How to Specify a Sigma Profile (Complete Structural Guide)

Cold-formed structural section

Complete Engineering & Procurement Guide

A Sigma profile is a:

Cold-formed structural section
Advanced purlin alternative
High-strength, high-efficiency member

It is commonly used in:

Long-span industrial buildings
Heavy roof structures
Mezzanines
Solar mounting frames
Structural steel systems

Sigma sections provide greater bending resistance than standard Z or C profiles due to their complex flange geometry.

1️⃣ What Defines a Sigma Profile?

A Sigma section includes:

✔ Web depth
✔ Primary flange
✔ Secondary return flange
✔ Stiffening lips
✔ Offset geometry
✔ Thickness
✔ Steel grade
✔ Punch pattern

The cross-section resembles the Greek letter “Σ” (Sigma), hence the name.

Because of its multiple bends, full dimensional specification is mandatory.

2️⃣ Standard Section Sizes

Common web depths:

150 mm
200 mm
250 mm
300 mm
350 mm
400 mm

Flange widths vary significantly:

60–100 mm common

Lip sizes:

15–30 mm

Sigma profiles typically have:

More bends than Z or C.

More bends = more forming load.

Never specify only “Sigma 250” — provide full geometry.

3️⃣ Thickness Range

Sigma sections are often heavier than standard purlins.

Typical thickness:

1.8 mm
2.0 mm
2.5 mm
3.0 mm
3.5 mm
4.0 mm

Heavier structural versions may exceed 4 mm.

Machine must be designed for:

Maximum thickness + maximum yield strength.

4️⃣ Material Grade

Common grades:

G350
G450
G550

Sigma profiles frequently use G550 to maximize strength-to-weight ratio.

Higher grade increases:

Forming force
Springback
Roll wear
Punch tonnage

Grade must be declared before tooling design.

5️⃣ Coating Specification

Common coatings:

Z275
Z350
Z450

Industrial or coastal zones may require heavier coating.

Coating affects:

Roll surface wear
Punch wear
Corrosion life

Always specify coating mass.

6️⃣ Typical Coil Width

Coil width is calculated from:

Web + multiple flange elements + lips + bend allowance.

Example (simplified):

Web 250 mm
Primary flange 75 mm ×2
Secondary return 25 mm ×2

250 + 150 + 50 = 450 mm
Add bend allowance → approx. 470–500 mm

Exact developed width must include:

Bend radii
Thickness compensation
Springback correction

Sigma sections have more bends, so developed width calculation is critical.

7️⃣ Structural Load Requirements

Before selecting Sigma size, define:

✔ Span length
✔ Wind load
✔ Snow load
✔ Deflection limit
✔ Load combination

Sigma is often chosen when:

Z purlin cannot meet deflection limits.

Section size must be structurally calculated.

8️⃣ Punch Pattern Specification

Sigma profiles often require:

✔ Bolt holes
✔ Cleat holes
✔ Service holes
✔ Slotted holes

Define:

Hole diameter
Hole spacing
Edge distance
Tolerance

Because Sigma has multiple flanges, punch positioning must be very precise.

Punch misalignment causes installation failure.

9️⃣ Length Specification

Common lengths:

6 m
9 m
12 m
Custom

Length tolerance typically:

±2 mm

Long sections require straightness control.

Sigma twist must be minimized.

🔟 Machine Engineering Requirements

Sigma profile machines are heavier than C/Z lines.

Typical configuration:

18–26 forming stands
90–110 mm shafts
37–75 kW motor
Heavy-duty frame
Servo punching system
Hydraulic or flying cut

Because Sigma has more bends:

Forming load is significantly higher.

Frame rigidity is critical.

1️⃣1️⃣ Production Speed

Typical speeds:

10–20 m/min

Thicker and higher grade reduce speed.

Punch cycle often limits production rate.

1️⃣2️⃣ Tolerance Requirements

Typical tolerances:

Web depth ±1–2 mm
Flange width ±1 mm
Twist control critical
Straightness tolerance defined
Length ±2 mm

Sigma asymmetry increases risk of twist.

Roll alignment must be precise.

1️⃣3️⃣ Sigma vs Z Comparison

Feature	Sigma	Z
Bending Strength	Higher	Moderate
Section Complexity	High	Moderate
Span Capability	Longer	Medium
Machine Complexity	Higher	Lower

Sigma provides improved structural performance but requires stronger equipment.

1️⃣4️⃣ Export Market Considerations

Sigma sections are common in:

Middle East industrial buildings
European long-span structures
Solar mounting systems

Always confirm local structural code:

EN
AS
ASTM

Section geometry must align with structural engineer calculations.

1️⃣5️⃣ Common Specification Mistakes

❌ Not defining full geometry
❌ Not specifying thickness
❌ Ignoring steel grade
❌ Underestimating coil width
❌ Not controlling twist
❌ Selecting Sigma without structural calculation

Sigma mistakes are expensive due to tooling complexity.

1️⃣6️⃣ Developed Width Reminder

Developed width must include:

✔ Web
✔ Primary flanges
✔ Secondary returns
✔ Lips
✔ Bend allowance
✔ Thickness compensation
✔ Springback correction

Multiple bends amplify calculation error risk.

Never approximate.

1️⃣7️⃣ Frame Rigidity Importance

Sigma forming generates:

Higher torque
Higher roll separating force

Machine frame must resist deflection.

Insufficient rigidity causes:

Dimension drift
Twist
Punch misalignment

Machine engineering must match structural requirement.

1️⃣8️⃣ Final Sigma Specification Checklist

Before tooling or machine approval:

✔ Confirm web depth
✔ Confirm flange widths
✔ Confirm lip size
✔ Confirm thickness range
✔ Confirm steel grade
✔ Confirm coating
✔ Calculate developed width
✔ Confirm coil availability
✔ Define punch layout
✔ Define length tolerance
✔ Confirm structural load requirement
✔ Confirm production speed target

Only then proceed.

FAQ Section

Why use Sigma instead of Z?

Higher structural efficiency and longer span capability.

Is G550 common for Sigma?

Yes — often preferred for strength.

Is Sigma harder to form?

Yes — more bends increase forming load.

Does machine need to be stronger?

Yes — heavier shafts and frame required.

Is twist a major issue?

Yes — asymmetric geometry increases twist risk.

Can one machine run Sigma and Z?

Possible with adjustable tooling, but machine must be designed for Sigma load.