|The Economist – Cities & Tall Buildings
Tall buildings Apr 7th 2005 |From the print edition
Hong Kong has 7,417 skyscrapers, more than any other city, according to Emporis, a firm that tracks the construction of high-rises. By its definition, a building must be over 35 metres (115 feet) tall to qualify as a skyscraper. New York ranks second with 5,444 skyscrapers; Los Angeles has just 450. Chicago's Sears Tower has more floors than any of its rivals, though other skyscrapers are taller.
Price Per Square Foot Construction Cost for Multi Story Office Buildings
Posted by Dean Dalvit • July 14, 2011 • Printer-friendly
Time again to look at the current construction market activity and analyze construction costs per square foot for various types of office buildings. How much does it cost to build an office building? Below is some data from RSMeans construction cost data that we keep a close eye on in order to estimate construction costs for all of our office building projects. This data is sorted by region from the most expensive to the least expensive.
price per square foot construction cost for two to four story office building
For the most common office building size, two to four stories tall, the range is from just over $130 per square foot in Winston-Salem to over $230 per square foot in New York. The spread here is largely due to the local cost of labor and regulations that allow various construction types that are allowed in low rise construction. For example, in some cases where wood frame construction is still allowed, depending on location and occupancy, this would help to keep costs lower. In areas that are restricted to non-flammable construction, price per square foot will go up.
By taking advantage of savings provided by vertical construction, you will see approximately a 4% savings in cost per square foot by increasing the stories to between five and ten stories. While one might expect a larger savings for that economy of scale, several new requirements come with the mid-rise building that are often not dealt with on the low rise buildings. For example, elevator shafts and service corridors get more complicated as well as HVAC systems.
Price per square foot construction cost for five to ten story office building
The geographic spread in cost per square foot is identical to the low-rise data. This is still principally driven by local factors such as labor costs and local regulatory requirements.
Finally, the high rise buildings see the most economic cost per square foot. For buildings between eleven and twenty stories tall, there is approximately an 11% savings over the mid rise buildings and 15% over low rise. this is largely due to the fact that similar elevator, HVAC and service equipment requirements are required for mid and high rise, resulting in more economy of scale for going up.
price per square foot construction cost for eleven to twenty story office building
Again, the geographic spread is still the same, telling us that the cost per square foot is very sensitive to geographic location. Note that over twenty stories starts getting into more unique building characteristics that will drive costs in various ways. For more information on estimating the cost of your office building during the early planning stages, contact any of us here at EVstudio and we can help scope the right size project for your pro forma.
Design and construction
Main article: Skyscraper design and construction
The design and construction of skyscrapers involves creating safe, habitable spaces in very tall buildings. The buildings must support their weight, resist wind and earthquakes, and protect occupants from fire. Yet they must also be conveniently accessible, even on the upper floors, and provide utilities and a comfortable climate for the occupants. The problems posed in skyscraper design are considered among the most complex encountered given the balances required between economics, engineering, and construction management.
One common feature of skyscrapers is having a steel framework from which curtain walls are suspended, rather than load-bearing walls of conventional construction. Most skyscrapers have a steel frame that enables to build taller than load-bearing walls of reinforced concrete. Skyscrapers usually have particularly small surface area of what are conventionally thought of as walls, because the walls are not load-bearing and therefore most skyscrapers are characterized by large surface areas of windows made possible by the concept of steel frame and curtain walls. However, skyscrapers can have curtain walls that mimick conventional walls and a small surface area of windows.
The concept of a skyscraper is a product of the industrialized age, made possible by cheap fossil fuel derived energy and industrially refined raw materials such as steel and concrete. The construction of skyscrapers was enabled by steel frame construction that surpassed brick and mortar construction starting at the end of the 19th century and finally surpassing it in the 20th century together with reinforced concrete construction as the price of steel decreased and labour costs increased.
The steel frames become inefficient and uneconomic for supertall buildings as usable floor spaces are reduced for supporting column and due to more usage of steel. Since 1960 Tubular designs are used for high rises. This conception reduces the usage of material (more efficient in economic terms - Willis Tower uses 2/3 of the steel as Empire state building), yet allows greater height. It allows fewer interior columns, and so create more usable floor space. It further enables buildings to take on various shapes.
