Tag Archives: Multi-family

Designing Sound Isolation in Multi-family Living

Acoustic_Rendering Twenty 20 Credit CBT Architects

Twenty20 is a 20-floor, 355-unit luxury apartment building in the NorthPoint section of Cambridge, Massachusetts. In addition to the residences in the tower, the project features 807 m2 (8690 sf) of retail space. Image courtesy CBT Architects

by Alicia Larsen, Benjamin Markham, LEED AP,  and Jeffrey A. Zapfe, PhD
Imagine moving into a new condo, only to realize the TV next door, the dog barking across the hall, and the neighbors walking around upstairs can all be easily heard. Acoustical consultants would love to help, but unfortunately there is little that can be done at this stage without significant cost and intrusion. Sound isolation issues are most effectively addressed before construction, during the design phase.

What can designers of multi-family buildings do to meet the sound isolation expectations of their client and future residents? Acoustical consultants use several techniques to address these issues. However, before delving into them, one must first understand a little bit about sound.

Every sound isolation problem has three elements—a source, a path, and a receiver. The ‘source’ is the noise generator. It could be anything from a fourth-grader practicing her saxophone to a piece of building mechanical equipment. In many cases, the most effective means of mitigating a noise concern is to choose a quieter source if possible (e.g. quieter mechanical equipment), or to increase the distance between the source and sensitive receivers (i.e. lengthen the path).

Unfortunately, many times the source is an element that cannot readily be changed. No matter how many rules you put in place, there is no guarantee the student will stop practicing her saxophone at odd hours or the man next door will not fall asleep with his TV on again.

The ‘path’ is the element designers have the most control over, and so it is what frequently receives the most focus during the design. Both the path’s length and the building constructions intervening between source and receiver (position and composition) can be controlled. The performance of those building components is predictable and does not rely on residents’ behavior to be successful.

Finally, the ‘receivers’ in a multi-family building are the residents. Individual sensitivity to noise or vibration varies, so designers have little control over this piece of the sound isolation puzzle. One exception is the background noise level in a residence—designers can control this aspect by introducing steady, broadband (full-spectrum) ambient sound that masks the intruding sound, similar to the ‘white-noise’ machines some people use in their bedrooms to help them sleep. The bottom line is ‘quiet’ does not equate to ‘private’—in fact, it is just the opposite: there is greater privacy when there is a steady (but pleasant) level of ambient sound present.

Given the design team has the greatest control over the path, this article focuses on this design aspect—namely, how to design building components that provide substantial sound isolation. To add just a little more complexity, there are really two types of sound transmission to be considered in multi-family buildings: airborne and impact.

Airborne sound transmission
Airborne sound is generated, and primarily travels to the receiver, in the air. This is most of the sound heard on a daily basis, with examples including people talking or a loud stereo system. In a room-to-room situation, airborne sound can transmit either through a gap in the construction (e.g. a door undercut) or directly through a wall by causing the intervening construction to vibrate and re-radiate the sound energy on the other side.

Figure 1

Acoustic_Acentech_Figure 1

Sound transmitting through a partition. Image courtesy Acentech

For example, with a single-stud partition, one side (the source side) can be thought of as a microphone, and the other side (the receiver side) as a loudspeaker (Figure 1). The sound energy travels through the air and gets picked up by the ‘microphone,’ transmits through the structure, and is re-radiated by the ‘loudspeaker’ into the air on the other side of the partition.

In this case, there are three primary methods to improve sound isolation:

  1.  Seal all gaps, cracks, and leaks. This is the easiest and most effective means to isolate sound. Sound will always find the weakest path—other attempts to improve sound isolation will be ineffectual if the gaps are not sealed first.
  2.  Increase the mass of the construction. This makes it more difficult for the airborne sound to cause the partition to vibrate. In the microphone/loudspeaker analogy, this corresponds to making the microphone and loudspeaker less effective or efficient.
  3.  Introduce decoupling into the construction. This allows one side to vibrate without transferring the vibration as easily to the other side. This is analogous to cutting the physical connection between the microphone and loudspeaker.

In many cases, all three methods are necessary.

Closing gaps
ASTM C919, Standard Practice for Use of Sealants in Acoustical Applications, contains recommendations on how to apply caulk in acoustical applications. Ideally, all penetrations should be sealed. Back-to-back electrical boxes should be staggered, preferably in different stud bays, and caulked. Walls should be caulked with one bead on each side of the partition. (A second bead of caulk on one or both sides of the partition does not result in a substantial improvement in sound isolation.)

