Composite structural member for a building structure

1. A method for use in designing a composite rafter for a building structure, wherein the composite rafter is required to have a first member with a first end region and a second end region, a second member with a first end region and a second end region, the first end region of the first member connected or connectable to a support portion of the building, the second end region of the first member connected or connectable to the first end region of the second member, and the design of the rafter is required to be such that, in use, the second end region of the second member is located higher than the first end region of the first member, the method comprising:

assessing bending moments which the rafter is required to be capable of resisting at various points or regions along the length of the rafter;

determining at least one point or region along the length of the rafter which is to be provided where the bending moment which the rafter is required to resist is maximum;

identifying a first type of structural component with a first bending moment bearing capacity which is capable of resisting said maximum bending moment, and selecting a structural component of the first type to use as the first member of the rafter;

determining at least one point or region along the length of the rafter which is to be provided where the bending moment which the rafter is required to resist is substantially smaller than said maximum bending moment;

identifying a second type of structural component which is different from the first type of structural component and wherein the second type of structural component has a second bending moment bearing capacity which is capable of resisting said smaller bending moment but not necessarily capable of resisting said maximum bending moment, and selecting a structural component of the second type to use as the second member of the rafter; and

determining a position, along the length of the rafter which is to be provided, at which to provide a join or transition between the first member and the second member, wherein the position of the join or transition is such that bending moments in portions of the rafter formed by the second member do not exceed the second bending moment bearing capacity.

2. The method as claimed in claim 1 , wherein the position of the join or transition is such that bending moments in portions of the rafter formed by the second member do not exceed 90% of the second bending moment bearing capacity.

3. The method as claimed in claim 1 , wherein the position of the join or transition is such that bending moments in portions of the rafter formed by the second member do not exceed 80% of the second bending moment bearing capacity.

4. The method as claimed in claim 1 , wherein the position of the join or transition is such that bending moments in portions of the rafter formed by the second member do not exceed 70% of the second bending moment bearing capacity.

5. The method as claimed in claim 1 , wherein the position of the join or transition is a position at which the length of the first member is no more than 150% greater than the minimum length that the first member can have, with the second type of structural component used as the second member of the rafter, without the bending moment in portions of the rafter formed by the second member exceeding the second bending moment bearing capacity.

6. The method as claimed in claim 1 , wherein the position of the join or transition is a position at which the length of the first member is no more than 100% greater than the minimum length that the first member can have, with the second type of structural component used as the second member of the rafter, without the bending moment in portions of the rafter formed by the second member exceeding the second bending moment bearing capacity.

7. The method as claimed in claim 1 , wherein the position of the join or transition is a position at which the length of the first member is no more than 60% greater than the minimum length that the first member can have, with the second type of structural component used as the second member of the rafter, without the bending moment in portions of the rafter formed by the second member exceeding the second bending moment bearing capacity.

8. The method as claimed in claim 1 , wherein the position of the join or transition is a position at which the length of the first member is substantially the minimum length that the first member can have, with the second type of structural component used as the second member of the rafter, without the bending moment in portions of the rafter formed by the second member exceeding the second bending moment bearing capacity.

9. The method as claimed in claim 1 , wherein the first type of structural component and the second type of structural component are different by virtue of being made of different types of material and/or by virtue of being of different constructions.

10. The method as claimed in claim 9 , wherein the first type of structural component and the second type of structural component are made of different types or grades of steel.

11. The method as claimed in claim 9 , wherein the first type of structural component comprises a hot rolled steel member.

12. The method as claimed in claim 11 , wherein the first type of structural component comprises a beam fabricated from hot rolled steel with an I-shaped or H-shaped cross-section.

13. The method as claimed in claim 9 , wherein the second type of structural component comprises a cold formed steel member or a timber (wooden) member or a carbon fibre member.

14. The method as claimed in claim 13 , wherein the second type of structural component comprises a beam fabricated from two metal sections connected along their length.

15. The method as claimed in claim 14 , wherein the second type of structural component comprises two cold formed steel C-channel section beams attached to one another back to back along their length.

16. The method as claimed in claim 1 , wherein the second member is at least 1.5 times the length of the first member.

17. The method as claimed in claim 1 , wherein the second member is at least twice the length of the first member.

18. The method as claimed in claim 1 , wherein the second member is at least four times the length of the first member.

19. The method as claimed in claim 1 , wherein the position of the join or transition is determined such that the cost of the rafter, and/or the cost of multiple of the rafters produced at or around the same time, is largely minimized based on per unit length costs and/or other costs associated with the types of structural components used as the first member and the second member and/or the number of rafters to be produced.

20. The method as claimed in claim 1 , wherein

the expected loading on the rafter when the rafter is in use is such that the magnitude of the bending moment in the rafter changes along the length of the rafter and the direction (sign) of the bending moment in the rafter changes at a point of contraflexure, and

if the point of contraflexure is located between the first and second end regions of the second member, the distance from the first end of the second member to the point of contraflexure is such that, based on the expected loading on the rafter, the magnitude of the bending moment does not exceed the second bending moment bearing capacity anywhere in the second member.