Raising the Roof

Perhaps the most difficult way to increase the size of a building is to extend upwards yet sometimes this is the best way. One such case occurred recently and the work is des­cribed by A. W. Ward, C.Eng, M.I. StructE, Project Engineer of Ken­chington, Little and Partners who were the consulting engineers.

Problems are inevitably posed when a business outgrows its situation – even, paradoxically, for a firm that makes its living relieving people’s space problems.

Such a firm is Gibbings-Harrison Lim­ited, a bulk warehousing company with main warehouse blocks on the crowded Terminus Road Trading Estate, Chichester. Its premises, confined on all sides, and densely developed with single ­storey warehousing, seemed destined to ‘burst at–the seams’ unless added storage capacity could be found to cope with its steadily increasing commitments.

An analysis of the firm’s problems re­vealed two alternative solutions to expansion on the existing site, short of wholesale redevelopment.

Initial thoughts centred on the possible redevelopment of one of the company’s warehouses into a two-storey unit, thus doubling the floor area and adding some 25% to the usable storage volume. This solution had major disadvantages, how­ever, in the form of direct additional over­heads created by extra goods lift and fork lift truck operatives, the consequent double handling of goods, and high cost. A dramatic increase in the provision of additional car parking areas on site further reduced the potential of the scheme – this seemingly inherent plan­ning difficulty being skilfully overcome by the Architect in negotiating the planning consent for the adopted scheme. The requirement therefore, was one of pure increase in storage volume with no dramatic increase in running costs.

Gibbings-Harrison undertakes the storage of almost anything – from canned fish to bulk plastic granules. As the majority of these goods are palleted and mechanically handled by fork lift trucks, stacking densely from central gangways, the height of the existing building (three metres) was the critical factor.

The brief to the design team was to prepare a method of almost doubling this height, using as much of the existing five ­year-old building as practicable, in as short a time as possible, while allowing continued storage in at least half the building at any one time. Extended and difficult deliveries of structural steelwork made the extensive re-use of existing building materials even more imperative.

The solution adopted was to jack up the roof from three metres to six metres in a two-phase operation. In this way, the client’s continuing storage requirements could be met, and the single-skin asbestos roof sheeting and all electrical installations could remain, the latter merely being extended at the vertical drops in the gables.

Lateral stability of the old building under wind loading was maintained by its multi-­bay portal construction, but the proposed new height required a new approach to stability. This was overcome by employ­ing internal vertical bracing, staggered on opposite and alternate bays, coupled with existing lateral bracing in the roof.

The two phases were isolated by strip­ping out one bay across the width of the building. Jacking was carried out some­what unconventionally by being ‘pulled up’ from the underside rather than from above. Synchronised hydraulic jacks on trestles – two per line of columns, 14 jacks/phase – operated on lifting chan­nels fixed to each line of columns, just above ground level. The dead load was taken by the jacks, and internal stan­chions burnt off 230 mm above ground level. External stanchions were extended above roof level, the external concrete panel walling remaining in position tem­porarily braced to the warehouse floor.

Jacking operations to each phase took place in two stages, the trestles needing to be extended at half lift due to head­room restrictions, and lifting was carried out at a rate of about one metre/hour. On completion of the full lift the stanchions were permanently extended using bat­tened channels back to back, welded to the stub at ground level and to the raised upper section of the existing stanchion. New sheeting rails were added to the perimeter, and vertical stability bracing internally. Coloured aluminium profiled external sheeting with double asbestolux internal lining formed the infill walling to the raised elevation.

The main structural and cladding works was completed in sixteen weeks, while working around stored goods and in two phases. The writer estimates, however, that this time could have been halved if the warehouse had been an empty shell.
Total costs for the 1,400 sq. m. ware­house conversion were in the region of £25/sq. m., representing only a fraction of the cost of a new building and taking no account of the continued storage enjoyed during the conversion.