Speculative flexibility and reliability

Gresham street
Even if speculative office buildings are not at the cutting edge of services design, better-than-statutory standards of energy efficiency can be achieved — as one of the City of London’s biggest speculative developments in recent years demonstrates.Exciting though energy-efficient projects using sophisticated and advanced building services might be, there are occasions when the perceived risk and lack of reliability prompts the use of tried-and-tested, commercially proven solutions. Such is one of the biggest speculative developments in the City of London in recent times. Its 36 000 m2 of office accommodation is designed for financial operations and aimed at the higher end of the property market. Developed by Land Securities, 30 Gresham Street is a 10-storey building designed for maximum flexibility. Energy efficiency Reliability and energy efficiency are not mutually exclusive, however, and the building achieves higher levels of energy efficiency than were required by the Building Regulations in force at the time applications were submitted. The building was not required to meet Part L of the 2002 regulations, but the client funded additional insulation measures on the project — the modelling of which indicated that the building would have passed Part L had it been applicable, primarily due to the sophistication of the control system. Air leakage An air-leakage test was carried out, and the building easily met the Part L requirements. As a consequence, the building achieved a ‘very good’ BREEAM rating. The second and third floors are designed to cater for financial/commodity dealing, with office accommodation on the remaining floors. Office space is planned around a central core, linking directly to double-height reception areas at the entrances from Gresham Street and Trump Street. There are also three retail units on the ground floor. At the heart of the design concept developed by FaberMaunsell was the awareness that financial companies are not keen on occupying buildings with risk. Reliability and integrity thus took precedent over energy efficiency. Cooling for the office areas is provided by ceiling-mounted 2- and 4-pipe fan-coil units with airside control. Chilled water is generated by four air-cooled screw chillers, each rated at 1250 kW. There is space for two more chillers if required by a tenant. A high level of robustness is built into the design. The chilled-water system is arranged as two normally separate stand-alone systems serving the west and the east of the building. Primary chilled water is then provided at 6°C to the air-handling units in the basements and plate-heat-exchanger units on the floors. These plate-heat-exchanger assemblies, with secondary chilled-water pumps, then provided chilled water at the elevated temperature of 10.5°C to the fan-coil units. The primary chilled water has 2-port control on the loads, and chillers are brought on line as required by the loads. Each chiller has its own duty/standby shunt pump, so the volume of primary chilled water varies with the building load to reduce energy consumption compared with a more conventional constant-volume pumping arrangement. Mike Hatter of FaberMaunsell explains, ‘This was less energy efficient, but more reliable than total variable pumping, but as the chillers are business critical, this was a more acceptable arrangement for this project.’ This being a speculative building, the ductwork onto each of the tenancies is provided with motorised fire dampers on the supply and extract, enabling unused or unoccupied areas of the building to be closed off and only ‘flushed’ for a minimum time each day. Inverter drives serve the fans in the air-handling units and extract fans. The control system turns them down to match supply and extract air flow rates to closely meet the occupant requirements. Each supply spigot is provided with a pressure-independent volume controller to ensure that the air supplied to each area does not vary as other floors are brought in or switched off. The chilled-water supply to each floor is part-load switched in a similar, with the supply to the primary/secondary plate-heat-exchanger assemblies on each tenancy of each floor being isolated when the floor is unoccupied, though with a separate branch serving 24-hour loads. For reliability, the electrical system is totally duplicated. During the shell-and-core phase of the project, back-up generators were installed to provide life safety. There is also space in the plant room for additional generators, fuel and interruptible power supplies if required by tenants. Flexibility Being a speculative building, the design incorporates a high degree of flexibility. Each of floors two to eight could accommodate four separate tenants, though it is expected to attract larger companies. Too meet all possible requirements, there are highly configurable controls for heating and cooling. All lighting is linked to a lighting-control system that can be extended to meet each tenant’s needs. Major technical challenges were presented by the need for the building to remain below the sightline of St Paul’s Cathedral, while also creating the amount of lettable space required by Land Securities. Given the high level of servicing required for such an IT-intensive building, co-ordination of services was very complex, so FaberMaunsell created a 3D model of the services to demonstrate to the client that they could be fitted into such a small space. This intricate use of 3D modelling won FaberMaunsell the Engineering Consultant Award in the Bentley Success Awards. A prototype model of the fifth-floor ceiling void, the worst-case scenario with a total depth of only 770 mm (including structure and ceiling zone), was created for this demonstration. The void houses the air-distribution system, fan-coil units, heating and chilled-water pipework, structural elements and cavity barriers. This model was then used to create an AVI movie to provide an interactive walk-through for Land Securities. Once the plans had been approved, 2D plans, elevations and sections were generated from the model. Cost efficient 30 Gresham Street has been designed and constructed in a very cost-efficient manner without compromising the quality of the accommodation offered to tenants. The fact that it was completed on time, despite the complexity of the building and the presence of 60 archaeologists on site for nine months while work proceeded, is testament to the teamwork that developed during the project.

A novel approach to design and procurement

While the services in 30 Gresham Street are conventional and low-risk, the approach to services design and procurement was innovative and effective. Although it was a design-and-build project, with Sir Robert McAlpine as the main contractor, Land Securities wanted to retain control of the design. The client thus took the unusual step of appointing one company for the design and monitoring of each discipline. For the building services, consultant FaberMaunsell carried out design for the main contractor and monitoring on behalf of the client. At the tender stage, the original FaberMaunsell team split into two — one novated to Sir Robert McAlpine, and the other as a monitoring and checking team for Land Securities. Mike Hatter of FaberMaunsell explains, ‘This arrangement brought considerable benefits to the project, as it meant that FaberMaunsell was involved from the initial concept right through to completion. Being aware of this approach from the start, we were able to plan for the split from day one in terms of the original design team. Both teams stayed stable throughout the project, so we had a mass of knowledge available. ‘There was very little rebriefing required at tender stage as we all knew what we wanted to achieve, and there was a less adversarial approach throughout the project. ‘Of course, the client was knowlingly buying a one-company reviewing process, but the arrangement was clearly more cost and time effective compared to using completely independent reviews. In fact, this proved to be a very efficient way to deliver a building of this size.’ So effective was the process that the building reached practical completion on the day originally required. FaberMaunsell Ltd, Marlborough House, Upper Marlborough Road, St Albans, Herts AL1 3UT.
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