WIND-INDUCED CREAKING NOISE IN TALL BUILDINGS

Introduction

During the last decade increasing numbers of apartment tower residents have reported disturbance from creaking noises seemingly generated within the building structure during high winds.

Tracking news articles on the subject across the globe shows significant activity in Australia in the mid 2010s and in the USA a few years later.? No causal link has been established to any specific building technique or material change, and the spread of the phenomenon seems as likely linked to growing awareness of an issue affecting slender residential towers of increasing height, overlain on the proliferation of these towers in certain regions.

The noise complained of is often described with reference to sailboat analogies, such as a pirate galleon rocking in a bad storm, or a tall sailing ship straining against its moorings. ?Online searches quickly return numerous videos of buildings around the world that are affected by this curious noise.

Although residential and commercial buildings are both affected, the implications are generally often more critical in dwellings in which sleep disturbance and annoyance are more critical. ?It has been suggested that property values can be affected, leading to disputes between the stakeholders in real estate transactions. ?The matter is, therefore, particularly sensitive, resulting in guarded discussions on the underlying causes. ?This paper seeks to fill some of the knowledge gap to the extent that it is helpful and appropriate.

Scope

CSA has been actively involved in assessing the severity of the effect over a number of years and studying the mechanism(s) generating the noise, with a view to evaluating mitigation options.

This paper provides insight based on this project involvement as well as the current status of a Ph.D research project[1] being pursued by the author .

This paper is intended to serve as an informative for acousticians, contractors and other parties invested. ?The information presented is deliberately generic, however, and may not apply to all situations. ◆

Description of the Noise Generating Mechanism

Local meteorological conditions, structural requirements, building shape/fa?ade design and building materials all play a significant role in the occurrence of the noise, however the exact relationship between each element and the noise is difficult to define given the complexities of each system.? It is clear, however, that affected buildings tend to be tall and slender, bending and flexing in high-wind conditions. ?This movement is not only perfectly safe, but necessary in order to maintain the structural integrity of the buildings in varying environmental conditions.? The bending and flexing of a building is not linear in elevation, resulting in each floor slab behaving slightly differently to that directly above and below.? This differential movement is typically referred to as inter-storey drift.

As each slab moves relative to its neighbours, friction fit internal fit out elements can rub against each other, releasing relatively low levels of energy as vibration. ?This vibration will then resonate efficiently through other rigidly connected building elements and generate the audible creaking sound. ?

The regularity of the creaking sound can range from single ‘clicks’ as individual, minute incremental movements overcome static friction and then ‘stick’ again (referred to as ‘slip-stick’ movement) to a series of such movements combining to form a rhythmic creak during peak conditions.

The creak regularity can then be linked to the natural frequency of the building’s response to wind-loading conditions that cause the building to oscillate. ?Wind-loading can result in several types of building movement, the most relevant to this topic being along-wind movement and across-wind movement. Along-wind movement is the building movement parallel to wind direction, whereas across-wind movement is the building movement perpendicular to wind direction It is generally agreed amongst structural engineers however, that across-wind motion results in the highest overall wind-loading on tall buildings, inducing a large dynamic response.[2]

Across-wind movement is generally attributed to a phenomenon known as vortex shedding in which, due to aerodynamic properties of buildings and the fluid behaviour of air, air is directed to a side of the building, resulting in negative pressure forming on the other. ?This results in air flow alternating between each side of the building. ?As this process continues during periods of high-wind conditions, the building will oscillate in the most efficient, or ‘natural,’ frequency.

Literature Review

Due to its sensitivity, little research on the topic has been widely published. ?However, a number of news articles are available, which are helpful in illustrating the scope of the phenomenon as well as characterising the effect on occupants of affected buildings.

A technical review of a specific mitigation system is partially published as part of the Acoustics 2017 conference proceedings, which is beneficial in confirming the effectiveness, in principle, of mitigation measures.

Disappointingly, as a result of understandable hesitancy over disclosure, there is a great deal of misinformation surrounding the topic. ?In the course of our research, we have seen the phenomenon wrongly attributed to a number of unrelated mechanisms such as the stack effect,[3] loose material in ductwork or louvres, or from the expansion and contraction of fa?ade structures due to temperature fluctuations, among others.

The noise from expansion and contraction of fa?ade elements can generate noise due to differential thermal expansion and contraction, which might have some similarities to that of the creaking described herein, in that it can be percussive and difficult to precisely locate.? Its character, however, is typically limited to a single, or brief series of, periodic ‘popping’ sounds as the fa?ade either expands in the heat or contracts in the cold.[4] ?In contrast, an important distinguishing factor for the creaking is the potential rhythmic component of the sound.

