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Introduction
In this small parish church at Manilla, in central northern
NSW, architect Ian McKay embraced plywood and exploited its
natural finish and structural capacity as a shear skin in
the elements of roof design.
The Congregational Building Committee at
Manilla required a church to seat 160. The site was flat but
on deep alluvial black soil that has the potential to move
as much as 5cm across the building area. An initial design
of solid inclined timber rafters set on deep piered footings
proved too expensive, so a light, earth integrated solution
evolved. The timber rafters were replaced with much lighter
plywood clad trusses. Earth filled and concrete encased
berms, tied through the floor slab, replaced the expensive
piered system. This flooring system spread the building load
over a wide area and provided a strong but flexible
structure, tolerant of building movements. The berms
provided stability for the structure and enclosure for the
congregation. The trusses, clad as if they were a folded
plate of plywood, provide an economic, light and flexible
roof, and a naturally finished ceiling that steps down from
the crown of the church to focus attention on the centre of
worship, the altar.
Description
A number of structural principles have been cleverly
interwoven in this small parish church, to create a simple,
intimate space for worship. The symmetrical space of the
church is divided in section into three parts. The Nave
forms the building's core with its rising and falling
ceiling and skylight creating a lofty worship space. The
Narthex, or entry, and the Vestry, the preparation space for
services, service the Nave at either end. The walls become
the ceiling in each of these spaces as the roof framing
springs from the berms and pitches to a ridge, creating an
earth integrated A-frame structure.
The berms which anchor and support the
entire building, are a construction of consolidated earth,
encased within reinforced concrete, and finished with a
cladding of brick. They are tied across the building by the
reinforced concrete of the floor slab. This triangulates the
structure of the building. The lateral forces from the roof
trusses maintain tension in the floor slab, which works to
counter some of the effects of soil movement.
The nave framing consists of a series of
pitched, parallel chord trusses supported on the berms and
spanning across the width of the church. Increasing in span
as the supporting berms angle out from the Narthex and
Vestry towards the width of the Nave, each truss steps up
above its predecessor. As each step is equal to both the
depth of the trusses and the spacing between them, a series
of regular 90 degree junctions are formed.
Ladder frames span between each of the
stepped Cypress trusses, connecting each top chord to the
bottom chord of the higher adjacent truss. Both frames and
trusses are clad internally with 6mm plywood, and this
creates the impression of a complex folded plate ceiling.
Battens, supporting the roofing tiles, are fixed diagonally
across the top chords of each of the trusses, providing
further triangulation of each of the roofing members. Each
of these components works efficiently to perform both a
structural and, in the case of the ladder frames and stepped
trusses, a finishing function.
The narthex and vestry are of simpler
construction. Solid timber rafters pitch to the ridge height
of the lowest nave frame to create an A-frame hallway at
either end of the central space. Ply facing is fixed
alternately to the top or bottom edges of pairs of rafters
to form an undulating ceiling surface.
A continuous glazed roof-light splits the
building across its widest and highest point and accentuates
the plywood surface through a play of light and
shadow.
A unique space was created through this
form of construction, using standard 90 and 45 degree angles
for all of the major timber junctions. This allowed for a
high degree of detail standardisation that was efficient in
construction time and in cost.
While the church is acknow-ledged as a
fine place of worship, the building has several problems.
The original plywood fascia to the building faced into the
western sun and deteriorated quickly. It was clad with metal
sheeting in about 1978 but this too has buckled. Ironically,
the stiffness of each of the roof sections has caused stress
through movement to be localised in the glazed roof-light.
This is the weakest element in the building and has required
ongoing repair.
A Strategy for Design with
Timber
Shear plywood skinsÉ structure as a
surface finish - The properties of plywood allow for the
design and construction of single skinned, stiff structures
that resist the affects of lateral forces.
Plywood is constructed of a series of
veneers laminated with the grain direction of adjacent
veneers at right angles to each other. To ensure a stable
material, free from warping, an odd number of veneers is
used. This assembly gives plywood great strength as a
lightweight, dimensionally stable cross-bracing sheet
material. A single skin of ply of appropriate thickness,
fixed to a timber frame, forms a strong, cross-braced
structure that resists deformation through the development
of shear across the panel. The ply skin can also become the
interior or exterior surface, and dependent upon the
application may only require clear finishing. Finishing
veneers are available with a wide range of colours or
characteristics, making ply an excellent natural surface
finish.
The design of the parish church combines
these two important characteristics of ply. Ply is used as a
bracing skin as it is fixed to the ladder frames and members
of the nave trusses, and between the narthex and vestry
rafters. Its aesthetic properties allow it to act as an
attractive internal surface finish.
Lightweight Structures
Due to its high strength to weight ratio, plywood, when
combined with a solid timber frame, creates a strong and
stiff, lightweight structure. The designer of the church at
Manilla used plywood to make a lightweight structure, in
order to avoid difficulties with the unstable soil
conditions on site. There are various other situations in
which this characteristic of plywood would be particularly
advantageous:
[1] Constructions that require
prefabrication, and transport to site - cost is often
reduced in proportion to weight.
[2] Building with small construction
teams - a sheet of structural ply can be lifted, and
machined by two people (a 9mm ply sheet - 2400 x 1200 weighs
about 16kg)
[3] Constructions on top of existing
buildings where additional building loads need to be
minimised.
[4] Building in remote or inaccessible
areas, where lifting equipment is not available.
Standardisation of
detail
While the profession of architecture has a reputation for
custom designed buildings, fittings, and fixtures, unique
structures can be generated through the use of standard
details.
This building has an unusual spatial
configuration and built form, but it is generated through a
simple geometry of 90 and 45 degree junctions. The stepped
space of the nave is achieved simply by reducing the height
of the same triangular section, and allowing the plan to
follow the corresponding reduction in span. Maintaining
regular spacing of structural members throughout the
building allows for further standardisation. There are
numerous advantages to this approach to design:
[1] Prefabrication can occur with
construction elements, as there is a reduced requirement for
components to be fitted and cut on site.
[2] A simple range of tools and jigs can
be employed for set-out, cutting and erection when standard
angles and dimensions are used. This can allow for
prefabrication off site, and ease on site construction.
[3] There is a reduced potential for
material wastage, if a standard set of angles and dimensions
are employed for a whole building. For example: short pieces
remaining after docking timber at 90 or 45 degrees can be
utilised in other junctions in the building.
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