space with flexible layout options;
University at Albany Center for Excellence in Cancer Genomics
Another method of increasing the STC of a wall
assembly is to increase the mass by using concrete masonry units (CMU)
or concrete wall panels. An 8-inch solid grouted CMU wall provides an
STC of 55.
In order to be effective, partitions meant to
block sound from traveling to an adjacent space must maintain a complete
seal against all edges. This may be difficult at tops of partitions
that abut irregular surfaces such as slabs on metal deck, but if not
properly sealed, flanking of noise from one space to another can occur. A
one-inch square hole through a wall can ruin the STC rating of a
rigorously designed assembly. When sealing the top of non-load-bearing
partitions, install materials that allow the floor system to deflect
without loading the walls. Do not grout the space between the top of a
CMU wall and the underside of the floor deck unless the wall is designed
to be load-bearing.
Noise Reduction Criteria (NRC) refers to the
sound absorption qualities within a space. Noise, defined as unwanted
sound, can be controlled within a room by carpeting the floor or
installing acoustical ceiling tiles. These materials absorb some
frequencies and reduce reverberation.
Note that modifications to room elements to
improve the NRC are made to benefit the occupants of the room, and have
no effect on the transmission of sound from one space to another. For
example, adding an acoustical ceiling in a mechanical room does not
effectively shield the occupant in the floor above from noise.
Both the STC and NRC deal with airborne sound,
and apply to a limited frequency range found in human speech (125 to
4000 Hz.) As for structure-borne noise, Impact Insulation Class (IIC)
measures the isolation effectiveness of floor-ceiling assemblies to
prevent the transmission of noise created by dropped objects and
footfalls from an upper space to a lower space. The IIC measures sound
pressure levels over a similar range as the STC (100 to 3150 Hz,) and
the higher the IIC, the greater the capacity of the floor to ceiling
assembly to prevent transmission of noise.
Construction methods of improving the IIC are
similar to improving the STC - provide isolation joints through floor
slabs to create breaks in noise pathways, mount ceilings on resilient
clips, and caulk all joints. Keep in mind that many of the techniques
used to improve isolation of adjacent spaces effectively sever floor
diaphragms and shear walls, making close coordination of noise control
measures with lateral force resisting systems imperative.
Measurements presented thus far focus on a
limited range of frequencies and primarily deal with standard methods of
controlling noise transmission between adjacent spaces. While these
concepts must be addressed, floor vibration and the prevention of
structure-borne noise from multiple sources represent more significant
challenges for the design team.
Columns along both sides of the corridor;
University at Albany Center for Excellence in Cancer Genomics.
Floor Vibration
Many laboratory buildings we design consist of
steel frame construction, with floors of steel beams acting compositely
with concrete slabs on metal deck. Floor beams in such an assembly
vibrate at a natural frequency when an impact force is applied. This
frequency depends upon the beam span, spacing between beams, and depth -
properties that affect the stiffness of the beams. Methods to calculate
the frequency of floor framing systems are presented in publications
such as the AISC Steel Design Guide 11, Floor Vibrations Due to Human
Activity, and most structural analysis programs include vibration
algorithms.
Vibration perceived by building occupants is
mitigated by damping, which acts to reduce the energy in a vibrating
system. Damping in floor systems is provided by the self-weight of the
floor system, as well as superimposed dead loads such as partitions and
millwork. Therefore, the components most under the control of the
design engineer are stiffness of the floor beams and mass of the floor
slab.
Vibration Sources
Vibration comes from a wide variety of sources
including vehicular traffic on adjacent roads, airplanes, footfall
traffic on supported floors, and mechanical equipment. The impact
resulting from vibrations causes an equally wide variety of problems.
For example, while touring a manufacturing plant, we wondered why a
lathe operator was staring out the window with tools in hand and the
machine running. We learned that she was not daydreaming on the job -
she was waiting for a train very close to the building to pass by
because the vibrations were strong enough to ruin a delicate process. In
addition, we recently completed emergency foundation repairs for an
historic building in New York City where vibrations from subway trains
contributed to subsidence of soil beneath existing footings. Both of
these examples represent serious impacts to productivity and safety by
vibration problems.
Although all sources of vibration must be
considered, by far the most significant causes of problems in laboratory
buildings stem from foot traffic and mechanical equipment. Evaluation
of design options, damping, source vibrations, and functions sensitive
to vibration disturbance leads to basic layout and planning principles
that we follow for all laboratory buildings.
Microscope lab; Hamilton College.
Layout and Planning
Corridors
The simple act of walking can produce troublesome
vibrations within a bay of floor framing. In a typical building with a
central corridor and rooms on both sides, it is structurally efficient
to place columns along only one side of the corridor. This results in a
long span from the corridor line of columns to the farthest exterior
wall. The long span supports both the rooms and the corridor, and
vibrations caused by footfall traffic in the corridor are directly
transmitted into the rooms sharing the corridor support beams. In order
to mitigate this problem, place columns along both sides of the
corridor. Although this requires additional columns and foundations,
isolating corridor traffic with separate framing prevents vibrations
from propagating into adjacent spaces. Furthermore, the shallower
members that frame the short corridor span provide extra depth for
utilities that compete for precious laboratory plenum space.
