Shopping centres

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This month we examine the main options for services installations for shopping centres with enclosed malls.

Shopping centre developments involve a complex balancing act between the needs of the developer/landlord, tenants and customers. For the services engineer, there are many challenges, related to the economic provision of ventilation and cooling in the mall, complex life safety measures, substantial decorative and feature lighting, the control of air infiltration and the cost-effective design of services supplies to tenants.

There are a number of key issues that need to be considered when developing an optimum design.

Environmental control of enclosed malls
As a result of Planning Policy Guidance Note 13, most shopping centres are being developed in urban locations, and as a result, mechanical systems with or without cooling are generally the only options available.

The large roof area of a typical shopping centre provides opportunities for the economic location of main air handling and heat rejection plant. The absence of space constraints also means that it is possible to specify discrete plant for each air conditioning zone, providing diversity and simplifying the control system. There are two principal design options for ventilation systems.

Extract at mall level through retail units or service corridors. Based on this strategy, the design of shop fronts must provide for a permanent air return route.
Extract at roof level, taking advantage of the stack effect within the mall.

Due to the volume of supply air involved, the incorporation of energy saving features into the design is very important. Options available to the designer include maintaining a high proportion of air re-circulation preferably utilising CO2 control, together with heat recovery in the air-handling units. Other options include the maximisation of free cooling control strategies and broadening of the acceptable temperature control bands to minimise energy usage. One of the priorities for the designer is to keep the design of systems simple and self contained. Strategies used on shopping centre schemes include:

The specification of indirect gas-fired air handling units in lieu of the use of an lphw heating coil. These units are cheaper and do not need central boiler plant, hot water circuits or frost protection.
The use of individual dx units close coupled to the air handling units, rather than central chiller plant.
Plant and component standardisation.
Functional and robust automatic control systems.

The introduction of the revised Part L to the Building Regulations will require greater consideration of the carbon output of mechanical plant, and it may be necessary, in the future, to specify a more centralised plant strategy to meet overall carbon emission targets.

Smoke extract from enclosed malls
The development of life safety and smoke extract strategies requires careful consideration of the interaction of architecture and services design. Trade-offs can be made, although these need to be determined on a case by case basis. The incorporation of smoke reservoirs into the design of the mall and rooflights and balconies is a key feature. Natural smoke dispersal via opening vents in the mall roof is the preferred option due to its simplicity, low cost and minimal plant space requirements.

In some instances, factors such as the number of levels in the centre, adjacent high buildings or other architectural reasons may require mechanical smoke extract using either roof level fans or direct extract through the rear of the smaller retail units. A key element of either smoke extract strategy is that the supply of make-up air should not disturb the smoke layer or plume. This is achieved by introducing fresh air through doorways or through vents from adjacent zones.

Services strategies to retail units
The servicing strategy for retail units will depend on the size of the unit and the overall mix of units in the development.

Tenants of anchor stores and large shop units will typically be expected to provide a fully self contained services installation. This will include a separate electrical intake, dedicated main plant and distribution, together with stand-alone alarms, sprinklers and other protective installations. Adoption of this strategy means that a centre's major tenants have complete freedom to design their services installation, independent of any constraint imposed by the overall sizing of the landlord's plant. It also means that the tenant has full control over the continuity of services provision.
Tenants of small units will typically be provided with the following services:
- A capped off cold water supply
- A three-phase 415 V electricity supply terminated either at a shop unit distribution board or a mains isolator.
- A condenser water supply to which a packaged cooling unit can be connected within the shop unit. This is the cheapest option and it is also easier to design and control the supply of condenser water than to provide direct cooling. Alternatively, where tenants are able to use their own dx systems, all the landlord provides is a roof platform and riser route for pipework and power.
- A sprinkler mains service, terminated at a control valve for each shop unit, together with an alarm interface back to the centre management system.
- A fire alarm interface back to the landlords system.

One of the major problems associated with the provision of services to tenants is the need to be able to control the loads placed on the system. This can be done in two ways:

By limiting the nature of the service provided to the tenant (eg condenser water rather than a central chilled water supply).
By controlling the loads available to the tenant in the tenancy agreement (eg the power supply, where typically a tenant must demonstrate that they have not exceeded their allocated supply before power is switched on).

The advantages to the landlord of these approaches are that the upfront investment in central plant is minimised, the administration of service charges to tenants is simplified, and the landlord's liability for service downtime is reduced.

Electrical services and mall lighting
The landlord's power requirement are extensive, and in addition to supplies to main plant, lifts and escalators, main mall and car park lighting and landlord's small power, provision also needs to be made for items including power and lighting to features in the Mall, external lighting and signage and power to cradle runway system for window cleaning etc.

The most important element of the electrical installation is the mall and external feature lighting. Not only does the scheme have a critical effect upon the attractiveness of the mall and the visibility of shop units, but also has a substantial impact on energy usage and running costs.

This requires careful attention and the standardisation of components is essential for effective post completion management. One of the main issues for the lighting designer is high level lighting of the glazed mall, which at night can make the glazing appear opaque and featureless.

Furthermore, a careful integration of architectural features such as fritted glass and louvres, and the lighting system will also help ensure that the mall roof remains a focal point at all times.

Security and protective installations
Security and the safety of the public are key issues for the landlord. Extensive cctv is used to monitor all areas involving back of house and service areas. Access control is also important, segregating malls, car parks and other public access areas from landlord and tenant accommodation.

Sophisticated zoned fire alarm systems are required in large enclosed shopping centres to provide effective life safety management. Two stage alarm systems are generally specified to assist with the management of evacuation and the avoidance of false alarms.

In addition to conventional detectors and call points, infrared sensors or air aspiration systems will be required at high level in mall areas. All fire alarms in landlord's tenant areas will be connected back to the centre management suite.

Sprinklers will be provided in back of house areas, where a fire load is present, and a supply will be provided to retail units. In the mall however, unless there is a significant fire load, sprinklers will not be required.

The impact of Part L on shopping centre design
The introduction of the new Part L to the Building Regulations in April 2002 could have a significant effect on the design of the fabric of shopping centres, with lower fabric U-valves and the need to control solar gain, in particular affecting the design of the mall and its roof lighting.

For services engineers, issues which will need to be addressed include:

Demonstrating the effect on carbon emissions reduction of existing energy saving strategies, and including these results in the overall assessment of compliance.
Demonstrating compliance of the complete shopping centre before the scope of fit-out works is determined.
Minimising air-leakage through service corridors, service yards and other penetrations through the building fabric.

Due to the complexity of shopping centre design and the opportunities to set-off performance of the building envelope and building services, the carbon emissions method will give the greatest flexibility to the designers to develop optimum design solutions.

Cost breakdown
The costs are based on a two-level shopping centre with a feature glazed mall in an urban location. The centre has a gross internal floor area of 70 000 m², 70% of which is occupied by retail units. The costs of car parking and the additional services associated with a food court are excluded from the analysis. Costs are current in June 2002.

Exclusions