HOUNSLOW EAST TIMBER LAMELLA ROOF STRUCTURE

 

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Source

Buro Happold


Credits

Date of completion: 2002
Client: London Underground
Architect: Acanthus Lawrence and Wrightson (AL&W)
Engineer: Buro Happold
Main Contractor: Gleesons Mabeys Construction Ltd
Roof fabricator: Cowley Structural Timberwork


Outline

...> Case study presentation
...> Architectural concept design
...> Background on lamella structures
...> Structure


Case study
presentation

London Underground’s Hounslow East Station has been redeveloped in order to upgrade staff accommodation and improve the station facilities.  In early 1999 Acanthus Lawrence and Wrightson (AL&W) won an architectural design competition. The project was tendered in July 2000, the Contract won by Gleesons Mabeys Construction Limited (Roof fabricator, Cowley Structural Timberwork), and completed in 2002 for a Contract sum of approximately £5.5m.  This case study examines the origins and precedents for the design and describes the final structural result. Engineering issues relating to the lamella configuration, jointing, fabrication and construction are presented.

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Architectural concept design

The architectural design concept was to create a sweeping ‘green embankment’ of a roof leading the passengers from the street, up to the higher platform level (see Figure 1). The main entrance and accommodation is in the larger Westbound platform building (maximum footprint dimensions of 45m length by 18m width), with a smaller Eastbound platform building opposite (22m length and 10m width). Both roofs are barrel vaults in sectional shape but irregular on plan to suit the site area.  The sectional shapes of the two roofs are of constant barrel vault, with a radius of 23.8m. The leading edge of the roof extends some 2.5m from the façade line to offer shelter on the platform. The two sides are linked by a tunnel running below the Piccadilly Line.

From the outset, timber was the preferred material for the roof, being eminently appropriate for the suburban setting. Timber's environmental credentials and the potential to create interesting structural forms were also important considerations. The design team convinced London Underground of the longevity and robustness of the design.

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Background on lamella structures

The choice of a lamella structure

Through discussion of various preliminary designs, the preferred aesthetic solution involved a ‘skin’ of constant thickness using a diagrid of structural elements.  There were various ways to achieve this, but a ‘lamella’ structure was investigated and preferred over other options for a variety of reasons, including cost, buildability and a clean, uncluttered structure.

Historical background to lamella structures

Lamella structures owe much to Friedrich Zollinger (1880-1945), Town Building advisor (1918-1921) for Merseburg / Saale, Germany who patented his “Zollbau” timber lamella housing roofs in 1921.  He later developed the system for larger span roofs (typically around 30m span in a tied arch format) which were adopted widely in the 1920’s and 30’s in Europe and America. One of the largest examples was the St. Louis Exhibition Hall (1929), Texas, where the timber lamella arch was of 50m span. Since that time, there have been relatively few examples - The Festival of Britain in 1951 featured a timber lamella in a pavillion known as "Lion and Unicorn" (see Figure 2), and at Oko-Zentrum in Hamm, Germany, a multiple span tied arch was constructed in 1995.  All of these lamella roofs adopted very simple bolted connections and the roof plane was primarily carrying in-plane compression. Steel lamella roofs also enjoyed a period of popularity during the 1930’s for larger span structures such as warehouses and aircraft hangers, but this did not continue after the Second World War.

Benefits of lamella structures

Although lamella structures have not until recently been fashionable, they do offer some significant benefits:

• Many small members make up the larger continuous structure – these are much easier to transport and erect at site (pieces can be carried by hand) (see Figure 3).

• Each member is identical (or two types – one a replica of the other), which enables a high degree of repetition and off-site pre-fabrication.

• Aesthetically pleasing appearance of the exposed grillage.

• Good acoustic performance due to the ability to break up sound waves and give a uniform sound pattern (in an auditorium for example).

• In the case of localised damage to a connection or area of roof, the system offers structural redundancy and alternative load paths, which will help guard against progressive collapse.

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Structure

Spans

The leading edge of the roof is high above the platform and so there was no opportunity to adopt arching action as in the earlier examples of lamellae. This meant that the longer spans of the main ticket hall needed further support in the form of a central ‘tree’ structure (see Figure 4).

