Despite the fate of the World Trade Centre, developer interest in tall buildings hasn’t diminished – and will increase if the mayor’s plan for London is put into effect. Here Davis Langdon & Everest, Arup and Mott Green and Wall summarise the key issues and examine the costs of building a 48-storey tower in central London
IntroductionDespite the events of 11 September, there is no shortage of developer interest in tall buildings. Since Davis Langdon & Everest’s last tall buildings cost model was published in 1997, the demand for tower development in the UK has increased dramatically. This process has just been given a push by the Greater London Authority’s draft London plan and the determination of a number of sensitive planning applications, including the approval of the Heron Tower.
Supporters argue that high-quality towers will be commercial and visual assets to London, will increase occupancy densities and make more efficient use of the public transport. In opposition are bodies concerned about overall development densities and preservation of London’s skyline and streetscape.
There is also lively debate and conflicting research as to whether tall buildings are required to meet the needs of London’s businesses. Research into the economics of tall building and occupier perceptions, published by the British Council for Offices, and based on work undertaken after September 2001, contradicts the London Planning Advisory Committee report, High Buildings and Strategic Views in London, published in 1998.
In particular, the BCO identifies two discrete groups of occupiers that are driving demand for distinctly different types of tall office building. These are large corporations relocating to single buildings, which drive demand for “fat” towers with floorplates of up to 3000 m2 gross. These relocations are planned to amalgamate operations, generate synergy between business units and reduce facilities management costs. With space requirements commonly in excess of 50,000 m2, many organisations, particularly banks such as HSBC, Citigroup and Barclays, are relocating outside of the City. Most have moved to Canary Wharf, which offers the right combination of size, floorplate, quality of space and critical mass of complementary businesses.
Another driver of change is smaller, often international, companies with a requirement for a prestige location, which have created a demand for space in multi-tenanted tall office buildings. A common characteristic of these buildings is their comparatively small floorplate of between 1500 and 2000 m2 gross. Tenants in these buildings value the prestige, high-quality shared facilities and opportunities for interaction with inter-related businesses, demonstrated by their low vacancy rates and rental premiums of up to 15%.
In addition to these two established markets, developers are looking at the opportunities for developing mixed-use towers in major cities, incorporating retail, office, hotel and residential uses within one building.
Design considerationsThis section outlines some of the key factors that need to be considered in selecting principal design options.
Planning Where planning restrictions exist, they can be a major determinant of the form and appearance of a tower. In the City of London, for example, the combination of irregular plots, protected sight lines and other site constraints, together with the aspirations of developers and planners, are encouraging the development of complex and articulated tower designs.
Design for multi-occupancy By comparison with a single-use tower, mixed-use buildings present complex problems and opportunities. The following issues require careful consideration:
- Stacking the tower to make optimum use of floorplate sizes and minimise the number of lifts required. Ideally, uses that are served by the smallest number of lifts, such as residential or a hotel, should be located towards the top of the building, reducing the core size on lower floors.
- Resolving means of escape issues, including phased evacuation, the efficiency of the core and the requirements for security and privacy of users.
- Optimising floorplate design to suit different requirements for floor space depth and different riser configurations, using either a stepped floor plate or tapered building profile.
- Using transfer structures to accommodate changes in the structural grid.
- Providing dedicated services to each occupancy.
- Isolating occupancies for security, fire separation and acoustic reasons.
- Core design Cores in tall buildings are much more complex than in conventional buildings and their design is fundamental to the development efficiency and operational effectiveness of a tower. The key elements of core design are as follows.
- Lifts: Minimising the overall size of the core while maintaining an acceptable lift service.
- Structural design: Where the core is an important element of the stability system, the service core should be designed to maintain the integrity of the structural core as much as possible, with risers being placed towards the perimeter to minimise the number of openings required.
- Services distribution: Similarly, duct branches should be located at the perimeter of the core to facilitate ease of installation and minimise effects on the structural integrity of the core.
- Lifting strategies The lifting design is a major determinant of the core size, net to gross ratio, and other aspects of efficiency, including space planning. It will also determine occupant travel times, which in office buildings should meet BCO requirements for handling capacity and maximum waiting times. For example, a 15% handling capacity within a five-minute period, and an average waiting interval not exceeding 30 seconds.
