What’s Shaking?

by Stacy Rinella | February 7, 2024 9:19 am

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From that annoying rattling sound disturbing your sleep to disruptive interference of sensitive laboratory equipment, vibration can have a significant impact on building occupants and uses. It is also one of the most overlooked and misunderstood aspects of building design. In both new building design and the renovation or repurposing of spaces in existing buildings, inexperienced designers and well-intentioned owners can make decisions with significant undesirable consequences. Without a good understanding of the dynamics of vibration, architectural, structural, and mechanical design choices can magnify vibration from existing sources or create new vibration issues within a building.

So, what is vibration? It is an oscillatory motion in materials that can be felt by people through surface contact, heard as a rattling of building components, or experienced as a low rumbling sound radiated by room surfaces responding to a vibration source nearby. The common sources of vibration within buildings include people walking, dancing, or exercising, mechanical equipment (pumps, fans, etc.), and elevators. Vibration may also originate from nearby transportation sources such as subways or trains under or near the building, elevated rail or roadways, vehicles in connected parking structures, and aircraft taking off and landing, or from construction activities in the vicinity.

Vibration can impact building occupants and uses in several ways. In residences and hotels, it can be annoying during waking hours and prevent restful sleep at night. In workplaces and schools, vibration can be disruptive of activities requiring communication and concentration. In healthcare and laboratory facilities, vibration can interfere with sensitive surgical, imaging, and laboratory equipment, and it can prevent patients from getting the rest they need to aid in recovery. In performing arts centers and recording studios, audible ground-borne or structure-borne noise during quiet moments in performances may annoy performers and audiences, or ruin digital recordings. While some vibration may be anticipated in transit centers, excessive vibration when trains or buses arrive and depart can be unsettling to passengers waiting on platforms.

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Now that we understand what causes vibration and how it can impact various uses, we can review the factors in building design that can exacerbate or mitigate it. Lightweight structures are more likely to respond to vibration, while more massive elements will tend to suppress it. Long-span beams are more susceptible to motion generated by human activity compared to shorter spans. Designs with adjacencies of vibration sources such as fitness centers next to or above quiet living and working spaces can create significant challenges. Equipment or machinery (such as mechanical components or elevators) without proper isolators can transfer significant vibrations into structures. In urban areas, it is common to have buildings constructed adjacent to or directly above transportation rights-of-way or subway tunnels. This situation can be particularly challenging for more sensitive uses such as residential, hospitality, healthcare, and performing arts functions.

Whether any of these factors are apparent or not, thoughtful consideration early in the design process can identify effective mitigation methods and avoid chronic problems or costly retrofits later. In new development, this might start with a vibration survey at the site if it is adjacent to a potential active source like a railroad or subway. With this data, experienced acoustical consultants can use software to estimate the vibration in a proposed building based upon the measured ground vibration. The quantitative output allows them to identify and assess potential modifications and advise designers on structural and architectural measures to reduce vibration. This could include the layout and sizing of columns and beams, the specification of resilient isolators at foundation connections to “float” the structure and break the pathway for vibration, or the selection of mechanical and electro-mechanical devices that dynamically counter the vibration within the building.

In existing buildings and as new building design advances, acoustical design experts can use simulation software to avoid internal vibration issues. This includes estimating the propagation of vibration generated from internal sources (e.g. fitness rooms) to vibration sensitive spaces elsewhere in the building, identifying locations in a building with acceptable levels of vibration for sensitive equipment, or conducting pre-installation surveys of floor vibration for new or relocated vibration sensitive imaging or laboratory equipment. The assessment could even get down to the level of floor finish selection or the location/relocation of fitness equipment or other vibration sources within a space to minimize vibration potential.

A recent project experience provides an example. The developer of an apartment building under construction wanted to study the feasibility of relocating an amenity fitness room from the ground floor to the second level, repurposing an apartment unit as the fitness room. The structural engineer proposed some modifications to the floor structure to accommodate the increased load, but additional analysis was needed to address vibration. The acoustic design consultant started by mounting accelerometers on the floor, and enlisting a volunteer to generate floor impacts in the areas where treadmills would be located, as well as repeatedly dropping a 15.9 kg (35 lb) kettlebell from several inches above the floor in multiple locations. The accelerometers recorded the signals from these impacts, which allowed an accurate estimate of the maximum vibration levels from anticipated fitness activities. These values were then plugged into software to create a dynamic model of vibration in the structure, allowing the simulation of structural modifications. The model indicated that additional structural reinforcing alone would not be satisfactory, thus prompting evaluation of a floating floor assembly that fit within height constraints and would sufficiently reduce vibration.

Vibration in buildings is not just a minor annoyance, it can impact the wellbeing of occupants and be disruptive to critical building uses, but once a building is completed, it can be difficult and costly to mitigate or eliminate sources of vibration. It is far better to recognize potential vibration issues before commencing design and integrate vibration control measures from the start.

Alan Scott is an architect and consultant with more than 35 years of experience in sustainable building design. He is director of sustainability with Intertek Building Science Solutions. James E. Phillips, MS, FASA, is a noise, acoustics, and vibration specialist with more than 35 years of experience. He is a senior consultant with Intertek Building Science Solutions. To learn more, follow Alan on LinkedIn at www.linkedin.com/in/alanscottfaia/.

Source URL: https://www.metalarchitecture.com/articles/columns/whats-shaking/

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