C and Z Purlins in Steel Buildings

1. Definition and Role

C and Z purlins are cold-formed, open-section steel members used as secondary structural framing in metal building systems. Purlins span between primary frames (portal columns and rafters) and support the roof cladding; girts perform the same function on walls, spanning between columns and supporting wall cladding. Both C and Z profiles are manufactured by roll forming from galvanized or zinc-aluminum coated steel strip, and they constitute the most widely used secondary framing system in industrial, commercial, and agricultural steel buildings worldwide.

The choice between C and Z profiles depends on roof slope, span requirements, connection details, and regional building practice. Z purlins with lapped connections dominate in Australia, New Zealand, and parts of Asia for their structural continuity over multiple spans. C purlins are common in North America and Europe, often used with sleeve connections or as single-span members on steeper roof pitches.

2. C vs Z Profile Geometry

Both C and Z purlins are cold-formed sections with a web, two flanges, and two lips (returns) at the flange ends. The lips provide local buckling restraint to the flanges and increase the section's torsional stiffness. The critical geometric difference is the orientation of the flanges relative to the web centerline.

2.1 Cross-Section Comparison

2.2 Standard Profile Dimensions (Typical Manufacturer Range)

Designation convention: letter (C or Z) + depth in mm + thickness code (thickness × 100, e.g., 15 = 1.5 mm). Exact dimensions vary between manufacturers; the table above reflects common Australian and Asian manufacturer catalogs.

3. Lapped Z Purlin Systems

Lapped Z purlin systems are the dominant roof framing method in Australia and New Zealand. In a lapped system, consecutive Z purlins overlap at rafter support points by 300–500 mm (depending on span and load), with the top purlin sitting inside the bottom purlin at the lap. Bolts through both purlins and the rafter flange connect the assembly. The lap creates structural continuity, allowing the purlin to act as a continuous beam over multiple spans rather than a simply supported member.

3.1 Lap Configuration Parameters

3.2 Continuous vs Simply Supported Capacity

A Z25015 (250 mm deep, 1.5 mm thick) purlin at 1,500 mm spacing under wind category N3 may achieve a simply supported span of approximately 4.0 m. The same profile in a 3-span continuous lapped arrangement can span 9.0–10.5 m total (3 × 3.0–3.5 m spans), representing a 50–75% increase in total coverage per purlin run. This efficiency is the primary reason lapped Z systems dominate large-span industrial buildings in Australasia.

4. Depth and Thickness Ranges

Purlin depth and thickness selection depends on the design span, purlin spacing, roof load (cladding self-weight, imposed load, wind uplift/suction), and steel grade. Deeper and thicker profiles provide higher section modulus and moment capacity but increase material cost and building weight.

4.1 Selection Matrix by Span and Spacing

4.2 Steel Grade Properties for Purlins

5. Wall Girts

Wall girts are horizontally installed C or Z sections that span between building columns and support wall cladding panels. Girts perform the same structural function as purlins but on vertical building surfaces, resisting wind pressure and suction on the wall cladding and transferring loads to the primary column frame.

5.1 Girt vs Purlin Design Differences

5.2 Girt Installation Heights

Girt spacing is determined by the spanning capability of the selected wall cladding profile and the requirement to align with door and window openings. A typical industrial building with 0.42 mm trapezoidal wall cladding on Z20015 girts at 1,800 mm spacing accommodates standard roller door heights (3.0–3.6 m) with a girt at 1,800 mm and 3,600 mm levels. Taller buildings may use 2,400 mm girt spacing with deeper cladding profiles.

6. Structural Design Standards

Structural design of C and Z purlins requires calculation of section properties, member capacity under combined bending and axial load, connection strength, and deflection limits. The governing standards vary by region but share common methodology based on effective width concepts for thin-walled sections.

6.1 Design Standard Reference

6.2 Key Design Checks

AS/NZS 4600 and AISI S100 use the Direct Strength Method (DSM) as an alternative to the Effective Width Method for checking local and distortional buckling. Most purlin manufacturer design software (such as Lysaght Pro, Metroll Design, and Stramit Design) implements these standards and produces span tables validated by physical testing.

7. Bridging, Cleats, and Connections

Purlins and girts require lateral restraint to prevent lateral-torsional buckling under bending. Bridging (also called fly bracing or sag rods) connects adjacent purlins at intervals along the building length, creating a lateral restraint system. Cleats and brackets connect purlins to rafters and girts to columns.

7.1 Bridging Systems

7.2 Cleat and Bracket Types

Adjustable cleats allow the purlin height to be set to match the roof pitch during installation. The cleat consists of a fixed angle bolted to the rafter and a sliding angle bolted to the purlin web, with slotted holes providing ±30 mm vertical adjustment. This adjustment accommodates frame fabrication tolerances and ensures the purlin top flange aligns with the roof slope for proper cladding bearing.

8. Roll Forming Production

C and Z purlins are produced on heavy-duty roll forming lines capable of processing 1.2–3.0 mm galvanized or AZ-coated steel strip at line speeds of 15–25 m/min. Purlin lines include hole punching stations for bolt holes and bridging connections, and flying saws for cut-to-length production.

8.1 Purlin Roll Forming Line Configuration

8.2 Roll Forming Parameters for Structural Purlins

High-strength G550 steel (550 MPa yield) requires heavier roll forming equipment and more forming passes than G450 material. The increased springback in G550 means roll tooling must be designed with greater over-bend compensation, and roll shaft deflection must be minimized to maintain profile symmetry. Most purlin manufacturers use dedicated roll sets for each profile size and steel grade combination rather than attempting universal tooling.

9. Application Scenarios and Technical Requirements

Purlin and girt specification for any project should be verified using manufacturer span tables generated in accordance with the applicable structural design standard. The tables above provide indicative configurations; actual selection depends on site wind classification, roof cladding weight, imposed loads, deflection limits, and connection details specific to the primary frame system.