Windows and entry doors should be effectively gasketed. Poorly sealed entry doors (particularly those that have a substantial undercut) can be a significant source of noise intrusion from corridors into residences.

Acoustic_Concord Park Residential Multi-Unit

Concord Park (West Concord, Massachusetts) houses senior residences in senior and assisted living accommodations for the Volunteers of America Organization. Recommendations were made for sound-isolating materials to lessen the train noise. In addition to the exterior sound isolation options, upgrades to the exterior windows were also suggested. Photo courtesy TAT/The Architectural Team

Increasing mass
A substantial increase in mass is necessary to have a meaningful effect on a partition’s sound isolation performance. (‘Substantial’ normally means in terms of a factor of two times heavier.) This could be accomplished by upgrading from one to two layers of gypsum board on each side of a stud partition, or using plaster rather than gypsum board. (Plaster has roughly three times the mass of gypsum board.) If partitions are concrete masonry unit (CMU), increasing the density or thickness, or filling the CMU cavity with grout, can effectively increase the mass.

Introducing decoupling
The goal is to eliminate rigid connections between one side of the construction and the other. The most effective way to do this is by using a double-stud or double-width construction with an air cavity between the two sides of the wall. When double walls are not possible due to cost or space constraints, other methods such as specialty resilient fasteners can be acceptable alternatives. In general, resilient clip products tend to be more reliable than resilient channels, but either product can be compromised in real-world applications, particularly if cabinets, TVs, or other wall-mounted elements must be anchored directly to the studs. Such complications are less prevalent in suspended ceiling applications in place to increase up-down sound isolation.

Impact sound transmission
Impact sound is produced by forces applied directly to the structure or partition. The most common example of this is people walking on the floor above, but things like chairs scraping along the floor and the thump of people ascending/descending stairs are also prevalent in multi-family buildings.

The sound isolation methods that are discussed for partitions can also  be used in order to mitigate impact sound. A few others are discussed in the following paragraphs.

Increase the structure’s stiffness
Floor stiffness is an important first step in mitigating impact sound (particularly the low frequency ‘thuds’). Unfortunately, increasing the structure’s stiffness can be challenging when the construction is already determined. As a rule, the stiffer the structure, the more effective the sound and vibration isolation will be, particularly at low frequencies. Structures that are not stiff do not generally isolate the low-frequency content of impact sounds, no matter what other mitigation means are employed.

Decoupling works like a shock-absorber on a car. Energy is applied to one side of the construction, but the way in which the sides of the construction are connected prevents the energy from being efficiently transmitted to the other side (the sides of the construction are often connected by a device such as a spring or a resilient pad). A car can go over a bump, but since the wheels attached to the shock absorber move up and down relatively freely, the passengers do not feel the bump to the same degree in their seats.

Acoustic_Berklee Tower - (c) Robert Benson Photography

Berklee College’s new multi-use 16-story tower houses 173 residence hall rooms for 369 students, 23 practice rooms, six two-story common areas, a fitness center, and a 400-seat dining hall that doubles as a performance space. The authors’ firm provided architectural acoustics for the tower, with emphasis on room acoustics, sound isolation, and mechanical systems noise control. Photo © Robert Benson Photography

Carpeting and other ‘soft’ floor finishes significantly cushion impact sounds, preventing the floor/ceiling assembly from becoming energized in the first place. Most impact sound problems occur with hard floor finishes like wood or tile. In most cases, addressing this issue with decoupling is the most sensible option, as it often has the fewest implications on the project.

The structure’s stiffness is typically decided in the earliest stages and changing it could have major cost implications. On the other hand, the finish floor hardness is often decided based on aesthetic considerations; changing the finished floor may or may not be an option.

Decoupling can be addressed on the floor side or on the ceiling side. On the floor side, a resiliently supported massive layer (a floating floor) is used to provide isolation. Floated floors can range from a separately poured concrete slab that rests on resilient isolators to a thin underlayment underneath the finished floor. The required scale of the decoupling depends on the amount of isolation needed.

One can also introduce decoupling on the ceiling side by installing a resiliently suspended ceiling. Again, depending on the application, this can range from hanging multiple layers of gypsum board on a network of spring hangers to attaching the gypsum board using resilient channels. (In best cases, decoupling is introduced both at the floor and at the ceiling.)

