If you are a specialist in residential development or hotels, you'll be well up on the many uses of off-site manufacture. But are there the same opportunities to build off site in the office sector? Simon Rawlinson of QS Davis Langdon discusses the current state of play
Office development is often at the forefront of innovation in construction. The combination of educated, proactive clients, high-quality project teams and the ever present drive to build better, faster and cheaper has resulted in some of the UK's most innovative projects: Broadgate and Canary Wharf (pictured), to name but two. Where office developers stand on modern methods of construction (MMC) is a useful barometer to how well the systems are being adopted and the directions that future development might take.
Modern methods in the office sector
It is important to get the terminology right. MMC encompasses off-site manufacture such as modular construction as well as management techniques including lean construction, supply-chain management, pre-construction modelling or use
of consolidation centres. Many MMC approaches are common on commercial developments and are being transferred to other construction sectors such as residential through projects such as English Partnership's key-worker housing scheme, the London-Wide Initiative.
The key terms that need to be understood are:
- Modular off-site construction. Modular units provide both the load-bearing structure and useable space, and are widely used in the fast food, student accommodation and retail sectors. However, because of the need for long spans and column-free space in most office buildings, the modular approach is uncommon for projects other than temporary accommodation.
- Volumetric off-site construction. Volumetric units also enclose space but are not load-bearing. Typical applications in commercial buildings include toilet pods, plant rooms or main risers. Given the size of floorplates in offices, getting pods to the workface without disrupting other work can be a major issue.
- Unitised off-site construction. Unitised components are pre-assembled elements such as curtain wall panels and boiler sets. The main distinguishing feature of unitised off-site construction is that a number of different components - ductwork and pipework for example - are brought together and assembled into a carrier frame before being taken to site.
- Component-based off-site construction. Components describe pre-manufactured products where final assembly and integration occur on site. Commercial construction is increasingly concerned with the assembly of highly developed components, examples of which include ceiling tiles, modular wiring or precast concrete floor planks.
How development priorities affect take-up
Office products are anything but homogenous - ranging from simple incubator units on a science park to iconic towers in Canary Wharf or the City of London. What most offices have in common is that they are developed for a tenant and investor market. As a result, the key investment criteria include:
- Development efficiency
- Institutional development standards
- Desire to manage the risk exposure associated with speculative development
- Perception of value of "flexibility" and the ability to respond to requests for tenant enhancements
Although the London office market is not representative of the UK market as a whole, emerging trends point towards the increasing use of components rather than a volumetric or modular route to wider adoption of off-site manufacture. These trends include:
- Floorplate size and the requirement for clear structural spans
- Continuing pressure on the overall area of the cores
- Use of leading-edge designers to secure planning with highly expressive designs
- Development up to the site boundary, with potential impact on the regularity of the structural and cladding grids
- Constraints such as rights of light, requiring the development of irregular profiles and floorplates.
Although office construction relies on the on-site assembly of components, extending the scope of OSM to volumetric or modular is relatively rare
Key elements of off-site construction
Frame, upper floors and stairs Steel frames and composite decks have always represented a high degree of off-site manufacture. Recent innovations include:
- Precast edge beam and column assemblies for use with in situ concrete post-tensioned flat slabs. The edge beams reduce the need for formwork and have pre-installed anchorages for post-tensioning tendons. Flat slabs also have simpler first fix requirements for high-level building services.
- Corefast. A steel sandwich panel system that forms a structural core when filled with in-situ concrete. Use of Corefast takes the core off the critical path and accelerates the commencement of the frame, envelope and following works.
External walls and windows Most office buildings use some form of cladding or curtain wall system. There is a big split involving different contractors and procurement approaches between curtain wall assembled in-situ from a kit of parts and the fully pre-fabricated unitised or semi-unitised systems. Unitised systems are built to factory levels of quality and dimensional tolerances, and have advantages related to the speed and simplicity of on-site installation. Other OSM applications for external walls include precast concrete panels, brick cladding systems and, of course, windows and rooflights.
The 2006 revision of Part L will boost demand for facades with low levels of air infiltration, which is easier to achieve with unitised systems. Solar control at the perimeter is also critical and double-wall facades have been specified on some high-end projects. Schemes on a tighter budget use various forms of solar shading that typically need to be fixed in a separate on-site operation. Some of the programme and risk benefits of a unitised solution may be lost if the solar shading installation is not fully integrated - particularly if the building is too tall to permit the use of mechanised access platforms.
