Mechanical, electrical and plumbing systems are a particular challenge for tall buildings, with their own considerations and commercial drivers. Alinea Consulting details the factors and costs for office towers
01 / The world is getting taller
Tall buildings are being delivered in greater numbers, at greater heights, and in more locations around the world: more than 70 of the world’s tallest 100 buildings were completed in the last decade. This premier league of towers has an average height of around 370m (the corresponding figure in 2000 was 285m).
Last year, towers of least 200m high were built in 65 cities across 23 countries, with 13 cities seeing their first 200m-plus building. As Antony Wood, executive director of the Council on Tall Buildings and Urban Habitat (CTBUH), puts it: “Tall buildings are becoming the accepted global model for densification as more than a million people on the planet urbanise each week.”
London is not a premier league player in terms of numbers, but its skyline has changed dramatically over the past 15 years, and its array of high-quality tall architectural forms and engineering solutions are admired around the globe. The experience gained by the various professionals involved has enabled them to export their talents internationally. This includes London-based M&E engineers, who can demonstrate their skills in making a landmark tower’s services as effective and efficient as possible – from perspectives of cost, value, operation and environment – all against the inherent challenges such buildings present.
This article outlines the key considerations and commercial drivers in the design of mechanical, electrical and plumbing (MEP) services for tall buildings, and provides a cost model for these elements for a high-rise London office building.
02 / Early strategic decisions will determine efficacy
Towers contend with a difficult balance between risk and reward, and the margins between commercial success and failure can be small. Success relies on achieving the optimal balance between cost, floor area and construction efficiencies, which must be considered in the initial development of massing and geometry. This applies to mechanical, electrical and plumbing (MEP) services just as much as it does to any other element. MEP services generally constitute 20% to 25% of the total elemental (trade) shell and core cost of a tall office building. With plant and risers accounting for up to 10% of total gross internal floor area, the stakes are high.
The design of MEP services will require some key strategic decisions to be made early on, with careful consideration and analysis of all relevant factors. Not least of these is the environmental strategy and how it can support the orientation of the building, the centralisation or decentralisation of services, the location of mechanical plant, the distribution of incoming power, and the arrangement of services up and around the core. These decisions should factor in not only capital costs and their various drivers (such as buildability), but also operational costs, ease of maintenance, space-take and carbon impacts.
With a limited pool of trade contractors having the capabilities and capacity to undertake larger and more complex towers, procurement requires careful thought too, with early engagement advisable.
03A / Key decisions - plant location, hydraulics and air distribution
The predominant MEP considerations for tall buildings are the selection of main plant locations and how services are distributed up and down the building.
Tall buildings typically have multiple basements, which tend to suit the accommodation of the electrical high-voltage (HV) incoming switchgear, water-cooled chillers, cold-water tanks and sprinkler tanks. This invariably requires the fitting together of puzzle pieces to achieve the most efficient fit with the smallest possible gaps. The competition against other demands for these below-ground spaces has become more intense in recent years with the proliferation of bicycles. The London Plan, and its calculations based on the total gross internal area of a building, defies tall commercial buildings to fit in vast numbers of cycle spaces, lockers and showers.
However, it is at the other end of the tall building where space is tightest. Ideally, plant such as cooling towers, boilers and generators would be roof-mounted, but most tall buildings have little or no such roof space to locate these key pieces of plant, at heights that offer premium office rents and opportunities for other valuable functions such as restaurants and viewing platforms.
Part of the answer often lies in interstitial plant floors, which could mean that a 60-storey building, in terms of MEP services provision, is the equivalent of three buildings stacked on top of one another.
Generally, the maximum vertical distribution distance for water systems tends be around 20 floors, before a hydraulic break is required on low-temperature hot water (LTHW), chilled water (CHW) and condenser water systems. The requirement of hydraulic breaks introduces an additional expense of heat exchangers and pump sets to distribute further distances to higher floors.
If more than 20 floors are served without the use of hydraulic breaks, higher-pressure-rated pipework and associated valves will be required – which could result in the introduction of on-floor hydraulic breaks between the shell and core and fit-out installations.
The same issues apply with plumbing services. Drainage in tall buildings has to be dealt with in a particular way, with several high- and low-rise stacks rising next to each other serving different sections of the building. There could be two to three times the amount of pipework compared with a building of the same total area but less height. Water services can serve a maximum of 150m because of the limitations of WTAS approved fittings and the practicalities of pressure on larger towers: pressure breaks will therefore be required.
