Fire Protection 1975

From Building with Steel, February 2025

A building now nearing com­pletion is the Berkeley Hambro Bank in Bishopsgate and Roy Thompson of Cascade Insulation Services Ltd here describes the various types of pro­tection given to this 29-storey block.

This important project, fully described in Constrado Project Study 2, includes in its construction 3,200 tonnes of structural steel, the fire protection of which repre­sented a challenge to Cascade Insulation Services Limited, the firm nominated to execute the work.

In conjunction with the architects, con­sulting engineers, main contractor and curtain walling sub-contractor, a series of specifications was evolved based upon the following criteria:

• The steelwork to the offices generally was to be protected to meet a two hour fire resistance rating.
• The steelwork to the basement floors and mid tower plant rooms was to be pro­tected to meet a four hour rating.
• Services were to pass through the webs of the universal beams in the office areas.
• As much of the internal work as pos­sible was to be carried out ahead of the installation of mechanical services with­out the need for making good damage caused to soft materials.
• The perimeter beam was to be clad on the external face with a decorative fin­ish and fire stopping between the beam and curtain walling was to be maintained.
• The perimeter beams and column cladding was to be weather-resistant.
• The general installation was to meet the programme of one tower floor per week to keep ahead of the mechanical services installation and the casing of the perimeter beams was to meet a similar programme to keep ahead of the curtain walling installation.
• No asbestos-based materials were to be used within the office ceiling void.

It was obvious that a variety of materials would need to be used as no individual specification provided sufficient versatil­ity. The requirements were met by using Vicuclad board for beams and columns, protected on the perimeter by Decadex Firecheck, leaving areas adjacent to ser­vices holes in beams until the installation of the services was complete. A spray team using Ceramospray ceramic fibre/mineral wool then followed to complete the fireproofing, ahead of the suspended ceiling. Although design refinements and variations caused dis­turbance to the programme, the produc­tion of one floor per week was met in the office area. The perimeter spandrel panel was designed around an open box of vermiculite board and asbestos board pre­painted a special colour to blend in with the tinted glass on office floors. The pre­fabricated boxes were hung from the con­crete slab by back-fixed brackets screeded over. Smoke-sealing the build­ing tolerances is by a special grade of mineral wool used as a gasket.

Exposed fire cladding to plant rooms in basement and mid-tower is protected against tenants’ mechanical damage by a wet-laid membrane of Decadex Fire­check reinforced with glass fibre cloth to both vertical and horizontal surfaces.

Smoke extract ducting from the compart­ments away from the perimeter in the basement levels have been fabricated from Unistrut frames clad on both sides with vermiculite board. In the mid-tower plant room large volume smoke outlets from the low-rise lift motor rooms have been fabricated in a similar manner and routed from the core outlets at 14th floor level up to the underside of the 16th floor slab, out to the perimeter where the cross-section is quadrupled to maintain the volume through the bird repellent grille.
Similar fire protection cladding has been fabricated for the roofs of plenum units and transformer rooms within plant room areas.

Non-combustible acoustic and thermal insulation cladding has been provided for non-structural steel elements and con­crete soffits below office floors by min­eral wool slabs fixed into expanded metal trays on hangers extended to form a void.

Fire and smoke stopping of service duct risers has been provided both in the con­crete core, where vertical and horizontal stopping has been installed using mineral wool, expanded metal and insulating con­crete, and in the perimeter service ducts.

Staircase pressurisation is provided by a vertical duct in vermiculite-based board with access panels for cleaning purposes.

The complex range of fire-protected cladding installed in this building to satisfy the specification and to be able to be executed to the required programme could only be provided by a specialist sub-contractor handling a wide variety of techniques and materials. Some of these new materials have required special plant to apply them, thus it follows that a flex­ible approach is essential if performance specifications are to be meaningful.

The structural fire protection on this project represents the best of modern practice –lightweight, mostly dry, easy and fast to erect, readily adaptable, fully tested to meet the requirements of BS.476 and to the approval of the Dis­trict Surveyor. This can be compared very favourably with the older method of concrete encasement which would have added thousands of tonnes of dead­weight and months to the building programme.

The cost of this fire protection was approximately 27 per cent of the cost of the steel frame.

Similar problems have been solved recently in the construction of the 200 metres high Montparnasse office building in Paris, partially structurally fire protected by Cascade Insulation Services Limited, where a whole variety of materials was selected to meet the stringent require­ments. This proves that versatility in fire protection is now required internation­ally, and can be provided by specialist sub-contractors who are able to marshal the necessary monetary and manage­ment resources.