PITCHED ROOFThe design and construction of skylight architectural structures requires meticulous planning. Structural and support elements, building details, drainage systems, decorative fittings and safety elements throughout the structure must all be carefully considered. In these pages, you will find practical information that will help you successfully plan roof constructions using Polygal structured sheets.
Terminology and Definitions:
- Sheet Length - the dimension parallel to the ribs of the sheet
- Sheet Width - the dimension perpendicular to the ribs of the sheet
- Deflection - the distance at the midpoint of the sheet area, between a state of no load and after load is applied.
- Rafters - Beams in a construction, which support the purlins and in many cases support directly the sheets along their sides (parallel to their ribs).
- Purlins - Beams in a construction, which support the sheets along their width (perpendicular to their ribs).
- Distance between Rafters or sheets supported- the distance measured from the center of one rafter or the sheet support to the adjacent one.
- Distance between Purlins - the distance measured from the center of one purlin to the adjacent one.
Installation of Polygal Sheets Onto a Structure
In architectural applications, Polygal sheets are normally installed within aluminum or Polycarbonate profiles, which are supported along their length and width by the Rafters and Purlins of the construction. In greenhouse applications, the sheets are connected directly to the greenhouse construction using screws.
Polygal sheets are asymmetrical in their geometry of length and width, and as such, their performance under loads differs according to changes in these dimensions.
How Changes in Distance Between Purlins Influence Sheet Resistance to Bending
If, for example, we take a square sheet (equal in length and width), which is supported at all four sides by a steel frame, and apply a load of a certain type, this will cause the sheet to deflect. Now, let's keep those same connection parameters and increase the length of the sheet, then apply load to get the same deflection as before. We'll find that, up to a specific, asymptotic sheet length, the load required to reach the same deflection will gradually decrease. Once this asymptotic sheet length is exceeded, no changes in the load level will be required to reach the original deflection level. Thus, we can conclude that, when exceeding a certain sheet length, the purlins that support the sheets along their width do not contribute to the sheet's resistance to bending, and as such, are unnecessary. Under these circumstances, we can say that the sheets perform as if they were connected to the construction in a Side/Side Configuration.
How Changes in Distance Between Rafter or Sheet Support Influence Sheet Resistance to Bending
If we increase the width of the sheet (which is connected to the construction on all four sides), the result will be that the load required to reach the same deflection will decrease incrementally as the sheet width increases. After a certain, critical width, the load will not need to change at all in order to reach the original deflection level, and the rafters along the length of the sheets will not contribute to the sheet's support.
Bending Deformation and Failure Mechanisms
If we take a sheet connected in any configuration and apply extreme load, the sheet will either buckle and be permanently damaged, or else it will pop out from inside the profiles.
Elastic and Plastic Bending Buckling
When a sheet which is attached within a frame is subjected to a heavy load, it can develop a wave-like deformation along its widhtwise axis and the sheet walls will buckle. This is called Elastic Buckling because once the load is removed, the sheet will return to its original condition without incurring any permanent damage.
If we continue to increase the load level, the sheet will deflect until it finally buckles (Plastic Buckling) causing permanent damage to the sheet. The deflection point at which a sheet buckles is referred to as the Buckling Deflection, and is a function of the geometric structure of the sheet and the amount of material it comprises. Under Long-term load, the sheet may initially deflect to a depth that doesn't cause the sheet to buckle (either Elastic or Plastic), but under constant load, the creep effect can cause an increase in deflection over time, which can eventually lead to Buckling Failure.
The deflection at which the sheet pops out from inside the profile is referred to as the Popout Deflection value. Popout failure depends solely on the shortest dimension of the sheet (in most cases, this is the width of the sheet) and the glazing depth of the sheet in the profile (for further explanations, see Sheet Connection Within Profiles and Desired Glazing Depth under Preferences). Under long-term load (i.e. snow), the sheet may initially deflect to a depth that doesn't cause the sheet to pop out of the profile, but under constant load, a creep effect can result in an increase in deflection over time, which can eventually reach Popout Failure
Elastic Buckling "Bubbles"
In some sheets, "bubbles" are formed when the lower wall of the sheet is compressed under loads or a too small cold bent radius. These bubbles disappear when the load is removed or when the cold bending is released and leave no mark on the sheets. This phenomenon becomes an aesthetic problem when the sheet functions as a skylight or other arched application. This problem should not occur if the minimum radius limits are maintained.