Internal partitions and doors Most partitions, whether in block or wallboard, continue to be erected on site, Pre-finished doorsets, often in steel rather than timber, are now widely used, particularly in landlord's areas.
Finishes Services fittings including luminaires, smoke and presence detectors, diffusers and floor grommets are pre-assembled as part of floor and ceiling assemblies.
Toilets Although the use of bathroom pods has become standard practice in hotel and student accommodation, take-up is less widespread in the office sector, partly due to the continuing perception by tenants of poor build-quality taken from 1980s developments. The main benefit of using toilet pods comes from the reduction of trades working in confined areas on site, together with improvements in assembly quality and simpler on-site management and inspection. Toilets can also be constructed using prefabricated wall panels with services already installed. Typically, the use of pods "de-risks" the programme and protects the finished works, although overall project durations are not usually reduced. As the planning of cores is a key aspect of development efficiency, the use of large modules can result in a less well-planned core layout.
The 2006 revision of Part L will boost demand for facades with low levels of air infiltration, which is easier to achieve with unitised systems
Mechanical services systems provide a wide range of opportunities for prefabrication at a number of different levels of integration. These systems benefit from the assembly-line approach in terms of supply-chain management, productivity, quality control and testing and commissioning.
Some aspects of off-site manufacture, such as the packaging of boiler and pumpsets on skids are now common practice. More extensive prefabrication - modular plant rooms, for example - are used less often. This occurs in part because the benefits of moving to the ultimate level of off-site manufacture do not justify premium costs, and because of the perceived need to finalise the mechanical systems plant design at an earlier stage than would be necessary when integrating systems in situ.
The generic levels of off-site manufacture available in mechanical services are as follows:-
- Prefabricated plant rooms These involve the complete fabrication of the plant room systems in a volumetric enclosure, including all interconnecting pipework, ductwork and so on. Plant room installations are pre-commissioned and are designed for plug-and-play operation with final connections to incoming and outgoing pipework, ductwork and so on. The plant room module can be designed to be fully weatherproof for rooftop installation. Cost premiums for fully prefabricated plant rooms range from 10% to 15%.
- Skid-mounted systems These are boilers and pumpsets pre-assembled with associated valvesets, pipework headers, insulation, controls interfaces and power supply, mounted in a box frame for ease of transport and installation. These systems reduce on-site installation to a minimum as part of a conventional plant room solution and can also be used for refurbishment projects.
- Services distribution modules Distribution modules comprise carrier-frame-mounted pipework, ductwork, busbars and containment. In commercial offices, modules tend to be used in risers and other landlord's areas rather than on office floors where the high value given to flexibility to respond to tenant requirements outweighs the potential benefits of improved productivity and better coordination. Multi-service beams incorporating chilled beams are an approach that is largely specified for owner occupiers. The beams incorporate all high-level services including sprinkler systems. Once costs of omitted ceilings are taken into account, the cost premium for these systems is £15-25/m² of GIFA, compared to a conventional fan-coil based system.
- Valve modules Valvesets comprising valves, test points and couplings are prefabricated for fancoil units, plant connections and many other applications. When designing systems with prefabricated valve sets, it is good practice to over-provide valves and connections so there is capacity in place for tenant enhancements.
Prefabrication of electrical services has focused on modular components with push-fit connections, serving lighting and small power distribution. Most of the benefit of these components is secured in general office areas.
An area currently under development, which could have a significant effect on the wiring costs of fire alarms, access control, CCTV and building management system sensors is the use of internet protocol networks for transmission of signals. Substituting dedicated cable networks for flood wiring has the potential to create very significant efficiencies in installation and operation.
Lifts are mostly prefabricated. A recent development aimed at further reducing on-site work is the motor-room-free lift, available at a similar cost to conventional traction units.
Commercial office development is often used as an exemplar for modern methods of construction. The widespread use of pre-assembled and standard components illustrates the extent to which the OSM agenda has become standard practice. The less widespread take-up of more complex forms of volumetric and modular OSM reflects the current priorities of developers, who particularly value risk reduction in terms of flexibility in the design and procurement process, diversified supply and use of familiar products. At present, there is not enough benefit in terms of gains in quality and overall programme and cost reduction to drive forward the wider adoption of these systems.
Skyline May 2006
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