This justifies a classic cost-benefit analysis, with the capital cost and space-take implications of the alternatives to be worked through and compared, so that a decision can be made and schematics developed.
There are several ways of distributing air around a tall building, with no obvious, preconceived answer for any project (which is why both the current and proposed batches of London’s tall buildings have adopted a variety of solutions).
That said, there are two principal options:
- Centralised air handling units (AHUs) – possibly with interstitial plant floors
- On-floor AHUs.
Figure 2 shows how these can be respectively configured to supply fresh air to the office floorplates. Each of these strategies carries some important issues:
- Have larger ductwork risers for the movement of greater air volumes up and down the building
- Rely on a few larger pieces of kit to serve more floors, raising questions of resilience (particularly in a multi-tenanted environment)
- Make it more difficult to install large AHUs, with thought required on issues of logistics and buildability – especially for the plant at the top of the building.
- Require the increased capital cost of additional CHW/LTHW connections, power supplies, and additional points and interfaces on the building management system
- Involve connections through the facades for air intake and exhaust
- Need access to their on-floor plant rooms, and maintenance access may need to be allowed through tenants’ floor spaces
- Should make commissioning a simpler process.
There are advantages and disadvantages with either of these two options, but perhaps the greatest factor in the decision is the effect on net internal floor area.
The options will usually involve more than two strategies, as the extremes of a completely centralised versus a floor-by-floor solution are supplemented by the option to provide intermittent plant floors with their individual AHUs.
03B / Other key decisions
Incoming electrical load
Tall buildings demand a high overall electrical load. Given the additional requirements for resilience, the incoming power is often fed directly to the building from one of the main 132kV substations at 33kV, sometimes by dual supplies for resilience.
Utilising a high-voltage (HV) 33KV incoming power supply not only adds considerable capital cost but also requires a large area to be set aside for a UKPN 33kV transformer, which in turn steps down the supply to 11KV before the power can be distributed around the building to the various distributed substations.
Introducing power at 11KV into the building is more cost-effective but is deemed less resilient. There may also be capacity issues with UKPN, but if these can be resolved at affordable levels then this is a valid option.
The location of the UKPN transformers requires careful consideration as the supplier insists on 24/7 access to their transformers; recently we have seen UKPN pushing for these transformer rooms to be located at ground floor level.
In most low-rise buildings the HV/LV plant is placed in the basement and power is distributed by low-voltage (LV) busbar up the building. In a tall building it is normally commercially better to distribute up the building at HV, in fire-protected routes, to substations at a higher level in the building. The LV distribution thus becomes a combination of supplies taken from these substations and distributed both up from the basement and down from the higher substations.
There is often the need for expensive power distribution controls (“supervisory control and data acquisition”, referred to as “SCADA”) to regulate and manage the start-up of the generators and the complex distribution of electrical loads.
Tall buildings normally have to be designed for a number of possible tenancies, large and small. MEP systems therefore need to be designed with multiple service distribution risers to serve these tenancies, requiring multiple:
- Soil and vent stacks for kitchenettes
- Cold water services for kitchenettes
- Chilled water risers
- Low-temperature hot water risers
- Supply ductwork risers
- Extract ductwork risers
- Electrical busbar distribution
- IT containment risers.
All these distribution systems have not only a capital cost associated with them, but they also demand extra space, eating into net internal floor areas, which must be balanced with the perceived value (lettability) that they offer.
Enhanced specifications and tenant provisions
It is difficult to find space for potential tenants’ plant and it is equally logistically difficult to add plant at a later date. These challenges, together with the multi-tenancy letting profile of most tall buildings, mean that they may be provided with a higher specification of MEP plant, including:
- fresh air provision
- electrical load
- cooling provision
- full building standby electrical back-up.
These specification and tenant enhancements not only have a capital cost implication but they also pose some space planning challenges within the building.
An example of one of the challenges associated with such enhancements is the provision of full building electrical back-up. Generators would typically be installed at the top of the building, with oil storage located in the basement, resulting in distribution pipework and pumps to supply the generator sets, together with significant acoustic treatment and fire protection.
Increasingly, the developers of commercial towers look to incorporate amenity spaces, terraces, viewing galleries, restaurants, and other functions throughout the building, to differentiate their product and invest in features that have a direct influence on occupiers’ health, happiness and productivity.
