3. How the Structures Work

The entrance to the zoo is formed of five undulating reinforced concrete slabs that appear like a wave above individual ticket kiosks which are built of brickwork and timber. Each roof slab is 175mm thick and approximately 7m x 7m on plan. The slabs have a projecting edge beam on three sides, a device that avoids water staining from the run-off of rainwater. The slabs are supported on nine 90mm diameter solid steel posts on a 3m square grid. These posts bear onto shallow concrete pad foundations. The two layers of 1/2 inch (12mm) plain round bars that we could see close to the soffit of each slab indicated that they were designed as two-way spanning slabs. Overall stability is provided by a moment connection at the column heads. The entrance is on a slope and the design cleverly exploits this by having the end of one slab overlapping the next. At each overlap the three edge columns pass through the lower slab and into the slab above; a detail that provides an additional restraint against lateral movements.

The shop is a more conventional structure with 100mm thick reinforced concrete walls and three lines of circular reinforced concrete columns. These columns support the roof which is formed by a 125mm thick slab and upstand beams. Three of the internal columns had been removed and replaced with steel beams and columns. We found out during the works that these alterations frame a lowered floor slab which was the dance floor from when the building had a brief life in the 1970s as Bentleys Night Club.

The two levels of walkways to the Bear Ravine are formed by flat slabs with the upper level supported on mushroom-head columns with thin reinforced concrete walls and hollow steel columns around the stairwell. The cantilevered viewing platform is a more complex structure. The 130mm thick slab is supported at the rear by the concrete walls to the bear pens below and propped at mid-span by a 350m deep x 200mm wide downstand beam. The outside edge of the slab is connected to the concrete parapet wall that acts as a deep beam which is anchored back to the main structure at each end. The structural action of the parapet as a tie beam was confirmed when we saw lines of twisted reinforcing bars close to the top of the beam. These are Isteg bars, a patented reinforcement system which was developed in Germany and introduced to the UK in the early 1930s. The system used pairs of plain steel bars twisted together so that the cold working increased the yield point of the steel by about 50%.  Spandrel panels below the parapet at each end act as a prop to the parapet walls and also provide a support to each end of the downstand beam. Both faces of all the parapets have a ribbed finish formed during the casting stage by corrugated shuttering.

The small kiosk near to the Bear Ravine uses a combination of reinforced concrete walls and hollow steel posts to support the 150mm thick elliptical roof slab.