183 Project Details Location: Elm Road, Wembley, Middlesex HA9 Size: Approx plan area is 130m long by 30m wide Height: 1 to 2-storey underground car parks with 4 to 5-storey timber framed buildings, 4-storey reinforced concrete framed buildings and a 10-storey reinforced concrete tower block. Cost: Approx £17 million History The Client tendered for the Elm Road Project from the London Borough of Brent as part of a regeneration package. The site was previously used as a poor quality ground level car park on semiderelict land. The agreement between the Client and London Borough of Brent was to develop the site for affordable housing with the provision of underground car parking. Furthermore, the site was surrounded by three roads, two of which were poor quality surfaced service roads. The Client, as part of the agreement, upgraded the two poor quality service roads to highway works standards which will later be adopted by the Local Authority following final completion of the project. Project Overview The underground car park covers the entire site area (i.e. between gridlines 01 to 18 and A to F). As a consequence of the site being on a fairly steep slope, the underground car park comprises two independent car parks each on separate floor levels (i.e. lower and upper underground car parks). These independent underground car parks overlap in the central portion of the site hence creating a two-storey underground car park. Both lower and upper car parks have entrances at ground level. The concrete framed superstructure occupies just under a third of the site plan area (i.e. between gridlines 13 to 18). The timber framed superstructure and ground floor landscaped gardens extend over the remaining area of the site (i.e. between gridlines 01 to 13). Timber Frame Buildings Support Area The timber frame building support area comprises 1 to 2-storey underground car park supporting 4-storey timber frame buildings referred to as Blocks A to E and a 5-storey timber frame building referred to as Block Z. The remaining area of the site consists of a ground level landscaped area. The timber frame buildings will be used for residential purposes only. The timber frame buildings are supported on a 550mm thick reinforced concrete transfer slab. The one to two storey underground car park is below the transfer slab hence supporting the timber frame buildings. The single storey underground car park comprises a piled foundation raft with concrete columns supporting the transfer slab. The two storey underground car park comprises a piled foundation raft with concrete columns supporting an intermediate car park flat slab and further columns supporting the transfer slab. Thomasons undertook the detailed design of the concrete frame structure; this concrete frame structure was modelled using ESA-Prima Win finite element analysis. Concrete Framed Buildings Area The concrete frame buildings area comprises 1 to 2-storey underground car park supporting 4-storey concrete frame building and a 10-storey concrete tower block. The concrete frame buildings will be used for residential purposes only. The concrete frame building and tower block are supported on a 900mm thick reinforced concrete transfer slab. The one to two storey underground car park is below the transfer slab hence supporting the concrete frame building and tower block. The concrete framed tower block and underground car park form a single independent structure; this structure is entirely a reinforced concrete structure supported on a piled foundation. The floors for both the concrete framed buildings and tower block are of flat slab construction. The single storey underground car park comprises pile caps with concrete columns supporting the transfer slab. The two storey underground car park also comprises pile caps with concrete columns supporting an intermediate car park flat slab and further columns supporting the transfer slab. The car park basement slab was designed as a suspended slab spanning between the pile caps. Thomasons undertook the detailed design of the entire concrete frame structure; this concrete frame structure was modelled using ESA-Prima Win finite element analysis. Design Challenges 1) Void formers were not adopted below the suspended foundation slabs in order to reduce the excavation depth. The reduced excavation depth was a requirement for a number of reasons which included (a) reducing excavation process, (b) reducing the spoil quantity that would need to be removed from the site and (c) reducing the height of cantilever retaining contiguous piled wall along the entire perimeter of the site in order to limit the ground deflection due to the close proximity of nearby existing buildings. As a consequence of omitting the void formers, the foundation slabs were designed for ground heave pressures. As the site is on a fairly steep slope, the ground heave pressure varies uniformly across the site. Using ESA-Prima Win, we were able to accurately simulate this varied loading directly onto the FE model foundation slab hence provide a more efficient foundation slab design and a more accurate pile design for the piles in tension. Furthermore, the reduced excavation depth enabled the adoption of a cantilever contiguous piled retaining wall along the entire perimeter of the site therefore reducing construction costs by keeping the site clear from any temporary prop obstructions. 2) The timber frame loadings were produced by the timber frame building consultant. There were many timber frame loadings on the transfer slab comprising of point loads and line loads. Nevertheless, although a time consuming and tedious process, the application of these loads using ESA-Prima Win was easy to apply. However, after completion of the detailed design of the concrete frame supporting the timber framed buildings, the timber frame loadings were changed therefore the whole exercise had to be redone. Fortunately it was easy and quick to reapply these new loads to the ESA-Prima Win FE model. 3) Architectural changes included re-profiling a corner of the transfer slab within the landscaped area. The transfer slab was modelled as a single 2D element with all the timber frame loadings applied to this single 2D element. Nevertheless, using ESAPrima Win we were able to modify the corner profile of the transfer slab without loosing any of the numerous timber frame loads. 4) The introduction of large openings within the slabs during late stages of the design process required modifying the FE model. This was easily undertaken with ESA-Prima Win and the areas affected were re-assessed and re-designed accordingly. 5) One of the tower cranes was founded on the piled foundation raft below the timber framed buildings in order to utilise the piled foundations. The crane base foundation was analysed using ESA-Prima Win which resulted in an economical crane base slab design and a reduction in the number of additional piles that would have otherwise been needed for an independent crane base. 6) ESA Prima was also used to model the complex core pilecap below the concrete framed tower block. This resulted in an economical pilecap design and pile design. 7) After reviewing the results from the ESA-Prima Win finite element model we were able to reduce the reinforced concrete transfer slab thickness hence provide considerable cost savings to the Client. 8) By modelling the tower block using ESA-Prima Win finite element package we were able to accurately assess the deflections of the cantilever concrete balconies. As a result we were able to maintain the long cantilever balconies detailed by the architect whilst meeting the maximum deflection requirements. Where deflections were excessive, in particularly at the glazed corners of the tower block, structural steelwork posts were provided to limit cantilever balcony deflections to acceptable magnitudes. 9) The degree of accuracy using the ESA-Prima Win finite element package enabled us to reduce the project cost by some £2 million, which was instrumental in making the project viable. Multi-storey residential complex in Wembley UK
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