GB2094372A - Steel trusses for pitched roof - Google Patents

Abstract

A lightweight steel truss for use in the construction of a pitched roof in which the rafter 1 and tie members 2 are constructed of steel strip rolled to a lipped zed section provided with a protective coating. The braces 3 are of channel or L-section. <IMAGE>

Description

SPECIFICATION Steel trusses for pitched roof construction This invention relates to steel trusses for use in the construction of pitched roofs for domestic and similar building applications and to a roof including such trusses.

During the past decade factory produced timber trusses for such roofs have to a significant extent replaced on-site built trusses in the domestic housing market.

A truss in pitched roof construction is used to support weather-proofing material, e.g. roof tiles, supported generally on timber or steel battens and to transfer loads imposed by e.g. snow, wind etc.

to the structural supporting walls of the building.

Conventionally, timber trusses are manufactured from stress graded timber fixed at the node points with proprietary steel connector plates with integral nails which are pressed into the timber thus connecting the members converging at the node.

Such trusses suffer from a number of disadvantages, these including: (a) Node point fixings may become loose from handling and during transport to site.

(b) Preservatives used for the protection of the timber may attack the steel node fixing plates.

(c) Timber strength is related to a moisture content and no guarantee can be given to the retention of moisture over prolonged periods.

(d) Timber is subject to attack by wood boring insects which ultimately weaken the timber.

(e) Timber is subject to twist and warp.

The present invention sets out to overcome these disadvantages.

According to the present invention there is provided a lightweight steel truss for use in the construction of a pitched roof in which the rafter and tie members are constructed of steel strip rolled to a lipped zed section provided with a protective coating. Preferably the steel strip from which the rafter and tie members are constructed is of a thickness less than 1 mm.

In one construction these members are manufactured from a steel to British Standard BS.

2989 :1975, hot dipped sheet and coil group Z35 coated with a class C galvanised coating and a minimum material yield stress of 350 n/mm2. In this preferred construction the member thickness approximates to 0.91 mm.

Bracing members provided between the rafter and tie members are preferably of angle L or channel section.

The invention further provides a pitched roof including steel trusses as described above.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which: Figure 1 illustrates a steel roof truss in accordance with the invention; Figure 2 is a section taken along line II--II of Figure 1; Figure 3 to 6 are scrap views of details of the roof truss illustrated in Figure 1; Figure 7 is a section taken along line VIl-VIl of Figure 3; and Figure 8 is a scrap view of a roof truss in accordance with the invention showing soffit and bargeboard fixing brackets.

A typical roof truss assembly in accordance with the invention is illustrated in Figure 1 and comprises two inclined rafter members 1 joined by a horizontal tie member 2. Bracing members 3 extend between the rafter members 1 and the tie member 2.

Each rafter member 1 and tie member 2 is constructed of steel to British Standard BS.

2989:1975, hot dipped sheet and coil grade Z35, with a class C galvanised coating. The minimum material yield stress is 350 N/mm2. Material of the appropriate thickness is cut into strips of a given width which is then fed through rollers to form the required lipped zed section. A typical section through a rafter or tie member is shown in Figure 2. The bracing members are also constructed of steel strip rolled or formed into "L" angle or channel section.

The thickness of each member 1 and 2 is, in the construction illustrated, 0.914 mm inclusive of its galvanised zinc protective coating. The thickness has been carefully chosen to enable building components such as plasterboard and tiling battens to be fixed to the tie 2 and the rafters 1 respectively by conventional means; thus tiling battens may be fixed to the rafter 1 by nailing and ceiling plasterboard may be fixed to the underside of the tie member 2 by self tapping screws.

The lipped zed section for the members 1, 2 permits simple fixings to be made at joints between the rafter and tie members and between these members and the bracing members 3.

For making each eaves joint 5, the top flange at the ends of the zed section is removed by cutting, thus permitting the vertical webs of both the tie and the respective rafter to overlap. The connection between the rafter and tie can therefore be made simply by spot welding through adjacent webs, or by bolting, or by riveting, or by arc welding. Such a joint is illustrated in Figure 3. A stiffening angle 6 is provided to facilitate fixing of the truss to a wall tie.

The overlap of the web at each eaves joint 5 provides an overlap at the ridge joint 7 between the two rafter members 1. In this case the bottom flange of one rafter member is cut away to provide the necessary overlap. The connection between the two rafter members at the ridge joint may be the same as that provided between each rafter member and the tie member. A typical ridge joint is illustrated in Figure 6. The bracing members 3 are connected to the respective rafter member 1 and the tie member 2 in the manner illustrated in Figures 3 to 6 using the same connections as are used between the rafter members and the tie member.

To enable standard building components such as barge-boards for gutter support and soffit to be connected as required to the truss, a support channel bracket 4 is attached to the lower cantilever end of each rafter member 1.

A typical bracket 4 is shown in Figure 8, this having a number of holes for use with a bolted connection to the rafter members 1. Each hole 8 is preferably elongate in the plane of its center line to permit on site adjustment to ensure plumbness of the bargeboard and fixing.

Claims (7)

1. A lightweight steel truss for use in the construction of a pitched roof in which the rafter and tie members are constructed of steel strip rolled to a lipped zed section provided with a protective coating.

2. A steel truss as claimed in Claim 1 wherein the steel strip from which the rafter and tie members are constructed is of a thickness less than 1 mm.

3. A steel truss as claimed in claim 1 or claim 2 wherein the rafter and tie members are manufactured from a steel to British Standard 2989:1975 coated with a galvanised coating and having a minimum material yield stress of 350 m/mm2.

4. A steel truss as claimed in any one of claims 1 to 3 wherein channel-section or "L' section bracing members are provided between the rafter and tie members.

5. A pitched roof comprising one or more steel trusses as claimed in any one of claims 1 to 4.

6. A roof as claimed in claim 5 including a support bracket connected to the lower cantilever end of each rafter member for supporting standard building components, the bracket having elongate fixing holes to facilitate correct alignment of said components.

7. A steel roof truss substantially as herein described with reference to the accompanying diagramatic drawings.