Elsewhere, spans between wall lines were acceptable in all cases. The platform cantilever is also supported (or tied down!) using timber struts springing from the steel columns of the façade (see Figure 5).

Construction of the lamella

The pattern of ‘lamella’ is shown in Figure 6 and Figure 7 – each rib is two ‘bays’ in length and overlapping the adjacent ribs to form a continuous grillage.

The lamella ribs were formed from Laminated Veneer Lumber (LVL) sourced from Finland, each piece of 360mm depth by 75mm width. LVL is a splendid material for this type of structure since it is supplied in large sheets (1.8m x up to 40m) from which any shape can be cut.  The surface of the barrel vault was divided into nodes at every 4.3° arc and the surface thus divided into a grid with side length of 1.5m. Hence each rib is 2.5m in length.  Looking along a line of ribs (see Figure 8) one can envisage how the ribs need to be progressively twisted in order to stay perpendicular to the roof deck (this requires some careful thought!).

Since each lamella is straight, the twist is achieved in two ways:

1. There is a mismatch in rib orientation across each joint of + and – 4.6mm (approx. + 1.5°).

2. The top and bottom surfaces of the ribs are planed such that the decking always lies flush to the timber surface.

Cowley Structural Timberwork devised ingenious methods to achieve this fabrication to extremely tight tolerances.  

The lamella ribs are all joined together by the structural decking which triangulates the system and acts as a continuous diaphragm. It is formed of tongued and grooved 450mm wide LVL sheets 27mm thick screwed to the top surface of the ribs.

The perimeter beams (see Figures 8) are complex elements curved in both plan and elevation. These were formed from thinner sheets of LVL pressure glued together in special jigs that were designed and made by Cowley Structural Timberwork.

Lamella connections

The lamella connectionss make use of the patented Cowley Shearlock Connector in pairs to the end of each beam. These are screwed into couplers embedded in the receiving beam. This gives totally concealed jointing throughout.  Connections were tested at the University of Bath, in a series of tests involving pure shear, pure bending and combined bending/shear loads in order to verify their performance and the analysis assumptions (see Figure 9 and Figure 10).  Where the shear capacity of the standard connections was insufficient, an increased number of shearlock connectors/couplers were used. Three and four coupler connectionts were also tested (see Figure 11).

Two trial assemblies were carried out in the fabrication yard, to check assembly issues and to agree some details and finishes (see Figure 12).

Roof erection

The roof was assembled rapidly, rib-by-rib, on site. Purlin rails along grillage node lines were pre-set to the appropriate level (see Figure 3).  Shearlocks were screwed in through inclined pilot holes to achieve the concealed detail.

Connection between roof and façade elements

The connection between the timber roof and the steel façade elements was generally achieved using wall plate elements as seen in Figure 13. This was the arrangement above the ticket hall curved façade, although along the platform elevation, this was not possible and brackets within the depth of the ribs were configured.  Great care was taken in the configuration of strut connections. Figure 14 shows how a platform laminated oak strut (120mm diameter) interfaces with the façade steelwork externally (steelwork fabricator – S. H. Structures).  The strut end grain is protected by a stainless steel collar. The bar is epoxy bonded into the end of the strut and the threaded clevis attachment gives sufficient adjustment for construction assembly.  Figure 15 shows a larger 225mm diameter main tree struts node. This passes through to the centre line of the rib with a simple bolted bent plate connection.

Nosing cantilever and roof build up

The nosing cantilever (1.2m long) is formed using 33mm thick LVL ribs at 450 centres, bracketed off the perimeter beam (see Figure 5).  The roof build up includes waterproofing, a vapour barrier, insulation, and a further ply deck on battens. This pre-patinated copper standing seam cladding.  The copper cladding is wrapped over the timber nose piece and returned to the strut support line.

Fire protection system

A fire protection system is included. This incorporates a darker stain for the decking diaphragm, and a clear stain for the ribs, which provides an interesting contrast (see Figure 4, and Figure 13).


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Figure 1

 

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Figure 2

 

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Figure 3

 

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Figure 4

 

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Figure 5

 

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Figure 6

 

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Figure 7

 

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Figure 8

 

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Figure 9

 

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Figure 10

 

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Figure 11

 

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Figure 12

 

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Figure 13

 

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Figure 14

 

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Figure 15

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