Introduction This development consists a multi-storey residential building where facade prefabrication is being considered. This report highlights the main advantages of precast and the enclosures provide specific budget and programme advice. Proposed Precast Cladding Systems In order to achieve an economical precast concrete cladding solution there are various considerations, but above all it is most important to incorporate panels as large as is reasonably possible. An ideal size would be storey-height x grid width, say 7m x 3m this will reduce the cost of on-site activity, contribute to the speedy enclosure and release site craneage at the earliest possible time. Repetition of panel types is also a bonus in terms of cost but not as prominent as the use of large units. Where possible, windows can be installed into the panels at our factory - the obvious benefits are reduction in the number of site operatives and again, a quicker enclosure. Precast has the edge over other cladding materials is its build ability. Here are some of its advantages: • Quality achieved by prefabricated manufacture in controlled environment, unaffected by weather and labour shortages. This permits rigorous selection and inspection before installation, removing causes of delay on site • Prefabrication and phased delivery to site accelerates the construction programme and achieves a weather tight building enclosure at the earliest opportunity • On-site `wet trades' are reduced; if internal precast elements are, pre-finished, wet trades can be virtually eliminated • Site installation by single team of skilled workers. No scaffolding required • Glazing, fixings, thermal insulation and vapour control layer can be incorporated in the factory before delivery to site • Fabric energy storage potential of precast; thermal mass, especially when used as exposed precast floor slab, helps to control building temperatures • Precast cladding panels produce a thinner external wall than conventional double skin walls, increasing the lettable floor area. Sandwich panels may also be considered, this generally involves an outer concrete `skin' approximately 80mm thick, an inner `skin' approximately 100mm thick with insulation say 80mm thick sandwiched between the two. The advantages of such a system is that apart from a simple internal finish a complete wall is erected in one `hit'. Probably the biggest disadvantage is the increased weight of the panels, which on average will be 33.33%, and this will impact on the site crane capacity.
Generally, the panels will be delivered to the site vertically, on steel `A' frames and taken directly from the transport to their final position on the building with deliveries being made on a `just in time' basis. We understand the current finish required is similar to the acid etched white, grey and black recon concrete as used recently at Masshouse in Birmingham. We have attached a take off and budget price from the drawings provided which are as follows: F088-01-013 rev A, R-AL-1-01 Rev J, R-AL-1-02 Rev K, R-AL-1-03 Rev F and R-AL-1-04 Rev G. Programme Our normal lead in period for this type of project is approximately 25 working weeks from a receipt of an instruction and the average rate of fixing is 18 panels per week per gang. We have attached an outline indicative programme for this specific project (Prog number 0635) There are many advantages to be gained in efficiency an early stage with the design team.
Case Studies Masshouse, Birmingham Masshouse is a 14 storey residential building under construction in the centre of Birmingham. It is one of a number of buildings currently under construction adjacent to the Bullring.
The building construction consists of a steel frame with insitu composite metal deck floorings. The stability we understand is being provided by a combination of core cores and sway frames. The main portion of the facade is to be clad in precast panels, typically bay size of 7.8 x 3.055 with a thickness of 150mm. The panels are supported from the steel frame and restrained laterally with traditional fixings to the frames. The calculations deal with the structural capacity of the panel to deal with the applied wind loads, the eccentricity of loading and the design or the restraint brackets to the structure (see sketch). The panel-to-panel joints are sealed using a double mastic seal, either silicone or polyurethane and the principle adopted is that of a two lines of defence. Design of the concrete is carried out to BS 8110 using a concrete with a characteristic strength of 50 N/mm. The colour is achieved in the panels by choice of cement, grey or white and natural colour of the aggregates and possibly the addition of some pigmentation.
Apartment building, Salford, Manchester The "Waterside at the Lowry' development is an 11-storey tower housing 165 apartments and penthouses on a site overlooking Salford Quays and just across the water from the Lowry Centre. It is clad with panels of precast concrete and rests on a two-storey podium of retail shopping and car parking which is clad with natural sandstone ashlar masonry. Techrete, the precast manufacturer, worked with architect DLA of Wakefield to design a finish with a colour and texture, which would be sympathetic to the large range of colours inherent in the natural stone on the podium below. The final mix contains limestone and quartz with a pigmented white cement matrix. The moulds were cast at Techrete's Brigg factory on steel vibrating tables; this is an alternative system to the traditional method of casting - in timber moulds; it allows changes in panel sizes to be made very easily, making casting faster and reducing cost. Each precast panel was fitted with two windows, pre-glazed at the factory. They were lifted in place by crane and the projecting balconies were installed at each floor level immediately after the cladding panel, with the precaster sealing the interface between panel and balcony.