In either case, it is best to begin with a stiff structure. When faced with an existing structure that lacks stiffness, it is typically necessary to add structural members (e.g. more beams) or increase mass (e.g. pour more concrete).

Wood-frame construction is fairly limp to begin with, so addressing the stiffness can make a large difference on the effectiveness of the sound isolation. At least 25 mm (1 in.) of gypsum concrete in wood-frame constructions goes a long way to stiffen the structure and add mass. Steel and concrete buildings tend to be stiffer, but sometimes even they require additional steel framing or concrete to provide adequate impact isolation.

Quantifying sound isolation
Various metrics put forward in the following ASTM standards are used to quantify the sound transmission between spaces and assign a single-number rating to the airborne sound transmission or the impact sound transmission:

  •  field: ASTM E336, Standard Test Method for Measurement of Airborne Sound Attenuation Between Rooms in Buildings;
  •  laboratory: ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements;
  •  field: ASTM E1007, Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission Through Floor-Ceiling Assemblies and Associated Support Structures; and Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine.

Airborne sound transmission is typically quantified using the sound transmission class (STC), which is a laboratory rating that cannot be measured in the field. Field equivalents include apparent STC (ASTC) and noise insulation class (NIC). The major difference between the two field ratings is ASTC is normalized to account for the room acoustics in the particular measurement scenario.

Using ASTC, the performance of two different constructions, in two different locations, with different source and receiver room types, can be compared. ASTC is a test that would be used to measure compliance to a design standard. NIC, on the other hand, does not normalize for the room conditions—as such, it better represents what the occupants actually experience rather than simply how the partition is performing.

In all cases (i.e. STC, ASTC, and NIC), higher values indicate better sound isolation.

Impact sound transmission has an analogous set of metrics:

  •  impact insulation class (IIC), the lab rating;
  •  apparent IIC (AIIC), the normalized field measurement used to determine compliance; and
  •  impact sound reduction (ISR), the non-normalized field measurement that correlates well to occupant experience.

Again, higher values indicate better sound isolation.

Physically, a partition’s sound isolation depends on the frequency of the sound. In most cases, partitions tend to isolate higher frequency sound better than low frequencies. Unfortunately, the single-number metrics all combine this frequency-dependent behavior into a single numerical rating. This means the ratings alone may not fully describe the performance.

For example, the two tests shown in Figures 2a and 2b have the same IIC rating, but very different properties of impact sound isolation across frequency. The first fits the AIIC 54 curve closely, whereas the second shows deficiencies at low frequencies and good performance at mid- and high-frequencies. With the floor-ceiling construction associated with 2b, one would be able to hear a 13.5-kg (30-lb) toddler jumping up and down, but not the click of his mother’s high heels.

Figure 2

Acoustic_Acentech_Figure 2 (2)

Impact insulation class (IIC) test data for two different constructions.

A change in STC or IIC of one or two points is not an appreciable difference in sound isolation. However, a change of five points is significant, and a change of 10 points typically corresponds to a largely significant difference—on the order of a doubling or halving of the perceived loudness of the intruding sound.

Code requirements/guidelines
Many states have adopted the International Building Code (IBC) into their state building code requirements. This calls for a minimum STC 50 laboratory rating, or 45 if measured in the field. Similarly, impact sound isolation requirements are IIC 50 or 45 if measured in the field.

It is important to understand these code requirements do not necessarily equate to occupant satisfaction, and certainly do not indicate inaudibility. Higher values are recommended for more sensitive applications. Figure 3 shows a summary of various sound isolation guidelines commonly referenced in the industry.

Figure 3

Acoustic_Acentech_Table 1 (2)

Comparison of sound isolation guidelines.

Ground-borne vibration and sound from rail lines
Multi-family residential structures are typically found in densely populated urban areas. Many of these urban areas also have extensive public transportation systems, some of which incorporate underground rail lines. Anyone who has visited New York City, Chicago, or Boston has likely encountered the characteristic low frequency rumble from a passing subway train.

Interestingly, vibration is actually the mechanism responsible for the rumble sound. Micro-forces due to imperfections at the wheel-rail interface produce vibration that travels through the soil to nearby buildings. Once inside the building, the vibrations cause the walls, floors, and ceiling to vibrate and radiate sound much like giant loudspeakers. It is this radiated sound people hear when the train passes. With the exception of air vents and other openings, the acoustic sound produced by the train in the tunnel is effectively trapped inside the tunnel.