- In tall buildings, the main components of the lifting strategy are the number of lifts per group, and the number of lift zones. A typical strategy is to divide a building into a number of zones, each served by an appropriately sized group of lifts. Getting the right balance between number of zones, size of groups, core size and lift performance to meet cost, area and performance objectives requires specialist input. The maximum number of lifts in a group should not exceed eight, and each group can normally serve up to 16 floors. Accordingly, a 45-storey office tower with a large floorplate will typically require three groups of lifts serving low, medium and high-rise zones, in addition to goods lifts and fire-fighting lifts.
- On towers of more than 50 storeys, the size of lift core required to serve all floors directly from the ground floor becomes prohibitive. As an alternative, a sky lobby served by express lifts can be introduced, reducing the overall number of lift shafts travelling through the lower parts of the tower. The space taken up by lift cores can be reduced by a further 30% through the adoption of double-deck lifts, although these require the introduction of carefully designed two-level lift lobbies at ground and sky lobby levels.
- The use of sophisticated controls in tall buildings is important to minimise waiting periods and to optimise the use of lifts during off-peak periods. This allows lifts to be taken out of service without affecting waiting times, reducing energy consumption and maintenance costs.
- Building environment strategy Compared with medium-rise buildings, towers can have a higher wall-to-floor ratio, which calls for special consideration of the performance of the facade and building services installations. Key issues include selecting the appropriate systems, minimising space requirements for plant and risers, and considering the effect of orientation and building shape.
- Facade performance: The thermal performance of the envelope is the starting point in the design of an efficient and sustainable solution. Cladding systems on tall buildings need simple and robust detailing, to resist high wind loads and building movement. Although most towers are designed as sealed, air-conditioned buildings, increasingly robust active facade solutions are being developed, utilising either natural ventilation or passive controls. The new Part L of the Building Regulations requires improved facade performance, and enhancements adopted may include high-performance glass, solid insulating panels or mid-pane blinds.
- Selection of heating and cooling systems: The system chosen will have an impact on the design of the core. Water-based systems such as fan coil units require 25% to 35% less core space than air-based alternatives such as conventional variable air volume units. Fan coils are a popular option because of their capacity and flexibility. Where a high-performance facade is specified, the use of chilled beams and underfloor ventilation could also be considered.
- Space requirements for plant and risers: Heating and cooling plant in towers needs to be centralised in basement and roof level plant rooms. By contrast, air-handling plant must be decentralised, located either on individual floors, or in a more efficient centralised arrangement of intermediate plant floors, serving between eight and 14 storeys. Typically, a 40-storey building can be served by one intermediate double-plant floor, and on taller buildings, the air-handling plant floor can be combined with sky lobbies or structural zones for outriggers or transfer structures.
- Orientation and building shape: The potential for a high wall-to-floor ratio and the height of tall buildings means that solar heat gain and glare need to be carefully considered at an early stage. Where opportunities exist, careful orientation of the building can be of use in the mitigation of these problems.
- The effect of 11 SeptemberThe events of 11 September 2001 have changed the way safety and security issues are perceived by building designers, builders, owners and occupiers. The objective of design is to understand the ultimate behaviour of a building, in terms of its own safety performance and the assurance it provides occupants and owner should an extreme event occur. The issues involved are complex and interrelated, and include:
- The behaviour of materials and fire protection systems in a fire, including cellulosic and hydrocarbon fires.
- The potential benefits of whole building evacuation, in combination with phased evacuation solutions, taking account of escape route locations and integrity, together with emergency training and communications.
- The detailed understanding of the behaviour of the structure and its dynamic robustness in an extreme event.
- The potential benefits of greater ductility in structures.
- The performance of glazing and facades in blast situations.
- The robustness and redundancy of building services and the possible protection of air inlets.
A risk-based approach to the design of significant buildings is advisable, together with an assessment of the appropriate level of protection. This assessment could apply to the risk of chemical and biological attack, and the need for enhancements to the building services installation, as well as the robustness of the structure and adequacy of means of escape. In many cases it is possible to arrive at measures that can be implemented at little extra cost but have the potential to significantly enhance safety.
