Canopy Design: pt#1 (Basic Procedure)

Free standing canopy (carports etc..) design is similar to the requirements for shed design. The primary difference is that with no wall cladding the wind load pushing against the side of the canopy is less. There is still a side ways load on the canopy, it comes from the horizontal component of the loads on the roof structure, and from drag loads across the roof cladding and against the face of the columns.

As there is no wall cladding nor girts to support such cladding, it also means that there is no lateral restraint to the columns and as a consequence the columns have less lateral torsional buckling (LTB) resistance than otherwise would have. Further people generally don’t want any cross-bracing in the sides of the canopy, therefore the posts have to be cantilevered in two directions, and so subject to biaxial bending. Therefore rather than the posts being the same as the rafters the posts are typically square hollow sections (SHS), or rectangular hollow sections (RHS).

Depending on other factors a different structural form may be adopted for canopies compared to the rigid portal frame used for sheds. This alternative structural form comprises of a perimeter frame mounted a top the columns to which the rafters are then simply connected. That is the rafters are not connected rigidly to the perimeter frame, the rafters can rotate at the ends, and typically would need to be larger than if part of a portal frame.

This alternative structural form is common for timber framed canopies, it is also common for cold-formed steel framed canopies when the spacing of the rafters is less than the spacing of the columns.