Each of these areas needs servicing with specialist air-conditioning plant, ventilation, separate power requirements, drainage, specialist waste disposal, gas, water and toilet provisions, and dedicated areas such as cool rooms and catering facilities – all needing to be incorporated into the design as efficiently as possible. Even if these areas are provided as shell-only with the incoming tenant responsible for the fit-out, the developer has to allow for all the shell and core provisions to be incorporated into the overall services design, not least the infrastructure provisions for servicing these spaces (including discrete access and egress). The logistics of these fit-outs, and any potential overlap with the base build programme, are a further factor to contemplate.
Tall buildings have the opportunity to invest in smart infrastructure. We are starting to see links to security access, lift calling, navigation around the building, control of the air-conditioning and lighting, all from a personal mobile device to give people a seamless experience and personal control. This is part of the trend for individuals to be able assess the quality of their working environment, a subject covered in a Building article on the WELL Building Standard published in May 2017.
To facilitate smart buildings, more communication systems are having to be brought into the base specification. These include:
- Landlords’ fibre backbone
- Wifi within landlord areas
- Wifi within lifts
- Mobile boosting technology
- Meeting room booking systems
- Audiovisual display screens in landlord areas, lifts, and so on.
Another provision found in tall buildings is the inclusion of a comprehensive blind control system linked to the cladding system (particularly where “active” facades are promoted). These can be controlled individually or set by elevation as part of the BMS installation.
04 / Preliminaries and site construction issues
The extended construction programmes of tall buildings trigger higher preliminaries costs for the main contractor and trade contractors – not least MEP trade specialists, who also contend with the involved co-ordination of their respective works with other contractors. Further, extended warranties are typically required as plant is usually brought to site relatively early in the programme in order to be incorporated into the basement construction. This means that the plant requires maintenance up to practical completion and full warranty periods after it. At the other end of the programme, roof-mounted plant and equipment will not be available until near the end of construction, causing additional costs associated with water treatment and additional commissioning.
Tall buildings also attract additional cost premiums due to site constraints and the simple fact of their height, causing significant non-productive time in getting labour up and down the building, and from the workface to welfare facilities. Some of this non-productive time has been reduced in recent times with the introduction of jump lifts: they climb with the core structure as it progresses, assisting the transportation of both labour and materials. Jump lifts do come with a cost premium of £250,000-£300,000 per jump lift, depending on the solution and number of levels served, but this cost can be offset by programme reductions. Main contractors have also realised the importance of providing canteens and toilets at regular intervals throughout a tower, reducing downtime inefficiencies.
Higher plot ratios (more building area placed on a site) may ultimately create value for backers, but they also imply restrictions on areas for storage and staging. This has encouraged a “just in time” approach to deliveries, supported by the use of consolidation centres off-site. This does require trades to have adequate resources in place to ensure that their supplies of materials and plant are managed correctly. Pressures on programme may also compel some deliveries to be made out of hours, which again attracts additional costs not typically seen on other projects.
The use of prefabrication is another way in which site constraints can be mitigated and the programme protected, by reducing site installation times, in turn easing the number of operative hours on site. Careful planning by the main contractor is needed with MEP prefabrication to ensure there is adequate hook time to drop prefabricated risers and plant into position. It has been known for prefabricated modules to lose out in the competition for hook time to items that are on the critical path. This would obviously erode the time advantages, as a cost premium has already been paid for breaking down the plant into modules and using multiple deliveries, plant handling, installation and retesting on site.
The fundamental commercial measures that underpin a tall office building – cost, time and floor area efficiencies – are of equal importance in residential tower developments. They may have their respective areas of focus and their own cost and value profiles, but both can reference the key ratios of wall/floor and net/gross as clear indicators of viability (even if they perform differently in these terms, not least due to the more slender form of residential high rises as they seek to optimise apartment sizes and daylighting).
In MEP terms, residential towers tend to demand a greater co-ordination between the base build and fit-out designs. Allied to this is the co-ordination of vertical services with unit mixes and the configuration of apartments around the core. Stacking is critical to achieving cost and construction efficiencies.
Other notable differences between the two typologies include:
- Envelopes for residential towers typically focus on amenity spaces (such as balcony options) and daylighting. This contrasts with the approach of office towers, which are more likely to focus on making an architectural statement that also works as part of the environmental strategy (in particular addressing solar gain).
- Localised ventilation through the facade is typical for residential towers, as opposed to the more centralised form of ventilation typically used for offices.
- A low-voltage power network is used in residential towers, compared with the combination of high and low voltage used in office buildings.
