Urban Engineering: Pringle Richards Sharratt in Coventry
Article Reference: AT183p84
Date: 1/11/07  
Architect: Pringle Richards Sharratt
Building: Herbert Art Gallery & Museum, Coventry
Title: A major extension to the Herbert Art Gallery & Museum is constructed from engineered timber.

Designed by Pringle Richards Sharratt and due to complete early next year, the Herbert Art Gallery & Museum extension in Coventry comprises an underground city archive, two new galleries and a state-of-the-art history centre. 

At the heart of the £11.5m project is a public arcade measuring 50 metres long by some ten metres wide and up to 12 metres high. This not only acts as a powerful organising device, but also extends the presence of the gallery towards the city's principal tourist attractions: the ruins of Coventry Cathedral on University Square and the Basil Spence designed new cathedral. 

Central to the design of both the arcade and adjacent history centre is the use of Finnforest engineered timber. The former is expressed as a dramatic glulam gridshell while the latter is topped by a curving glulam beam and solid timber panel structure supported by tree-like clusters of spruce columns. These key elements provide the gallery with a vibrant new civic identity that is both highly contextual and sustainable. 

The Herbert was opened in 1960 as the city's first purpose-built museum and art gallery. Designed by Alfred Herbert & Son, the building's primary frontage faced south onto Jordan Well - a major route into the city from the ring road. Since its completion, University Square and the cathedral have come to dominate the local context to the north, meaning that the museum's main focal point is now on the opposite side to its original entrance. 

The brief was two-fold: to provide a modern facility to conserve and improve access to the city archives, museum and art collections, and to make a significant contribution to the 'new' civic context at the rear of the site. The development programme is being carried out in three phases. The first, which was completed by Haworth Tompkins Architects in 2005, combined alterations to the existing building with a new entrance and cafe on Bayley Lane. Phases two and three are being run concurrently and comprise a new 1,720 square metre extension to the north, as well as an extensive refurbishment programme providing additional galleries, education spaces, storage and offices. 

Pringle Richards Sharratt was appointed in November 2002 following a design competition run by Coventry City Council. The scheme started on site in October last year and is due to complete in February 2008. Principal John Pringle explains that the concept was to turn the back of the building into a new front by establishing a pedestrian route connecting it to the existing main entrance. 

The strategy also seeks to improve the contextual setting of University Square by enclosing its 'leaky' south-eastern corner with a positive urban statement. The scheme is planned around a 450 square metre arcade running north-south across the site. This links the new entrance and public spaces to the existing building while also forming a buffer between the outside and the controlled gallery environments. 

The partially glazed, glulam diagrid roof is designed to act as a filter, gradually reducing natural light levels from around 70 per cent at the main entrance to ten per cent at the entrance to the galleries. A diagrid roof was chosen for reasons of structural efficiency and visual resemblance to the interior of the cathedral roof. The diagonal-based geometry also enables the roof to be terminated at 45 degrees, thereby ensuring the glazed entrance to the arcade addresses University Square and the cathedral. 

The galleries are located on the east side of the arcade and comprise a permanent exhibition on the ground floor and temporary exhibition spaces at first floor level. In contrast to the arcade, the galleries are constructed in precast white concrete to make use of thermal mass for internal temperature control. 

On the opposite side of the arcade is the 342 square metre history centre. This faces west onto Bayley Lane and includes a reading room, search room and offices. The single-storey structure has a sloping mono pitch roof formed from glulam beams and LenoTec solid timber panels. The timber beam and slab structure curves upwards as it meets the edge of the arcade, before bifurcating into a barrel-vaulted diagrid. The city archive is situated at basement level directly below the arcade and galleries. 

The use of engineered timber - namely glulam beams and LenoTec solid timber panels - for the arcade/history centre roof was favoured for three main reasons. First, timber - if correctly sourced - is one of the most sustainable building materials available. Second, inherent qualities such as lightness and strength, combined with the team's prior knowledge and experience of the material, made it the ideal choice for a structurally and aesthetically challenging design. Last but not least, its visual and tactile qualities were felt to be particularly suited to a public project of this nature. The architect's close involvement with Finnforest Merk on schemes such as Shrewsbury music school (AT120), Sheffield Winter Garden (AT135) and Carlisle Lane flats (AT161) made the Finnish manufacturer a natural choice as the supplier/specialist subcontractor for the roof structure. 

All the timber products supplied by the company came with PEFC (Programme for the Endorsement of Forest Certification) chain of custody guarantees. Finnforest Merk was responsible for developing the detail design of the roof, prefabricating the glulam and LenoTec components at its Aichach plant in southern Germany, and building the structure on site. Finished with a class-one, clear fire-retardant coating, the structural components were supplied pre-sized and pre-drilled ready to receive fixings and anchors on site.

The use of prefabrication combined with 'dry' construction methods provided advantages in terms of reduced costs, improved quality and increased speed. Project architect Karsten Weiss says the use of Tekla Structures CAD software played an important role in developing the roof concept. The programme, which allows users to construct and view structural models from any angle, has the ability remove visually obtrusive elements, thereby revealing interfaces and surfaces normally hidden from view. 

This allowed the team to investigate complex three-dimensional junctions in a way that would have been virtually impossible using standard two-dimensional drawings. The BIM (building information modelling) characteristics of the system ensured that Finnforest Merk was able to fully exploit the Tekla Structures CAD model, linking it to CNC machinery used to fabricate the individual timber components. 

