New independent research confirms that concrete offers big cost advantages to the schoolbuilder. On the different designs tested, concrete beat steel for cost and lead times every time, reports Francis Ryder, head of costs at The Concrete Centre
The findings of a recent independent cost study show that, when it comes to the design and construction of schools, concrete offers the best overall economic solution.
The study, which was undertaken by consultants Architecture plb, Arup, Davis Langdon and Costain and commissioned by The Concrete Centre, compared the costs of constructing a typical secondary school using a variety of short-span and long-span reinforced concrete and steel frame options, taking into account construction times and the effect of programme times on cost.
The objective of the study was to provide an independent comparison between concrete and steel frames for secondary school buildings of two- or three-storeys on brownfield sites in town centre locations. Particular emphasis was placed on cost. Identical specifications were required, with the only permissible variations being those which were directly attributable to the material used in the structural frame.
A design was commissioned for a typical secondary school to enable a comparison of the relative costs of concrete and steel structural solutions to be carried out.
Streets, courts and campuses
School design involves many different responses to specific sites and initial consultation with Architecture plb produced a study of architectural precedents, identifying the following three main typologies commonly seen in contemporary school design:
- The street: comprising different subject areas connected by a long covered space for circulation. This space accommodates a variety of uses and can provide the principal social space for the student community.
- The courtyard: an area of classrooms arranged in groups around open areas, usually at low level to allow for good lighting to the classrooms.
- The campus: often found when there is a difficult site with different levels, when it is advantageous to group areas together and create a more open environment for learning.
From its initial study, Architecture plb interrogated the features that could constitute a typical brief for the design of a secondary school and identified the key design drivers and relevant guidance dealing with them.
In response to these drivers, an initial outline design was produced, providing a school of about 16,000m2 gross external area (13,500m2 gross internal area) for 1,400 pupils. The architectural scheme, layout and specifications were based on contemporary practice, prevailing DfES funding guidelines and current design guidance and regulations.
The school is a hybrid arrangement, combining courtyard and street typologies, which is the most common model at present and is currently being used by both schools and larger city academies. The form of plan is scaleable for future expansion and size of institution (see image above).
Structural designs were developed for alternative framing solutions and the designs were taken to normal outline design stage, the only differences being directly attributable to the structural frame material. The scheme was developed to a level that allowed a full structural analysis to be undertaken and a budget cost plan to be produced.
In respect of structural options, the building divides naturally into two types of space requirements.
Shorter-span areas house the general teaching classrooms, larger specialist teaching areas such as design and technology rooms and laboratories, administrative offices, shared teaching resource areas, special educational needs rooms, library, learning resource centre, support services areas, kitchens, toilets and changing rooms. In these cases the general structural grid varies within the range of 8.25m × 5.53m, 8.25m × 7.75m, 8.25m × 8.05m and 8.25m × 8.20m, according to the specific type of space and the taper of the building.
Long-span double-height areas are located within the north block and comprise the assembly hall, with a grid of 8.25m × 16.6m, the gymnasium hall and dance studio, with a grid of 8.25m × 14.15m, and the sports hall, with a grid of 8.25m × 18.30m. In these cases, long spans were catered for by the use of cellular steel beams, regardless of the structural solution adopted for the shorter span spaces.
In terms of overall construction cost, the most economic concrete-framed solution, the post-tensioned flat slab, was found to be up to 6.4% less expensive than the steel-framed solutions, even after adjusting time-related preliminaries for construction programme difference.
The most significant differential for both buildings occurred using a Slimdek steel-framed solution, for which the overall construction costs were found to be 6.8% more expensive than a concrete post-tensioned flat slab solution, after adjusting time-related preliminaries. When only the costs of the frame were considered, the concrete post-tensioned flat slab solution was found to be 39% less expensive than the Slimdek option.
With regard to speed of construction, the construction programmes for the concrete solutions were between 65 and 69 weeks, while those for the steel-framed solutions were between 66 and 67 weeks. The fastest concrete-framed solution was the post-tensioned flat slab at 65 weeks and the fastest steel-framed solution was composite at 66 weeks. When procurement and lead times were taken into account, as well as construction duration, based on an eight-week procurement programme and contractor lead times of four to eight weeks for concrete-frame construction and 12 weeks for steel-frame construction, the overall programme for the concrete-framed solutions is shorter, as summarised below:
- Concrete-framed options: 81-83 weeks
- Steel-framed options: 86-87 weeks
In addition to the cost and programme advantages, there is a range of further concrete benefits particular to school construction.
Every year over 2,000 schools suffer from serious fires. Concrete offers inherent fire resistance of up to 4 hours, removing the time, cost and separate trades required for fire protection. Added value benefits include enhanced property safety, potential for lower insurance premiums, re-usability of the structure and considerably reduced downtime after a fire.
Concrete's mass and damping qualities can easily meet the required Building Regulations acoustic performance BB93, minimising or even eliminating the need for additional acoustic finishings, generating savings in initial cost, programme and project financing, together with lower maintenance, repair and replacement costs.
Educational demands and the requirements put upon schools often change. The use of concrete construction automatically ensures many of the qualities that aid flexibility in school design. Design solutions are often based on columns, which can be embedded in walls, with the options of an in-situ concrete slab (choose from flat slab, ribbed slab or one-way slab), or precast floor units spanning onto beams. Concrete crosswall solutions, with large openings (of up to 75% of the width of the classroom) are another way to provide flexibility in joining classrooms. Alternatively concrete floors can span between walls on facade and corridor lines. The adaptability of concrete construction provides structures that have long-term economic viability. Flat slabs permit fully adaptable horizontal services distribution.
Reduced operational costs
A concrete structure has high thermal mass.By exposing the soffits, this thermal mass can be utilised through Fabric Energy Storage (FES) to reduce plant costs by minimising or eliminating the need for air conditioning and so reducing lifetime operational costs. Exposed soffits reduce or eliminate the need for suspended ceilings, thereby providing further initial cost and programme benefits, as well as reduced lifetime operation and maintenance costs.
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