If the vibration is sufficiently severe, people may feel it, but more often than not, the vibrations cannot be felt even though the resulting sound can still be heard. Since the vibration is propagated through the soil, the resulting sound and vibration inside the building are commonly referred to as ground-borne. Surface rail systems also produce ground-borne sound and vibration, although this is often masked by the direct airborne sound from the passing train.

The Federal Transit Administration (FTA) provides guidelines for acceptable levels of ground-borne vibration and sound in residential settings where people normally sleep. While these levels strictly apply to new transit projects near existing communities, they also can be used as a reasonable guideline for a new residential structure near an existing rail line. Unfortunately, FTA does not specifically say what the effects will be if its limits are exceeded.

More insight into this aspect is available from the TCRP D-12 study. The D-12 project studied the relationship between ground-borne vibration and sound and community annoyance and provided a method to estimate the likelihood of annoyance based on the vibration or sound exposure level.

Like airborne sound, mitigation of ground-borne sound and vibration can be conceptualized in terms of the source, path, and receiver. In addition to effective maintenance of the track and rolling stock, source mitigation treatments usually involve a resilient track support. Such supports come in many varied forms, but the basic concept involves the placement of a resilient element (rubber, neoprene or even steel spring) between the rail and the tunnel floor. Figure 4 is one example of a resilient track fastener that is located between the rail and the wood tie. Resilient track supports can be very effective at reducing vibration, however, they also require the direct involvement of the transit agency, whose cooperation can be challenging to secure.

Figure 4


Resilient track fastener. Photos courtesy Acentech

Path mitigation has limited effectiveness for ground-borne vibration. Trenches are often proposed as a mitigation option, but in addition to being of limited practicality in an urban setting, their effectiveness is modest at best.

Mitigation at the receiver is a viable option, particularly for new construction. Buildings have a natural vibration attenuation of one or two decibels per floor as one moves up away from the source. There is also distance attenuation as one moves farther from the rail line. Designers can take advantage of this by locating the most sensitive receivers on upper floors, saving the lowest levels for less-sensitive uses such as parking, mechanical, and retail.

More active steps can also be taken to reduce the amount of vibration entering the building at the foundation. Base isolation systems are used to support the building resiliently. Base isolation is typically done at the column base (Figure 5), although continuous pads under mat foundations can also be employed. The isolation performance of the isolation is specified based on the source levels (train) and the design goals for the living spaces. Vibration reductions associated with a base isolation system are comparable to what would be expected from a track isolation system.

Figure 5


A look at the base isolation pad.

Mitigation is also possible within the building on a room-by-room basis. ‘Room-within-a-room’ construction using a floated floor and resiliently supported walls and ceiling can be effective provided constructability challenges such as differential floor heights can be addressed.

Sound and vibration isolation comes down to mass, stiffness, and de-coupling. In multi-family buildings, it is necessary to consider wall constructions, floor/ceiling assemblies, and environmental noise and vibration sources with these factors in mind. It is sometimes said good fences make good neighbors. In a multi-family building, good building constructions can make for good neighbors, too.

Sound Isolation Pitfalls
Acoustic_Acentech_Figure 3

Beam parallel to wall partition. Image courtesy Acentech

1. Some ‘resilient channels’ are not really resilient. Two-legged resilient channels do not provide a meaningful acoustical benefit.
2. Shear panels in wood-frame construction can be tricky. When a resilient channel/clip is applied between a shear panel and a finish layer of gypsum board, it creates a narrow airspace at the resilient element. This narrow airspace of entrapped air is actually quite stiff, preventing the resilient element from flexing. When introducing resiliency, the resilient element should look into as deep an air space as possible.
3. Resilient elements are often pinned in place. This occurs commonly at ceiling perimeters – a resiliently suspended ceiling might be anchored to the walls at the perimeter with a rigid wall angle; this pins the ceiling in place, limiting efficacy of the resilient hangers.
4. Some conditions are difficult to seal. For example, the structural beam running parallel to the partition in the photo at left makes it difficult to get the beam with a caulk gun to adequately seal the partition, exposing the partition to sound isolation problems.

Alicia Larsen is a consultant in acoustics at Acentech, a multi-disciplinary acoustics, audiovisual systems design, and vibration consulting firm. Her special interests include sound and vibration isolation for mixed-use buildings, community noise issues, and acoustical measurement and analysis. Larsen has presented her graduate and consulting work at the Acoustical Society of America (ASA) and Institute of Noise Control Engineering (INCE) national conferences, and chaired ASA’s Greater Boston Chapter. She can be reached at alarsen@acentech.com.

