17 March, 2004
Design Demands of New Academies
Following the completion of two recent Academy projects, the relative merits of two different pre-cast floor systems employed are discussed by Angus Palmer of Buro Happold in relation to their use within a steel framed construction.
In September 2003, two of the first new Academies opened. Both schools lie within the M25, one at Bexley in Kent and the other at Brent in north London. These flagship schools are targeted to raise standards in inner city schools, by replacing existing schools with state-of-the-art facilities where new teaching methods are employed.
In 2000 the DfES (Department for Education and Skills) announced plans to construct about 50 Academy schools around the country by 2007, Bexley and Brent being part of the first wave. Each school is around 10,000m2 in size and has a construction budget of around £25m. All new Academy schools have external sponsors, who contribute up to 10% of the project costs. These sponsors help to bring in new ideas and encourage each Academy to focus on a specialism, in addition to their wider role in the community. Bexley has a business focus, whereas the Brent Academy is intended to be a centre of excellence in sport.
Over the past 20 years very few secondary schools have been built, with the result that expertise in their procurement, design and engineering is limited. Buro Happold is, however, currently working on its16 th City Academy project and, as a result, is one of the few engineering consultancies to have gained significant experience in the challenges faced and solutions possible in building 21st century schools.
In addition, the company was involved in two teams working on designs in the DfES¡¦ Exemplar Designs initiative, which aimed to produce hypothetical designs for a variety of school types and environments, presented in February. At the same time, the DfES also unveiled its Building Schools for the Future programme, which has earmarked an initial £2.2 billion to fund the construction and refurbishment of 180 new secondary schools in the UK for 2005-2006.
Technically, Academy buildings are very demanding as they serve as ¡§mini cities¡¨ with up to 1,500 pupils accommodated in a single location. Each school not only provides general teaching classrooms, but also science and technology laboratories, art rooms with kilns, music studios, auditoria with retractable seating, national standard sports halls, kitchens and restaurants providing an all-day meal service (including breakfast), generous atria and circulation spaces and ITC facilities providing a 1:3 ratio of computers to pupils. This range of functionality demands a highly flexible design solution which allows for future modifications, and which can evolve to accommodate design changes as the educational brief becomes defined. An accelerated design and construction programme put further demands on the design team.
Both schools were constructed in steel to provide for aesthetic, programme and flexibility advantages. Typical grids of 9m x 7.2m were adopted that worked with the classroom modules.
Exposed pre-cast concrete floor planks were used within the main classrooms to provide thermal mass for environmental benefits; these are pre-cooled by cross ventilation at night to help moderate the daytime temperatures. Two different pre-cast plank solutions were used for the two schools. At Bexley, a pre-cast pre-stressed hollow plank system was used, whereas Brent utilised a pre-cast flat-form plank system. Both systems provided a similar end product with acceptable finish qualities, but they required quite different design and construction methods.
Hollow core plank
Planks are generally a one-way spanning, so any significant holes require trimming steel to be added. Where the design and construction programme is so tight, this led to the addition of trimming steel very late in the process, either as a shop drawing change, or as an on-site modification.
This not only added considerable additional secondary steel, but delayed work in some local areas. However the majority of the construction proceeded very rapidly.
The supply and installation of the pre-cast planks were included in the steelwork fabricator¡¦s package, and this helped alleviate co-ordination issues.
As the planks were pre-cambered, there was some discrepancy in level between adjacent planks. This was mitigated by matching planks with similar pre-camber within each bay. The final finish was further improved by using mastic between the joints and painting the soffits white.
„h The hollow cores retained water when left exposed, and this took time to drain away.
Flat-form plank
The flat-form plank solution was chosen for Brent as it was felt that this provided a slightly better quality of finish. A reliable and economic source was found in Holland, within the necessary time constraints. As the building was curved on plan, the planks were set to span circumferentially to avoid the need for trapezoidal-shaped units.
These planks have a thick, two-way spanning topping which is able to self-trim large builders work holes although holes mid-span required careful consideration to limit the number of re-bars to be cut. For known holes these bars were diverted in advance during the pre-casting operation.
As the building is set on a curve, each plank within a bay is of a different length with non-parallel short sides. Even with a reduced bearing some of the planks did not fit and required site trimming.
A top rebate was omitted: this would have allowed for easier installation between the steel beams, especially for the last, longest plank.
A reduction in temporary stirrups was made as a cost saving that prevented the planks from self spanning during installation. This required the planks to be fully supported and propped during installation, casting and curing. This slowed the installation and delayed work locally due to prop obstruction. This was alleviated by changing the construction sequence; working from one end constructing to the full height, rather than constructing a floor at a time as originally planned.
The drawbacks to this system were exacerbated by a break in the supply chain; the steelwork fabricator was only responsible for the installation – not the supply and installation – of the planks. This meant that some tolerance and buildability issues were not fully considered and needed to be solved on site.
The final finish needed no further work as the propping matched adjacent planks and the use of steel moulds gave a good finish.
Conclusion
In general both systems gave acceptable results as a cost-effective solution to provide exposed soffits for use in a naturally ventilated environment, within the tight timescales needed. However, issues of continuity and flexibility to accommodate late changes should be considered in assessing the merits of a particular system. In particular, a single source supply chain is recommended for the supply and installation of the steelwork frame and planks.
Authors:
Angus Palmer and George Keliris of Buro Happold
Credits:
Bexley Business Academy:
Architect: Foster and Partners
Engineers: Buro Happold
Contractor: Exterior
Steelwork Fabricator: Graham Woods
City Academy, Brent:
Architect: Foster and Partners
Engineers: Buro Happold
Contractor: HBG
Steelwork Fabricator: Bone Steel
Background information for editors:
Press office and practice information Buro Happold is a multi-disciplinary international practice of consulting engineers established in 1976 offering civil and structural engineering, mechanical and electrical engineering, quantity surveying, building services and environmental engineering, health and safety management, infrastructure and traffic engineering, ground engineering, façade engineering, fire engineering, computational fluid dynamics analysis, disability design consultancy, project management, urban design and a range of specialist CAD services.
Elspeth Wales
Telephone: 01225 320 627
Fax: 0870 787 4148
Email:elspeth.wales@burohappold.com elspeth.wales@burohappold.com
