Cold-formed steel design for acoustic code compliance

by nithya_caleb | October 30, 2018 11:35 am

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By Steven R. Fox, PhD, P.Eng.

The North American steel industry has completed a research program to develop the necessary tools to meet code requirements for acoustic separation in cold-formed steel frame construction.

For decades, Canada’s building codes for residential construction only took sound transmission through separating walls or floors into account when assessing acoustic separation and performance. Known as sound transmission class (STC) ratings, this system led to innumerable situations where inadequate acoustic separation between neighbours was blamed on design or workmanship of those assemblies, often focusing remediation efforts in the wrong place. This problem was especially evident in multifamily housing projects where privacy and sound transmission between units are a major concern for owners and residents.

The National Building Code of Canada (NBC) has evolved to consider additional paths for sound waves, including flanking transmissions through shared ceilings and floors. “Flanking transmissions” simply means sound waves in a room engaging with all of the room’s surfaces, including ceilings and floors. When those ceilings and floors are shared with adjacent rooms, the transmitted vibrations are expressed as sound next door regardless of the acoustic separation designed into the shared wall between the two rooms that was the focus of previous NBC editions.

As 2015 NBC is adopted throughout the country, architects and specifiers are designing to this more holistic understanding of sound transmission, referred to as apparent sound transmission class (ASTC) ratings, for their assemblies. This evolution in treatment of sound in residential construction can reduce sound transmission between rooms and units, but it also creates new challenges and requirements for architects and specifiers.

Those challenges are frankly why it has taken so long for building codes to catch up to what has been widely understood for decades. ASTM E336, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings, providing an ASTC rating, was first published in 1997. However, while measuring acoustic performance is one thing, offering architects the tools to reliably predict the performance is another. While some European building codes have used the 2005 International Organization for Standardization (ISO) 15712-1, Building acoustics — Estimation of acoustic performance of buildings from the performance of elements — Part 1: Airborne sound insulation between rooms, these methods have not found broad adoption in North America. This is because they largely provide reliable estimates for buildings constructed from heavy, homogeneous building elements, and not for structures constructed from lightweight-framed elements widely used in mid-rise construction projects in North America. Additionally, from a practical perspective, ISO standards for building acoustics have many differences from the ASTM standards used by the North American construction industry—both in terminology and in specific technical requirements for measurement procedures and ratings.

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To address these challenges, the National Research Council Canada (NRC) and the Canadian Sheet Steel Building Institute (CSSBI) undertook a project with co-funding support from the Steel Market Development Institute (SMDI) and other steel industry partners to support the transition of construction industry practice to using ASTC rather than existing STC ratings for sound control objectives in NBC. In addition to supporting code compliance, the study report also facilitates design for enhanced levels of sound insulation in applications where desired, and should be generally applicable to construction with cold-formed steel-framed assemblies in both Canada and the United States.

Thanks to this research, architects and designers will be able to use the NRC’s soundPATHS[3] prediction tool for the calculation of direct and flanking sound transmission between adjacent rooms. This web application incorporates data from this research to help builders accurately predict ASTC performance. For more complex environments, the NRC report titled “Apparent sound insulation in cold-formed steel-framed buildings[4]” outlines the steps of the process and standard measurement data required for the calculations necessary to translate the normal calculation procedure of ISO 15712-1 to ASTC in compliance with 2015 NBC requirements.

Researchers tested a variety of cold-formed steel wall and floor assemblies at the NRC’s wall and floor sound transmission facilities in Ottawa, according to ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements. To measure all potential permutations of flanking sound transmissions between various rooms, an eight-room structure (two floors of four rooms each) was constructed in the facility. Researchers tested combinations of loadbearing and non-loadbearing cold-formed steel stud walls, cold-formed steel C-section floor joists, gypsum board, insulation, resilient channels, gypsum-concrete floor toppings, and floorcoverings.

The full report offers detailed discussions of experimental factors and findings. It also provides specific STC test values for almost 50 different common wall assemblies and more than a dozen floor or ceiling assemblies.

In addition to these specific measurements, researchers identified some larger trends and best practices that can inform broader strategies for residential design.

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Broad trends

One key experimental takeaway is continuous surfaces between rooms are a threat to effective acoustic separation.

Experimental results across a variety of constructions make it clear a subfloor which is continuous across the junction can cause serious flanking sound transmission via the floor surfaces of a wall/ceiling junction with continuous subfloor, when compared to a wall/ceiling junction with discontinuous subfloor. With a continuous subfloor, the ASTC values were at the lower limit of acceptability. Breaking the subfloor with an intervening wall assembly can provide significant ASTC value, on the order of 15 to 20 points.

The flanking transmission at wall/wall junctions also depends significantly on the way gypsum board is attached to the wall framing. Gypsum board continuous across the junction was shown to decrease the ASTC rating by about 30 points compared with gypsum board interrupted at the junction. Continuous gypsum board on flanking walls provides an effective path for sound transmission between adjacent rooms and can significantly affect the apparent sound insulation, independent of the design of the separating wall. When the gypsum board and wall studs are well-separated, ASTC values for the wall-to-wall path can be reduced by almost half.