Benjamin Markham, LEED AP, is the director of architectural acoustics at Acentech, and an acoustician involved in projects concerned with performance spaces and other commercial, residential, and civic facilities. He also teaches classes in architectural acoustics at Massachusetts Institute of Technology (MIT) and Cornell University. Markham can be contacted via e-mail at bmarkham@acentech.com.

Jeffrey A. Zapfe, PhD, is Acentech’s president. His expertise is in the area of vibration, structural dynamics, vibration sensitive facilities and equipment, and vibration isolation. Zapfe’s work includes the analysis of structures at the design stage as well as the measurement, analysis, and mitigation of vibration in completed buildings. He can be reached at jzapfe@acentech.com.

How Thin is Too Thin?

Evaluating slab thickness in reinforced concrete flat-plate construction
by Dimitri Papagiannakis, PE

Typical flat-plate construction.
Photos courtesy SGH


In the project’s early stages, structural engineers are often asked by architects and owners how thin the slabs in a flat-plate system can be. The question is usually motivated by a desire to achieve taller floor-to-ceiling heights, which can be an important selling feature to end users. There are building code provisions that address minimum slab thickness as a function of the span length and span condition (e.g. continuous versus discontinuous, etc.). There are also practical and economic factors that often influence the design of concrete flat-plate slabs.

The design of reinforced concrete structures is governed by American Concrete Institute (ACI) 318, Building Code Requirements for Structural Concrete, which provides minimum thicknesses for one- and two-way slabs supporting structural and/or nonstructural building elements. These are intended to limit deflections that may result in serviceability issues with the structure or that may damage architectural building elements.

The prescriptive minimum thicknesses are a function of the span length, continuity conditions, and end restraints of the slab; they are intended to provide a slab section that conforms to code-prescribed deflection limits without the need for the engineer to perform detailed deflection calculations. However, the code also permits the design engineer to specify thinner slabs when calculations are performed showing short- and long-term deflections will not have an adverse effect on structural or nonstructural elements attached to or supported by the slab.

Clusters of mechanical/plumbing penetrations through flat-slab. Slab design must be checked for required additional reinforcement at penetrations.


Pros of a thinner slab
There are several benefits to specifying thinner slabs from a structural perspective. One obvious advantage is less concrete is required. Consequently, a reduction in concrete also decreases the gravity loads on the vertical load-carrying elements. This will usually result in smaller columns with less reinforcement, and thus a savings in material costs.

A reduction in building mass also has a direct effect on the seismic loads to which a building is subjected. The seismic base shear of a building structure is directly proportional to its seismic weight—a reduction in the seismic weight of a building generally results in proportional decrease in the seismic-load demands to the lateral-load-resisting elements of the building structure, and thus a more cost-effective design. Additionally, reduced building loads may also yield a less-expensive foundation design depending on the proposed system.

Plumbing sleeves placed near columns.  This requires careful review of slab shear capacity.


Cons of a thinner slab
Depending on the horizontal spans that must be achieved, minimum slab reinforcement may not provide enough strength to support code-prescribed loads. Therefore, additional reinforcement may be required within the slab, negating some of the aforementioned material cost savings.

Thinner concrete sections are also susceptible to punching shear failures and must be carefully evaluated. Under certain circumstances, the avoidance of the punching shear limit state can preclude the use of smaller column cross-sections. The potential for overstressing the slab at the slab/column interface is further exacerbated by the use of slab-column moment frames often employed as part of the lateral-load-resisting system (where permitted by code). The magnitudes of the unbalanced moments and shear stresses at the slab-column connections are highest at the moment-frame locations, and may require use of thickened drop-panels at the columns to resist the applied loads. Alternatively, shear studs may be placed at the column heads to provide the required strength, or larger beam sections may be used around the perimeter to develop moment-frame action in lieu of the slab. These options result in added labor and additional cost for the project.

Flat-plate construction requires a great deal of coordination between the structural system and the mechanical, electrical, and plumbing (MEP) components. Slab penetrations for vertical mechanical and plumbing risers must be evaluated for potential additional required reinforcement. Riser penetrations located around columns must also be carefully coordinated and evaluated, as they can have a significant impact on the punching shear and flexural stresses near the columns, and may require additional flexural or shear reinforcement.