The amount of steel, concrete and glass needed to construct a single skyscraper is large, and these materials represent a great deal of embodied energy. Skyscrapers are thus energy intensive buildings, but skyscrapers have a long lifespan, for example the Empire State Building in New York City, United States completed in 1931 and is still in active use. Skyscrapers have considerable mass, which means that they must be built on a sturdier foundation than would be required for shorter, lighter buildings. Building materials must also be lifted to the top of a skyscraper during construction, requiring more energy than would be necessary at lower heights.
Furthermore, a skyscraper consumes a lot of electricity because potable and non-potable water have to be pumped to the highest occupied floors, skyscrapers are usually designed to be mechanically ventilated, elevators are generally used instead of stairs, and natural lighting cannot be utilized in rooms far from the windows and the windowless spaces such as elevators, bathrooms and stairwells.
Elevators are characteristic to skyscrapers. In 1852 Elisha Otis introduced the safety elevator, allowing convenient and safe passenger movement to upper floors. Another crucial development was the use of a steel frame instead of stone or brick, otherwise the walls on the lower floors on a tall building would be too thick to be practical. Today major manufacturers of elevators include Otis, ThyssenKrupp, Schindler, and KONE.
Basic design considerations
Good structural design is important in most building design, but particularly for skyscrapers since even a small chance of catastrophic failure is unacceptable given the high price. This presents a paradox to civil engineers: the only way to assure a lack of failure is to test for all modes of failure, in both the laboratory and the real world. But the only way to know of all modes of failure is to learn from previous failures. Thus, no engineer can be absolutely sure that a given structure will resist all loadings that could cause failure, but can only have large enough margins of safety such that a failure is acceptably unlikely. When buildings do fail, engineers question whether the failure was due to some lack of foresight or due to some unknowable factor.
Loading and vibration
Taipei 101 endures a typhoon (2005)
The load a skyscraper experiences is largely from the force of the building material itself. In most building designs, the weight of the structure is much larger than the weight of the material that it will support beyond its own weight. In technical terms, the dead load, the load of the structure, is larger than the live load, the weight of things in the structure (people, furniture, vehicles, etc.). As such, the amount of structural material required within the lower levels of a skyscraper will be much larger than the material required within higher levels. This is not always visually apparent. The Empire State Building's setbacks are actually a result of the building code at the time, and were not structurally required. On the other hand John Hancock Center's shape is uniquely the result of how it supports loads. Vertical supports can come in several types, among which the most common for skyscrapers can be categorized as steel frames, concrete cores, tube within tube design, and shear walls.
The wind loading on a skyscraper is also considerable. In fact, the lateral wind load imposed on super-tall structures is generally the governing factor in the structural design. Wind pressure increases with height, so for very tall buildings, the loads associated with wind are larger than dead or live loads.
Other vertical and horizontal loading factors come from varied, unpredictable sources, such as earthquakes.
A shear wall, in its simplest definition, is a wall where the entire material of the wall is employed in the resistance of both horizontal and vertical loads. A typical example is a brick or cinderblock wall. Since the wall material is used to hold the weight, as the wall expands in size, it must hold considerably more weight. Due to the features of a shear wall, it is acceptable for small constructions, such as suburban housing or an urban brownstone, to require low material costs and little maintenance. In this way, shear walls, typically in the form of plywood and framing, brick, or cinderblock, are used for these structures. For skyscrapers, though, as the size of the structure increases, so does the size of the supporting wall. Large structures such as castles and cathedrals inherently addressed these issues due to a large wall being advantageous (castles), or ingeniously designed around (cathedrals). Since skyscrapers seek to maximize the floor-space by consolidating structural support, shear walls tend to be used only in conjunction with other support systems.
The classic concept of a skyscraper is a large steel box with many small boxes inside it. The genius of the steel frame is its simplicity. By eliminating the inefficient part of a shear wall, the central portion, and consolidating support members in a much stronger material, steel, a skyscraper could be built with both horizontal and vertical supports throughout. This method, though simple, has drawbacks. Chief among these is that as more material must be supported (as height increases), the distance between supporting members must decrease, which actually in turn, increases the amount of material that must be supported. This becomes inefficient and uneconomic for buildings above 40 stories tall as usable floor spaces are reduced for supporting column and due to more usage of steel.
