C & Z Style Purlins

Stramit Exacta® C & Z Purlins & Girts

Stramit Exacta® C&Z Purlins come in sizes of 150, 200, 250, 300 and 350 are subtly but importantly different to standard Stramit® Z&C Purlins.
The distinction between the two types is that Stramit Exacta® C&Z Purlins have been performance optimised for the most commonly used spans. This simple change, enabled by Stramit’s state-of-the-art manufacturing equipment and high standards of quality control, can lead to capacity improvements of up to 18%. 

This Stramit Exacta® C&Z Purlins product technical supplement incorporates both design capacity tables (load tables) and design member moment capacities. The latter enables engineers to input properties and capacities into their preferred structural analysis software.With this capability it is practical to select purlin (and girt) configurations that are outside those listed in the design capacity tables.  In particular it allows the analysis of irregular purlin configurations and complex loading patterns. 

All data within this technical supplement are based on the limit state AS/NZS4600:2005 ‘Cold-formed Steel Structures’.  Computations have been made using dedicated software developed by the University of Sydney. The procedures within the supplement show all of the design checks necessary in the calculation of purlins using the unique Member Moment Capacity method..
How to Use

The procedure for using the Stramit Exacta® C&Z Purlins and Girts Product Technical Manual is largely selfexplanatory.  Please ensure that you are familiar with the assumptions and conditions by reading the manual fully.

Once these are understood the basic sequence is:

1. Establish the required inward and outward loads, normally using AS1170.
2.Turn to the design capacity tables for the preferred span configuration
3. Select a suitable purlin from the outward design capacity page (always the  left-hand opening)
4. Check for inward capacity and deflection on the adjacent page at the same section, span and bridging case
5. Check that the bridging and bolt requirement is specified.

Complementary Software

Stramit® has EX-facta™ moment capacity design software available for alternative purlin span configurations. Spans can be mixed to any length and also include cantilevered ends. Even more importantly the software allows interactive repositioning of bridging positions, changing of lap lengths and mixing of section thicknesses to allow engineers to really optimise their purlin configurations.

The EX-facta™ program shows nominal section efficiencies within each span helping the user to visualise critical and over-designed sections. See pages 31 to 33 for worked example with screenshots.

Related Literature 

This document can be used in conjunction with Stramit Exacta® C&Z Purlins and Girts and Bridging detailing and installation guide which contains information on hole sizes, locations and punching, laps, expansion joints, accessories, bridging assembly options, detailing, procurement and installation for all Stramit® Purlins including Stramit Exacta® C&Z Purlins

Intellectual Property 

This document is subject to copyright. The design method utilised within this manual is protected by patent. 

Technical Support 

Stramit have Technical Service staff in each region of Australia to assist with all technical issues.  This enables Stramit to provide advice that reflects local conditions and practices.
  • High Tensile Steel - for high strength and low weight. 
  • Z350 Zinc Coating - for durable but economic protection.  
  • Available in the durable ZAM™ coating for more severe environments. 
  • 150, 200, 250, 300, 350 sizes - a comprehensive range to cater for almost all applications.
  • Full Thickness Range - with Stramit's proven record for manufacturing and supply.
  • Integrated Bridging - boltless design specifically for Stramit® sections.
  • Full range of Accessories - from brackets to bolts to ensure easy use and installation.
  • Limit State Design in accordance with AS4600 - suitable for all government projects.
  • Downturn Lip available - for projects requiring this feature.

Stramit Exacta® C&Z Purlins and Girts
are used to support roof or wall sheeting primarily in sheds and large industrial and commercial buildings. All uses of these sections require engineering design to ensure suitability.


Stramit Exacta® C&Z Purlins and Girts are manufactured from hi-tensile GALVASPAN® steel, with a minimum Z350 galvanised coating (350g/m2) conforming to AS1397 and ZAM™ coated steel. Other coatings, grades and materials may be available, subject to inquiry.  The mass and steel grade for each standard Stramit Exacta® C&Z Purlin section is shown on page 2 of  the brochure, please download.

Adverse Conditions

Stramit Exacta® C&Z Purlins and Girts will give excellent durability in most applications.  GALVASPAN® steel is not recommended for use within 450mm of moist soil.  Unwashed areas subject to salt-laden air or other corrosive matter may need additional protection.  In many cases such protection can be provided by using ZAM™ coating.

Contact between Stramit Exacta® C&Z Purlins and Girts and copper (e.g., pipework) must be avoided as premature corrosion will occur. 


Maintaining the correct specification of purlins, girts and bridging is very important. Even a small change in specification can lead to substantial reduction in performance.  A suggested specification is:
“All purlins and girts shall be Stramit Exacta® C or Z Purlins and Girts used with Stramit® Bridging or approved equivalent, supported by submission of section properties, purlin capacity calculations, and a performance warranty, produced and detailed for this project.  All sections shall be produced from (either - galvanised steel to AS1397 with a coating mass of at least 350g/m2, or ZAM™ coated steel) and designed in accordance with AS4600:2005. All sections should be installed in accordance with the manufacturer’s instructions with particular regard to bridging and bolt locations and lap sizes.”  “Where required for structural or installation purposes, Stramit® Bridging shall be installed using pre-made components to manufacturer’s instructions.  All other purlin accessories shall be supplied by Stramit.” 