The procurement of tall buildingsThe size, complexity and duration of tower projects mean there are a limited number of ways to design and construct them, both in terms of overall strategy and the range of firms with the skills, experience and resources to do the work. Key issues to be considered in developing a successful procurement strategy include:
Speed and efficiency Tall buildings are usually efficient because they lend themselves to standardisation and repetition. Achieving and maintaining speed is critical to successful project delivery, and package-based or two-stage procurement strategies are essential, as only the overlapping of design, procurement and construction can achieve acceptable project durations.
It is vital to invest in strategic construction planning and buildability reviews at an early stage to optimise the programme and to eliminate sources of delay and disruption.
Key elements in the programming of tall building projects include:
- The management and co-ordination of trade activities on site to maintain the floor construction cycle.
- The management of extended design and procurement programmes to make the most of standardisation and minimise floor construction cycles.
- The optimisation of standardised and prefabricated components.
Specialist contractors Their procurement is strongly influenced by the size of packages involved and, given the importance of co-ordination, it can be beneficial to bundle related trades into a single package with united management. The core area fit-out or services installation would benefit from this approach. The specialists should be appointed early to make the most of their design skills and get a good degree of co-ordination. For example, a concrete trade contractor may influence the choice of core construction method, which could result in changes to its design.
Logistics The provision of craneage and hoisting. Co-ordination of deliveries and craneage slots within the construction programme is essential to ensure that speed is maintained. Getting operatives to their workplaces and keeping them there is also crucial to achieving full use of labour. This means that the provision of generous hoisting and welfare facilities is necessary, as the location of toilets and canteens at regular intervals up the tower will minimise downtime and prevent unnecessary journeys on hoists.
Health and safety considerations A major factor when working at height. One benefit of working on large buildings is that trades do not need to work on top of one another, and the risk of minor incidents is potentially reduced.
Structural design considerationsFrame options
The resistance of lateral force is fundamental to the structural design of a tall building. A major consideration is the dynamic motions caused by wind loads on towers, which is accentuated by their height and slenderness. The accelerations associated with these motions can cause discomfort, and the design may incorporate stiffening or dampening features to reduce the accelerations.
There are many factors affecting the selection of the structural design solution for towers, but the main ones are: building height, shape and plan; location of the core; spacing of the perimeter columns; and the overall cost and efficiency of the structural solution.
Most stability systems can be placed into three categories (see pictures, left):
- Core systems Lateral stability is provided by the core alone. The core can be constructed using coupled, reinforced concrete shear walls or braced steel frames. There are limits to the height that can be achieved, but the design of the perimeter structure is less constrained, as it is only required to carry gravity loads.
- Braced and moment-resisting frame systems The frame is typically placed around the perimeter of the building to form a tube, which resists lateral loads. Moment-resisting frames generally require a closer perimeter column spacing, and braced frames impose diagonal members across the facade. In other alternatives, the outer tube can be used in combination with a core to form a “tube within a tube”, or moment frames can be used to form a set of “bundled tubes” that act compositely – although this often involves the introduction of internal columns, as at the Sears Tower in Chicago.
- Core and outrigger systems These combine the lateral shear resistance of the core with the overturning resistance of the perimeter structure by linking the two elements at discrete levels using outrigger trusses or beams. The beams usually need to have significant depth to create the required stiffness, but with careful planning they can be integrated into plant room or sky lobby levels. This type of system can mobilise the overturning resistance of a minimal number of heavier vertical elements at the perimeter of the building, freeing up the remainder of the facade.
- The frame systems given above are generic, and do not include some of the hybrid and advanced structural solutions that can be developed for individual projects. As further innovations in the architectural form of towers are developed, in parallel with advances in materials, structural analysis and environmental design, so new stability systems are being used, such as the Commerzbank Building in Frankfurt.
- Floor structure options
- Composite floor construction is the preferred option in the UK, but other systems are available and can prove attractive in certain circumstances.
- Composite floor systems These are common in tall buildings, and consist of steel beams and trusses linked in shear to the concrete floorplate so that they work in combination with the slab. This solution offers the option of prefabrication, it can assist integration with services, and it provides a relatively light form of construction that reduces the size of the vertical structure and foundations.
- Insitu reinforced concrete This has been used widely in places such as Hong Kong, where labour costs are low and where geology enables high foundation capacities to be achieved.
- Prestressed, precast and post-tensioned concrete floors These help to minimise the overall structural depth, and have been used in many buildings in Australia over the past 20 years.
