Tag Archives: Multi-family

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

Reinforced concrete flat-plate construction is popular among mid- and high-rise residential construction projects. It provides a great deal of flexibility in the placement of the structure’s vertical load-carrying elements (i.e. columns and walls) without sacrificing the efficiency of the floor framing—as could potentially be the case with steel or masonry.

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.

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.

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/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.

Conclusion
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.

Improving Floor/Ceiling Sound Control in Multifamily Projects: Sound Testing Practices

by Josh Jonsson, CSI

The sound transmission class (STC) and impact insulation class (IIC) are ASTM-derived single number ratings that try to quantify how much sound a stopped by partition being tested.

Laboratory testing involves an ideal setting for the floor/ceiling assembly—it is isolated from the walls, and there are no penetrations for HVAC, plumbing lines, sprinklers, can lights, or electrical boxes. In the field (i.e. F-STC and F-IIC), the floor/ceiling assembly often sits on load-bearing walls, is connected to the structure, and contains many ceiling and floor penetrations for the items just mentioned. Consequently, the code allows for a lower rating for field scores over those in the lab.

The STC rating essentially tells how much noise is stopped from going through a wall. The test involves blasting loud noise at all the measured frequencies in a room. A Level 1 sound meter measures this exact noise in that room level at all frequencies, as well as the sound in the room on the other side of the partition. These two different levels are then essentially subtracted from each other, with some corrections made for background noise.

The IIC rating is not a comparative test like the STC. Rather, it uses an ASTM-specified tapping machine that sits directly on the floor—more specifically, directly atop the finished floorcovering. (Consequently, an IIC rating always lists the floorcovering with which it was tested.)

The machine has five steel hammers that spin on a cam shaft, falling onto the floor from the same height, no matter what or who is testing. These hammers put a consistent energy into the floor. The sound level meter is taken downstairs below the tapping machine and the sound level is measured at all the frequencies called out in the ASTM standard. These sound levels are plugged into the equations in the standard; a single number is generated summarizing how much sound was stopped.

To read the full article, click here.

Improving Floor/Ceiling Sound Control in Multifamily Projects

DSC_1207-2

All images courtesy Maxxon Corporation

by Josh Jonsson, CSI

In recent years, demand has increased for better floor/ceiling acoustics in multifamily construction. This has been driven by consumer desires, new guidelines from code bodies, and stricter enforcement of existing codes. How do design professionals keep pace as the traditional approaches to multi-unit residential sound control evolve?

This article reviews important new guidelines that must be taken into account by architects and specifiers, and examines how construction manufacturers have created new products or enhanced existing ones in the pursuit of achieving higher acoustical performance.

Thanks to product technology improvements and more stringent regulations, the wood-frame multifamily industry is paying increasing attention to the acoustics of the floor/ceiling assembly.

Thanks to product technology improvements and more stringent regulations, the wood-frame multifamily industry is paying increasing attention to the acoustics of the floor/ceiling assembly.

Two of the principal measurement standards for acoustics in multifamily construction are:

  • sound transmission class (STC), which pertains to the amount of airborne sound contained by a given building element (i.e. walls, doors, windows, and floor/ceilings); and
  • impact insulation class (IIC), which deals with impact noise (i.e. footfall, chair scrapes, and dropped objects) transmitted through a floor/ceiling system.

Both these single-number ratings apply to the full assembly of building materials used to separate tenants, including floor/ceiling assemblies.

For more than 50 years, these measurements have helped architectural project design teams quantify the acoustic levels of floor/ceiling assemblies. In fact, the Department of Housing and Urban Development (HUD) wrote A Guide to Airborne, Impact, and Structure Borne Noise: Control in Multifamily Dwellings in 1967, helping reinforce the importance of sound control in multifamily construction. This document, along with the Uniform Building Code (UBC), helped project teams recognize an acoustical threshold was needed in multifamily construction. UBC required an STC and IIC rating of 50 (or 45 if field-tested as F-STC or F-IIC). The higher the rating, the better the performance. (See “Sound Testing Practices.”)