Electrical/plumbing conduit placed within slab.  Coordination is required to avoid over-congestion of conduit (such as shown here) and delays associated with modifying conduit locations in the field.


Electrical conduit is also typically placed within the slab, at mid-height. Sufficient cover must be provided around the conduit and between the conduit and slab reinforcement. The conduit diameter and spacing must be kept within certain limits to prevent it from degrading the slab’s strength or becoming the focus of shrinkage stress cracks. Design and coordination of these items becomes more challenging—and potentially more expensive—as the slab’s thickness, and thus the space within which to fit the components, is reduced.

For thinner flat plate slabs, the increased surface area-to-volume ratio makes it more susceptible to early drying due to a reduction in the heat of hydration (i.e. the reduced concrete mass retains less heat—a key component to the curing process). Higher drying rates increase the likelihood of early-age cracking and, in turn, the slab’s deflections.

This reduction in heat of hydration also becomes a factor in cold-weather conditions, where the freshly poured concrete may be more susceptible to freezing due to lower concrete temperatures than would otherwise be present to help protect the slab. Thinner slabs are also more prone to early-age cracking from the shoring and re-shoring loads typical of rapid construction cycles.

Selecting the most appropriate slab thickness is a critical aspect of a reinforced concrete flat-plate project. Modern engineering methods and the availability of finite-element software provide useful tools for quick and efficient evaluation of flat-plate systems.

The design engineer should assess the feasibility of reducing the slab thickness beyond the prescriptive limits provided by the code, and should communicate to the owner and design team the implications of doing so (e.g. additional reinforcement, connection detailing requirements, coordination issues, etc.). As mentioned, there are numerous pros and cons to reducing design slab thickness, and each must be evaluated to arrive at the most appropriate conclusion.

DimitriPDimitri Papagiannakis, PE, joined Simpson Gumpertz & Heger (SGH) in 2011 with nearly a decade of structural engineering experience. A registered professional engineer in New York and New Jersey, his work includes design of new building structures and subdivisions, as well as renovations, alterations, repairs, and investigations of existing buildings. He can be reached at dpapagiannakis@sgh.com.


by Josh Jonsson, CSI






To read the full article, click here.

Improving Floor/Ceiling Sound Control in Multifamily Projects


All images courtesy Maxxon Corporation

by Josh Jonsson, CSI











CS_July_2014.inddUpdating acoustical recommendations




Components of acoustical design

  • hard-surfaced flooring;
  • wood subfloor;
  • wood floor trusses or joists;
  • insulation;
  • resilient channel; and
  • one layer of gypsum board.



Improving IIC ratings with sound mats



CS_July_2014.inddImproving IIC ratings with resilient clips and channels



How to design for desired acoustical performance





Acoustic qualities of various fl ooring assemblies.

Acoustic qualities of various flooring assemblies.



Designing Floors for Optimal Performance: Understanding the impact of product choices and installation methods

All images courtesy Weyerhaeuser Trus Joist

All images courtesy Weyerhaeuser Trus Joist

by Tomo Tsuda, P.Eng, PE

When designing a wood-framed floor system for multi-family projects, building to meet the prevailing building code is only one step in the performance spectrum. Less clear-cut are the issues surrounding occupant comfort—how much the finished floor bounces under everyday use, or in special cases related to increased load or foot traffic.

The size and thickness of floor framing materials, the spacing of members, and the manner in which they are installed directly influence how stable the floor feels, and how well the finished flooring on top performs over time. Although these elements may not be required to ensure the building’s strength or durability, they are often critical for ensuring occupant satisfaction and perceptions of overall quality.

This article outlines the physics behind floor vibration, identifies problem areas where perceived movement is most likely, and examines strategies for avoiding performance issues.

The challenge
Perceptions of unacceptable floor performance have challenged designers for years. Normal working loads, usually from the movement of occupants, sometimes result in floor motions considered annoying by others. This can occur even on floors where design loads are extremely large compared to the forces generated by a person walking. Conforming to static deflection criteria, as dictated by building codes, does not always eliminate this potential problem.

Manufacturers have investigated performance issues for floors framed with joists and structural composite lumber products. In one case, extensive floor performance survey results were linked with research and theory to the perceptions of floor users. The results of this research can aid specifiers in determining which factors are most likely to contribute to performance and how to make adjustments based on a balance with cost. However, it is first necessary to examine the physics behind the problem—the fundamental dynamic properties and their relative effects on a floor..