In loadbearing junctions, the continuity of the joists affects both the flanking sound transmission via the floor surfaces and the transmission via the ceiling. Joists continuous across the junction allow sound to travel uninterrupted, and therefore should be avoided where possible. In the study, ASTC values for the ceiling-to-ceiling path for discontinuous joists were 10 points higher than for continuous joists.

Continuous cavities are also conducive to sound transmission. Fortunately, many best practices in design to limit smoke and fire penetration also contribute to sound insulation by eliminating cavities that can provide acoustic flanking paths. Adding a fire block at the wall-to-floor junction does improve the STC flanking rating for floor-to-floor and ceiling-to-ceiling transmission, but its impact on the rating is often minimized by limited performance at some frequencies.

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Wall assemblies

Researchers measured direct sound transmission loss through wall assemblies comprised of a frame of cold-formed steel studs with gypsum board attached on both sides of the studs. The gypsum board was either fastened directly to the studs or supported on resilient metal channels. Most of the tested assemblies had sound absorbing material in the cavities between a single row of loadbearing cold-formed steel studs. (A complete description of all tested assemblies with individual STC test ratings is included in the report.)

Some significant takeaways from the research process include:

• adding extra layers of gypsum board in the construction detail had the most obvious effect on sound transmission loss values of cold-formed steel-framed walls;
• the change in STC rating due to filling the inter-stud cavities with sound absorbing material is similar to the improvement observed when the layers of gypsum board are doubled on both sides (partially filling the cavity provides most of the effect);
• although differences are evident among the sound transmission loss data with the three tested types of sound absorbing materials (glass, mineral, and cellulose fibre), the differences are insignificant in the sound frequencies dominant for determining STC ratings;
• adding sound absorbing material to the wall cavity has a major effect if the gypsum board on at least one side is mounted on resilient metal channels;
• thickness of the material used to fabricate the cold-formed steel studs has only a small effect (the STC rating increases by one to two points if studs with thinner steel are used);
• increasing the spacing between the cold-formed steel studs from 406 mm (16 in.) on centre (o.c.) to 610 mm (24 in.) o.c. increases the STC rating by one to two points for the walls used in this study, and a similar effect was observed when increasing the spacing between resilient metal channels;
• changing the depth of the steel studs from 92 to 152 mm (4 to 6 in.) studs can increase the STC rating by up to four points on walls with resilient channels; and
• the use of flat straps and bridging channels in loadbearing cold-formed steel-framed walls was found to have a negligible effect on the STC rating.

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Floor and ceiling assemblies

Researchers tested floor assemblies with cold-formed steel joists spaced 406 mm o.c., and all had resilient channel spacing of 406 mm o.c. or less. These common features of the floors rule out demonstrating the acoustical benefits of changing these parameters, but ensure adequate fire resistance.

Some important takeaways from the research include:

• the mass per area of the top floor surface was the primary factor in sound transmission loss;
• regular concrete’s 16 per cent higher density when compared to gypsum concrete gave significant increase in mass per area and hence higher sound transmission loss;
• the subfloor is secondary to sound transmission (while thinner joists and increased spacing between resilient channels provide small improvements, they are overshadowed by mass per area of the
  top surface);
• doubling layers of 15.9 mm (⁵/₈ in.) fire-resistant gypsum board on a ceiling consistently increased the STC rating by about four points;
• filling about 40 per cent of the cavity volume between floor joists increased STC ratings by almost 10 per cent (increasing the fill to almost 100 per cent provided a benefit only a third as large, but in total, filling the cavity with absorbing material was roughly equal to the percentile increase in STC rating from doubling gypsum board layers);
• as with walls, the study suggests no significant change in the STC rating due to the type of sound absorbing material used;
• in studies between cold-formed steel-framed floors with a joist depth of 254 mm (10 in.) and a nominally identical cold-formed steel-framed floor with a joist depth of 317 mm (12 in.), STC ratings differed by one point at most (cavity depth does not significantly influence sound insulation);
• while carpet and laminate flooring perform better than vinyl flooring at higher frequencies, the impact for all three in STC ratings is small because of poor low-frequency performance; and
• adding gypsum board to ceilings increases not only the direct sound transmission loss for the floor-to-ceiling assembly, but also the flanking transmission loss for paths including ceiling surfaces.

The ability to use thinner studs when building with steel gives the material an inherent advantage over thicker alternative materials in terms of acoustic performance. These findings, detailed in the NRC research report and the associated tools, will help builders maintain compliance with updated ASTC rating requirements in the national codes while taking advantage of the many benefits of cold-formed steel construction, from sustainability and resiliency to design flexibility and adaptability and changing occupant demands.

[8]Steven R. Fox, PhD, P.Eng., is the general manager of the Canadian Sheet Steel Building Institute (CSSBI), the national association representing the structural sheet steel building products manufacturers in Canada. In this role, he is responsible for the management of all CSSBI’s technical and promotional programs. Fox has more than 35 years of experience in the construction industry, most of which has been with CSSBI. He can be reached at sfox@cssbi.ca[9].

Source URL: https://www.constructioncanada.net/cold-formed-steel-design-for-acoustic-code-compliance/

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