Mason UK meets stringent specification with proactive engineering

Park Modern is a residential development with luxury apartments, exceptional facilities, and views across Hyde Park. Due to the proximity of the structure to the London Underground, extensive vibration control was required. This article describes how vibration control specialist Mason UK tackled these challenges and delivered a challenging specification.

Park Modern includes 57 high-end apartments, with prices for these apartments ranging from £2 million for a one bedroom to £60 million for a striking nine-bedroom penthouse. The development also includes a wellbeing floor, with a residents-only spa, treatment rooms and private healthcare, a 25-metre swimming pool, a gymnasium, and a 16-seat cinema that residents can reserve for private use. For contractor Ant Yapi, this was their sixth project in the UK and their largest to date.

The building rises nine levels above the ground with the majority of apartments overlooking Hyde Park. The design and building project required extensive ground retaining structures in close proximity to the London Underground Central Line to support the open excavation to 20 metres below ground level for the lowermost basement level. In addition to the facilities listed above, such as the swimming pool and gymnasium, the basement levels also contain plant areas.

Enter Hoare Lea

With the London Underground Central Line so close to the basement, vibration from the tube would be easily transmitted into the building’s structure, causing unacceptable noise within. Martin McNulty, a vibration consultant with Hoare Lea, provided the specification to guide the contractor and meet the client’s requirements.

In addition to comprehensive site surveys, Hoare Lea carried out a number of computer simulations to predict the vibration levels at the basement level and throughout the development. The results confirmed high levels of vibration would be present in the primary structure. McNulty recalled that ‘‘a view was taken to ensure a premium-quality approach throughout all noise sensitive spaces, including the basements. In this scheme, sensitive spaces were not confined to above ground residential spaces, as is common, but also below ground in amenity and health care zones. This presented a challenge, as a one size fits all approach for building with significant basement zones is difficult.’’

Ultimately, this required a two-stage approach, controlling both groundborne vibration, comprising building isolation to mitigate dwellings at Level 1 and above, and a box-in-box strategy for the floors below. Box-in-box solutions would be required at all basement floors down to the lowest floor of development. Further, the client’s aspiration was that certain noise-sensitive spaces below ground were subject to the same performance standards as those above ground. This was particularly challenging as proximity to the tube tunnels meant vibration levels below ground were far in excess to those which would be above ground.

‘‘To meet this challenge, we developed a high performing box in box system and also insisted upon the contractor testing at key points throughout construction to ensure the designs were optimal. This also had the added benefit of permitting some value engineering opportunities to benefit the client,’’ explained McNulty.

To deliver the specification, Ant Yapi turned to vibration control specialists Mason UK. ‘‘We needed Mason both for the structural isolation of the entire building, and for box-in-box isolation at the basement level,’’ explained Baris Bayraktar, Construction Manager at Ant Yapi. Ant Yapi also brought on board another acoustic consultant, Cahill Design Consultants (CDC), to ensure that Hoare Lea’s recommendations were implemented on site.

The isolation line

As depicted in Figure 1, the isolation line for the project ran horizontally across level one, while dipping down the two elevator cores to the bottom of the basement (B3). This is standard when providing structural isolation for a building with elevators, as cutting through building cores or reacting ground bearing pressures compromises acoustic performance. 

This method meant the elevator cores at the bottom of B3, along with the entire building from level one upwards, were isolated on a system of 8Hz rubber bearings. ‘‘I had previously been involved in projects where bearings were used for seismic activity, so I was familiar with the theory. However, this was the first time I had worked on a project where they were required for acoustics,’’ recalled Bayraktyar.

In total there were approximately 350 vertical load bearings, ranging in size from 130x130x95mm thick to 460x460x95mm thick to suit the various loads.  The implementation of this design interrupted the vibration transmission path from that generated by tube trains into the building’s structure, where it could manifest as noise disturbance in the apartments above.

However, with the bearings in place there was a need to provide lateral restraint. Columns and building cores provide extensive lateral restraint and prevent a building swaying under wind loading. However, measures have to be implemented, both to provide lateral restraint but also to ensure that the acoustic performance of the bearings is not compromised. For this project, this was achieved through the use of a system of shear pins. These are simple, inexpensive and acoustically excellent as they do not engage unless wind loads are present.

Furthermore, the introduction of bearings required working closely with the structural engineers at AKT. Adding bearings to a building effectively introduces movable connections which would otherwise be rigid. It was essential that Mason provided preliminary bearing designs well before ground was broken to AKT so the structural model could be updated, and loads redistributed. Charlie Ashton, a structural engineer with AKT, described a ‘‘long winded, iterative approach,’’ involving lots of coordination between AKT’s engineers and Adam Fox, Director at Mason UK.

