Feature

Unshakeable

Many new laboratory buildings have been developed in recent years, dedicated to advancing subject areas such as life science, materials science, engineering and physics. This research often involves the use of scientific equipment to study very small organisms and structures in microscopic detail. This equipment is sensitive to vibration and hence this must be taken into consideration in the structural design of the building.

The Sainsbury Wellcome Centre for Neural Circuits and Behaviour at University College London, designed by Ian Ritchie Architects, became operational during 2016 and houses several laboratories. The nature of the research and the use of imaging equipment, including two-photon and electron microscopy, which is sensitive to the smallest of movements, required a low-vibration environment. A fundamental aspect of this was the selection and design of the reinforced-concrete foundation, superstructure frame and floor slabs.

Vibration criteria

Vibration criteria must be identified early in a project because, in addition to strength, architectural, services and other considerations, they inform selection of structural forms. Industry-published or generic criteria are suitable for this purpose as they cover broad classes of equipment, and are therefore compatible with the early stages of a project when certainty about specific equipment cannot be attained. Industry vibration criteria published by ASHRAE and more recently IEST (see Further Reading, below) are referred to as VC criteria and their purpose is to define limits on vibration transmitted to the equipment to give acceptable functional performance (see graph, below). The VC criteria involve vibration magnitudes smaller than a human can perceive and are hence referred to as sub-perceptible. VC-A is applied to typical laboratory floors which might support many researchers and equipment often on suspended floor structures. VC-D is typically applied to specialist imaging suites, such as electron microscopy, which tend to be located on foundation slab structures.

Meeting the relevant criteria is important as it enables the building to realise its full value. Since the criteria essentially apply to the equipment, they may be attained through structural design or some combination of design and mitigation, often in the form of isolation. While vibration originates from a range of sources, the transmission of vibration is related to the structure and it is therefore necessary to evaluate it at the design stage. Typical external vibration sources – i.e. outside the project boundary and where vibration is transmitted through the ground to the foundation – are underground rail and highway. Typical internal vibration sources – i.e. acting directly on the structure within the building – are footfall and mechanical and electrical plant systems.

Structural design

For the Sainsbury Wellcome Centre, a vibration survey was undertaken at an early stage to evaluate the ambient site vibration from prevailing external sources. Traffic on the streets around the site boundary was the main source. While the Victoria and Northern London Underground lines un close by, they are too distant to feature prominently in the vibration at the site.

The new centre has two basement levels which include provision for an imaging suite at level –2, common support space at level –1 and four upper floor levels. Levels 1 and 3 are laboratory floors and levels 2 and 4 are designed to be adaptable so that they may become laboratory floors in future. Many aspects of the structural design influence vibration performance. The material selected throughout was in-situ reinforced concrete (RC), including the primary structural frame and floor slabs.

The selection was made primarily on the basis of cost and constructability; however, RC also has dynamic modulus, density and damping properties that are compatible with low-vibration environment structural design. Grid size was a critical parameter for the project in terms of the architecture and space utility but also for vibration. Studies at the concept design stage indicated that a grid size in the region of 6.5m would be needed to achieve acceptable laboratory floor vibration performance. A piled slab foundation form was selected to control differential settlement and to enable the building gravity loads to be taken down to the Thanet Sand layer.

A secant pile retaining wall structure was selected. This provides some limited “screening” for the structural piles from vibration transmitted from highways. When the vibration arrives at the piles, it is transmitted to the foundation slab by a combination of friction and direct transmission at the pile toe level and into the pile structure generally. RC slab thickness is a key design parameter for strength and especially for vibration. Low vibration is best achieved by designing the slab to have a primary vertical mode with natural frequency above the range of the main footfall harmonics. This frequency is in the region of 10Hz. In this way resonant vibration response from footfall – i.e. where vibration builds up with each footfall – is avoided. A slab thickness of 400mm for the laboratory floors at levels 1 and 3 was found to prevent significant cracking from developing under serviceability loads. There was therefore no loss of stiffness or frequency reduction and the potential for resonant response was avoided. A thickness of 350mm for floor plates at levels 2 and 4 ensured they would have high vibration serviceability now and also strong potential for adaptability in future.

Assessing vibration

Design tools were used to help steer the structural design and to demonstrate that it would meet the criteria. For footfall in buildings, finite element (FE) methods are typically used to evaluate and assess vibration of structural designs. Arup’s GSA FE code was applied to this project, it being consistent with UK concrete industry guidance and national standards concerning the effect of vibration in buildings on the occupants. These methods are adaptable to situations where the vibration output is to be considered in the context of equipment functional performance as opposed to perception. This is discussed in the CCIP-016 guidance (see Further Reading below). Walking speed is an important input in this evaluation and particularly for this design, with corridors alongside the large laboratory areas. Walking speed within laboratory environments is typically 1.8Hz and in corridors it is a maximum of 2.5Hz. The single person walking load case for corridors which run alongside laboratory spaces was evaluated as a priority, with vibration tending to increase with faster walking speeds. This computational work showed that the vibration from footfall for the proposed structural design was within the criteria for levels 1 to 4.

Vibration transmission from external sources was evaluated at the design stage using computational models in conjunction with measured data. This evaluation showed that vibration performance was within the criteria for receiver locations on the foundation and suspended slabs. Where this was not the case, mitigation was developed in the form of equipment isolation or change to the structural design. Mechanical, electrical and public health (MEP) plant vibration is a common aspect of all building design. However, for this building, with MEP plant at the basement and roof levels, the potential for transmission of vibration through the structure to sensitive occupancies is particularly important.

A vibration isolation specification was developed to define how much isolation was needed to limit MEP plant vibration transmission to slabs. Isolation vendors then proposed either a system with elastomer bearings or helical steel springs to deliver the specified isolation for the running speed range of the MEP plant. For large plant items, which produce high dynamic loads – for example, generator sets and combined heat and power units – a concrete slab supported by helical steel springs was specified. ASHRAE and CIBSE guidance on the isolation type for ranges of machine types, powers, running speeds and slab forms was used to inform the initial selection of isolation.

Laboratory and other technical buildings such as hospitals typically house vibration-sensitive equipment and occupancies. This article discusses a specific project, but many, if not all, of these aspects of vibration control would need to be assessed at the design stage for any vibrationsensitive building project. The key aspects of this process – criteria selection, identification of internal and external vibration sources, structural design to control vibration from footfall and MEP plant isolation – are typical of such a design. The concrete frame in this project was ideal to help realise a low-vibration environment because of its dynamic modulus, density and damping properties, as well as providing an efficient, economic and buildable structure.

Further reading

ASHRAE HVAC Applications Handbook (2015)

IEST-RP-CC012.2: Considerations in Cleanroom Design, Institute of Environmental Sciences and Technology (2012)

A Design Guide for Footfall Induced Vibration of Structures, CCIP-016, by MR Wilford and P Young, The Concrete Centre (2007)