Another consideration for corridor design is the
speed at which people will walk. The expected vibrational velocity of a
floor subject to slow walking is about 1/15 of that for fast walking.
Therefore, try to arrange corridors in ways that discourage fast
walking, such as making them shorter or interrupting them with turns. If
this is not feasible, incorporate visual breaks in floor patterns and
lighting that emphasize elements transverse to the length of the
corridor. Providing visual variety calms traffic, and people tend not to
rush through pleasant spaces as fast as unpleasant ones.
Span
Bay size plays a critical factor in the vibration
characteristics of a floor system. Since the stiffness of a beam varies
as the cube of its length, shortening the span is a very effective way
to adjust its flexibility. Even though we have the technology to span
long distances with high-strength members, laboratory buildings benefit
from closer column spacing.
Note that not all spans in a building must be
short. Designating sensitive equipment zones can provide vibration-safe
areas and maintain a certain amount of flexibility for moving equipment
within the zones, but not penalize the entire structure with closer
column spacing everywhere. Similarly, not all floor framing systems must
satisfy the most stringent equipment requirements. Criteria is
available that categorizes sensitivity to vibration based upon
equipment, such as magnification power of microscopes; or by use, such
as micro-surgery, and can help the structural engineer fine-tune spaces
for known uses.
Mass
The mass of floor slabs affects the vibration
characteristics of the space. Office building floors often consist of
3-1/4 inches of light-weight concrete over the metal deck top flutes,
yielding a 2-hour fire rating. By comparison, 4-1/2 inches of
normal-weight concrete is required over the top flutes of the deck to
achieve the same fire rating. Using lightweight concrete allows the use
of lighter beams, and can reduce footing sizes. However, laboratory
buildings benefit from the enhanced damping effect provided by
normal-weight concrete, and its use is standard practice for labs in our
office. Even if the building code requires less stringent fire
separation and a thinner slab would suffice, constructing a heavier slab
improves the performance of the floor system from a vibration
perspective.
Layout
A footfall impact at midspan of a beam produces
greater vibration than the same impact near a column. Furthermore,
vibrations dissipate as they cross column lines, walls, and framing.
Consequently, sensitive equipment placed close to columns and far away
from corridors will perform better than equipment placed near bay
centers and close to sources of excitation. From an overall planning
perspective, hold early discussions to identify critical equipment or
functions, and decide their appropriate locations within the building.
For example, particularly sensitive equipment may want to occupy
isolated slab-on-grade space rather than an elevated slab level.
Inertia base suspended with elastic straps; New
York University.
Special Considerations
Floor Isolation
As an extension of the concept of separating
corridor supports from lab space supports, a walking structure supported
independently from the floor framing may provide an increased degree of
isolation. Corridor beams that connect only to the columns and not to
adjacent floor framing could accomplish this separation.
Another method of isolating an entire floor is
constructing a room within a room. A secondary slab floating above the
base structural slab on neoprene pads provides effective isolation of
discrete areas. Combined with high-STC walls and an independent ceiling
structure, this type of construction creates a well-protected shell.
However, support of the base structural slab must still meet basic
deflection limits, and the frequencies of the intended isolation must be
determined. This type of construction typically involves an acoustical
consultant.
Mechanical Equipment Isolation
Modern installations of mechanical equipment
include vibration isolators, flexible couplings, and resilient hangers
designed to prevent transmission of equipment vibration into the
structure. These are typically designed by the equipment manufacturer,
and not the project engineer. However, there is a useful concept to know
about equipment isolators. Neoprene pads effectively prevent
transmission of high-frequency vibrations, such as those produced by a
running motor. However, they are ineffective at preventing transmission
of low-frequency vibrations such as jolts created when a motor starts
and stops. Springs are needed to handle that type of action. Be aware of
the limitations of each type of isolator, particularly when
troubleshooting.
Sensitive Equipment Isolation
Some laboratory equipment comes standard with its
own isolation system designed to prevent transmission of
structure-borne vibrations into the equipment. Many of these systems
include some type of inertia damper, and the structure must be designed
for the additional weight.
Commissioning
More and more building owners realize the many
benefits of commissioning, particularly with the demand for achieving
LEED™ certification. The intent of commissioning is to verify and ensure
that fundamental building elements and systems are designed, installed
and calibrated to operate as intended. Discovering improperly installed
or short-circuited isolation devices during the commissioning process
can avoid complaints from users and potential equipment damage.
Common Sense
Structural engineers have the ability to make
just about anything work. We can design an elaborate isolation system
allowing the placement of an upper floor eye surgery suite directly next
to an aerobics room. But why would we? We have the responsibility to
use our Earth’s resources and our client’s money efficiently. Close
cooperation among lab planners, architects, and engineers of all
disciplines in an integrated fashion empowers us to subdue vibration
problems intelligently as a team.▪
Franklin D. Lancaster, P.E., RA, LEED™ AP
is Principal in charge of structural engineering at Einhorn Yaffee
Prescott Architecture & Engineering PC. Mr. Lancaster has nearly 20
years experience in the repair, strengthening, and alteration of
existing structures, as well as the design of new structures. He can be
reached via email at flancaster@eypae.com.