Holding up the past
June 24, 2009 \ Heritage & Conservation
by Justin Fenton
Our seminar, ‘Holding up the Past’ revealed the backroom thinking and decision making in relation to listed structures in four of our recent conservation projects. Structural engineers from both Jacobs and Scott Wilson assisted in the presentations.
During work with historic buildings early indepth investigation, research and analysis is required to gain an understanding of how structures and compositions work, what are original elements and what are later additions and alterations, and what are the most significant and sensitive features.
Each case study was presented to discuss a particular detailed element of the works:
• Malcolm Mitchell teamed up with Colin Morrison of Scott Wilson to talk about the urgency of the repairs to the disintegrating roof structure at Kelvinside Hillhead Parish Church following storm damage to the ornamental flèche.
• Justin Fenton spoke about revealing and clarifying the original sequence of rooms at the McManus Galleries, Dundee made more legible by the addition of a contemporary lift and stair tower in the eastern wing.
• Alistair King described the reordering of another Gilbert Scott creation at Glasgow University and the work to transform the refectory and kitchens housed in the former library back into a study space for postgraduate research.
• At St Vincent Street Church the big conservation question that Ian Hamilton of Page Park and Stuart Nisbet of Jacobs faced in the roof structure was whether to intervene or leave the under-strength trusses alone and, considering the construction of the trusses, how that intervention could be achieved?
In each project we faced decisions about how to repair or adapt the original building fabric, when should we make visible contemporary interventions and when should our work should blend into the surroundings?
Kelvinside Hillhead Parish Church
Malcolm Mitchell provided a summary of the architectural significance of Kelvinside Hillhead Parish Church and its derivation from St Chapelle in Paris. The challenging roof structure sits atop slender piers and massive window openings to provide a bright and airy interior without columns.
The magnificent flèche perched over a steep roof was displaced by a storm in January 2008 which left the roof structure in a precarious state. The massive trusses were left hanging perilously onto the edge of a timber wallhead weakened by rot.
Colin Morrison opened with a description of the emergency propping measures used to prevent further collapse. They found that the purlins, rafters and sarking formed a stiff membrane, like an open book and this had compensated for the truss ends which had failed. The massive baulk trusses are reinforced with diagonal timber bracing under the flèche and are augmented with steel ties to reduce thrust. The three dimensional dynamics had to be carefully analysed before a permanent repair could be devised. Steel reinforcement had to be sized to minimize disruption to ceiling finishes and to allow the members to be manhandled in the confined roofspace. A strategy to support and raise the sagging roof structure had to be developed directly with the contractor.
St. Vincent Street Church
Completed in 1859, Alexander ‘Greek’ Thomson’s St. Vincent Street Church is Category A listed and of international importance, sitting as a classical ‘temple’ on the plinth of massive stones flanking Pitt Street.
In 2007 the owners commissioned Page Park, with engineers Jacobs, to investigate and submit proposals to remedy persistent problems of water ingress through the main ‘temple’ roofs. Before contemplating any form of repair the design team needed to understand the building’s structure. Because no original drawings survive, most hidden details, particularly the methods used to span openings, were unknown. Intrusive investigations revealed that Thomson used a variety of materials for beams and lintels, including timber, stone and cast iron, but in 2007 the form of the roof structure still remained unknown.
Investigations by internal and external scaffold in 2007 confirmed that the main beams of the roof trusses consisted of two timber joists with a gap between and subsequent structural calculations showed that this structure was theoretically incapable of withstanding the original imposed loads. The big ‘Conservation Question’ was whether to intervene, or leave the trusses alone given the building’s significance and, after much consideration, it was decided to strengthen them.
The recently completed remedial works contract involved fixing steel reinforcement plates either side of the roof beams, fabricated and decorated to match so as to leave minimal evidence of the structural intervention. The seminar explained the background and process undergone to achieve these sensitive strengthening works.
Post Graduate Accommodation, University of Glasgow
The new Centre for Post Graduate Studies occupies the former Lower Library Hall of Sir George Gilbert Scott’s Gothic Revival style A-listed University Building. The Hall’s balcony had been removed in the 1970s and replaced with a mezzanine for a two-storey kitchen and refectory. This had obscured the visibility of the original structure of ornate cast iron columns and beams.
Page Park’s insertion extended the mezzanine but by including judicious glazed panels and openings through the core accommodation and floors, increased visibility of the structure from across the space has been achieved. Conceived as a distinct object containing meeting spaces and breakout, the new core and mezzanine structure acts as a container for air, heating and cabling routes. Careful detailing has enhanced the differentiation between this contemporary insertion and the original Victorian fabric adding legibility to the architecture.
McManus Galleries, Dundee
Continuous use and the inevitable evolution to meet ever-changing demands and statutory requirements had resulted in a building that had lost its clarity. Access by the public at all levels was a key element of the project. Rearrangement of support and access functions re-established prominence and dignity to the rooms concerned.
An improved setting for the Category ‘A’ listed McManus Galleries building was paramount as was the incorporation of a ‘green lung’ for the city centre. Improved pedestrian access from retail areas was important and service access to galleries and surrounding businesses had to be maintained. Inscribing the building with a vesica piscis – the geometric construction that generates the Gothic arch – provides a dynamic setting for the gothic building.
The establishment of a new arrival point was key to providing a new organisational logic to the entrance sequence and to providing a focus at the heart of the building. Critically, this move connects the McManus Galleries back into the heart of the city, an objective taken further by the creation of an external terrace allowing the café to spill-out and animate the building edge attracting people into the Galleries. The monumental aesthetic of the Gothic has been maintained whilst gaining permeability through the incorporation of a dynamic new lift and stair core serving all levels. The project at McManus Galleries has made the building organisation clearer, better serviced and more accessible. By the removal of 20thC clutter and by undertaking 21stC structural interventions the disparate elements of the historical plan are brought together.