This cost model focuses on MEP services: a separate article could be dedicated to vertical transportation in tall buildings. Lifts and escalators are a major part of the design and a key influence on net:gross floor area efficiencies as well as operational efficiencies. Together with MEP they can constitute up to a third of the total net trade costs in a tall building construction project.
Getting the right solution from a number of strategic options (single or double decks, twin lifts, express lifts) and sub-options (number, speed and arrangement of lifts; destination control) is vital in optimising cost and space-take, interfacing with architecture, structure and services to achieve a tight core and workable main entrances.
As occupancy rates and tenancy expectations have increased in recent years, this has put more pressure on lift systems to meet the current BCO waiting times. The advent of additional amenity provisions and specialist areas in towers often requires separate express passenger and goods lifts, which also need to be considered in the core design and often need their own lobby areas separate from the main passenger lifts, with distinct servicing.
05 / Summary of the key cost drivers
MEP services in tall buildings play a critical part in the commercial performance of the scheme. Making the correct early decisions on some crucial strategic options, and paying close attention to the development of these design routes, will go a long way towards mitigating the inherent challenges of high-rise schemes, ensuring best fit between capital costs, area efficiencies, construction programme and operational effectiveness. Tall building MEP key cost drivers can be distilled into:
- Plant location – centralised versus on-floor plant versus interstitial plant rooms; efficient design of extensive plant in the basement and on the roof, aiming to alleviate the increased distribution of ductwork, pipework and electrical systems
- Hydraulic separation between risers and plant or landlord and tenant areas, and hydraulic separation versus higher ratings of pipework and valves.
- Increased plant provision due to a lack of available space for tenants’ plant, which prompts an enhanced specification of air provision, electrical loading and cooling provision, and very often 100% standby generation (and power management controls to manage the complex power distribution systems)
- More landlord areas to the larger cores and split lift scenarios, staircases, entrance areas and lift lobbies, which all need servicing
- Greater proportion of bike provision and associated facilities and additional basement plant often results in multiple basements and extensive servicing and smoke extract systems
- Construction issues produce higher contractors’ preliminaries costs due to extended programmes and initiatives to reduce them: the use of jump lifts and extensive welfare facilities, prefabrication techniques, just-in-time deliveries using consolidation centres, extended warranties and maintenance and commissioning.
- Smart building technologies such as blown fibre, wifi, phone boosting systems, blind control systems and specialist waste disposal systems are often incorporated into the base specification to reflect a building of the highest quality and comfort for its occupants.
06 / Regulation changes
The gestation period of tall buildings leaves them open to changes in legislation, regulation and guidance. Furthermore, the impacts of such developments may be substantial: given the size of the endeavour, large quantities and the fact that components can be repeated thousands of times through the building mean that scale can act as either an economy or a diseconomy.
A recent example affecting MEP services, which has a substantial cost impact on commercial tall buildings – because of the volume of basement space usually involved – is the change to the regulations for smoke extract ductwork (BSEN1366). This regulation has meant that the capital cost of fire-rated ductwork (per metre run) has increased by about 50%, together with the additional scope requirements across both supply and extract ductwork, including higher levels of insulation.
07 / About the cost model
This cost model is based on a generic 50-storey office tower (plus two basement levels) in central London, using current day fluctuating prices. It assumes a total gross internal floor area of 90,000m² with a net internal floor areas of 60,000m², and is designed for an occupancy of one person per 8m² NIA. The building has 11kV incoming power, on-floor air handling units, 100% standby generator provision, water-cooled chillers in the basement and roof- mounted boiler plant and cooling towers.
It is a base building cost that excludes any enabling works and services infrastructure/diversions, fitting out beyond shell and core construction and tender price inflation beyond first quarter 2018.
