Acoustic Quirks: Stacking Multiple Uses Vertically

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Sunday, February 8th, 2015
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As our cities get more populated and land is at an ever bigger premium, the obvious answer is to build upwards. However, stacking different uses vertically can bring sizable noise impacts above and below.

Acoustics are an essential element to get right, but when done properly, there is no limit to how buildings can be put together.

A new 16-storey development in Melbourne’s Docklands is one such example of the growing trend for multi-use spaces stacked vertically. Containing a mix of offices, retail space and food outlets, its particular quirk is the inclusion of a multi-purpose outdoor sports court on top of the commercial space.

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Acoustics were always going to be a key component of the design solution for the high impact basketball court. A strong likelihood that the commercial space was to be used as a medical practice, however, meant ensuring noise was kept at a satisfactory level so as not to compromise occupant comfort and well-being became even more important.

“Our initial advice to the client was that if this adjacency were to work from a noise transfer perspective, a secondary isolated floating slab should be considered,” explained Alex Campbell, Asia Pacific acoustics manager at WSP. “However, as this had obvious cost implications, we explored a number of alternatives as part of proactive cost management.”

“Firstly, the obvious solution was to try and move the court to avoid the adjacency, however this wasn’t possible within the landscaping/urban design brief. Secondly, we investigated a simple resilient layer solution below the playing surface. Site measurements to inspect example installations showed that this wouldn’t have the level of acoustic performance required in our scenario. Finally, we considered building the spaces below as isolated ‘box-in-box’ designs. However, this would place a lot of constraints on the fit-out of the space below, as well as having the added risk of structure-borne noise from the basketball court re-radiating elsewhere in the building which would be near impossible to resolve retrospectively. In the end, it was agreed that the secondary floating slab would be the best overall solution.”

While there are a number of basketball courts designed in this manner internationally, there are not many others known in Australia, and certainly no outdoor courts. The design constraints of the building which challenged the design of the floating floor were:

  • The structure spanned across the loading dock entry road to Etihad Stadium. This meant a long-span solution was required, which from a structural perspective meant a relatively lightweight 150-millimetre composite structural slab with large transfer beams was adopted. Lightweight slabs and long spans usually lead to more readily excitable structures in terms of vibration, bad news when you’re trying to stop structure-borne noise (vibration).
  • The top of any secondary floating slab had to be constrained to a maximum of 150 millimetres above the level of primary structural slab in order to meet the same finished floor level at the surrounding ‘park’ level. This would impact the options available for spring mount selection and slab thickness.

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  • The structure had to allow for a maximum design live load of five kilopascals.
  • The outdoor environment lead to drainage and waterproofing issues not normally encountered in such installations.

The transfer of vibration (or noise) in any structure increases at the resonant frequency of that structure. When there is a secondary slab suspended above a normal slab there are two separate systems each with their own resonant frequencies.

  • The resonant frequency of the first element is that of the primary structural slab. This is very hard to change once the structure is designed, so should be considered ‘fixed.’
  • The resonant frequency of the second element is that of the secondary floating system slab above the primary structure. This can be tuned by appropriate spring selection and the mass of the secondary slab.

In order for the system to be effective, these two resonant frequencies should in no circumstances be the same. When these resonant frequencies are the same, a huge increase in the transfer of noise and vibration is experienced, which would possibly bring the performance of the whole structure worse than just providing no treatment.

In extreme cases, this ‘resonance matching’ can lead to vibration causing problems with structural integrity and stability. As such, there is a minimum safe buffer distance that the resonant frequency of these two systems must be designed to be apart.

In addition to this, the designers should also avoid the frequency of excitation of whatever will be on the structure. In this case, it meant avoiding typical frequencies of the footfall of people running.

All of this meant that in order to safely avoid the resonant frequencies of the structure and of the excitation frequency of footfall, the secondary slab system was required to be designed to a very narrow frequency band of between 3.2 and four hertz.

“The final design solution comprised a 100mm floating slab with an air gap of 50 millimetres. 330 jack-up spring mounts at 1.5-metre spacing cover an area of 630 square metres,” explained Campbell. “Due to the high deflection and low damping, 25-millimetre deflection springs (Embelton XW-1624) were chosen for the isolation element to get the best possible performance at audible frequencies.”

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Basketball backboard/hoop posts were secured to the hob around the perimeter. Embelton SuperShearflex rubber pads with bolt isolation sleeves and rubber washers were then installed between the basketball posts and structural slab to reduce vibration from the ball hitting the backboard/hoop transferring in to the slab.

The design, of course, then needed to be tested to measure the sound caused by ball bouncing, jumping and shooting.

Impact noise tests in accordance with ISO and Australian Standards were performed, but the most telling result was from undertaking real-world activities and an example game on the court, then measuring the noise levels below. During the sample game, the court was found to be comfortable to use with no adverse effects from the flexibility of the slab.

During the tests, the commercial space below was still in its base-building ‘cold shell’ state, with no fit-out works or services, etc, contained within. This space has now been subsequently fitted out with the clinic rooms and ceiling, along with associated MEP equipment.

“To limit disturbance to the occupants, we developed a criteria that meant that maximum noise levels (LAmax) during basketball games should not exceed the design ambient noise level (LAeq,T) in the fitted out clinic space below. The design ambient levels for medical spaces are defined in Australian Standard 2107:2000,” Campbell said.

“We found that the worst case maximum noise levels were caused by shots at the backboard/hoop and players jumping on the court. These results were within seven decibels of the maximum criteria established. Running and ball bounces produced noise levels below the established maximum noise level. With the space below fitted out, the inclusion of a plasterboard ceiling in the clinic space further reduces maximum noise levels in the clinic by at least 10 decibels, meaning the criteria is achieved. In addition to this, the installation of mechanical services also creates a masking noise level which will mean that activity on the court will be audible only at very low levels in the clinic space – if it is audible at all.”

Tall buildings continue to flourish. As they do the concept of the vertical city is evolving. Intelligent engineering, from structures to acoustics, is allowing all types of uses to be contained successfully in single buildings and sit comfortably and noiselessly together.

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