PEP - Using Orion concrete design on a 134-million-dollar scheme in London
Clapham One project, a 134-million-dollar PPP scheme, was commissioned by London Borough of Lambeth in order to create new public facilities, a leisure center and luxury apartments as part of wider regeneration of Clapham High Street. PEP Civil & Structures Ltd was appointed as the structural engineers for this landmark project, working with architect Studio Egret West and Joint Venture Developers, Cathedral Group & United House.
As part of this major redevelopment project the former Mary Seacole House office building on Clapham High Street was demolished and replaced with The Library Building; an award winning, iconic and sustainable building which is significant in both size and complexity. The Library Building comprises up to ten stories of luxury residential apartments constructed over a three storey circular shaped public library, medical centre and a single storey basement car park.
A focal point of the scheme at the lower level is a striking and innovative spiral ramp within the library, which rises three stories and connects the medical centre, café, performance space, community rooms and other facilities.
PEP’s brief was to deliver an economical design, therefore the need to value engineer the scheme to achieve the optimum solution was paramount, as was the necessity to provide fast and effective responses as the scheme evolved.
The engineering challenge
The Library Building is structurally very complex, and the architect’s exciting form created a number of interesting engineering challenges including the extensive use of curved facades, complex load paths and transfer slabs resulting from differing structural grids, culminating in challenging foundation arrangements. By taking advantage of the integrated building design approach with Tekla’s Orion package, PEP was able handle the challenges quickly and with confidence.
The library ramp
The public library has a signature oval ramp spiraling up from basement level to second floor, partially suspended from a 950mm (3 ft) thick transfer deck above. The architect’s original vision was for a column-free space but with spans of up to 22 meters and the significant transfer loads from 10 stories of apartments, the most effective and commercially acceptable solution was to incorporate eight columns around the inner perimeter of the ramp. PEP used Orion to investigate alternative solutions before settling on the final solution comprising five 1,200mm x 400mm rectangular columns and three 750mm diameter circular columns to the glazed main entrance.
Flat slab design
Within the residential apartments PEP adopted a 225mm thick reinforced concrete flat slab solution throughout.
The footprint of residential apartments took a very different shape to the library below, resulting in the 10-story apartments over-sailing the library significantly and a 950mm thick transfer deck required to carry the enormous loads. Away from the library, the remainder of the structure comprised seven stories of apartments sitting above a single-story medical centre at ground level, with a single-story basement car park and plant rooms below. This mixed use of the building led to differing structural grids and column arrangement at each level thereby requiring PEP to design and detail a 425mm thick transfer slab at the first floor and a 600mm thick transfer slab at the ground floor.
The challenge PEP faced was to achieve an economic solution by balancing overall slab thickness versus reinforcement, acceptable deflection estimates, punching shear requirements and the ramifications on the foundations. Clive Lungley, PEP Associate Director, found that by using the Finite Element capability in Orion, he was able to iterate and optimize the slab design quickly to establish the most efficient solution.
Load chase down and foundations
Faced with an awkward footprint, eccentric and discontinuous cores and many transfer levels, Orion’s unique Finite Element load chase-down was essential in helping quickly predict load paths and accurate foundation forces early in the process, thus enabling designing an economic foundation solution at tender stage. The basement slab was optimized to be a 250mm thick suspended slab supported onto perimeter secant wall and internal bored piles; these were typically 400mm diameter but under heavily loaded areas such as the cores, the piles increased to 600mm diameter.
Prior to the demolition of Mary Seacole House, PEP established the location of existing foundations using as-built drawings and designed the new foundations to fit around these. However, due to inaccuracies in as-built drawings, during installation the piling contractor found over 100 piles in unexpected locations resulting in clashes with the new pile arrangements. With temporary halting of works, PEP were under pressure to modify the design and reposition the new piles quickly, which they achieved using the integrated foundation capability within Orion.
Originally the building height was kept below 30m to meet the agreed planning consent, but later commercial pressures resulted in the developer adding an additional two floors of luxury apartments which increased the overall building height to 36m. These were initially schemed in steel/timber to minimize ramifications on the already designed foundations, but the contractor opted to construct the apartments from concrete. The significant increase in weight resulting from this required PEP to carry out further checks on the design and using Orion they could establish quickly that the proposed foundations were still adequate for the additional loading but that the main transfer slab needed to be thickened by 50mm.
With a building of such irregular shape and stepped height, determining the lateral stability and twist was a complex task. Three 200mm thick slip-formed RC cores provided lateral stability while two of these were full height from basement to roof level, the third terminated on the 950mm thick transfer deck over the library, transferring lateral forces through the slab into walls below. Using Tekla’s Orion saved yet more time for PEP by fully automating the application of the notional horizontal loads in accordance with the local standards, as well as predicting the resultant drift and twist of the structure each and every time modifications were required.
“PEP needed to react to major changes throughout the project, partly due to the problems encountered during piling but also because the scheme evolved continually. Orion played a pivotal role in ensuring we could manage the project variations from a single design model quickly, economically and accurately,“ says Clive Lungley, PEP Associate Director and continues: “Orion enables us to compete with the heavyweights of the industry with escalating success. The time savings we achieve and the gains in productivity and efficiency are impressive. We continue to reap the benefits of our investment in these building design tools.”
Director of Tekla’s Engineering segment, Barry Chapman, says: “The Library Building at Clapham One is an exciting and prestigious project which has clearly provided many technical challenges to the design team. It’s gratifying to see our tools help a medium sized practice deliver such a large and complicated project by using the right software tools efficiently.”