Fire performance: assessing concrete structures for reuse

With increasing numbers of concrete-framed buildings being adapted for new uses, it is vital to determine the fire resistance of the structure. Tony Jones of The Concrete Centre and Octavian Lalu of BRE outline the key assessment methods

It is hardly surprising that the reuse of concrete structures is becoming more common: the reuse of a structure is almost always the lowest embodied carbon solution, and concrete buildings can often remain useful well beyond their initial 50 or 60-year design life.

But if the structure is to be significantly modified, or the use of the building changed, it is often necessary to assess the structural capacity of the building, including its resistance to fire. In some cases, it may be enough simply to establish what the fire resistance is. However, the situation can become more complicated. If the use of the building is changing, it may be necessary to increase the period of fire resistance.

Updates to building codes may also mean that the information used to design the structure no longer represents best practice. Indeed, the simpler methods in newer codes often take a more cautious approach because they cover a wider range of situations, which can lead to uncertainty.

Information required

The ideal situation for the designers of a repurposed building is to use existing as-built drawings; however, these are often not available and may not accurately represent the current condition. To carry out an initial calculation, the minimum information required is the dimensions of the element (beam, column or slab) under consideration and the distance from the concrete surface to the middle of the main steel reinforcement, known as the axis distance. This information can normally be obtained from a geometrical survey and a cover survey, which can be confirmed by inspecting localised “breakouts”. If it is not possible to justify the element’s use based on this information, a structural fire assessment will be required, and this will need an understanding of the amount of reinforcement in the element, its strength and the strength of the concrete.

Simplified methods

When the fire resistance is unknown, an initial assessment can be made with minimal structural information. This can then be refined using historical test results from standard fire resistance tests, where they exist. The most straightforward way to determine the fire resistance is from the tables in EN 1992-1-2, the current design code for fire design of concrete structures. These tables give a required section size and axis distance for a given fire resistance period. In some cases, the tables also include a factor that represents the degree of utilisation of the section during a fire – without further information, this should be assumed to be 0.7. If no information is available on the reinforcement layout, then initially slabs and beams should be considered as simply supported.

If the information on concrete strength and reinforcement strength and quantity is available, it may be possible to show a lower utilisation than 0.7, leading to smaller permissible section sizes, or to demonstrate that higher temperatures are permissible in the reinforcement, leading to a lower axis distance requirement. Both refinements are relatively easy to implement using the methods in EN 1992-1-2.

Although the sections considered in EN 1992-1-2 are generic, the withdrawn British Standard BS 8110-2 contains similar information for more specific types of construction that were common in the UK – for example, hollow pot floors.

Where information in the withdrawn standard is superseded by the Eurocode, the latter should take precedence, but for these specific types of flooring, the approach in BS 8110-2 may be informative. For certain floor systems, test information has been published and summarised in BRE’s Information Paper 9/12. Following a site inspection todetermine the main parameters of the floor system, this test information may be used to justify the fire resistance directly.

The test information includes various historic precast and prestressed floor systems, hollow pot systems and wood wool slabs. It is important to establish the relevance of the test data to the existing floor being assessed. Similarly, test information to support some of the tables in BS 8110 is summarised in reference document BR 468. If the section being investigated was within the parameters of the testing when constructed, this may provide an alternative justification.

Advanced methods

If relevant test data is not available, advanced numerical models can be used to more accurately calculate the fire resistance of existing structures. These complex computer codes require significantly more information about the existing structural materials – for example, the thermal and mechanical properties and the correct boundary conditions.

Advanced calculation models are often used when the part of the structure under assessment has a complex geometry or the complete structure needs to be analysed. The many assumptions and approximations in advanced calculation models are usually of a higher order of refinement than in simple calculation methods. This means that a higher degree of accuracy can be expected – which in turn can provide the opportunity to develop more economical designs, while maintaining acceptable levels of life safety.

Advanced numerical analysis is usually carried out in two stages: thermal and structural analysis. The temperature rise within a member is calculated in the first stage through heat transfer analysis. This first step is important since the temperature developed within the defined section will determine the capability of the member to carry the applied load. In the second stage, the time-temperature history is used as an input to the structural model to determine the mechanical response.

The heat transfer calculations require knowledge of the geometry of the element, temperaturedependent thermal properties of the materials and the boundary conditions applied in the model. For materials with low thermal conductivity, such as concrete, it is important to determine the thermal gradients developed within the concrete section since these can influence the temperature rise of the main reinforcement, moisture migration, and development of thermal stresses and creep deformations. The computer models can be validated against test evidence to show that the boundary conditions and the material properties selected are appropriate for the end-use application.

Enhancing fire resistance

In some cases, particularly when an existing building undergoes a change of function, the fire resistance of a structural concrete member may need to be increased. This can be achieved by adding finishes to reduce the temperature of the main reinforcement bars. Where further measures are required, cementitious sprays, intumescent coatings or board types can be used. Care should be taken to ensure the protection layer retains its integrity for the duration of the design fire exposure. Guidance on finish types (plaster or sprayed fibre) was provided in BS 8110-2. However, standard fire resistance test methods are now available to determine the contribution of applied fire protection materials to the performance of concrete structural elements in fire.

The approach determines an equivalent thickness of concrete (in terms of thermal insulation) to be used in subsequent analysis. Again, further benefits may be achieved through the more advanced analysis methods.  A more in-depth version of this article is available: Fire Performance: assessing concrete structures for reuse.

Key references

• BS EN 1992-1-2, Eurocode 1: Actions on structures - Part 1-2: General actions -Actions on structures exposed to fire, BSI, 2002 BS 8110-2, Structural use of concrete - Part 2: Code of practice for special circumstances, BSI, 1985 (withdrawn)
• BS EN 13381-3, Test methods for determining the contribution to the fire resistance of structural members. Applied protection to concrete members, BSI, 2015 BR 468, Fire Safety of Concrete Structures: Background to BS 8110 fire design, BRE, 2004
• NIST, Best Practice Guidelines for Structural Fire Resistance Design of Concrete and Steel Buildings, Technical note 1681, 2010
• IStructE, Guide to the advanced fire safety engineering of structures, The Institution of Structural Engineers, 2007
• Lennon T., Assessing the performance of existing reinforced concrete flooring systems, IP9/12, BRE Press, 2012
• Lennon T., Structural Fire Engineering, ICE Publishing, 2011
• Kodur V. R., et.al., Structural Fire Engineering, McGraw-Hill, 2020

Photos from top to bottom 

1. Ollie Hammick - At The Archives in north London, architect ROAR has converted a reinforced-concrete industrial warehouse into a creative and social hub, with a cafe, climbing centre, workspace and performance space
2. Dirk Lindner - Gort Scott’s The Magistrates project in Walthamstow, north London, repurposed a brutalist 1970s courthouse as shared workspace, offices and a cafe
3. Eden Retirement Living - Ted Cullinan’s grade II*- listed RMC headquarters in Thorpe, Surrey is being turned into retirement housing by architect Ayre Chamberlain Gaunt
4. MICA Architects - MICA Architects’ reinvention of Centre Point in central London as an apartment building involved extensive analysis of the concrete structure and cladding