Tube structural systems
See also: Tube (structure)
The Willis Tower showing the bundled tube frame design
Since 1963, a new structural system of framed tubes appeared. Fazlur Khan and J. Rankine defined the framed tube structure as "a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation." Closely spaced interconnected exterior columns form the tube. Horizontal loads (primarily wind) are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes allow fewer interior columns, and so create more usable floor space. Where larger openings like garage doors are required, the tube frame must be interrupted, with transfer girders used to maintain structural integrity. Tube structures cut down costs, at the same time allow buildings to reach greater heights. Tube-frame construction was first used in the DeWitt-Chestnut Apartment Building, completed in Chicago in 1963. It was used soon after for the John Hancock Center and in the construction of the World Trade Center.
A variation on the tube frame is the bundled tube, which uses several interconnected tube frames. The Willis Tower in Chicago used this design, employing nine tubes of varying height to achieve its distinct appearance. The bundle tube design was not only highly efficient in economic terms, but it was also "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings." The bundled tube structure meant that "buildings no longer need be boxlike in appearance: they could become sculpture."
The tubular systems are fundamental to tall building design. Most buildings over 40-storeys constructed since the 1960s now use a tube design derived from Khan’s structural engineering principles, examples including the construction of the World Trade Center, Aon Center, Petronas Towers, Jin Mao Building, and most other supertall skyscrapers since the 1960s. The strong influence of tube structure design is also evident in the construction of the current tallest skyscraper, the Burj Khalifa.
Changes of structure with height. The tubular systems are fundamental for super tall buildings.
Since 1963, the new structural system of framed tubes became highly influential in skyscraper design and construction. Khan defined the framed tube structure as "a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation." Closely spaced interconnected exterior columns form the tube. Horizontal loads, for example from wind and earthquakes, are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes allow fewer interior columns, and so create more usable floor space. The bundled tube structure is more efficient for tall buildings, lessening the penalty for height. The structural system also allows the interior columns to be smaller and the core of the building to be free of braced frames or shear walls that use up valuable floor space. Where larger openings like garage doors are required, the tube frame must be interrupted, with transfer girders used to maintain structural integrity.
The first building to apply the tube-frame construction was the DeWitt-Chestnut Apartments building that Khan designed and was completed in Chicago in 1963. This laid the foundations for the framed tube structure used in the construction of the World Trade Center.
The John Hancock Center, designed by Skidmore, Owings and Merrill with chief designer Bruce Graham and structural engineer Fazlur Khan. The building was completed in 1969.
Trussed tube and X-bracing
Khan pioneered several other variations of the tube structure design. One of these was the concept of X-bracing, or the "trussed tube", first employed for the John Hancock Center. This concept reduced the lateral load on the building by transferring the load into the exterior columns. This allows for a reduced need for interior columns thus creating more floor space. This concept can be seen in the John Hancock Center, designed in 1965 and completed in 1969. One of the most famous buildings of the structural expressionist style, the skyscraper's distinctive X-bracing exterior is actually a hint that the structure's skin is indeed part of its 'tubular system'. This idea is one of the architectural techniques the building used to climb to record heights (the tubular system is essentially the spine that helps the building stand upright during wind and earthquake loads). This X-bracing allows for both higher performance from tall structures and the ability to open up the inside floorplan (and usable floor space) if the architect desires.
In contrast to earlier steel-frame structures, such as the Empire State Building (1931), which required about 206 kilograms of steel per square metre and Chase Manhattan Bank Building (1961), which required around 275 kilograms of steel per square metre, the John Hancock Center was far more efficient, requiring only 145 kilograms of steel per square metre. The trussed tube concept was applied to many later skyscrapers, including the Onterie Center, Citigroup Center and Bank of China Tower.
One of Khan's most important variations of the tube structure concept was the "bundled tube," which he used for the Sears Tower and One Magnificent Mile. The bundle tube design was not only the most efficient in economic terms, but it was also "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings."