No widely agreed assessment methodology is currently available, nor are there any guidelines on thresholds of acceptability. ?However, in the course of our research, it has been found that, in lieu of a more appropriate methodology, references to WHO Guidelines on community noise 1999, or derivatives thereof, such as those tabulated in Table 4 of BS8233, are common.

Assessment Considerations

The primary effects of the noise relate to annoyance and the potential for sleep interference. ?As such, although average sound pressures can be helpful references, the sound pressure level of individual events may be the most relevant factors, whilst considering the uncertainty in maximum sound pressure level data.

When seeking to relate creaking noise to wind conditions, the specific micro-climate must be considered to understand the actual forces giving rise to the building motion on a second by second basis.? A high correlation between increased wind gust speed and the creaking has been identified. ?This should be paired with directional data to account for the nature of building motion induced in its surrounding microclimate.

General Guidance

Our research suggests that a dataset consisting of less than 3 hours of sound monitoring data collected during high wind conditions may be insufficient in appropriately categorising the effect of the phenomenon. ?To reduce uncertainty, sound level monitoring can be undertaken following procedures cited in the ANC Guidelines 2020: Measurement of Sound Levels in Buildings. In addition to monitoring of sound pressure levels, other elements are critical to characterise the total effect. These are described below.

(a)? Regularity of creaking events

The regularity of creaking events can be defined as the elapsed time between the start of two consecutive creaks from the same epicentre. ?Once the movement is activated, it may accelerate as wind conditions advance and decelerate as wind speeds decrease. ?The representative value(s) used to describe this element should be derived from the data when the phenomenon is at its peak. ?Typically, this may occur when the building is oscillating at its maximum amplitude or when wind gust speeds are at their highest.

(b)? Duration of individual creaking events

The duration of creaking events can vary significantly from building to building, ranging from a few consecutive ‘ticks’ to an almost continuous rhythmical creaking sound. ?The duration of individual creaking events is, therefore, important in contextualising the overall impact.

(c)?? Sound pressure level of individual creaking events

A typical creak should be considered in the context of the total dataset for a given period (or periods). ?Special consideration should be given to the inherent uncertainty in Lmax datasets. ?It may, therefore, be more representative to consider statistical data types, such as LA01 over the duration of relevant dataset.

(d)? Wind-gust speed threshold to trigger the creaking

As wind patterns vary dramatically around the globe, it is important to identify the wind-gust speed threshold that triggers the phenomenon within the specific test specimen. ?Close analysis of the data may be required for this element, as a single creak during relatively low speed gusts may not necessarily represent the onset of a creaking period.

For buildings that are more affected by winds traveling in specific directions, it may be necessary to undertake several studies to determine the wind direction that most greatly affects the building response, in order to identify suitably robust representative values for each element.

Due to the non-linearity of the building sway, as well as other factors such as structural loading, the data may vary along the elevation of a building.? For this reason, testing in one room in a building is unlikely to be representative of other spaces throughout the building.

Although there are no thresholds or guidance values for each of these assessment elements, it is clear that lower numerical values are desirable for elements b and c (duration of individual creaking events and sound pressure level of individual creaking events), whilst higher numerical values may be desirable for elements a and d (regularity of creaking events and wind-gust speed threshold to trigger the creaking).

Mitigation

Some strategies are available for mitigation in new-build structures as well as some that can be retrofitted to existing buildings. ?Some are proprietary products, whilst others are builderswork techniques that can be implemented. ?In either case, no single strategy can be dubbed as a full solution due to the complexity and limited published data available for each.

All strategies, however, rely on the simple principle of mechanically decoupling the offending connections. ?In practice, this is far from simple as an array of considerations must be made when pursuing any individual strategy, including fire safety, material resilience, financial cost, installation time, and sustainability, among others.

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Clarke Saunders Acoustics is an independent consultancy practice specialising in applying both rigor and pragmatism to real world challenges. We are actively engaged in development of best practice guidance and standards across the acoustics industry, collaborating with colleagues, stakeholders, and decision makers. To continue this discussion on Creaking noise in tall buildings, or any of the other multitude of areas in which acoustics touches all our lives please contact us at [email protected] | LinkedIn.

[1] The Ph. D Research project entitled Wind-induced creaking noise in tall buildings is supported by The Association of Noise Consultants and Clarke Saunders Acoustics

[2] Günel M H and Ilgin H E. 2014. Tall Buildings Structural Systems and Aerodynamic Form. Routledge. Abingdon, Oxon.

[3] Stack effect relates to the passage of air through the elevation of a building via unsealed gaps and thereon into risers within the cores. ?The noise from the stack effect is generally described as a ‘whistling’ noise.

[4] A description of this phenomenon is presented in the judgement from Tejani v Fitzroy Place Residential Ltd. (2022)