Spring Curving
Spring curving, also known as draping or arching of roofs, is a method of providing continuous lengths of roof cladding over a curved roof structure without pre-curving the sheets. It is best suited to corrugated profiles, or symmetrical roofing profiles of low rib height and narrow pan width, which can follow a curve without excessive panning or distortion. Asymmetrical rib products have greater minimum radii, which are limited by their pan width and rib height.
Because symmetrical profiles do not have a large rain-water carrying capacity they are limited in maximum radius and length. Maximum radius is limited by the need to provide adequate drainage at the top of the curvature and minimum radius is limited by the need to avoid distortion without pre-forming.
Continuity over a minimum of three purlins is required for successful spring curving and therefore any interruption such as a penetration may affect the ability of the sheet to drape curve.
Purlins must be accurately positioned with the top faces tangential to the radius of the arch and should be within a 5 mm tolerance to avoid purlin creasing. Roof traffic should be restricted to avoid damage, particularly in the low pitched region or in highly visible areas, particularly in the low-pitched region or in highly visible areas. Some purlin creasing or canning is to be expected with stronger profiles at minimum radii.
For convex roofs, the minimum radii should be adhered to because the pans are in compression, whereas with concave roofs the pans are in tension and the panning or distortion of these roofs will be less.
Typical Radius for Spring Curved Asymmetrical Profiles
The tables above for recommended radii assume the cladding is draped over an arc where the base chord is parallel to the ground. When the base chord is on an incline, the maximum radius can be increased.
The two top purlins should be spaced to enable the sheeting to follow an arc that minimises purlin marking.
Draped curved roofs or curved ridges should be fixed by fastening each sheet first to one side of the roof and then pulling it down to be fixed on the other side. Where sheets are end-lapped, alternate sheets should be laid in sequence to avoid cumulative errors and be laid from opposite sides of the roof to ensure squareness is maintained.
Because extra uplift load will be imposed on the end fasteners of convex spring curved roofs, through the torsional action of the sheets on the fasteners, screws and load spreading washers should be used on the penultimate and the last purlins. This torsional uplift must also be considered when designing purlin to rafter connections and other structures below.
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