A key part of Ashton’s job was updating the finite elements model to incorporate Mason’s bearings. After the model had been run (a process that typically would take twelve hours) it would usually require Mason to make changes to the system of bearings in light of fresh calculations about loads. ‘‘Every time you run the model, you get a different set of reactions,’’ explained Ashton. ‘‘In addition to calculations about what load a bearing can handle, you also have to take into consideration the impact on acoustic performance.’’ The project was therefore a lot more challenging than normal projects, with static loads and more simple structures.

Mason’s largest floating floor

Below the isolation line, everything was isolated independently using isolating floating floors. For Mason UK, this was the largest floating floor system, by surface area, the team had ever installed, totalling 2,000m² across the three basement levels.

The company has extensive experience of installing floating floors, but it is more common to have a floating floor for smaller, isolated areas such as gyms or a cinema. In contrast, the complexity of the Park Modern project required the total footprint of the building on basement level one, two and most of basement level three to be supported on a floating floor system. Although the floating floor system itself was relatively standard, the scale of this undertaking was enormous and complex given multiple level changes, imposed loads, plus variation in floating floor and air void thickness.

Steve Hart, Director at Mason UK, is one of the most experienced engineers in the industry when it comes to the design and installation of floating floor systems. ‘‘This is the largest floating floor system we have ever done by surface area, so although the system itself was relatively straightforward, it was an exciting project to be involved with. Handling all of the logistics for the concrete as well was also a challenge.’’

At basement level, the floating floors were installed as part of a box-in-box method of isolation. This comprises a floating floor, isolated walls and a suspended ceiling. All elements are therefore supported or suspended using isolators, ensuring no solid connection between the existing structure and the interior of the room. This method prevents vibration from the tube below passing into the structure, but it also prevents noise and vibration from the basement levels from emanating outwards into the rest of the building.

Proactive engineering

One of the most challenging aspects of this project was providing effective isolation at B3, the level closest to Central Line. In the original design, following stringent requirements from Ant Yapi, Hoare Lea had specified higher performance spring isolators and a ‘‘double’’ floating floor to achieve maximum performance.

In comparison to rubber, springs generally have a lower natural frequency and are therefore more efficient at reducing vibration transmission. However, when using springs as part of a box-in-box system you can introduce potential problems. For example, as the springs by necessity compress by a higher degree, to 25 millimetres is typical, there is the potential for the structure to move too freely, leading to potential problems across thresholds and finishes as internal walls and finishes are added. Another challenge is the presence of rigid connections coming through such as pipework, requiring isolated slip joints capable of absorbing travel.

While these challenges are not insurmountable, these additional complications mean rubber isolation can save cost as well as complexity. Although the simple option would have been to follow the existing specification, the Mason team were therefore keen to explore the viability of alternative options that would avoid some of the challenges above, saving the contractor space, materials and money. ‘‘Using springs would have had certain drawbacks, especially given the space constraints,’’ recalled Bayraktar.

Mason therefore proposed to Ant Yapi and CDC that the correct use of rubber isolators could satisfy the requirements of the specification, without the additional complications and expense introduced by spring isolators. To test the viability of this solution, Mason constructed a mock up test floor for the third level of the basement on site. The test floor, which measured approximately ten square meters, allowed them to successfully demonstrate that the rubber system would uphold the specification. The low dynamic stiffness of the rubber formula and the attention to detail during installation were absolutely key in ensuring the correct performance levels.

This was not the only instance where a proactive approach saved space and money, while still maintaining the requirements of the specification. There was also a swimming pool on B3, which was originally planned to be constructed on a floating floor. To save space and materials, Mason suggested an isolated deck rather than a concrete floor and worked meticulously with the specialist contractor to develop this.

The Mason contribution

For those involved in providing vibration isolation, it was a uniquely challenging but rewarding project. For McNulty, who has a reputation in the industry for tackling complex vibration isolation requirements, this project presented some unique challenges and required an especially stringent specification. However, he recalled how ‘‘we were confident that Mason would be able to deliver a system which not only met our requirements, but that they as suppliers would also be skilled to ask the right questions of both design and contractor as the scheme progressed to ensure the strategy was as intended.’’

‘‘The involvement we had in developing the structural design of the building and variations to the floating floor specifications, showed the need for early engagement with the design and contracting teams,’’ reflected Adam Fox.  ‘‘We committed to years of effort to support the client from concept to delivery and we are very proud of the result.’’

London is home to some of the world’s most eye-watering residential developments. Many challenges must be overcome to bring these buildings to life. For those involved in acoustics and vibration control, the creation of deeper basement levels, the proximity of many buildings to major tube or train lines, and the presence of extensive facilities from gymnasiums to cinemas, are all challenges that are becoming increasingly common. Thankfully for building contractors, those working in this sector are becoming more accustomed to finding the optimal solutions to these unique problems.

Mason UK specialises in acoustic and vibration isolation solutions for architectural noise control, M&E and HVAC isolation. To read more about our projects, visit www.mason-uk.co.uk

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