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Located above a busy underground railway, the new East Buildings – Britomart complex in Auckland, New Zealand, has an extraordinary noise-minimisation feature: full-building rubber pads. TIM BERESFORD from Norman Disney & Young explains the acoustic merits of the system.
Due to the ever-increasing demand for premium office space, designers and engineers have progressively found themselves working on developments where individual site locations present design challenges that require ‘non-standard’ solutions. Such was the case with the East Buildings, Britomart, Auckland, where a group of three medium-rise buildings totalling 40,000 square metres GFA (gross floor area) of A-grade office space is nearing completion above Auckland’s underground Britomart train station and tunnel.
The key challenge with this site was to address vibration from rail movements in the tunnel below, which had the potential to produce unacceptably high noise levels, if left untreated, for occupants within the building.
As the lead acoustic consultant for this project, Norman Disney & Young (NDY) was required to find a solution that would reduce such vibration on a large scale – across all floors of all three of the East Buildings. Several options were considered, but ultimately it was decided the best solution was to vibro-acoustically isolate the buildings. The concept behind this solution is simple: isolate the building with rubber pads, as is done for many vibration problems. But how does one achieve this for a whole group of buildings, specifically a group of this size? Add in the fact that the buildings are located in a seismically active area, requiring them to be seismically restrained, and you have a real design problem on your hands.
ASSESS THE PROBLEM
The first stage in the design process was to assess the extent of the vibro-acoustic problem. A detailed ‘vibration map’ of the site was created to identify any hot spots. Holes were excavated through the carpark asphalt topping, which was located above the rail tunnel, to expose the tunnel ‘lid’ onto which the East Buildings’ foundations would sit. Accelerometers were attached and rail movements in and out of the tunnel were tracked. Analysis and predictions of the vibration spread throughout the proposed building indicated a widespread problem of audible low-frequency noise when particular rail movements occurred.
The design of the isolation bearings or ‘pads’ required careful consideration. It was important to ensure loading of the pads was not too great and that bulk shear strain within the pad was kept within acceptable limits. This was vital to ensure the life of the pad matched or exceeded the life of the building. As one can imagine, it would be no trivial task replacing a failed pad on an already completed building.
PAD DEVELOPMENT
Development of the pad was a complex procedure involving a collaborative effort between NDY and the bearing manufacturer, combining knowledge, experience and a good deal of trial and error. NDY worked closely with Trelleborg, trialling different combinations of rubber compounds and metal shims to make up the final design. Trelleborg had test facilities that allowed the pads to be tested under simulated loads of up to 150 percent of the predicted working load.
The testing of the pads was considered an essential part of the process; however, it also presented its own limitations – the test rig could not test individual pads that would be large enough to safely support the weight of the building at each of its foundation points. So NDY worked with the structural engineer to develop a distributed bearing pattern for each foundation so the load could be distributed among up to four smaller pads. This meant each individual pad could be tested, and it was known that the group of pads could collectively bear the weight of these buildings’ foundations.
Another consideration was the need to seismically restrain the entire building due to its location in the seismically active Auckland region. During the developed design stage, different restraint options were investigated, including high-tech shock absorbers manufactured in the US. With the high price tag associated with these and many of the other available options, it was decided that bespoke snubbers would be designed and incorporated into the structural steelwork at the foundations.
MEASURING VIBRATION
The East Buildings are very near to completion at the time of writing. In August, 2009, NDY completed vibration measurements in the worst-case locations of the partially completed buildings to determine the effectiveness of the isolation pads. The resulting graph (see below) indicates the sound levels present within office areas of the building with and without vibro-acoustic isolation, based on predictions from our initial site survey (no isolation) and from measurements within the partially completed building (with isolation). The noise criterion for this office space is for sound pressure levels to be less than or equal to the PNC 40 curve. As can be seen noise levels without isolation would have been in excess of PNC 55. With vibro-acoustic isolation, internal noise levels have been decreased to below PNC 25.
TARGETS MET
The developer, Cooper and Company, was delighted with the outcome of the vibro-acoustic isolation. On-floor noise levels due to rail movements are well below the project threshold throughout the East Buildings, fulfilling the requirement for the design and delivery of an A-grade office environment on this challenging site.
Tim Beresford is an acoustic engineer who has worked with Norman Disney & Young for the past five years, following his graduation from the University of Auckland as a mechanical engineer. During this time he has been involved in a wide range of significant acoustic and vibration designs throughout New Zealand, including work in the fields of building services for commercial and residential applications, aircraft noise modelling and spatial acoustic design. Beresford enjoys music, playing piano and electric guitar, and has been actively involved in several bands. He sees music as the link between his recreation and profession and for this reason has a keen interest in good acoustics for performance spaces. Recently he accepted the opportunity to relocate to Australia to manage the acoustic section at NDY Melbourne.