In the frame

At the start of each project, a decision is made conceming the form and material of the structural frame. It is a decision that is often influenced by cost or the frame type previously specified. However, familiarity may not always offer the best structural solution for that particular project. The

Design tools

The Concrete Centre has developed a number of design tools to enable fast and effective examination of a wide range of concrete structural options. These include Concept.xls, an invaluable design tool that can produce 13 reinforced concrete conceptual designs within five minutes, RC design spreadsheets that allows the rapid production of clear and accurate calculations and a number of design and best practice guides. All of these enable the best structural solution in concrete to be examined and developed, rather than just the most familiar.

In addition to the most efficient structural design, there are other matters to be considered. A major issue, particularly for owner-occupiers and PFI consortia, is whole-life value. Here, the 'hidden' benefits of concrete such as fabric energy storage, fire resistance and sound insulation mean that concrete buildings tend to have lower operating costs and lower maintenance requirements. A new construction guide from The Concrete Centre entitled Concrete Framed Buildings examines the range of benefits to be realised from specifying concrete frames.

Construction costs and programme

Construction costs and programme are also important. These days, in-situ concrete-framed buildings often take no longer to construct than steel-framed buildings - indeed, they can be even faster. Rationalising reinforcement, designing and detailing for prefabrication, precasting or part precasting and modern quick-release form methods can further enhance the speed of concrete construction on site. Importantly, concrete construction lends itself to a fast and safe programme. It provides a safe working platform including floors and gives semi-internal conditions that allow services installation and follow-on trades to start early. Furthermore, design changes can be incorporated late in the day without a negative impact on the programme.

In terms of costs, a full cost study undertaken by four professional teams examining 31 different designs for commercial, residential, school and hospital buildings has shown that the difference between steel and concrete frames is insignificant. Although foundations for the heavier concrete solution are more expensive, the difference still only represents a small percentage of the whole project costs and can easily be offset by using post-tensioned slabs that are typically 15% lighter.

Using concrete can also reduce cladding costs - the thinner the overall structural and service zones, the less cladding is required. Cladding can represent up to 25% of the construction costs so it is worth minimising the cladding area. Indeed, the cladding can even double up as the structure, representing further savings.

In addition to estimating first costs, best value calculations should include the structure's ability to meet Standards without excessive additional finishing. Concrete's built-in fire resistance, sound insulation and vibration resistance means that concrete requires little or no additional coating, finishing or damping to meet building regulation requirements. Furthermore, concrete lends itself to reduced long-term operational and maintenance costs. Its thermal mass properties reduce the need for heating and air-conditioning; its robustness is resistant to accidental damage and vandalism.

Therefore, initial construction costs are not the only issue to be considered. All of the above needs to be taken into consideration as a whole package to ensure that the structural design looks beyond the familiar and provides the optimum solution.

Concrete structural solutions

There are a wide range of concrete structural solutions that should be considered, including:

* Flat slabs: highly versatile and widely used structural elements that provide minimum depth, fast construction and allow flexible column grids.

* Ribbed and waffle slabs: providing a lighter and suffer slab than an equivalent flat slab, thereby reducing the foundation requirements. They can also prove extremely good at meeting strict vibration criteria for hospitals and laboratories.

* Beams and slabs: involving the use of one- or two-way spanning slabs onto beams spanning in one or two directions. Beams can be wide and flat or deep and narrow. Commonly used for irregular grids and long spans and also for transferring columns, walls or heavy point loads to columns or walls below.

* Post-tensioned slabs: these offer the thinnest slab type. They can use bonded or unbonded tendons. This is a fast and cost-effective structural approach.

* Hybrid concrete construction: combining both precast and in-situ concrete and thereby offering the benefits of both. Many different forms of hybrid concrete construction are possible as different parts of the structure can be precast.

* Precast concrete: precast concrete can form all types of structures from crosswall to 'stick' frame with columns beams and slabs. Precast is well suited to projects where speed on site or a fine fair-faced concrete finish is required.

* Tunnel form: used to form cellular construction and recognised as a modern method of construction, tunnel form offers excellent productivity and quality benefits with speeds of 300m^sup 2^ of floor per day being possible.

The availability of new innovative high-performance and high-strength concretes, self-compacting concretes and early-strength testing further enhances the benefits of modem structural concrete.

Concrete for schools and hospitals

In addition to Concrete Framed Buildings, The Concrete Centre has two construction guides that look at the benefits of concrete in the context of specific building types: High Performance Schools - using concrete frames and cladding and High Performance Hospitals - using concrete frames and cladding.

For schools, concrete construction has a number of particular benefits. One of the most notable is its inherent fire resistance, given that every year over 2000 schools in the UK suffer from fires, many of which are the result of arson. A further benefit is a high level of sound insulation, particularly useful for the often noisy activities of school life.

During the hot summer of 2006, many schools in England had to close and send pupils home as classroom temperatures rose to over 36?C. Proper use of the school building's thermal mass could have significantly reduced the internal ambient temperatures without having to install energy-intensive and expensive air-conditioning. This approach is advocated by both the Chartered Institution of Building Services Engineers (CIBSE) Technical Memorandum 36, which highlights the need for solar shading, ventilation and thermal mass to combat overheating, and by the Department for Education and Skills (DfES) Building Bulletin 101.

Unlike other building materials, concrete has a high thermal mass capacity. It acts as a thermal sponge, absorbing heat during the day and so cooling and moderating the daytime peak temperatures. The stored heat is released at night in readiness for the next day. This moderation of peak temperature is called Fabric Energy Storage (FES). This is a natural process that minimises the need for energy-intensive air-conditioning with all its resultant carbon dioxide emissions. It is predicted that climate change will result in hotter summers. Designing and building schools with adequate ventilation, solar shading and use of thermal mass will help prevent occupants from overheating.

Concrete construction is also beneficial to help 'future proof schools. Future flexibility and adaptability are key requirements for schools. Extended linear room plans allow efficient modification of room sizes and functions over the life of a school. Adaptability can be built in with, for example, three 60m^sup 2^ classrooms that can be changed to one of 70m^sup 2^ and one of 110m^sup 2^. Basing school layouts on columns with no beams and using fiat soffit concrete slabs provides an extremely flexible design solution for both space configuration and services installation.

For hospital construction, concrete's superior vibration performance is a major benefit. Vibration control is especially important in areas such as operating theatres, night wards and intensive care units. Concrete can be designed easily for the comprehensive vibration control required for these areas, often without the need for significantly thicker floor slabs. A study by Arup, Hospital floor vibration study: Comparison of possible floor structures with respect to NHS vibration criteria, examines the vibration performance of different structural forms and their additional mass and stiffness requirements to upgrade a basic office structure to meet the more stringent criteria of hospitals. The study found that concrete solutions can meet these criteria with minimal increases in mass and depth, and therefore cost, compared with other structural materials.

A further benefit of concrete construction particular to hospitals derives from the flat soffits. With concrete, the flat soffits enable partitions between rooms to be easily and effectively sealed. This helps to prevent airborne cross-contamination between rooms. Therefore, sealing walls at the soffits of the floor above is particularly important The use of flat slabs simplifies this and reduces partition costs by up to 4% of the frame cost. Concrete flat slabs are ideal for the highly serviced areas in hospitals. They allow complete freedom to prefabricate, install and maintain services without having to thread under or through intrusive downstand beams.

Concluding remarks

Concrete has an unrivalled range of inherent benefits and possible structural solutions. The Concrete Centre has developed a range of resources that enable the optimum concrete solution for a particular project to be effectively and efficiently examined.