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Wednesday30 July 2014

Velvet Mill rooftop pods by David Morley Architects

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David Morley Architects’ conversion of a Bradford landmark for Urban Splash introduces innovative zinc-clad rooftop structures.

Location Manningham, Bradford
Architect David Morley Architects
Structural engineer Price & Myers
Completion date March 2012

Lister’s Mills have dominated Bradford since the 19th century. The textile manufacturer was a major employer in the city, and its vast 11ha site – once the largest silk factory in the world – is still a notable landmark, with its 76m-tall chimney.

The mill, built by Samuel Cunliffe Lister in 1870-73 in the city’s Manningham district, is a vast and imposing complex comprising two six-storey mill buildings, the chimney, a weaving shed and a boiler house.

In the North Mill, Lister’s velvet was manufactured, the inspiration for the building’s recent change of name to the Velvet Mill as part of its refurbishment and conversion into apartments by David Morley Architects.

The mill closed in 1992, as Britain’s textile industry declined, and the grade II* listed buildings lay vacant and derelict until 1999 when developer Urban Splash took on the site’s regeneration.

The South Mill, now known as the Silk Warehouse, was converted into 131 apartments and a community space, which completed in 2007.

The Velvet Mill has now also been converted into 166 apartments, but here the work involved the addition of 10 two-storey “pods” on top of the existing mill, providing 29 penthouse apartments split into different configurations.

The form is inspired by twisted strands of yarn, a nod to the mill’s weaving heritage.

“It also looks a little like a curvilinear metal fabric wrapped into a plait,” says founding partner David Morley. “At the fold lines the fabric is cut away to create windows and balconies.”

He adds: “This generates a rigorous geometry which harmonises with the structure below and creates space for the apartments which face out diagonally towards stunning long views of Bradford in one direction and the moors in the other.”

The zinc-clad double curvature pods – composed of a steel and timber structure – are complete and waiting for fit-out. The top-floor apartments are accessed via a centrally placed entrance in the original mill building, leading to the ground-floor lobby.

A new central atrium creates a full-height slot, containing stairs and a lift, which provides access to all the floors including the rooftop pods while drawing daylight into the circulation core.

Section through pods

SECTION THROUGH PODS

1 Steel balustrading
2 Aluminium curtain walling
3 Zinc standing seams
4 Zinc cladding
5 Timber cassette
6 Bedroom Level 7
7 Bedroom Level 6
8 Steel transfer structure
9 Roof membrane

Creating the rooftop structures

David Morley Architects initially sketched an outline of the rooftop pod’s shape but its complex form made it difficult to define its geometry. Price & Myers Geometrics was appointed to resolve this using parametric software which developed the form into a series of repeating loops.

“The only way to economically build the pods was to make them in such a way that their complex form could be fabricated using a set of simple, yet intricately shaped flat parts,” says Tim Lucas, partner at Price & Myers. “These fit together in a single way to create the three-dimensional form.”

The structure of the pods comprises two main elements: the steel frame or hoops and the curved timber ribs. The steel frame was made from hollow box curved sections using mild steel. All the nodes and joints were welded offsite and bolted together on site to make a curved sway frame with no cross bracing.

The hooped form of the apartments was created with the steel elements, and the gaps between each apartment, where the private stairs are located, are enclosed above with faceted double-glazed glass panels.

The timber structure is composed of a combination of plywood and oriented strand board (OSB) and is distributed around the hooped frame.

Price & Myers developed a system of curved ribs that could be prefabricated, taking a lead from the low-cost plywood furniture that is cut from flat sheets using a CNC router and sold in flat pack form.
Lucas says the way the timber structure fits together was inspired by the very thin plywood toy dinosaur skeletons he constructed as a boy.

Exploded timber cassette

EXPLODED TIMBER CASSETTE

1 18mm OSB interlocking timber ribs
2 Individual timber ribs
3 23mm x 70mm plywood flanges
4 Slots to accommodate continuous flanges
5 11mm OSB web plates
6 Notches to fit timber on steel frame
7 Slots to ensure correct orientation of assembled timber parts

 

Each pod has 60 structural timber ribs, which span 6.5m between the steel edge frames. The ribs were broken down into component parts that could be cut from standard 2,440mm x 1,220mm plywood sheets.

“The flat cut parts were used to form beam webs, flanges, noggins and skin units,” says Lucas. “They were cut using a CNC controlled router as the geometry of the structure repeats across the building and each component is duplicated 12 times.”

Six or seven ribs and five noggins were brought together to form a 6m long x 2.5m wide cassette. The noggins perform a structural and practical purpose by preventing the ribs from buckling, and they also set the ribs out in the correct orientation. This was achieved by slotting the components together so there was no need for any further cutting.

Strips of 18mm-thick OSB span each rib to clad each cassette and the curved form of each strip was developed into a flat sheet to allow it to be manufactured.

“The shape of the developed panels varied with the geometry of the pod,” says Lucas. “Where the curvature is more acute the developed strips were more bowed to compensate.”

The skin or OSB elements were glued and screwed onto the 25mm-wide rib flanges to create the finished modules. This action both braced the cassettes and added to their strength by creating a “stressed skin” – a skin that helps to carry the force within a structure, like an aeroplane wing.

The 120 cassettes were then bolted on site to the steel frame using a steel angle plate.

 

Zinc cladding

Kalzip metal standing seam had been considered for the final roofing material to clad the pods but it was too difficult to build on site.

Zinc proved to be a better choice, given its malleability, but the architects had to work hard in persuading the subcontractors to fit the zinc with a cross seam rather than a staggered seam.

A zinc test panel was made by a skilled installer and because the quality of the panel was so high, it demonstrated that it was possible to fix the zinc using continuous cross joints and therefore set the precedent for the standard of finish for cladding the apartments.

Before the zinc was laid, softwood battens were fixed to the timber and steel structure of the pods and a breather membrane added.

The rolled flat zinc strips were fixed by skilled subcontractors and zipped up on the welts, creating the appearance of even, parallel seams spanning across the roof.

David Morley's Velvet Mill

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