The last major buildings engineered by Khan were the One Magnificent Mile and Onterie Center in Chicago, which employed his bundled tube and trussed tube system designs respectively. In contrast to his earlier buildings, which were mainly steel, his last two buildings were concrete. His earlier DeWitt-Chestnut Apartments building, built in 1963 in Chicago, was also a concrete building with a tube structure.
The influence of Khan's tube structure design can be seen in numerous buildings built since the 1960s. Tube structures have since been used in many skyscrapers, including the construction of the World Trade Center, Petronas Towers, Jin Mao Building, and most other supertall skyscrapers since the 1960s. The strong influence of tube structure design is also evident in the world's current tallest skyscraper, the Burj Khalifa in Dubai. According to Stephen Bayley of The Daily Telegraph:
Khan invented a new way of building tall. [...] Which created the unconventional skyscraper. Reversing the logic of the steel frame, he decided that the building's external envelope could – given enough trussing, framing and bracing – be the structure itself. This made buildings even lighter. The "bundled tube" meant buildings no longer need be boxlike in appearance: they could become sculptures.
The elevator conundrum
Elevators in the Empire State Building in New York City.
The invention of the elevator was a precondition for the invention of skyscrapers, given that most people would not (or could not) climb more than a few flights of stairs at a time. The elevators in a skyscraper are not simply a necessary utility, like running water and electricity, but are in fact closely related to the design of the whole structure: a taller building requires more elevators to service the additional floors, but the elevator shafts consume valuable floor space. If the service core, which contains the elevator shafts, becomes too big, it can reduce the profitability of the building. Architects must therefore balance the value gained by adding height against the value lost to the expanding service core. Many tall buildings use elevators in a non-standard configuration to reduce their footprint. Buildings such as the former World Trade Center Towers and Chicago's John Hancock Center use sky lobbies, where express elevators take passengers to upper floors which serve as the base for local elevators. This allows architects and engineers to place elevator shafts on top of each other, saving space. Sky lobbies and express elevators take up a significant amount of space, however, and add to the amount of time spent commuting between floors. Other buildings, such as the Petronas Towers, use double-deck elevators, allowing more people to fit in a single elevator, and reaching two floors at every stop. It is possible to use even more than two levels on an elevator, although this has never been done. The main problem with double-deck elevators is that they cause everyone in the elevator to stop when only people on one level need to get off at a given floor.
The first sky lobby was also designed by Khan for the John Hancock Center. Later buildings with sky lobbies include the World Trade Center, Petronas Twin Towers and Taipei 101. The 44th-floor sky lobby of the John Hancock Center also features the first high-rise indoor swimming pool, which remains the highest in America. This was the first time that people could have the opportunity to work and live "in the sky".
Skyscrapers are usually situated in city centers where the price of land is high. Constructing a skyscraper becomes justified if the price of land is so high that it makes economic sense to build upwards as to minimize the cost of the land per the total floor area of a building. Thus the construction of skyscrapers is dictated by economics and results in skyscrapers in a certain part of a large city unless a building code restricts the height of buildings. Skyscrapers are rarely seen in small cities and they are characteristic of large cities, because of the critical importance of high land prices for the construction of skyscrapers. Usually only office, commercial and hotel users can afford the rents in the city center and thus most tenants of skyscrapers are of these classes. Some skyscrapers have been built in areas where the bedrock is near surface, because this makes constructing the foundation cheaper, for example this is the case in Midtown Manhattan and Lower Manhattan, in New York City, United States, but not in-between these two parts of the city.
Today, skyscrapers are an increasingly common sight where land is expensive, as in the centers of big cities, because they provide such a high ratio of rentable floor space per unit area of land.
Cities throughout the UK are developing residential towers and landmark skyscrapers. Steve Watts and Neal Kalita of Davis Langdon consider the design and construction challenges of high-rise development and provide a cost model for a central London office tower
Davis Langdon’s last tall buildings cost model was published in September 2002, when the Heron Tower had been granted planning permission. Other landmark London towers, such as the Leadenhall Building, the Shard, 20 Fenchurch Street and The Pinnacle, were due to follow.