All sections will be produced within the following tolerances:    
  • Section length + 0mm / -10mm
  • Section web (dim. D) +/- 1mm    
  • Section flange (dim. B, E, F) +/- 1 mm    
  • Internal flange angle +/- 1°    
  • Internal lip angles + 5° /-2°    
  • Hole centres +/- 1mm  
Downturn Lips 

All Stramit Exacta® C or Z Purlins and Girts may be available with downturn lips for special projects.  Purlins of this shape cannot normally be lapped and are usually used in single or short double span construction. However, Stramit can supply “lappable” Z’s, subject to inquiry, providing a unique section that offers strength and economy as well as downturned lip.  Please contact your local Stramit office for details on availability, minimum order quantity and delivery lead times.  

Remember to allow additional cleat length for clearance between the downturn lips and the support beam.

Stramit Exacta® C & Z Purlins & Girts

Always follow the applicable OH&S regulations for your state. 

Good Practice

Stramit recommends that good trade practice be followed when using these products, such as found in Australian Standards HB62 part 1 and HB39. 


Stramit Exacta® C&Z Purlins, Girts and Bridging
are not designed to be walked on.  Residual oil may be present on these components from manufacturing.  Do not walk on purlins or bridging. 


Always use the correct size and grade of bolts.  Ensure that all bolts are securely tightened. 


Purlin laps must be bolted in the top web hole and the lower flange holes at both ends of the lap as shown below.  Bolting only in the web of lapped purlins does not provide full structural continuity and excessive loads could be placed on to roofing screws that penetrate both purlins within a lapped region. 

Endlapping with Standard Purlins

Stramit Exacta® C&Z Purlins
have different web and flange sizes to standard purlins and will therefore not end lap. 

Fly Bracing

If the lower web hole in a lap is used for attaching fly bracing ensure that an additional bolt is used.


Stramit® Bridging
can be installed either up or down the roof slope, but cannot be mixed within a bridging run.  However, as the starting and finishing components will be different, the direction of fixing must be established at the design/procurement phase. 

Girt bridging must not exceed its compressive capacity.  Where more than one row is to be installed always complete the bridging for each girt before commencing on the next (i.e. do not complete one row of bridging before starting the next). 


Stramit does not recommend the welding or hot cutting of purlins, girts or bridging.The heat produced in welding will affect the material properties of the high-yield strength cold-formed steel used by Stramit in its purlins. In many instances considerable stress concentrations are likely to arise, even with good quality welding. In addition, welding will locally remove the protective coating, leading to a potential eduction in durability.

Further Information

As well as our standard range of Technical Manuals,Installation Leaflets, Case Studies and other promotional literature Stramit has a series of Guides to aid design. These include:
• Concealed Fixed Decking
• Roof Slope Guide
• Foot Traffic Guide
• Roof System Selection Guide
• Bullnosing, Curving and Crimping
• Acoustic Panels
• Cyclonic Areas
• Spring Curving Guide
Please contact your nearest Stramit Building Products location for any of these guides, or other literature. 

Other Products

Stramit offers a wide range of building products, including:

        • Formwork decking
        • Roof and wall sheeting
        • Lightweight structural sections
        • Truss components
        • Gutters and downpipes
        • Fascias
        • Custom flashings
        • Insulating products
        • Fasteners 

Girt Bridging Capacity

Stramit® Bridging is only designed to allow purlins and girts to resist wind loads once the sheeting has been attached. Purlins, girts and bridging should not be subjected to loading from stacked materials, even when sheeting is attached, or from lifting assemblies of framing.  Bridging used with girts may be subjected to compressive (or tensile) loading due to gravity during installation.  These loads become cumulative with increasing wall height, unless a separation joint is included.T he capacity of Stramit® Boltless Bridging, Stramit® Bolted Bridging and Stramit® Large Series Bridging to resist these loads is given in the Stramit® Purlins, Girts and Bridging Technical Manual. These capacities are based on the mass of the girts plus a 1.1kN load to allow for riggers. 

WARNING – girt bridging capacity is usually limited by bridging end component strength rather than channel compression. Stramit® Bridging ends have been designed through testing to have a capacity substantially greater than most other bridging types. Accordingly this data must only be used for Stramit® Bridging.

EX-facta™ Design Software

Stramit Building Products offers highly interactive EX-facta™ design software for use by engineers.The key features of EX-facta™ software are:-

• fully variable span dimensions
• variable lap lengths
• visual display of span efficiencies
• up to 10 span continuous systems
• multiple combined load cases
• variable UDL within spans
• concentrated/point loads
• variable bridging positions
• mixed bridging numbers
• cantilevers
• any combinations of the above

For a copy of the EX-facta™ software program or for more information please contact your regional Stramit Technical Services Manager.

This product was added to our catalog on Tuesday 21 July, 2009.

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