History centre roof 
The history centre roof structure is organised on a 3.75 by 9.6 metre grid. At each grid line is a raised 400mm or 600mm diameter pre-cast concrete stub column. These support pairs and double pairs of splayed circular timber columns on the west and east sides of the plan respectively. The 175mm and 250mm diameter turned spruce columns are tapered at both ends and fitted with circular galvanised steel caps. These are connected to galvanised steel pin-joint assemblies, which connect to the tops of the stub columns and the underside of the glulam beams supporting the roof. 

Manufactured by aligning and gluing lamellas of wood in the direction of the grain, the glulam beams measure 170mm wide by 295mm deep and 13.5 metres long. On the west elevation they are tapered and project some two metres beyond the building line providing solar shading and a covered seating area facing Bayley Lane. On the eastern side of the roof, the beams are curved to a radius of 3.9 metres before bifurcating into the barrel-vaulted diagrid structure of the arcade. 

Made from Finnish spruce, the glulam sections used throughout the project have a density of 440kg/m3 (15 per cent moisture) and are strength grade GL32. Spanning between the glulam beams are 135mm deep LenoTec solid timber panels. Formed from adhesive bonded cross-laminated spruce boards, these strong and dimensionally stable panels can be used to form homogenous floors, roofs and loadbearing walls. Available in sizes up to 4.8 metres by 14.8 metres, the panels were favoured for their lateral strength (necessary to stiffen the frame), self-finished aesthetic and ability to form curves. 

Added to this, they combine good levels of thermal and acoustic performance. Immediately above the LenoTec panels is a geotextile fabric, 75mm of CFC/HCFC-free rigid urethane insulation fixed between softwood battens, then a layer of 18mm plywood, followed by an underlay/breather membrane and finally a terne-coated, stainless steel standing seam roof covering. Arcade roof The vaulted diagrid structure springs from the curved glulam beams and paired columns forming the eastern edge of the history centre roof. 

Measuring up to 2.2 metres long by 142mm wide and 270mm deep, the diagrid glulam beams are connected to 250mm diameter cast steel nodes using semi-recessed steel flitch plates (typically four per node). The ends and undersides of the beams have a deep rout, which articulates the flitch plate connections and gives the impression of double structural sections. 

On the eastern side of the vault, steel pin-joint assemblies transfer the structural loads from the steel nodes back to the concrete roof beams of the gallery. Welded to the top of the nodes are steel rods supporting 100x170x9500mm glulam timber purlins. These run north-south along the vaulted roof and are spaced at 1900mm centres. Fixed to and spanning between the purlins is an extruded aluminium framing system, which supports a combination of 1.8 metre square insulated stainless steel cladding panels and rooflights. 

The connections between the frames and the purlins are sealed for airtightness using neoprene tape. The solid cladding panels are white powder coated 3mm pressed aluminium cassettes filled with 100mm mineral wool insulation and covered with a 0.4mm thick terne-coated stainless steel external skin. 

The double-glazed rooflights comprise an 8mm toughened/heat-soaked outer pane, a 15mm cavity and then an 8.8mm laminated low-e inner pane. Insulated aluminium bridging sections with silicone gaskets provide a weathertight seal between the cladding panels and rooflights. 

The cladding extends over the curved glulam beams at the base of the arcade/perimeter of the history centre roof using the same support system. A 300mm high, insulated stainless steel flashing connects the two terne-coated roofs. The northern edge of the diagrid is partly supported by the glazed end wall/entrance to the arcade. 

The weight of the roof combined with substantial wind loading requirements for the facade necessitated a composite structure in order to minimise the size of the timber sections. Located at approximately 2.5 metre centres, the main vertical supports consist of 100x200mm glulam columns bolted to 100x330mm T-shaped steel flitch plates. 

Running between the columns are 60x120mm glulam transoms spaced at 2.5 metres above finished floor level, and 60mm diameter structural tubes at 2.2-2.5 metre vertical centres. Fixed to the external face of the mullions and transoms is a double-glazed aluminium curtain walling system. 

Project team 
Architect: Pringle Richards Sharratt; project team: John Pringle, Penny Richards, Malcolm McGregor, Valerie von Truchsess, Karsten Weiss (project architect), Andy Reader, Marlene Chaussé Sene, Mary Comerford, Michael Wilson Katsibas, Daniel Jones; 
structure: Alan Baxter & Associates; 
services: SVM Consulting Engineers; 
landscape: Edward Hutchison Landscape Architects; 
planning supervisor: Phillip Pank Partnership; 
access consultant: Jayne Earnscliffe - Access for People; 
acoustics: Sandy Brown Associates; 
project manager: Gardiner & Theobald Management Services; 
cost consultant: Turner & Town-send; 
lottery consultant: Focus Consultants; 
archaeology consultant: Northamptonshire Archaeology, Birmingham Archaeology; 
building survey: Richard Audley Associates; 
fire engineer: WSP Fire Engineering; 
exhibition designer: Event Communications; 
lighting consultant: Martin Klingler Lichttechnik; 
contractor: Galliford Try Construction; 
client: Coventry City Council. 

Selected subcontractors and suppliers 
Glulam beam/purlins, LenoTec panels, spruce columns: Finnforest; engineered timber roof/wall structure: Finnforest Merk; precast white concrete: Hibex; double glazed rooflights: Raico; vertical glazing: GLS; rigid urethane insulation: Kingspan; standing seam roof: CEL; terne-coated stainless steel roof: Uginox; gallery rooflights: Vitral.

Link: Architecture Today

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