- Sustainability issuesTowers have an increasing role in supporting the sustainable growth of cities. They can contribute to meeting sustainability objectives in the following areas:
- Tall buildings can help optimise the use of limited land resources, and enable development to be focused on existing transport hubs.
- Multi-use towers can help with the development of a more diverse city-centre economy and can help to reduce transport needs.
- Towers generate sufficient value to enable investment in extensive, high quality public spaces, amenities and infrastructure.
- Historically, towers have been perceived as inefficient users of energy, although this is difficult to prove on a like-for-like basis, because of concern about design characteristics such as low floor area efficiencies and high wall-to-floor ratios.
- In mitigation, many elements of sustainable design practice can be incorporated into high-rise schemes, including:
- More efficient lifts, lifting strategies and lift control systems
- Facade design to provide a balance between energy efficiency and good daylighting levels of over 2% daylight factor.
- Use of openable facades or mixed mode ventilation.
- Use of combined heat and power units on multi-occupancy buildings and other methods of heat recovery.
- Using layout, orientation, and features such as podiums, arcades, canopies and tree planting to reduce overshadowing, local wind turbulence and downdrafts that could be caused by a tower.
- Cost model: 48-storey tower, central LondonThe cost model summarises the shell-and-core and category A fit-out costs for a notional 48-storey office tower located in central London. The scheme comprises three basements, ground and 44 upper floors, including an intermediate plant floor and two plant levels at the top of the building. Retail areas constructed to shell and core are incorporated at ground and first basement levels. There is a mezzanine at ground-floor level which could be used for office or retail functions.
- The gross internal floor area is 118,450 m², providing 76,650 m² of net internal space (offices and retail) at an overall efficiency of just less than 65%.
- The unit rates in the model are based on price levels current in central London in the second quarter of 2002 for competitively tendered packages under a construction management arrangement. The costs exclude demolitions, enabling works, external works and services. The developer’s costs (professional and statutory fees, taxation, insurances and so on), are excluded, together with the costs of surveys, environmental impact assessments and so on. No allowance is made for works beyond the scope of the category A fit-out, such as tenants’ category B works, loose furniture or IT equipment.
- Adjustments to the unit rates should be made to account for location, site conditions, programme and procurement route.
- The costs of the model can be compared to the range of costs currently experienced on London office tower projects in the table below. The cost ranges in the table do not represent maximum and minimum thresholds, but rather indicate typical design and specification criteria.
- Indicative cost ranges for tall building construction (£/m2)
- Shell-and-core costs
- functional tower = 1850–2150
- cost model = 2160
- landmark tower - 2150–2450
- Costs are for shell-and-core construction only, excluding external works, fees and VAT, current at third quarter 2002.
- Functional office towers are efficient developments, designed for single-occupiers, with larger and regular floorplates, conventional cladding and building services, built on relatively clear sites. Landmark schemes have more complex and articulated designs, and are often built on constrained sites.
- Factors that influence the overall level of shell-and-core construction costs of office towers include:
- Height, shape and geometry The most sensitive cost drivers are the extent and the specification of the frame and envelope, which are determined particularly by planning issues, the floorplate design and the environmental strategy. All of these affect the wall-to-floor ratio, structural complexity, core design and envelope performance criteria.
- Services and lifting strategies
- Building uses The costs of single occupancy versus multi-let buildings.
- Multi-use The design complexities associated with accommodating multiple uses within a single building.
- Building ownership Drivers include the quality of design and materials, investment to reduce whole-life costs, sustainability issues and the requirements for flexibility and redundancy.
- Building Regulations Design responses to changes to Part L together with enhanced sustainability performance.
- Safety and security Drivers include the requirements for blast protection for cladding, structural collapse resistance, fire protection enhancements, evacuation improvements and provision of diverse support systems.
- Site constraints For example, adjoining structures and existing services.
What qualifies as a tall building?
Given the great variation in the development densities of cities such as London, Hong Kong, Chicago, and Frankfurt, it is impossible to define tall buildings using absolute measures, such as the number of storeys. Tall buildings are therefore best understood in relative terms as buildings whose planning, design, construction and occupation is influenced by height in ways that are not normally associated with more typical, local developments.
Davis Langdon & Everest would like to thank John MacArthur and Mohsen Zikri of Arup for their contributions to this article, together with the DL&E Tall Buildings Specialist Group led by Steve Watts.