Andrew Minson, head of structures at the Concrete Centre, discusses the role of concrete in designing and constructing big sheds.

THE EXPECTATIONS of overseas clients locating in the UK, a range of sustainability drivers, and concerns over the potential for extensive fire damage means that the market dominance of the ubiquitous steel shed could be under threat. Unlike the rest of the world where concrete sheds are commonly used, in the UK steel portal frames dominate the market for long span single storey buildings for warehousing, distribution centres or retail use. However, increasingly overseas companies investing or locating in the UK are questioning that dominance.

Throughout Europe concrete sheds are widely used not only for the provision of long spans – prestressed concrete beams of spanning up to 42m are used – and for high roof loads to incorporate heavy air handling units but also for the architectural solutions that they provide.

The precast, prestressed roof elements, which evoke memories of insitu shell structures from the mid 20th century, have evolved through many stages. For example, in Northern Italy they are now commonly used to provide 23m to 35m clear spans.

These elements have evolved from short panels that were used in place of purlins and sheeting to span between concrete primary roof beams. By increasing the structural depth and prestressing, the structural capacity of the panels is made sufficient to replace the primary roof elements. The long panels provide both cladding and primary structure.

Initially, these primary roof elements – like the short cladding panels before them – comprised of four planes in the geometry of a hip-ended roof. However, greater structural efficiency is gained by curving the section both longitudinally and laterally. In some cases, sister units are used alternatively to provide northern roof lights. Roof construction can vary from heavyweight, which uses all the advantages of concrete’s thermal mass to lightweight solutions using timber glulam or steel beams.

All the built-in advantages of concrete construction enjoyed in commercial and domestic buildings – robustness, minimum maintenance, fire resistance and minimum inuse energy consumption – are also of benefit in industrial sheds.

These benefits are certainly recognised by overseas warehouse occupiers who are more familiar with concrete sheds and want the same levels of performance for their UK buildings. For example, one of the latest occupiers to use concrete by choice is Amazon, whose 750,000 sq ft warehouse in Swansea has just been completed.

Every year there are some 2,500 warehouse fires in the UK. The size of modern warehouses which often hold the majority of a company’s inventory densely packed with flammable materials such as plastic shrink wrap, cardboard and polystyrene, can mean a significant loss when a fire occurs.

As a result, warehouse fires usually spread very quickly. Research by the Building Research Establishment (BRE) found that a fire can reach the top of a 10m high racking bay in as little as two to three minutes. The research also found that typically a local fire brigade could take 20 minutes before being operational on site by which time is it unlikely that the blaze can be controlled and the building and contents saved. Furthermore, as the prime objective of the fire fighters is to save lives and not property, they will often not venture into a building to fight the fire because of experience of steel portal frames collapsing on them.

By contrast, concrete can offer up to four hours fire resistance, well beyond the 30 minutes stipulated by the Building Regulations. This benefit is welcomed not only by the fire fighters but also by the building occupier who will want to repair the damage and bat back into business as quickly as possible after the fire, rather than see his premises having to be demolished and rebuilt, and of course by the insurance industry who may well want to reward such concrete shed owners with lower premiums.

A further built-in advantage, and one that is becoming a major driver for the specification of concrete construction, is that of sustainability. The main sustainability drivers for warehouse projects are typically energy usage and CO2 emissions. These can be addressed by insulation, air tightness and thermal mass.

More stringent air-tightness construction criteria have resulted in design teams and clients reconsidering their usual building solutions. Compared to the number and type of joints of some panel systems, concrete walls can provide the best practical solution. With sheds, the ratio of wall-to-roof areas is small so air-tightness for the overall building will be improved not only through the use of concrete walls but also significantly by the option of using concrete roof elements.

Concrete also provides high levels of builtin thermal mass, which evens out the fluctuations of internal temperatures by absorbing the heat of the day and releasing it during the night.

This can reduce initial plant costs and ongoing operational costs for air-conditioning. With summer temperatures in the UK predicted to rise due to climate change the thermal mass of concrete can play an increasingly important role in reducing the reliance on energy intensive air conditioning.

Climate change is also predicted to result in increased storm and flooding severity. Here, the robustness and longevity of concrete solutions will increasingly be appreciated.

Led by forward thinking developers and clients, there appears to be an increased interest in concrete shed construction as a real alternative for industrial, warehouse and retail sheds. The ongoing dominance of the steel shed is certainly beginning to be questioned.