Research has shown most people agree low frequencies, particularly in the range of 8 Hz or less, are uncomfortable. By adjusting the details of the floor system (as described in this article), it is possible to raise the frequency of a floor system to a level more widely acceptable.

Large floor movements are generally more noticeable, regardless of frequency. Amplitude increases with span.

This graphic demonstrates how added mass reduces damping.

This graphic demonstrates how added mass reduces damping.


If the wave motion caused by a moving load rapidly reduces, the movement is less noticeable regardless of frequency and amplitude. Damping increases with the addition of a ceiling and a solid partition transverse to the joists.

Adding mass reduces damping. Typical dynamic wave motions and damping related to mass are shown in Figure 1.

The addition of a poured topping on the deck increases the floor’s transverse stiffness, which is a positive effect. The added mass, however, decreases damping, which can have a negative effect. An appropriate method of accounting for these contradictory outcomes is still being investigated.

Assembly components
Various floor assembly components will affect a floor’s performance.

Basic stiffness
This is a combination of joist depth and span. Greater basic stiffness increases frequency and assembly stiffness. For a given span, increasing the joist depth results in the greatest increase in basic stiffness.

Joist spacing and deck stiffness
Reduced joist spacing or increased deck thickness generally improves floor performance by increasing assembly stiffness.

Composite action
‘Composite action’

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is a measure of how the assembly’s deck component interacts with the joist to effectively increase basic stiffness. Having this thicker deck, or use of construction adhesives, improves composite action for short-term dynamic loads.

Joists that are continuous over several supports generally enhance floor performance because they deflect less than the same joist in a simple span application. Care must be taken if such joists continue into an adjoining occupancy as these members can transmit vibration and sound through the floor assembly.

A directly applied (not suspended) gypsum ceiling or strapping—minimum 1×4 applied flat to the joist at 1.5 m (5 ft) on-center (oc) or less—improves floor performance. Assembly stiffness and damping are slightly increased.

Bridging/blocking and strapping properly installed at 2.4 m (8 ft) oc or less enhance floor performance. Bridging/blocking and strapping should be continuous from wall to wall (or support beam) and evenly spaced along the floor span. When interruptions from HVAC equipment (e.g. a duct running parallel to the joists in the floor cavity) and/or changes in joist depth occur, there must be proper consideration for detailing.

When joists are supported on beams, there is a small increase in deflection under normal working loads, which slightly reduces floor performance. Beams designed for relatively large tributary floor areas have less effect.

Additional contributing factors
Full-height framed partitions that are transverse to the joist and away from supports have the effect of damping vibrations, which improves floor performance. However, such partitions must be solidly connected to the floor assembly.

It is important to remember a floor assembly deflects even under light working loads. Bridging that splits during installation, and any ductwork rubbing against joists, can produce noise that may reduce the perceived quality of the floor.

The most effective and economical technique for ensuring good floor performance is the identification of the proper depth, series, and spacing for the floor joist during the design phase. A deeper, stiffer joist is the most economical solution for increasing floor performance for a given span.

In multi-family projects, bouncy floors may lead to occupant dissatisfaction. Long spans next to short spans, long spans under dead loads, and joists used to maximum spans are three areas that may be particularly prone to deflection, but that can also be easily addressed with minor floor system design modifications and product changes.



Performance versus cost
While it is desirable to obtain the highest possible rating for all floors, there are always economic factors. Proprietary software tools can account for the unique variables affecting performance to determine an appropriate rating; the program can also list the components required to reach this rating. It considers the floor assembly in addition to joist stiffness to rate the performance.

The rating system can be targeted to different client preferences, or even to individual areas of the floor, taking into consideration how different rooms will be used and occupied. For instance, different areas of a condominium unit could require different actions. As an example, a rec room designed for entertaining might be considered separately from the dining area or kitchen. Other considerations include whether rental apartment unit floors be considered the same as those in expensive condominiums from a floor performance standpoint. (This might depend on the size of the rental units and the target market.)

The system gives designers a comparison cost value based on an input cost of decking and the volume of floor joists in the assembly. This is not precise cost per area, but it gives a reasonable relative number. Some experimentation with varying components of the assembly offers the designer a good feel for how to obtain the best value in the assembly, from both cost and performance rating standpoints. For example, depending on building details, it may cost very little to increase joist depth. In other cases, a smaller joist section at reduced spacing may be the best choice.