These stemmed from high demand from the financial industry and residential sectors and increased developer confidence in the value of high rises. Tenants appreciate a landmark address and politicians are conscious of the symbolic role of tall buildings. The continued demand for residential property as a result of the buy-to-let phenomenon, low interest rates, steady house price growth and an imbalance between supply and demand have meant more than 130 residential towers (more than 20 storeys) are either being planned or are under construction in the UK.
Most commercial towers (more than 30 storeys) being developed are in London, perhaps because commercial rents in the regions would not support office towers and land values are lower.
02 Development and design challenges
Developers are more willing than ever to take on tall buildings, thanks to high demand and the benefits of high buildings, such as increased density and higher rent. But for the financials to stack up, the challenges of high-rise development have to be overcome:
• Income stream. Residents usually cannot inhabit towers during construction owing to single central core access, so developers can not realise their full income until completion.
In commercial developments, phased occupancy through multiple lift cores may be possible, enabling an accelerated and phased return (although problematic)
• Floor area efficiencies. Efficiencies are affected by height, as core and structural zones expand relative to the overall floorplate to satisfy the requirements of vertical circulation and resist wind loads
• Planning hurdles. The increased scrutiny of a tower’s architectural, environmental and economic impact means significant effort, time and money has to be invested to help it through planning and consultation processes
• Procurement strategy. The current state of the property and construction market is crystallising procurement strategies for large and complex projects. Capability and availability of trade contractors has to be considered. Particular attention should be paid to early and continuing involvement of specialists
• Programme. Towers take longer to build than short buildings. This costs money in construction and developer’s costs, produces uncertainty because of the difficulty in predicting future costs, changes to regulations and market demands
• Technical challenges. London’s schemes vary in shape and form, but all pose technical challenges related to developing commercially viable towers in constrained, sensitive locations, while satisfying stakeholders. In terms of safety and security, developers are taking a pragmatic approach, focusing on management issues and sensible enhancements to the base building.
A tall building faces more scrutiny than a lower-rise development owing to its visibility in the urban landscape. Architectural quality and iconic architecture are often cited as the main contributors to the success of the planning applications. This is recognised in PPS 1 and 3, which states the importance of design quality, and reinforced by Cabe and English Heritage.
In the UK, towers make headlines because of their interesting forms, which range from gherkins and cheese graters to lipsticks and walkie-talkies. Iconic architecture is a holistic term that should recognise the longevity of the design and not only fashion. 3D visualisation helps illustrate the impact of towers and is a prerequisite for a planning application. Standardising elements and high quality repeated details can pay dividends.
It is difficult to quantify the value of iconic architecture in securing planning permission and attracting tenants and purchasers.
However, a building being associated with a particular company limits secondary market potential, as in the cases of 30 St Mary’s Axe and the NatWest Tower (now Tower 42). While the former was sold on for a profit, the latter needed considerable refurbishment to make it appeal to occupiers, though it provided a successful second life.
• Floorplate efficiency: The shape and geometry of the building needs to satisfy the value and cost of the development equation. Floor area efficiencies go a long way to help the financials and are determined by the size of the floorplate and rationalised cores.
Slimmer towers are more expensive to build because of lateral restraints and wall:floor ratios, so they suffer from adverse floor areas efficiencies. Residential towers tend to be slender because of unit size requirements and daylight issues, and their designs respond to this trait.
03 Construction issues
Height comes at a cost and programme premium. To optimise both, new ideas need to be encouraged and experience from around the world should be taken advantage of. There are a number of issues that require thought and analysis, including:
Towers are built floor by floor so construction cannot be accelerated easily. Pace needs to be considered at the outset, through strategic programming and buildability reviews, to mitigate sources of delay. Procurement strategy is a critical function, not only in programme terms but to respond to market challenges.
The floor construction cycle contributes to programme pace and can be optimised by using standardised, prefabricated components that minimise the number of trades operating per cycle. Concurrent programming of design, procurement and construction activities can also achieve acceptable project durations.
The logistics of material supply focus on adequate craneage and hoisting. Co-ordinating deliveries and craneage slots is critical. To maximise labour efficiency, generous hoisting and welfare facilities must be supplied. This means locating toilets and canteens at regular intervals up the tower to minimise downtime.