In 1997, UBC gave way to the International Building Code (IBC) as the widely accepted model code. This shift brought greater awareness of acoustical ratings and their deemed thresholds in unit-over-unit construction, however these code levels remained aligned with the UBC’s established minimum requirements of STC and IIC 50 (or 45 if field-tested).

As the multifamily industry became more competitive, developers began offering upgrades in flooring and lighting to tenants as an amenity, yet little to no attention was paid to acoustical performance. This is astounding when one considers acoustics continue to be one of the driving factors in maintaining low vacancy levels, as well as one of the most litigated issues in this type of construction. To compound the subject, many of the amenity upgrades offered, such as hard-surfaced finished floors and canister lighting, can adversely impact a floor/ceiling assembly’s performance.

CS_July_2014.inddUpdating acoustical recommendations
In response to the need for updated acoustical guidelines, the International Code Council (ICC), along with several respected acoustical experts, created ICC G2-2010, Guideline for Acoustics. The guideline recognizes:

the current level and approach of sound isolation requirements in the building code needs to be upgraded. The requirements are currently insufficient to meet occupant needs.

As shown in Figure 1, the guide provides two levels of acoustical performance: ‘acceptable’ and ‘preferred.’ Both exceed code minimums for airborne and structure-borne noise.

These new levels now give a clearer direction on what levels should be targeted for desired acoustical performance, depending on the building type. As the names suggest, when one wants a building that has an acceptable level of acoustical separation, ‘acceptable’ is targeted. When one is designing a building on the higher end of market rate or luxury level, or has tenants or owners sensitive to noise, the desire should be for a ‘preferred’ level of performance.

Components of acoustical design
How do these new recommendations apply to the current approach for multifamily construction? As Figure 2 shows, a commonly specified design for many multifamily projects, which is also recommended by acoustical consultants, includes:

  • hard-surfaced flooring;
  • 25 mm (1 in.) or more of gypsum concrete;
  • 6.4-mm (¼-in.) entangled mesh sound mat;
  • wood subfloor;
  • wood floor trusses or joists;
  • insulation;
  • resilient channel; and
  • one layer of gypsum board.

CS_July_2014.inddThe typical rating for the design would be an IIC 51 to 55 and STC 56 to 60, depending on the floorcovering (e.g. laminate, tile, floating engineered wood), the acoustical performance of which would be listed by the consultant and verified by test reports. These numbers exceed code minimum and exceed the STC requirement for ‘acceptable,’ but only marginally—at best—meet an ‘acceptable’ level for IIC and any nuisance impact noises from upstairs tenants (IIC).

It is important to keep in mind these listed ratings would be achieved by selecting and installing all the components based on proper acoustical design. For example, the resilient channel would need to be a product similar to a proprietary one using steel measuring 0.5 mm (0.021 in.) thick by 38 mm (1.5 in.) wide, as opposed to a similar but lesser design lacking proper acoustical performance. To increase the IIC rating, an upgrade to the sound mat and/or the resilient ceiling system must be made.

Improving IIC ratings with sound mats
Traditional sound control mats with entangled mesh enhance IIC performance through the mesh being attached to fabric, which is loose-laid over the subfloor and then encapsulated with a gypsum concrete topping. The entangled mesh acts as a spring and produces an air space with little surface contact (i.e. three to five percent). Until recently, IIC performance was upgraded by using a thicker sound mat and deeper gypsum concrete.

Sound mat manufacturers have added new technology that allows for higher ratings while continuing to meet industry expectations for the corresponding thickness of the gypsum concrete. Traditional entangled mesh mats are now being manufactured with an additional acoustical fabric—Figure 3 depicts a 6.4-mm (¼-in.) entangled mesh mat with this upgrade. The acoustical fabric is laminated to the underside of the mat, creating an additional vibration break and absorptive layer. This improved product requires the same thickness of gypsum concrete as its standard counterpart. The IIC performance of the system is improved by two to five points without adding any measurable thickness to the floor system.

Another option is to employ the original 6.4-mm entangled mesh sound mat and a secondary topical mat placed between the gypsum concrete and the finished floor. If this option is selected, this secondary mat should be high-quality and thoroughly tested for sound ratings. (The sound test showing this type of product’s performance must be specific to the assembly that is being used versus a sound test from an unrelated design—for example, using concrete test data for a 2×10 joist system.)

