Purlin and Girts Size Calculator to AISI

Verified by the CalcTree engineering team on August 1, 2024
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Welcome to CalcTree's Purlin and girt design tool! You will be able to determine the member capacities for purlins and girts according to American Iron and Steel Institute (AISI) S100-16. The purlins are manufactured by AEP. For details on how to design purlins and girts, check out our design guide here.
🐍This page is powered by the sectionproperties python library.🐍  
All calculations are performed in accordance with AISI S100-16.
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Typical purlin and girt steel sections
CalculationAssumptions
This calculation only includes section sizes included in AEP's catalogue.  AEP provides a very extensive selection of section sizes. Other manufacturers may provide sections with different dimensions, in which case you can choose a section that is most similar. 
Inputs and outputs are all in imperial units.
This calculation is based on assumptions stated by the manufacturer. See the link in References. 
Webs do not have transverse stiffeners
Use one of the following pages according to the purlin geometry. 
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Cee section shape
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Zed section shape
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ExplanationFor details of how to design purlins and girts, check out our design guide here. 
Here is a brief summary to get you up to speed:
Purlins and girts are structural members used to support loads from sheathing and cladding in roofs and walls, respectively. 
The designer must consider various loads such as dead, live, snow and wind loads. Dead loads include the self-weight of the purlin or girt and superimposed dead loads, such as cladding, tanking, insulation, plaster, finishes, and services. 
Purlins and girts can be made from timber or steel, with steel sections being either C or Z-shaped and are usually cold-formed. 
The maximum distance between steel purlins and girts should be around 1200mm, with typical distances in residential buildings being about 600 or 800mm. 
The initial depth of a purlin or girt can be estimated by the span/32 rule of thumb. 
C-section purlins and girts are typically used in simple, non-continuous spans, while Z-sections are typically used in continuous lines of purlins for easy lapping at internal supports. 
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Bonus info
Here is a handy hint about how to orientate your Z-purlins on a sloped roof: When orientating purlins, remember “ducks always go uphill”!
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Object orientation on a sloping roof
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Orientation of Z-purlins on A sloped roof
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That is to say, when installing your purlins and when inspecting them on site, the 'beak' of the Z-shape should always face up the roof slope. Check this out in our 'Steel design rules of thumb' article for more hints
ReferencesManufacturer's table: AEP
AISI S100-16
Related ResourcesSteel Beam and Column Designer to AISC
Steel Beam and Column Designer to AS4100
Steel Design Rules of Thumb for Structural Engineers
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