All About Engineering

steel structure system

enas saleh

8 months ago

Renovation of Buildings using Steel Technologies (ROBUST)

Renovation of existing timber roofs to provide additional space is an important and expanding sector as it increases the building size without adding to its ‘footprint’. Existing timber trusses cannot easily be modified to provide this useable space, whereas steel open trusses can be assembled within the existing roof and the timber trusses cut away.
Light steel open roof may be of two basic forms: 

  • ‘A-frames’ consisting of rafters and floor beams and using C sections of different depths, as shown in Figure 1. In this case, the roof truss spans between front and rear façades

  •  Longitudinal lattice girders that support the existing timber rafters. In this case, the longitudinal trusses span between cross-walls or gable walls.

This paper addresses the structural design and practical application of open roof trusses in renovation or in new building. There are opportunities to use steel and plywood compositely to create a ‘plyweb’ beam that has advantages in terms of structural performance and thermal efficiency.

Open Roof Truss 
The steel open roof truss is based on the use of cold formed C or Z sections and the bottom chord of the truss provides the floor to the habitable roof space. The members are easy to assemble on site by bolting (although screws would also be possible) and is self-jigging for dimensional accuracy.
The form of the roof can be easily varied to suit the required living space and window openings. The case shown is for the use of Velux type roof lights. Where other window forms are required, the position and height of the vertical members can be modified in manufacture.

The range of application of the open roof system is:

  •  Spans of 6 to 10 m.

  • Spacing of 400 to 1200 mm, depending on the span of the tiling battens and plasterboard supports 

  •  Roof slope of 30 to 45° 

  •  Habitable space of 3.5 to 6 m width between the vertical members


Roof lights may also be incorporated by increasing the spacing of the trusses locally although, economically, the system is more appropriate for wider truss spacings (say 900 mm), which reduces the number of trusses and assembly time. Dormer windows can be introduced and in this case, the minimum height of the vertical member may be taken as 900 mm.
The roof may be insulated externally by closed cell insulation board (typically 70 to 100 mm thick) to which counter-battens and battens are attached or by insulation placed between the rafters. Slotted or perforated C sections for the rafters may be advantageous when mineral wool insulation is placed between the rafters.