Case Study Steel Concrete Composite Structures


Swiss Re – structural fire engineering for the newest tall building in London

Anthony Ferguson, Barbara Lane

Arup Fire, 13 Fitzroy St, London W1T 4BQ
©Arup Fire

Introduction

30 St Mary Axe, the building currently under construction for Swiss Re is located in the City of London between St Mary Axe and Bury Street. The structure was completed at the end of 2002 and the first occupants are due to move in at the end of 2003. Swiss Re, one of the leading re-insurance companies, chose Foster and Partners to design the building and Bennett Interior Design for the fit out of their office accommodation. Arup is the structural engineer and Arup Fire the fire-engineering consultant.

Building form

The development is a freestanding tower with 40 storeys plus mezzanine, above ground, and one basement level. The tower is circular on plan. The tower diameter varies with height, the maximum diameter being at level 17. Between levels 2 and 28 the majority of the office floors have 6 triangular lightwells equally spaced around the perimeter. These lightwells are generally 6 storeys high. This pattern is interrupted between levels 2 and 4, 10 and 15, where the lightwell cells are only two storeys high. Each floor plan is rotated by 5o with respect to the next storey. As a result the axis of each lightwell void is not vertical.

The steel framed structure includes the central core and a perimeter structure known as the “diagrid”, with circumferential ties in the external wall, linked by main beams. The floors are of composite metal deck and concrete construction.

Proposed use

The building is mainly office use. There is some retail space at ground and first floor level. Ancillary facilities are included, such as dining at the top of the building, and service spaces including goods handling and operational car parking, in the basement.

Prescriptive guidance

Being in the City of London and over 30m in height the Tower is subject to Section 20 of the London Building Acts as well as the England and Wales Building Regulations.

For office buildings over 30m the Building Regulations guidance document on fire, Approved Document B [AD B], prescribes 120 minutes fire resistance for elements of structure. For compartmentation purposes the standard is 90 minutes. The main frame, including the diagrid, would be 120 minutes on this basis with the compartment floors 90 minutes, except for the main floor beams which would be 120.

The Building Regulations enable alternatives to be used instead of the prescriptive guidance in AD B, and this was done at 30 St Mary Axe, as will be described below.


Fire Strategy

The unusual lightwell arrangement leads to a fire escape strategy based on a variation of phased evacuation. In this case all six floors linked by a set of lightwells are evacuated in the case of a fire on any one of them. Where only two floors are linked then those two constitute the first phase. So the lightwells are designed following the guidance for simultaneous evacuation, which allows them to be open to the accommodation. Because the lightwell base floors are protected by sprinklers on the overhanging soffits above, they can be used as office space too. A system of smoke curtains form smoke reservoirs in the lightwells, and others delay the transport of smoke from accommodation into the lightwells. Natural ventilation is used for smoke clearance for the lightwells and the accommodation.

The building is sprinklered, including arrays of window sprinklers on part of the façade of levels 2 and 3, to protect a glazed opening in the compartment floor of level 4, directly above. However sprinklers have not been fitted in the 12m high domed space that forms the very top of the building.

The top of the building will be a stunning space with 360 degree views of London from the mezzanine level. The sprinklers were omitted from this top space on aesthetic and practical grounds. Instead the mechanical ventilation system needed for environmental purposes has been upgraded to serve as a temperature and smoke control system. It is sized to prevent the temperature in a fire getting high enough to damage the glazing, which could be a danger to firefighters in particular, and to maintain a clear layer for an extended period, to assist escape and firefighting.

The Tower has two firefighting shafts with dedicated lifts. The lobbies are naturally ventilated using a variant of the “traditional” L-tube arrangement with the provision of a fire service control over the opening to the outside at the base of the shaft, and the use of dedicated smoke detectors in each lobby which cause the vent to open in that lobby, as well as at the top of the smoke shaft and the top of the stair. This control system follows the pattern recommended in the BRE research Project Report 27904.