08 / Cost model: Generic MEP cost plan for a 50-storey office tower
|Total||£ / m² GIA||%|
|Sanitaryware installation - included in toilet fit out package|
|Rainwater installation including attenuation tank : 90,000m² @ £5/m²||450,000.00|
|Soil waste & vent to sanitary appliances (final run-outs in toilet fit out package): 1600nr @ £360||576,000.00|
|Capped-off soil and vent stacks for tenants kitchnettes and retail units||99,000.00|
|Allowance for condensate drainage: 90,000m² @ £1.5/m²||135,000.00|
|Allowance for incoming MCWS, storage tank, booster, meter, water conditioner & distribution pipework||325,000.00|
|Cold water service to sanitary appliances (risers only, run-outs in fit out package)||280,000.00|
|Central hot water generation and risers to fit out area||650,000.00|
|Capped-off water supplies for future tenants kitchenettes and retail areas||80,000.00|
|Grey water installation to WCs (run-outs in fit out package)||300,000.00|
|Gas fired condensing boiler c/w primary pumps, pressurisation units, dosing pot & primary distribution pipework: 5300kw @£40 KW||212,000.00|
|Heat pump c/w primary pumps, pressurisation units, dosing pot & primary distribution pipework: 600kw @£250/kw||150,000.00|
|Flues from boilers to atmosphere: 120m @£600/m||72,000.00|
|Space heating and Air treatment|
|Water cooled chillers: 9,500kw @ £160/Kw||1,520,000.00|
|Closed circuit cooling towers: 10,900kw @ £110/kw||1,200,000.00|
|Condenser water installation: 90,000m² @£9/m²||810,000.00|
|Chilled water including pumps, pressurisation units, pipework, risers, valves and fittings: 90,000m² @£29/m²||2,610,000.00|
|LTHW water including pumps, pressurisation units, pipework, risers, valves and fittings: 90,000m² @ £23/m²||2,070,000.00|
|On floor supply and extract AHUs||2,350,000.00|
|Fresh air intake and exhaust ductwork to on-floor AHUs||300,000.00|
|Supply and extract ductwork for AHUs to office floors. including dampers||500,000.00|
|Air conditioning to entrance area||260,000.00|
|Heating to stairs and circulation areas||95,000.00|
|Supplementary DX cooling systems||75,000.00|
|Cooling to lift lobbies by FCUs||165,000.00|
|Central toilet extract system||560,000.00|
|Toilet supply via make up from the office floors||130,000.00|
|Supply and extract AHU and distribution serving basement shower area||135,000.00|
|Basement ventilation systems||900,000.00|
|Fire fighting lobby vent, mechanically-assisted BRE type system: 100 landings @£6000||600,000.00|
|Capped off ductwork for future tenants’ kitchenettes||60,000.00|
|A3 unit kitchen extract riser - fire rated||225,000.00|
|HV distribution: 11KV incoming mains; distribution through building: 90,000m²@£15/m²||1,350,000.00|
|LV distribution, including switchgear, cables, rising busbars, etc: 90,000m²@ £45/m²||4,050,000.00|
|Full building back up generation, including flues and oil storage: 4nr generators @ 2,650kVA @£400/kVA||4,240,000.00|
|Power management system (SCADA)||500,000.00|
|Power to mechanical services: 90,000 @ £3/m²||270,000.00|
|Landord’s small power installations: 90,000 @£2/m²||180,000.00|
|Lighting to landlord areas, circulation, lift lobbies, shower areas, lighting control, etc||2,200,000.00|
|Feature lighting to reception||450,000.00|
|Aircraft warning lights||80,000.00|
|Allowance for feature external lighting||275,000.00|
|Containment for security/ CCTV/ fire alarms/ communuications: 90,000m² @£8/m²||720,000.00|
|Earthing and bonding: 90,000m² @ £2/m²||180,000.00|
|Allowance for power interfces to PV installations||150,000.00|
|Gas installation to boilers and capped off services to retail unit||155,000.00|
|Wet riser installation: permanent and temporary||420,000.00|
|Sprinkler installations, complete with tanks, pumps, risers and coverage to landlord areas: 90,000m² @ £20/m²||1,800,000.00|
|Gas suppression in communications rooms; protection to generator room||100,000.00|
|Lightning protection: 90,000m² @ £2/m²||180,000.00|
|Communications, Security and Controls installations|
|Fire detection and fire alarm system: 90,000m² @ £12/m²||1,080,000.00|
|Voice alarm system: 90,000m² @ £8/m²||720,000.00|
|Landlord’s data backbone (Landlord’s mobile boosting and Wi-Fi coverage excluded)||220,000.00|
|Security / CCTV installations||350,000.00|
|Turnstiles in reception: 10nr @ £25,000 each||250,000.00|
|Disabled alarm, refuge and fire telephone installations||250,000.00|
|Building management and energy management system||3,700,000.00|
|Allowance for testing and commissioning @2%||836,000.00|
|Subcontractor preliminaries @15%||6,390,000.00|
|Allowance for renewable technologies @ 3%||1,470,000.00|
Acknowledgments: This article and cost model were prepared by Nick Mulholland, Rob Butler, Sophie Rogers and Kostas Dellas of Alinea’s MEP team, together with Steve Watts, Alinea partner and CTBUH chair.