CS_July_2014.inddImproving IIC ratings with resilient clips and channels
Properly installed, high-quality resilient channel will improve IIC ratings, but the resilient channel’s effectiveness can be easily lessened through faulty installation. To install traditional resilient channel, proper-length screws are imperative so as not to penetrate the joist or remove the channel’s resiliency.

Penetrations from the drywall into the joist through the resilient channel create flanking paths that transfer sound through a floor/ceiling assembly, as does having a channel affixed tightly to the assembly. For these reasons, new resilient clips that are difficult to install improperly have been introduced to the market. These clips can deliver equivalent performance to properly installed, high-quality resilient channel.

Hanging systems that provide spring and reduce or eliminate resilient channel contact with the joist offer even better performance. Figure 4 shows two such products: the ceiling wave hanger and a spring isolator. Either of these products installed in conjunction with a 6.4-mm (¼-in.) entangled mesh sound mat on the floor above would help the system exceed the ‘acceptable’ level, and approach ‘preferred’ levels for the IIC rating with hard-surfaced floorcovering. See Figure 5 for various assemblies and their acoustics attributes.

How to design for desired acoustical performance
As a specifier or architect team leader, one must first determine the level of acoustic performance to which to design. This should not be a matter of just meeting code—rather, the entire conversation must be approached in a new light. The following questions should be asked:

  1. When considering the amenities offered to tenants, how important are the acoustics of the unit? In other words, how important is the quality of life related to acoustical privacy?
  2. Does the project team want to just meet code because complaints and vacancy rates are unimportant or not a factor? Do they want ‘acceptable’ performance, significantly reducing noise complaints and removing sound control from the vacancy equation? Or, do they want ‘preferred’ performance to meet client expectation and greatly reduce potential for noise complaints?
  3. Once the level is determined, which method makes the most sense for achieving that performance level? Does the sound mat get upgraded to a very high-performing mat (manufacturers offer many styles with differing performances)? Does the sound mat get upgraded while keeping the system as thin as possible? Does the sound mat stay the same and the ceiling hanger system get upgraded? Is a secondary sound mat added while upgrading the primary sound mat and/or ceiling system to reach optimal sound ratings? Or, do the mat and ceiling get upgraded to reach better ratings?

CS_July_2014.inddEven after the desired level of performance has been determined, there are other factors that should be considered, such as whether the project will always be apartments or if they could become condominiums. There is also the matter of whether carpet and pad areas will always have carpet and pad.

Projects that start as apartments and then plan on being converted into condominiums should be approached as if they were condominiums from the beginning. Future owners may tear out carpet and replace it with hard-surfaced flooring.

Sound mat manufacturers receive a high volume of phone calls every year where a condominium project put sound mat only in the hard-surfaced areas. The new owners want hard surfaced flooring throughout and are being told they need to provide levels of performance similar to the ‘preferred’ levels while only being able to add a thin amount to the profile of the floor. As they can only do work in their unit, they are left trying to use a thin, lower-performing sound mat to reach the requested, more stringent criterion.

Acoustic qualities of various fl ooring assemblies.

Acoustic qualities of various flooring assemblies.

Conclusion
Throughout the United States, the wood-frame multifamily industry is paying increasing attention to the acoustics of the floor/ceiling assembly. Innovative architectural acoustic products continue to see greater use in existing metropolitan areas as well as in new areas of the country. It is important specifiers continue to be educated on new products and adapt their specifications to ensure they meet defined levels of sound control that tie directly to the end user’s satisfaction with their living space.

Josh Jonsson, CSI, is an acoustical specialist and West regional manager at Maxxon Corporation. He has more than 15 years of experience in the architectural noise industry and has worked for acoustical and vibration consulting agencies. Jonsson is a member of CSI, Acoustical Society of America (ASA), and ASTM International committee E33 Building and Environmental Acoustics. He can be contacted via e-mail at josh@maxxon.com.

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..

Frequency
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.

Amplitude
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.

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.

Continuity
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.

Ceilings
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
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.

Beams
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.

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.

CS_DECEMBER_2013.indd

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.

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.

Subflooring
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.

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

Conclusion
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.