Opening up at Covent Garden

A project to open up the space below the Grade II listed glass facade of the former Floral Hall at the Royal Opera House in Covent Garden involved a major exercise of structural engineering.

The load of the facade and retained lintel beam was supported via load cells through the brackets and posts to the flat jacks.

Fifteen years after a major redevelopment of the Royal Opera House in Covent Garden, London, which included knitting the adjacent Floral Hall into the opera house building complex, a £35 million project upgrade has been completed that includes a major structural exercise to open up the area below the hall.

The project is intended to enhance the experience of the opera house visitors, staff and artists alike by opening up the space below the historic 16 metre-tall, Grade II listed glass facade along Bow Street, dating from the 1850s and currently known as the Paul Hamlyn Hall. The project has been designed by architects Stanton Williams, working with construction managers Rise and Robert Bird Group (RBG) providing the structural engineering services.

The Royal Opera House has a long and varied history with buildings for a variety of theatre uses going back several centuries. In more modern times a third theatre, designed by Edward Middleton Barry, opened in 1858. This was a fireproof building in a regular classical design, and alongside it Barry built the Floral Hall in 1858–9. This was a glass and iron-framed structure intended to serve as a concert hall annexe and winter garden. The cast ironwork was manufactured by Henry Grissell’s Regent’s Canal Ironworks in north London and is dated (on the column bases) 1858.

The theatre became the Royal Opera House in 1892. While the main theatre remained little altered after its construction, the roof of the Floral Hall had to be rebuilt after fire damage in 1956 which resulted in the loss of its lofty glass vaults and dome. In the mid-1990s the major redevelopment programme was embarked on under the architects Jeremy Dixon and Edward Jones. As part of this the Floral Hall was taken down and put into store. The greater part of the main structure was subsequently re-erected in its original location on a new reinforced concrete frame, while its southern portico was moved to form a frontage to Borough Market in Southwark. The refurbished Royal Opera House reopened in 1999. The Paul Hamlyn Hall is now used as a bar and restaurant, interconnecting with the auditorium and other elements of the opera house complex.

The Paul Hamlyn Hall is designated as Grade II listed because:

it is a fine example of Victorian technological innovation, despite its dismantling and re-erection in the 1990s
it is noted for the high quality of its design and decorative elements
it is a fine example of the work of the eminent Victorian architect EM Barry
it has group value with the same architect’s Grade I listed Royal Opera House, which it was originally designed to complement.

The Open Up project’s principal objective has been to transform the public’s experience of the Royal Opera House. Stanton Williams’ brief was to offer a series of legible and flexible spaces that can be inhabited and curated by the Royal Opera House to bring a sense of the magic that is created each night on stage to the public areas.

The brief included:

a new glazed extension and entrance on Bow Street to provide passers-by with a clear view of the activities taking place inside
an improved entrance on to the Covent Garden Piazza, with additional foyer space to present and display the work of the Royal Opera House and to encourage visitors to enjoy the building throughout the day as well as the evening
the creation of the new Linbury Theatre, the west end’s newest, most intimate theatre within the shell of the former Linbury Studio Theatre, providing a space that has warmth, comfort and a distinct character, adaptable to different staging formats with the intention of bringing it to the fore as a world-class, mid-scale theatre, and an artistic laboratory for both the Royal Ballet and the Royal Opera Companies.

Structurally, the most challenging part of the project was to open up the space below the Paul Hamlyn Hall to create a light and airy foyer that can be seen by the large pedestrian flows passing by outside.

In addition to the usual project parameters, there were three key requirements:

the sequence of works must not cause any damage to the retained Grade II listed Paul Hamlyn façade - there was always the danger of the cast-iron frame fracturing and glass falling out.
the opera house should remain operational during all stages of the construction works which was vital for the opera house to maintain revenue and achieve project viability
the structural zone must be limited to only 1,100mm to provide the required clear height for the new foyer of the Linbury Theatre.

To address the architectural vision of opening up the space below the Paul Hamlyn Hall, RBG was appointed for the temporary and permanent works required to remove three of the seven existing piers from the 1990s concrete frame supporting the hall’s facade, and to find how to hold the facade in place over a clear opening of about 15 metres wide.

Due to the fragile nature of the historic facade, any structural solution had to achieve very tight and controlled movements to ensure that the facade was not damaged during the load transfer from the temporary structure (acting while the concrete piers were being demolished) to the future permanent support structure. This was made more complex by the absence of any reliable information about the existing loads coming from the Floral Hall facade on to the supporting concrete piers.

A number of options had been considered, including the installation of a pair of 16 metre long C-section beams that each weighed 16 tonnes. This was rejected because of the extra cost and disruption it would have caused to transport them in by road to the heart of Covent Garden, and the difficulty of placing them within an existing structure.

The solution decided on by RBG was to use a pair of lighter trusses (weighing seven tonnes each) that could be assembled on site to facilitate their installation within the logistics of this very confined site. Due to the paucity of information on the listed facade, load cells were introduced within the temporary works to measure the incoming loads. This information was used with a complex monitoring system to control the facade movement to within 2mm of its original position at all stages of the works.

As each of the concrete piers adjacent to the new void had to support additional loads from the future transfer trusses, the construction sequence involved the installation of mini-piles under each pier to increase its foundation capacity, followed by the attachment of the transfer trusses support brackets on either side of the piers via chemical anchors. The three columns to be removed were each provided with an assembly of four vertical posts that had at both ends brackets bolted to the concrete face. This temporary works assembly, also containing flat jacks and load cells, was intended to support the facade once parts of the concrete column were cut away.

The flat-jacks were then jacked in sequence with a nominal force to mobilise the vertical posts and load cells. At this point the loads in the existing concrete columns could be transferred to the temporary bracketry. The columns were then cut using a diamond wire cutting service. The loads from the facade and the retained concrete edge beam were now transmitted through the brackets and vertical posts back to the remaining part of the concrete piers. Vertical movement of the facade resulting from this operation was recorded and found to be negligible.

The 15-metre internal truss assembly was then delivered in sections and spliced on a crash deck at ground level before being lifted into position using a traditional block-and-tackle gear. The external truss was delivered in one section and installed using a mobile crane. Flat-jacks mounted on the trusses were then incrementally loaded through a controlled sequential operation until the full load was transferred from the temporary bracketry to the new transfer trusses. This load transfer operation was made within a +/-2mm overall movement envelope and 1mm between adjacent cast-iron columns. The operation was deemed complete when the load cells were fully unloaded. It was a highly iterative process undertaken outside normal working hours to avoid operational disruption, and requiring continuous control and monitoring by the engineers. The remaining parts of the intermediate concrete columns were then removed.

Additional improvements for this project included the design of a three-storey steelwork structure to replace the old Linbury theatre, a new steel support structure for the historic stonework Grand Staircase and the extension of the existing basement beneath the facade using a new secant pile wall and terrace slab.

This article originally appeared in IHBC's Context 157 (Page 41), published in November 2018. It was written by Barry McCormack, Mitesh Patel and Radu Enache from the Robert Bird Group.

--Institute of Historic Building Conservation