Investment in sufficient labour resource is also essential. Work on a tower enables trades to be kept apart, so the site can be flooded with workers.
Structural frame, cores and upper floors amount to 15-25% of the construction cost in a commercial tower and 10-15% in a residential one. The design of the building (shape, massing and height) determines the weight and therefore the quantity of material required, which affects cost.
However, complexity is as important, with the number of members, simplicity of connections, ease of fabrication and erection and other factors affecting cost.
Core integrity and wind Core layout and wind loading also present challenges.
Core layout is critical to development efficiency and operational effectiveness and affects how the structure copes with wind. The elements influencing design are:
Lifts. Factors include the number of lifts and their speed, size and arrangement, which affect space use and cost. In Broadgate tower, double-deck lifts were chosen, a solution that can reduce core size by 30%. However, this requires two level lift lobbies at ground and sky corridor levels.
Structural integrity. Maintaining core integrity by positioning risers and duct branches at the perimeter minimises the number of openings required, facilitating services installation and maintenance.
Air-conditioning. Air supply for fan-coil units are lower, so supply and extract riser ducts are more space-efficient than all-air systems.
Wind loads increase disproportionately with height, and slender towers are particularly sensitive to sway. To reduce this, a stiffer structural frame is required. Swaying can also be minimised by manipulating shape, geometry, surfaces and mass distribution. Use of dampers is a last resort on a structure below 200m.
The drag of airflow around the structure creates wind at pedestrian level, which can be mitigated by the use of additional elements such as fins on the edge of the form with canopies and planting at ground level.
Most tall buildings adopt unitised curtain walling. This entails storey-height elements fully assembled off site. Widths are typically 1.5m, enabling supply via hoists rather than cranes and reducing demands on hook time.
Part L says facade performance should be considered along with the mechanical and electrical systems that comprise the building’s environmental strategy. Residential towers are subject to Part L1, which concentrates on heat loss, so the facade must accommodate more solid panels than glass. Part L2 relates to commercial towers and concentrates on solar gain, which can be achieved by using louvres and photovoltaics, as in the Heron tower.
Facades contribute to controlling heat loss and gain. Active, ventilated facades afford high thermal performance and can respond to daily or seasonal changes too. A facade that enables natural ventilation or mix mode (both natural ventilation and air-conditioning), will achieve a cool interior. Where natural ventilation is the predominant mode, the facade can be designed to deal with the high wind pressures. The cost penalty is offset by reduced energy costs.
The facade needs to let in sufficient light while minimising glare for occupants and neighbouring buildings. It must also keep out noise and control reverberation.
Facades must be given particular attention when designing a tall building because of their aesthetic qualities, their contribution to environmental strategy and the range of costs possible. The costs vary for two principal reasons:
The envelope, when expressed as a cost per unit of floor area, is sensitive to changes in wall:floor ratio, determined by building shape and floorplate size
The wide range of architectural and specification options. The route to cost efficiency is through off-site prefabrication, simplicity and repetition of details.
In tall buildings, M&E design is focused on providing enough capacity for the population density and load. Maintaining hydraulic pressure for water and coolant requires pressure breaks and multiple plant. Similarly, Part B: Sprinkler requirements now requires a shut-off valve on the wet riser every third floor in commercial use. In residential buildings, the requirement is for sprinklers in storeys above 20m.
Lift strategy affects design on all towers. Furthermore, until the lift strategy is resolved, core and structure design cannot be finalised. One driver is the period of time for users to get from the ground floor to their destination. The British Council for Offices rule of 30-second waiting time for a lift is not achievable in towers regardless of the strategy used.
Another contributor to the M&E design of commercial towers is the impact of potential tenant enhancements. In conventional buildings, this is not usually an issue as it is often left to tenants to fit out extra equipment in the roof or basement at their own costs. Furthermore, this usually involves a small number of tenants.
In a tower, retrofitting distribution infrastructure along with installing a new generator, plant or chiller for one tenant alone is costly. Multiply this by the increased number of tenants in a tower and the space and distribution premium throughout the building, and this makes it unviable.
