There are two kinds of components at the Whipsnade House that function as supports: the walls that cross the short dimension of the plan and the vertical posts within the frame of the window wall. The cross-walls are made of four inch thick reinforced concrete, cast in-situ. The posts are also reinforced concrete construction, but were pre-cast elsewhere on-site and
subsequently installed on the plinth. Both the cross-walls and posts were monolithically joined to the roof as it was poured, although some portions of the roof are supported by reinforced
concrete beams, as in the case of the long header over the entrance, sidelights, and clerestory window in the corridor. A single steel post—which is visible in the original plans and photographs
but not included in the version published in L’Architecture d’Aujourd’hui—is not a supporting
component at all, but rather a downspout that drains the roof through the interior of the central entry hall.
When the project for the houses began, Ove Arup was still an engineer with the general contracting firm of J. L. Kier and Co, and because they were already on site working on the zoo buildings, Lubetkin hired them to construct the houses. Arup and Kier were also working on Highpoint One, which was currently under construction and on schedule to be completed by early
1935. Highpoint Two and the Finsbury Health Centre would be commissioned later that year, and Arup would also play an important role in these projects. The Whipsnade House, coming as it did midway through the construction of Highpoint One but just before the start of Highpoint Two and the Finsbury Health Centre, represents a significant moment in the collaboration of Lubetkin, Tecton, and Arup.
In particular, this moment is wrapped up with the adoption of reinforced concrete cross- wall construction for medium-to-high density urban housing in England, an idea that originated with Tecton’s work on Highpoint One (Fig. 3.71-Fig. 3.74). Godfrey Samuel had asked Arup to quote the structural work for Highpoint One as early as 1933, two years before the Whipsnade Houses. In reports of their early discussions, Lubetkin seems to have envisaged a post and beam structure with non-load-bearing concrete enclosures, as found at Le Corbusier’s Villa Stein (1927), but Arup suggested that, given the presumed use of reinforced concrete, they might as well use load bearing walls as vertical support. The suggestion entailed adopting the techniques of reinforced concrete slab construction for use in vertical supports. Furthermore, a monolithic connection between horizontal and vertical slabs would exploit the strength of the material, which made it possible to reduce the thickness of the load-bearing walls from six to four inches, which was a considerable savings considering that concrete accounted for nearly a third of the total cost of building materials (Arup, 1979: 316).
The monolithic connection between horizontal and vertical slabs was made possible in large part by a sophisticated machine for shuttering that Arup had used in his work on marine silos—an architectural invention in the grand sense of Filippo Brunelleschi or Domenico Fontana. The shuttering system, which approximates a contemporary slip-form method, allowed the same set of wall and floor formwork to be used for the entire building by suspending the forms from above and then moving them up in a sequence of three steps per story (Fig. 3.74-Fig. 3.75). Lubetkin, Tecton, and Arup would complete several large housing projects reusing the same formwork, including Highpoint Two, Spa Green Estate, and Priory Green Estate.
Arup named this “box-frame construction” because the vertical slabs that replaced the need for posts and beams were also the basis of the enclosure, like a box. The use of box-frame construction allowed for the possibility of omitting the interior columns on the residential floor plates of Highpoint One; however, building regulations in London only allowed for the calculation and construction of a reinforced concrete frame in which the vertical load was carried by columns, and so a local variance was required. Arup prepared a summary of the new calculations and Tecton created drawings that favorably compared the new system to its traditional antecedent, and a variance was granted. A principle part of the argument for the new system—aside from the economic benefits of the reduction in material or the acceleration of construction—was the appearance of freedom it offered to the distribution of each floor plan. Only a single beam would run the length of each arm of the plan, which could later be absorbed into the thickness of the interior partitions, and there would be no beams running transversally across the short dimension of the plan. At this point in 1933, the notion of a plan emancipated from the impediment of vertical structure became a guiding principle in the work of both Lubetkin and Arup.