Formby Swimming Pool, Formby, Lancashire

Introduction

This £7.3 million scheme has transformed a town centre football ground into a new public park and a swimming pool, fitness suite and cafe, giving the town and community a new focus and forming a civic presence in the High Street. The site is sandwiched between the ‘public’ environment of the town centre and more private residential areas. The scheme is designed so that the building relates to a sequence of landscape spaces which are designed to draw people through the site from both directions. Sustainability was a key issue for both client and architect and this, together with the architect’s experience of pool buildings, led to the choice of timber for structure and cladding.

 

Project description

The building consists of a copper-clad entrance block containing foyer, changing rooms and fitness suite, linked to the taller and more dominant pool hall containing cafe, a 25metre long five-lane swimming pool and a learner pool. It is designed on sustainable principles; the structure is timber and the walls are clad with Douglas fir and European oak, all from sustainable sources. The roofs are clad with copper.

A dramatic exposed timber structure forms the roof of the main pool hall. It is poised high above the water, supported at the west elevation on delicate timber columns at 10.8 metre centres, allowing the lower 2.1metres of the wall to be entirely glazed with frameless glass panels. This huge glass wall, 50metres long, looks out over the park, so that swimmers have views out and passers-by can glimpse the pool.

The roof is a series of canted bow-string trusses which span 15metres across the width of the pool. The 140 x 315mm top booms and 100 x 100mm central struts are formed of Kerto LVL (laminated veneer lumber), tied by galvanised steel tension rods and forked connectors. The trusses are set at 1.2metre centres and support an exposed deck of 15 x 1.2metre LVL boards; these run the full width of the pool and their joints lie directly over the top booms of the trusses, so that all edges are concealed.

From the café the roof rises, increasing in pitch to form a dramatic parabola over the deep end. This subtle twist is reflected in the geometry of the trusses; the LVL deck above was flexible enough to accommodate the twist and the slightly skewed edges are hidden at the concealed joints.

The low side of the roof rests on Parallam (parallel strand lumber) composite beams and is restrained laterally by the plywood diaphragm deck of the lower entrance block building. At the higher side of the roof, along the eaves of the west elevation, the trusses are flitched to exposed LVL posts – ‘sticks’ - which are bolted back to a series of concealed glulam trusses. The trusses run for 50 metres above the 2.1 metre high glazed wall and are supported on 400 x 200mm composite columns of galvanised steel and Kerto LVL. They are clad internally with vertical Douglas fir boards and externally with vertical European oak boards.

 

Structural engineer's account

The structural concept of the building was to be simple, regular and robust with a clear structural form and philosophy. Sustainability was a fundamental part of the design and following on from previous pool project collaborations with Feilden Clegg Bradley Studios, a timber frame structure was chosen. Timber performs well in the warm atmospheres of varying humidity encountered in the pool hall and changing ‘village'.

The pool hall roof structure is made almost entirely from Kerto LVL, a composite timber of 3mm laminates. Lamination produces a material which is significantly stronger and stiffer and has a lower shrinkage potential than the parent softwood. The pool hall trusses to the west elevation are fabricated from glulam with bolted flitch plate joints.

The columns on the west elevation are of a composite steel and LVL section. The central steel flitch plate is concealed by a Kerto infill piece. Its function is to stiffen the section about its major axis and provide some fixity at its connection to the pool surround slab about its major and minor axis. The composite section is created using a modified epoxy by Rotafix to bond the timber to the steel flitch plate with steel dowels introduced to resist peeling action at the base of the column. At the column/truss interface, the column is mechanically clamped by through bolting.

 

Geometry

The use of glulam, Kerto LVL and softwood is proven for roof structures. Fire resistance is not required, though a coating to prevent surface spread of flame is.

The upper roof comprises trusses at 1.2m centres. Each truss is made from a Kerto-S LVL top chord and a galvanised steel rod as the lower chord. A single timber strut acts as a web member and is normal to the centre of the truss. The rod is connected to the top chord via a steel flitch plate at each end.

The top chords of the trusses are profiled to allow the decking to stay into contact across the width of the chord. The shape is defined by the roof pitch which varies in two directions. Due to the shallow inclination of the tension chord in relation to the compression chord the resulting offset of the flitch connection means that the connections do not node out, therefore the additional moment due to joint eccentricity is taken into account in the design.

The pool roof geometry is defined by a constant level on the internal wall of the pool hall, but on the external wall the level rises towards the north. The bowstrings here sit on ‘sticks’ which extend down in front of and attach to a deep truss which is part of the external wall. The space between the ‘sticks’ is glazed to create a clerestory window.

 

Pool hall stability

Transverse loads are transmitted through the top chords of the Kerto bow strings to the supporting steel columns along the east side of the pool hall. These loads in turn are transmitted into the lower roof plywood diaphragm by a flitch plate connection to parrallam beams supporting the plywood diaphragm.

The diaphragm loads are taken out by concrete and masonry shear walls. The lower roof is split into 3 sections to reduce moisture induced movements and each section acts as an independently restrained diaphragm.

Longitudinally, loads are resisted by bracing along the east elevation and by sway frame action on the west elevation combined with the diaphragm action of the roof restrained by the transverse stability elements.

Wind loads applied to pool hall gables are resisted by Kerto LVL columns which span from pool surround slab to roof deck level.

 

Changing area stability

The low level roof is split into three segments and each segment has a plywood roof diaphragm supported by timber engineered I-beams/Parallam beams/glulam beams. The roof diaphragms are restrained by shear walls in both directions. Internal shear walls in wet areas are built in either blockwork or reinforced concrete; perimeter shear walls consist of timber studwork with OSB sheathing.

 

Local stability of the bowstring trusses

The bottom chords of the trusses included a facility to pretension the truss to remove dead load deflection and sag from the tie rods. This took place before the deck was fastened down to reduce stresses induced in the deck to chord connection. Whitbybird provided a deflection history to the specialist so that the correct amount of precamber could be installed according to the stage at which it was applied. The trusses are tied together by the Kerto-Q LVL roof deck, which acts as a diaphragm. The shear is carried through fixings which are limited to 4mm diameter as per the certification requirements for LVL. The Kerto LVL deck had to prevent buckling of the top chord of the truss; it also had to prevent snap-through arising from fabrication and erection inaccuracies where a notional horizontal component of the tension chord might be applied out of plane.

The out-of-plane rotation of the bowstring tie rods and struts is resisted by fixity at the strut to top chord connection created by the dowelled connection. The bowstring top chord is restrained against rotation by its connection to glulam beams running over the Kerto deck normal to the bowstrings.

Completion Date:

2006

Year Published:

January 2008

Building Type:

Swimming pool

Location:

Formby, Lancashire

Country:

England

Client:

Formby Land Trust

Architect:

Feilden Clegg Bradley Studios

Structural Engineer:

Ramboll Whitbybird, Bristol

Main Contractor:

Dew Construction/ ISG Construction

Timber Element(s):

LVL timber structure Cladding

Timber Specie(s):

Douglas fir and European oak cladding

Awards:

2007 Wood Awards Gold Medal, RIBA Regional Award 2007, RIBA National Award

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