On completion next year, Heron Tower will be one of London's tallest buildings. David Stow and Graeme Flint discuss the fire engineering work they undertook to help maintain the architectural vision of multi-storey open spaces in the building.
Heron Tower is high rise office building being constructed in the City of London, designed by architects Kohn Pederson Fox Associates (International) PA for the property development group Heron International. The building will provide over 68,000m2 of floor space, comprising mainly offices with a small amount of retail at the ground and first floors. A restaurant and bar will be provided on the 38th to 40th floors, to be open to members of the public. The 47-storey tower will rise to 203m in height, with a mast of 39m taking the highest point to 242m. Due for completion in 2010, it will be one of the city's tallest buildings.
The project design team included a multi-disciplinary team from Arup, including involvement from structures, acoustics, security, geotechnics, transportation, facades, IT and communications – as well as fire engineering. Arup Fire has been involved in the project since its inception in 1999, initially to provide fire strategy advice up to the planning application, but our role has since grown to include CFD modelling, structural fire engineering and an extreme events study.
The fire engineering design was largely completed in 2006 when conditional approval was granted by the City of London under Part B (Fire Safety) of the Building Regulations for England and Wales 2000 and Section 20 (Fire Safety in Section 20 Buildings) of the London Building Acts 1939.
Fire engineering and CFD
A key requirement of the architectural design was to maintain an open, interconnected feel to the building. This has been achieved by subdividing the tower into 3-storey and 6-storey villages, each with accommodation arranged around a central atrium. Each village is separated from the next by a 2-hour compartment floor and the building is fully sprinkler protected; hence the principle behind the fire safety design is to treat each village as being essentially a separate building. The building is also split vertically into two zones, with the accommodation and atria situated to the north of the building and the core zone containing combined firefighting/escape stairs and plantrooms situated to the south.
To increase the lettability of the building, the client wanted complete flexibility of the villages to allow tenants to either enclose the atria or leave them open to the accommodation. Because open atria would introduce a direct route for smoke to spread between levels, the fire safety design has been developed using a simultaneous evacuation regime within each village, and ensuring that occupants on all parts of the floors can always escape away from the atrium in order to reach the escape cores.
The British Standards recommend that a smoke reservoir be provided in the top of the atrium to delay the time it takes for the smoke layer to build down to a level where it could spread back onto the upper floors and hence potentially affect escape at those levels. In this case, in order to create a suitable reservoir, it would have been necessary to separate the uppermost level of the atrium with smoke retarding construction.
However, to achieve the flexibility of open or enclosed atria desired by the client, CFD modelling was undertaken to demonstrate that occupant evacuation at the upper levels would not be compromised by the smoke spreading from a fire at one of the lower levels via the open sided atria.
The CFD analysis was run in two parts. The first model was created to assess the conditions that occupants of the top floor of a village may face as a result of smoke spreading via the atrium from a fire on a lower floor. An axi-symmetric plume in the base of the atrium and a spill plume from the lowest level were modelled. It was demonstrated that for both scenarios, occupants would have adequate time to evacuate away from the atrium and into cores before the onset of untenable conditions due to visibility, temperature and carbon monoxide levels.
The second model was created to assess the conditions occupants might face on a single floor of the building if there was no atrium, i.e. a possible ‘code compliant' arrangement. The results of this analysis demonstrated that conditions would be significantly worse with a single storey arrangement without an atrium, when compared to the proposed village arrangement with an atrium. It was therefore demonstrated that the village concept would not compromise occupant life safety due to smoke spread, and that the design performed better than a possible code compliant arrangement. Close consultation with the District Surveyor early on in the design resulted in a smooth approvals process when the modelling results were presented. This was a key milestone for the client and provided confidence that the village concept would be acceptable.
Structural fire engineering
The main superstructure of Heron Tower is a vierendeel stress tube that wraps around the perimeter of the office floors. The office floors are supported by long span (up to 14m) solid section Universal Beams acting compositely with a 130mm deep re-entrant concrete deck.
Arup Fire designed an engineered fire protection layout, reducing fire protection to all primary members (beams and columns) from two hours to 90 minutes and leaving secondary beams unprotected. This was considered appropriate because of the robust structural form that had deliberately been chosen by Arup's structural engineer with structural fire engineering in mind.
To demonstrate that this would provide an adequate level of protection, a finite element analysis was carried out using the commercial modelling programme ABAQUS. The first stage was to agree a reasonable design base fire scenario. The parametric fire in Eurocode 1 Part 1.2 was proposed with the fire located at a single level only. However, due to the atria penetrating the normal floor to floor compartmentation, it was agreed that two models would be run in order to fully evaluate the structural response: a single storey model with the onerous parametric fire and a multi-storey model with a less severe parametric fire than the single storey model. The models were then created giving a realistic representation of the structure including non-linear temperature dependant material properties, which are necessary to capture the kinds of large displacements seen in structures under fire load.
In the single storey model, with the more severe fire, maximum deflections over unprotected beams were approximately 2m (Span/7.2). By comparison, the Cardington test series saw a maximum deflection ratio of approximately Span/10. The response of protected primary beams was much less extreme with maximum deflections of approximately 500mm (Span/20). The model demonstrated that stability and compartmentation were maintained. The multi-storey model indicated smaller beam deflections (approx. Span/10) due to the more reasonable fire. Even though columns were affected over a number of floors, there was no indication of column instability.
A code compliant fire protection layout of the single floor model was also assessed and showed considerable structural movement. It is commonly assumed that a building designed to code requirements will be relatively unaffected by fire. This analysis demonstrated weaknesses in the structural design that would not normally be observed. The finite element analysis therefore allowed us to demonstrate the robust nature of the building, rather than assuming that code compliant protection would be enough. A close relationship was maintained with the approving authorities and their designated third party checker throughout the modelling project, in order to ensure that they were happy with the modelling approach and its validity.
Approval was granted in December 2006, achieving significant savings for the client not only in terms of the cost and the consequential reduction in required future maintenance, but also the benefit to the project programme and better architectural finishes to exposed elements. Additionally by reducing the amount of spray-on intumescent, the environmental impact of the building and hazard to workers was reduced.
This is understood to be the first building in the UK that has been approved using a multi-storey fire analysis as a fundamental part of the approvals process, and is now widely seen as a benchmark for structural fire engineering in London.
Conclusions
Arup Fire, working closely with the rest of Arup's multi-disciplinary team, created an integrated engineering solution that enabled the architectural vision for multi-storey open spaces throughout the building to be realised. Advanced computer modelling was used to justify the design in a number of areas and formed a large part of the ongoing approvals process that was initiated at an early stage of the project. Arup's involvement brought significant benefits not only by increasing freedom in design and easing the approvals process, but also by contributing to the sustainability ethic of the project by rationalising the amount of fire protection materials needed.
David Stow is an associate and Graeme Flint is an engineer at Arup. The Heron Tower project won the Fire Safety Engineering design category at the Fire Excellence Awards in May 2009.
