Although the case for pre-fabrication still needs to be made on a case by case basis – clients need to weigh up potential cost and time savings – there is a growing demand for more extensive use of pre-fabrication.
Productivity
Low site productivity is recognised as a major problem affecting building services installations. BSRIA's 1997 report Improving M&E Site Productivity, which compares UK practice with European and American examples, concludes that average productivity in a sample of UK projects was only 37% of the best-observed practice.
According to the report poor performance in the UK is caused by a range of factors, including a poorly motivated, under trained, sub-contract workforce, high levels of ineffective supervision and inefficient on-site practices. Use of labour intensive detailing, lack of standardisation, interference from other trades and requirements for rework also contribute to poor productivity.
Site labour typically accounts for 30-40% of the overall services costs of a typical commercial project. Although improved site management and detailing can contribute to greater productivity, the transfer of site-based activities into factory conditions provides opportunities to address many of these problems in a controlled environment.
Applications for pre-fabrication and pre-assembly
Applications for prefabrication and pre-assembly include:
- pre-assembled plant rooms and lift motor rooms;
- the prefabrication of distribution systems;
- pre-assembly of terminal units including fan coils and vav boxes.
BSRIA's 1998 report Pre-fabrication and pre-assembly identifies significant productivity improvements associated with the use of pre-assembled plant modules, with a 30% improvement on observed best practice being recorded. In this instance, productivity improvements help to offset some of the additional costs associated with the assembly of small runs of bespoke units.
When pre-assembly options are assessed, a number of sources of additional cost need to be considered including costs of enclosures, cranage and on-site installation costs. These may add to the main contractor's costs rather than those of the fabricator. The appraisal should also consider indirect benefits, such as the ease of maintenance than can result from optimum layouts, and reduced defects resulting from off-site assembly and testing.
Pre-fabrication of risers, pipework, ductwork and other distribution systems is increasingly common, particularly for repetitive installations in large commercial developments. Modular pipework and ductwork solutions are economic where there is a high level of repetition and minimal requirements for fittings and junctions. In an effort to obtain the maximum benefit from off-site fabrication, multi service modules which comprise pipework, ductwork and electrical services are also available.
Productivity gains associated with pre-fabrication are less dramatic than those found for large-scale pre-assembly activities. In BSRIA's 1998 study, a 50% improvement in productivity, to 70% of observed best practice, was recorded in a case study. Additional benefits of cost and programme reduction together with a mitigation of the risk of delays associated with a congested site were also secured.
For the full range benefits to be obtained, the use of pre-fabricated distribution systems needs to be co-ordinated with structure and services design at an early stage. The sizing of riser shafts to accommodate prefabricated sections is a good example.
The pre-assembly of terminal units potentially provides great opportunities for improving the quality and co-ordination of mechanical and electrical second fix installations. A fan coil unit can be supplied with associated pipework, ductwork, controls and power wiring.
The greatest level of efficiency will be achieved with, the development of generic, fully tested proprietary modules that can be bought off-the-shelf. However, the need to accommodate various combinations of consultant specified equipment reduces opportunities for the development of proprietary solutions.
Procurement
Consideration of the application of pre-fabrication and pre-assembly has to be made at an early stage in the design process, allowing for co-ordination with other disciplines and the early involvement of contractors and manufacturers in design. Procurement routes that facilitate early appointment, based on two stage tenders or partnering, are particularly effective.
When appointing specialists, the pre-qualification and final selection of contractors and fabricators should take into account their greater involvement in design and quality control, and their design and fabrication resources.
The appraisal of alternative tender proposals should be based on a range of selection criteria including cost, programme, design and installation quality, to enable a full benefit of a proposal to be assessed. As part of a two-stage process, the preferred contractor's proposal will be developed with the rest of the project team before the design and cost is finalised.
A clear definition of design roles and responsibilities, warranties and appropriate insurances will need to be in place to ensure that design responsibility is transferred effectively.
Constraints on the adoption of pre-assembly and pre-fabrication
The application of pre-fabrication techniques and the greater use of modular components have increased in response to client demands for faster, cheaper and better quality construction.
However, usage has not reached its full potential, for a number of reasons including:
- Gradual take-up by designers. Designers value the speed of construction achievable using prefabrication. However, the adoption of prefabrication strategies may lead to requirements for an early design freeze, which may not be appropriate for all projects.
- Limited available capacity. There are only a small number of specialists who are currently able to do the work, which can result in extended lead-in times and reduced price competition.
- Perception of increased initial cost. The cost models demonstrate that increased costs do not apply in all cases. Furthermore, reduced on-site costs can offset the additional costs associated with pre-fabrication itself.
- Lack of an agreed method of demonstrating the overall benefits of prefabrication/preassembly. Assessments are currently made on an ad hoc basis. Lack of standardisation of design solutions.
- Lack of standardisation results in increased design and fabrication costs.
- Requirements for adaptations to contract documentation. Contracts and tender documentation which facilitate the early involvement of contractor and fabricator, and which effectively allocate design responsibility, need to be made more widely available to encourage the greater take-up of pre-fabricated engineering solutions.
Three cost models are included in this article, demonstrating the potential for savings on both pre-assembled plant and pre-fabricated distribution. It should be noted that the analysis does not include potential savings on preliminaries associated with reduced project durations.
Model 1: packaged plant room
This model compares the cost of a packaged plant room with traditionally installed services for a new 6000 m2 commercial development in central London. The rooftop plant room comprises a chiller enclosure, and lift motor room, and houses boiler and air handling plant.
The plant room came in seven sections, the largest weighing nine tonnes. Costs for separate cranage are included which reflect the weight of the units and the reach required. Although the pre-assembled plant room is the more expensive option, it was selected for programme benefits.
Model 2: pre-assembled boiler system
The pre-assembled 560 kW boiler system is an off-the-shelf module comprising boiler, pressurisation unit, pumps, valves and controls. The cost model compares the cost of this system with a conventional site installed equivalent.
The only site works associated with the pre-assembled system are the flues and supplies of gas, power and water, together with the connection of flow and return pipes.
The specification of the pre-assembled module reduces design, procurement and commissioning costs, and reduces site installation times from eight to two weeks.
Model 3: multi-service prefabricated module
The model features factory assembled modules, comprising lphw and chilled water supplies, a ductwork header, basket tray and a busbar.
The model compares the costs with a conventional design where secondary pipework, ductwork and power distribution are installed by different trades. Using the pre-fabricated modules, bracketry and support work is simplified, and cutting and jointing on site is eliminated.
Source
ÐÇ¿Õ´«Ã½ Sustainable Design
Postscript
Acknowledgement
Mott Green and Wall would like to thank specialists Plant Energy Systems Ltd, Skanska Rashleigh Weatherfoil and AC Engineering for their assistance in the preparation of this article.