Problem areas
Although it is important to ensure the entire structure meets strength and service requirements, there are a few areas of a floor more likely to attract attention from occupants. Therefore, these are the most logical points of focus.

Long spans next to short spans
In a room in which there are long span joists parallel to short spans, the occupant may perceive the floor to be more solid in areas with short spans. To accommodate these differences in floor performance, designers can tighten the spacing of the joists or employ stiffer joists.

Long spans combined with higher dead loads
In multi-family projects, open floor plans, combined with heavy kitchen islands or concrete toppings, can be another trouble spot. To reduce the possibility of unacceptable vibration, the size of members under the dead load can be increased, or spacing tightened up, even when the code allows for wider layout.

The size and thickness of floor framing materials and member spacing/installation influence how stable the floor feels, and how well the floorcovering performs in the long term.


Joists used to their maximum spans
When reaching the maximum strength or deflection limit for a certain joist, the floor system may be economical and strong enough, but it also may undergo more deflection or bounce than expected. Depending on the client’s expectations, it may be better to consider an alternative, stiffer floor assembly, and evaluate the effect on performance to make the best system choice.

Installation considerations
Along with product specification considerations, the floor system’s actual installation plays a significant role in overall performance related to occupant comfort. Framing contractors should adhere to the following standards to reduce the chance for squeaky floors:

Floor system
For the floor system, all subflooring fasteners must hit the supporting joist members, with all supports solid and level. Hangers, when used, must be installed in accordance with manufacturer’s instructions.

To provide proper spacing, oriented strandboard (OSB) and plywood subfloor panels should be spaced with a 3.2-mm (1/8-in.) gap at all edges and ends—this accounts for naturally occurring expansion, enabling avoidance of buckling. Many premium floor panels have tongue-and-groove edges designed to self-gap.

Wet lumber can lead to dimensional changes as the joists dry, resulting in nail pops and floor squeaks. Wood subfloor panels should be allowed to dry if they get wet, especially if installed under sensitive finish materials such as hardwood.

To avoid nail pops, pullouts, and shiners (all of which can cause squeaks), the correct nail size and spacing must be specified, ensuring the nails penetrate the floor joists and sink fully. Generally, nails (i.e. 6d ring or screw shank, or 8d common) should be spaced 152 mm (6 in.) oc along supported panel edges, and 305 mm (12 in.) oc on the panels’ interior supports, or as specified on the construction drawings (which may call out a longer nail for a thicker deck or may need higher diaphragm capacity). For panels thicker than 25 mm (1 in.), 10d nails should be used.

It is also important to remember glue-nailed construction techniques are optimal for ensuring a flat, stable floor. One should specify solvent-based glue meeting ASTM D3498, Standard Specification for Adhesives for Field-gluing Plywood to Lumber Framing for Floor Systems; in cases where latex subfloor glue is required, careful selection is necessary due to the wide range of performance between brands.

While it is desirable to obtain the highest possible rating for all floors, there are always economic factors that may have an impact on performance.


Contractors should apply glue per manufacturer’s specifications and ensure the joists are dry and free of dirt. Many manufacturers recommend applying a continuous (¼-in.) diameter glue bead to framing members, and using a serpentine pattern for supports that are (3½-in.) or wider. Two beads of glue should be applied to panel joint locations; a 3.2-mm (1/8-in.) bead at the tongue-and-groove joints can further improve floor performance. Since the glue should not be allowed to develop a skin or dry, it should be applied onto only one to two panels at a time.

It is also important to ensure installers are reminded not to use a sledgehammer to force tongue-and-groove joints tightly together, as this can damage the panels and close up the required gap. After the floor system’s installation, it is critical to walk

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it, checking for squeaks or bounce beyond what is expected. It is easier to remedy the problem now than after the interiors are finished.

Even when built to code, many floor systems still have room for performance improvement. Design choices upfront—such as joist spacing, stiffness, or continuity—can significantly affect the floor system performance for occupants. ‘Performance-focused’ design, combined with reasonable care during installation, can help avoid potential occupant dissatisfaction down the road.

Tomo Tsuda, P.Eng, PE, is an engineer for codes, standards, and product engineering with Weyerhaeuser Trus Joist. He has worked with the company for 17 years, including a decade in Japan. Tsuda is a licensed civil engineer in British Columbia and Idaho. He can be contacted at tomo.tsuda@weyerhaeuser.com.