PROFESSOR REMO PEDRESCHI
HND BSc PhD MICE CEng
20 Chambers
Street, Edinburgh EH1 1JZ, Scotland
Phone: 0131-650-2301
E-mail:r.pedreschi@ed.ac.uk
Steel and Plywood composite structures
The need to pre-fabricate and industrialise the construction industry is stimulating interest in new forms of construction that are lightweight and can be manufactured efficiently. Building on earlier research on light-weight metal systems attention has now turned towards the potential of lightweight structural systems using steel in combination with plywood. The structural elements consist of pre-galvanised cold-formed steel flanges attached to plywood webs using connections such as self-tapping screws. The potential advantages of this construction are:
-
Efficient use of material: the most highly stressed parts are the steel flanges, taking advantage of its strength and the consistency of its physical properties.
-
The use of plywood in the web provides very effective resistance to shear force and avoids problems of web buckling and crushing more likely to occur in thin deep cold-formed steel webs.
-
The fabrication of these structures is very simple and straightforward. Only hand held power tools are needed.
-
The combination of plywood and steel is highly adaptable to many different applications: the same basic components of flange and web, can be used to form a wide range of differently shaped trusses and beams.
-
Structures can be easily dis-assembled and the components re-used in new arrangements
-
The techniques are suitable for both factory and site construction.
The structures are suited to use in a range of practical applications, particularly where spans start to exceed the effective limit of solid timber joists or cold-formed steel beams. One important application is in housing to create ‘room in the roof’ attic constructions. There is a growing demand for usable attic space in new housing and also for non load-bearing internal partitions that allow more flexibility in planning and re-planning. The structure therefore has to be able to span the full width of the building, without support from internal walls whilst carrying normal residential floor loading.
To demonstrate the potential of this new form of construction a full-scale prototype roof was built and tested.
Attic Roof Construction
The prototype consisted of two attic roof trusses formed using standard plywood and standard cold formed steel C sections.
The overall dimensions of the prototype construction were 8.0 metres in length and overall height of 3.05 metres. The rafters were inclined at angle of 35 degrees. A room space inside the structure 4.48 m wide by 2.25 m. high was formed. The floor beam of the truss consisted of a steel plywood composite beam, 300 mm deep. The two trusses were spaced 600 mm apart and the internal wall and ceiling surfaces was lined with 12.5 mm plasterboard. 22 mm flooring grade was used to span between the floor beams.
The plywood and steel elements were connected using standard self-tapping screws, at 100 mm spacing along the centre line of the C sections.
Test Results
A series of load tests were carried out on the roof construction. Load was applied using standard dense concrete blocks with an average weight of 18.9 kg each. The load was applied in five stages. The tests are summarised in the table below.
| Load no | Position on truss |
Applied load kg |
Total load on truss |
Comments |
|---|---|---|---|---|
| 1 | both rafters |
604 |
604 |
Design working load on roof |
| 2 | floor |
775 |
1379 |
Design working load on floor |
| 3 | both rafters |
454 |
1833 |
Design ultimate load for roof |
| 4 | floor |
453 |
2286 |
Design ultimate load for floor |
| 5 | floor |
1153 |
3439 |
Overload test on floor |
Loads 1 and 2 represent the normal working loads applied to the residential buildings. Loads 3 and 4 represent the additional loads that both the roof and floor should carry to ensure an adequate factor of safety against collapse. Load 5 is the additional load in excess design ultimate load that was applied to determine exactly how strong the structure was. The floor was loaded to three times it normal design load without the structural failure, demonstrating the considerable reserve strength of the construction.
Truss carrying three times the design load for a house.