Platform frame construction

Construction principle

Platform Frame Construction

Fig. 45: Platform frame elements prior to erection

Bearth & Deplazes: private house (Willimann), Sevgein (CH), 1998

Fig. 45: Platform frame elements prior to erection

Bearth & Deplazes: private house (Willimann), Sevgein (CH), 1998

Platform frame construction is currently very popular in Switzerland. This is the outcome of marketing campaigns and engineering developments carried out by the timber building industry during the 1980s. The goal was to transform timber stud construction - which had been used widely since 1930 and itself had been inspired by the balloon frame system used in the USA and Canada - into a new building system. This new system had to exhibit a high degree of prefabrication and standardisation of the parts.

Consequently, platform frame construction is a further development of the tradition of improving timber buildings raised using traditional carpentry skills. The primary loadbearing system continues to rely on an arrangement of linear members which has been optimised and developed so that most of the work can be carried out in the factory. The degree of prefabrication has been gradually increased since the introduction of this system and has virtually reached the limits imposed by the system itself.

Thanks to its great flexibility and high degree of prefabrication, platform frame construction has been widely accepted by the building industry. However, it is itself now facing competition brought about by newly developed wood-based products which are tending to r ender the system of linear loadbearing members obsolete in favour of planar loadbearing elements (see "Panel construction - Current developments").

The system is based on a close grid of loadbearing linear members whose spacing can be varied depending on the given geometry, the format of the i nsulating material between the members, and the loads expected. Timber members with the same cross-section are used for the vertical studs as well as the horizontal head binders and bottom plates; their arrangement enables them to fulfil almost all structural requirements. The inner layer of sheathing stiffens the whole frame and leads to the whole providing a plate effect. All connections are generally nailed, but if necessary (tension-resistant) screws can also be used.

Fig. 47: Erection of platform frame elements with sheathing both sides

Bearth & Deplazes: private house (Willimann), Sevgein (CH), 1998

The use of standardised building materials is one of the prime advantages of platform frame construction. The majority of buildings employ timber members with cross-sections between 60 x 120 mm and 60 x 200 mm. These relatively small sizes result in little waste when being cut to size and are easy to store; they are ideal for kiln-drying and machine-grading.

It is advisable to fix sheathing to the prepared frames (so-called black box). To do this, battens, if necessary also counter battens, are fixed inside and outside and the sheathing attached to this, creating a "sandwich". The ensuing air cavities provide ventilation on the outside and space for services inside. The choice of material and surface finish is wide and only loosely dependent on the system.

Custom préfabrication

Unlike methods of construction that use batch préfabrication based on the use of standard basic elements (modular construction) or a fixed grid, timber platform frame construction is a method that allows custom prefabrication.

This means that, starting with a specific project which can be designed more or less as required (subject to the usual boundary conditions), a sensible breakdown into units can be achieved in conjunction with the manufacturer.

The individual elements of this "set of parts" are produced as self-contained "black box" assemblies in the factory and delivered to the building site as stable wall plates. These consist of a frame of linear timber members that is filled with i nsulating material and covered on both sides with suitable sheathing. The arrangement of the individual linear members within each element is chosen depending on the structural requirements and the geometry, taking into account any openings necessary in that section of wall.

The thickness and format of the i nsulating material chosen also influences on the spacing of the linear members and their sizes. The most common cross-sections in use lie between 60 x 120 mm and 60 x 200 mm because the thickness of insulation varies from 12 to 20 cm depending on the specification.

The assembly on the building site involves merely erecting these finished wall panels. The butt joints between the panels are either nailed or screwed depend-

ing on requirements. Normally, the elements are set up storey by storey, with the floors either being placed between successive wall panels or suspended from these inside.

Once completed, our assembled set of parts forms a stable, insulated building. To protect the building against the effects of the weather, it needs to be clad. There

Platform Framng

Fig. 49: Timber platform frame construction as a "building kit"

1 Individual project

2 Breakdown into sensible parts

3 Elements as self-contained "black box" assemblies (stable wall plates)

Fig. 49: Timber platform frame construction as a "building kit"

1 Individual project

2 Breakdown into sensible parts

3 Elements as self-contained "black box" assemblies (stable wall plates)

are hardly any limits to the type of cladding that can be chosen, but it must guarantee air circulation for the timber construction underneath. Timber platform frame buildings are mostly lined on the inside. This protects the inside sheathing to the black box (which, depending on the insulating material used, must provide a vapour barrier or vapour check) against mechanical damage and penetration. The lining permits individual interior design requirements to be met (plaster, wood panelling, etc.) and also conceals any electric cables subsequently installed (these may not be routed through the insulation).

Fig. 50: Custom préfabrication: Schemes 1-4, plans Schemes 1-4, sections

The design and construction sequence in timber platform frame construction

Dividing a specific project into sensible "building units" made up of wall and floor elements

Cooperation between architect, manufacturer and, possibly, engineer

Production of elements as self-contained "black box" assemblies in the factory

Arrangement of linear members based on structural requirements and insulating material format (50 x 100 mm or 60 x 120 mm)"

Sizing of linear members depends on thickness of integral thermal insulation (120-200 mm)

Joint design (butt joints with seals or overlapping as shown here)

Delivery of elements to building site and erection

Plan: vertical butt joints nailed (for compression only) or screwed (also for tension)

Section: stacking the elements

In order to guarantee the continuity of the vapour barrier, vertical and horizontal butt joints must be sealed accordingly

Attaching the external cladding (and internal lining)

The external cladding must guarantee a circulation of air for the underlying timber construction

The internal lining may be chosen to suit interior design requirements and can also conceal electric cables. There are no services (electrics, water, gas, waste water, etc.) in the platform frame elements themselves because otherwise they would have tc penetrate the vapour barrier.

Fig. 51: Timber platform frame element, layers and sheathing

1 Internal lining, 12 mm

2 Vertical battens (services), 50 mm

3 Wood-based panel (vapour-tight), 12 mm

4 Frame: head binder, 60 x 120 mm to 60 x 200 mm

6 Frame: bottom plate, 60 x 120 mm to 60 x 200 mm

7 Insulation, e.g. ISOFLOC, 120-200 mm

8 Bitumen-impregnated wood fibre insulating board, 18 mm (airtight)

9 Vertical battens, ventilation cavity, 40 mm

10 Horizontal sheathing, 24 mm

11 3-ply board with tongue and groove, impact sounc insulation

12 LIGNATUR box element

13 Airtight membrane over butt joint

14 Counter battens, 40 mm (needed to guarantee vertical continuation of ventilation cavity)

15 Horizontal battens, 40 mm

16 Vertical sheathing, 24 mm

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