The Royal Academy of Arts is undergoing a transformative redevelopment which will be completed in time for its 250th anniversary in 2018. Led by internationally-acclaimed architect Sir David Chipperfield RA and supported by the Heritage Lotterly Fund (HLF), the plans will link Burlington House on Piccadilly and Burlington Gardens for the first time, uniting and revitalising the two acre site. Full weather protection was required for the internal contents of The Royal Academy of Arts during the remodelling of the building and the refurbishment of the roof and external walls. A temporary works solution was devised which provided an independent perimeter access and support scaffold, onto which a roof spanning 40m could be supported. The scaffolding is currently wrapped by Yinka Shonibare RA's art work RA Family Album, 2016, which shows over 160 images of the people who have made the Royal Academy an inclusive place for artists, creativity and learning since its foundation in 1768.
The independent scaffold was constructed using tube and fitting materials, supported off the pavement where practical and off gantries when the scaffold was to be suspended above building extensions and protuberances. Access was provided at every level. Stability ties were placed in specific locations, controlled by the work being undertaken to the external walls and by the strength limitations at various levels. The independent scaffold was designed to BS12811 using the Design Guide TG20:13, incorporating client required access and working loads to each area of scaffold.
The original concept for the temporary roof was provided by HAKI who proposed a duo-pitched roof, utilising the HAKI tie bar system to enable the 40m span to be achieved. The proposed method of erection was to construct a fixed bay positioned mid-way up the height of the building, spanning across the width of the building, from which the roof panels were assembled and subsequently rolled out incrementally through the use of cranes positioned in the public highway.
The proposed HAKI design however was flawed and would prove to be costly and very difficult to organise. The duo-pitched roof design required an unacceptable vertical cantilever height for the supporting independent scaffold, the internal reinforcement required to enable to roof to span 40m resulted in a further increase in height so the temporary roof would clear permanent obstruction e.g. chimney pots, towers etc. and the method of erection would require a substantial secondary structure to be constructed, since it was not acceptable to crane the roof sections into place.
An alternative design for the roof was decided upon, resulting in a unique 3 pitched mansard roof, with a twin wire restraining tie to assist with managing the local forces within the frame. The alternative design reduced the height of the independent perimeter scaffold, reduced the height needed to clear existing obstructions, met the requirements to enable the roof to be rolled out and provided a simpler construction process with cheaper material costs – a 40% reduction against the HAKI solution.
The late addition of a gable screen to seal the open ends of the roof to prevent wind driven rain entering the newly exposed building was particularly challenging. The original solution provided a catenary wire suspension system that spanned across the 40m width in a single span. Whilst the design was technically feasible, the amount of horizontal billowing movement in the gable screen under a strong wind created gaps that would breach the integrity of the seal. A modification to the design was undertaken that introduced a lateral restraint to the catenary wires at the mid-point. This cut the horizontal deflections in the gable screen down to manageable levels without inducing additional load in to the existing roof structures. The roof and gable frame wind and snow forces were calculated in accordance with current Eurocode design standards, incorporating the UK Annexe specific loading criteria.
Shortly after the erection of the independent scaffold, temporary roof and gable framing, the structure encountered a 1 in 25 year storm wind, with speeds in excess of 75 mph. Under these conditions the scaffold and roof structures were severely tested. Other temporary works structures in the same vicinity showed signs of damage to cladding elements and secondary structures. The sealed element of the roof for The Royal Academy of Arts had no damage. Some localised damage to the gable and sidewall cladding was experienced at the corners, due to the very high localised pressure coefficient. Minor repairs were instigated and local details improved, after which there have been no other reworks required.