Future upgrades can be catered for by designing in resilience. This contributes to the cost increase in M&E design, but is the only way tenant facilities can be catered for.
For residential towers outside London, air-conditioning has not been specified widely as market values will not support its installation.
In London, cooling is a prerequisite in towers owing to agents and sales advice, the values achievable and market expectations. This is becoming harder to achieve owing to the onsite renewables requirement prescribed by the Greater London Authority (GLA).
Initial costs and operational costs of towers may be high, but it can be argued that tall buildings are sustainable. They make better use of land than a building of the same capacity spread over a larger space and locating the tower near a transport hub can support the growth of a diverse city-centre economy.
The Shard at London Bridge provides a mix of uses over one of the busiest rail terminals in the country, adding to its green credentials (pictured).
The Greater London Authority prescribes the need to generate 10% of energy on site, which is set to be increased to 20%. Most solutions currently being developed involve a combination of sources ranging from photovoltaics to biomass.
Biomass offers a one-source solution but there are logistical issues such as source, supply, storage and transportation of sufficient woodchip to generate energy.
Eighty tonnes of woodchip are required for every 1MW of energy generated.
A single articulated truck supplies 15 tonnes of woodchip, therefore to generate 1MW, six deliveries are required. For an average tower, the 5-6MW of energy required every week equates to about 30 deliveries a week.
Biofuels such as rapeseed oil are more efficient in terms of volume, but their use is curbed by the Clean Air Act, which only allows biofuel generators of 300kW of energy.
For residential towers, the Code for Sustainable Homes specifies mandatory minimum energy and water use levels at a percentage better than those specified in Part L (2006). For a Level 1 rating (the lowest) the use of energy and water must be 10% better than those specified in Part L.
The use of visible sustainable technologies such as photovoltaics and wind turbines are limited owing to small roof areas on residential towers, however Vauxhall tower and Castle House at Elephant & Castle have integrated roof wind turbines into their designs.
It is clear that a holistic approach needs to be taken when considering a tower’s performance in sustainability terms, as the issues are wider than simply integrating renewable technologies.
Developing tall buildings involves a number of challenges and the range of potential costs is large. In Davis Langdon’s experience, there are a number of general factors that can lead to success:
Understand the cost and value drivers at the feasibility stage
Keep a building interesting, but as simple and buildable as possible
Squeeze every square foot out of the floorplate
Perfect the details and repetitive elements
Settle the core design early, considering all factors that maximise efficiency
Get the early input of a constructor on planning, programming and logistics
Involve the main specialist trade contractors in the design process, obtaining their advice and buy-in to design strategy, detailing, methodology and so on
Encourage ideas and teamwork at every level.
06 Residential vs commercial
In comparison to the cost model overleaf, which is a commercial tower in central London, the “average” above-ground benchmark cost for residential towers in London is considered to be £2,960/m2 with a range of ± £800.
The reasons for the large range are:
Differentials in floorplate size
Form, height and location
On a national basis, the average cost will be significantly lower owing to the varying cost bases and market expectations.
07 Cost model
This cost model summarises the shell and core construction costs for a notional landmark high-rise office building in central London. It totals 97,550m2 (1,050,000ft2) gross internal floor area over 55 floors (including ground) and three basement levels. It provides a total net internal area
of 62,245m2. This is predominantly office space, with about 1,860m2 of retail and food and drink space at the lower levels.
It achieves a net:gross floor area efficiency of just under 64% and an above ground net:gross efficiency of about 68%. The typical floor to floor height is 3.90m and wall:floor ratio is 0.55 (there are 55m2 of facades for every 100m2 of above-ground gifa).
Unit rates are based on price levels in central London in the first quarter of 2007 for competitively tendered packages under a construction management arrangement – all assuming an immediate start on site.
All non-shell and core items are excluded (demolitions and enabling works, external works, incoming services and fitting out works). Developer’s costs are also excluded (professional and statutory fees, taxation, insurances, finance charges, disposal costs), as are the costs of surveys, monitoring works and environmental impact assessments.
Also excluded are professional fees, VAT and site abnormals. The rates may need to be adjusted for specification, site conditions, procurement route and programme.