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Natural History Museum unveils its £78M cocoon

A £78M extension to London’s Natural History Museum featuring the largest sprayed concrete curved wall in Europe opens this week.

Concrete sprayed onto an expanded metal mesh was used for the central “cocoon” of the extension. The cocoon houses 17M entomology specimens and 3M botany specimens in 3.3km of cabinets.

The construction method was chosen as it reduced the cost of constructing the 60m-long, 12m-wide and eight storey high amorphous shape.

“We considered steel, precasting concrete offsite and insitu concrete but each steel mullion, precast panel or piece of formwork would have been different as there is no repetition,” said Arup associate director Ed Newman-Sanders.

Reinventing solutions

Arup has used sprayed concrete before on the façade for Selfridges in Birmingham, but here it forms the structural shell, carrying loads from the floors.

“Sprayed concrete is traditionally a tunnelling solution, but in recent years it has been applied to buildings. Architects are gravitating to more amorphous forms,” said Newman-Sanders.

“Sprayed concrete has recently been applied to buildings. Architects are gravitating to more amorphous forms.”

Ed Newman-Sanders, Arup

The floor slabs and internal columns were cast first, with temporary columns supporting the slab edge. Reinforcement bars were placed spanning between slabs and the expanded metal mesh attached to them, onto which the concrete was sprayed.

The concrete shell also protects the collection from an unusual pest: “One of the problems was the collection was under attack from the museum beetle − a living beetle was dining on the collection,” said Newman-Sanders.

“The integrated pest management system was of utmost importance. Temperature in the cocoon had to be kept below 17°c so museum the beetle couldn’t reproduce and all surfaces had to be easily cleanable so we chose a flat slab and column option. A steel frame would have had nooks and crannies.”

The extension, designed by Arup and architect CF Moller was built by contractor HBG (now BAM). The main concrete frame was built by Getjar and the sprayed concrete by Shotcrete.  Turner and Townsend were the quantity surveyors for £78M project.


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