Structural fire engineering

During a fire temperatures can be such that the window glazing may break and thus allow cool air to enter and hot gas to escape. Alternatively, temperatures may be such that the fire has not engulfed a large area and is not severe enough to actually break the glass. In both cases the temperature reached in the compartment and the duration of a fire is dependent on the amount of ventilation, and it is assumed that sprinkler activation has not prevented the fire from growing.

Recommendations in Approved Document B for structural fire resistance take no account of different ventilation conditions. Given the high ratio of glazing in the facade of the building a fully developed fire on one of the storeys would be well ventilated. Therefore heat is dissipated from a fire compartment to the outside, which would limit the temperatures reached in the structural elements. Moreover in a sprinklered compartment, fire temperatures are unlikely to go above 100°C, ie. successful sprinkler operation extinguishes a fire in most instances and would certainly prevent flash over.

Therefore the fire resistance standard for some of the elements of structure was the subject of analysis, using the “equivalent time of fire exposure” method described in the “fire actions” Structural Eurocode. This document was available as an ENV with supporting National Application Document, during the design development of the building. Aspects of the analysis were the subject of a Determination by the Secretary of State. The completed building reflects the outcome of the Determination. Given the relative novelty of the Eurocode approach for real building design a sensitivity analysis on the calculation results using an alternative means of calculating ventilation was also provided. The analysis method is also supported by data from two large-scale office fire tests conducted on a steel building frame at Cardington.

Conservatisms in the analysis include the assumption that a severe time-temperature environment from a standard furnace test occurs for the entire period of exposure. In practice the time-temperature environment is likely to be less severe. The method assumes that with sprinklers the design fire load is reduced by a factor of 0.75; in the literature this factor is normally 0.5 or 0.6. It was assumed that all fuel in the compartment would be involved simultaneously in the fire. In practice in a compartment of this geometry all of the fuel would not typically be involved at peak intensity at the same time. The design fuel load is increased substantially by safety factors that consider the probability of occurrence of a fire for this occupancy type, and the consequence of failure of a building of this height. Finally the fact that the members of the diagrid will in practice be even cooler due their location on the perimeter of the fire compartment was not considered. The Cardington tests clearly indicated that such elements do not reach temperatures as high as internal elements.

Representative compartments were selected for calculations. The design fire load was determined for each representative fire compartment. The ventilation factor depended on the area of external glazing likely to provide ventilation to a fire compartment, as well as the compartment geometry.

The resulting analysis concluded with the following recommendations for passive fire protection ratings. Some additional localised checks were also carried out separate to the time equivalent method as noted as follows.

Retail areas – ground & level 1: 90 minutes fire resistance for load bearing elements of structure. Floor between retail and office use, being a compartment floor, 90 minutes. The ground floor slab meets the recommendations of AD B Table A2 with 60 minutes fire resistance.

Entry area – ground level: In the entry area at ground level there is to be a reception desk with associated chairs and paper. There are no other significant combustibles in the entry area, apart from seasonal decorations or items of personal baggage. Goods delivery is entirely separate, via the basement loading dock. A study of the effect on the circular hollow section columns of a sprinkler-controlled fire involving baggage concluded that the circular hollow section columns in the entry area require 30 minutes fire resistance.

Upper floors: The Eurocode analysis indicated that elements of structure in the office levels from level 2 upward need only have 60 minutes fire resistance. In view of the Determination however the standard for the structural frame (the diagrid and the main radial beams) was increased to 90 minutes.

The fire-fighting shaft was designed to the 2 hour standard as recommended in BS 5588 Part 5. The intermediate floors within the lightwell cell were demonstrated as requiring 60 minutes fire resistance. Where the diagrid is located in a 120 minute area, such as the basement vehicle space, it was protected to give 120 minute standard.

Conclusion

The performance-based design approach was able to show that reduced periods of fire resistance, compared to the prescribed values, were sufficient to meet the functional requirement of the Building Regulations.

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