Cold-formed Steel Roof Purlins

How to Take This Module

To take this module read the technical article below and click through to a multiple-choice questionnaire, once taken you will receive your results and if you successfully pass you will be issued automatically with a certificate to print for your record.

Introduction

Light-gauge cold-formed steel roof purlins were first introduced to the UK construction industry in the fifties, to replace heavier hot-rolled steel angle and channel sections.

A purlin is a secondary structural member that spans between the primary/main steel frame to support the roof cladding. The purlin transfers load from the roof cladding to the primary steel rafters. Similarly, a side rail is a secondary structural member that spans between the primary/main steel frame to support the wall cladding and transfer load from the wall cladding to the primary steel columns. Both purlins and side rails are used to provide restraint to the primary steel frame members. Light gauge cold-formed steel roof purlins are used on a wide range of building types including retail and leisure, industrial, warehouses and distribution, healthcare and education.

This CPD will examine the benefits of cold-formed steel roof purlins, common applications and key design considerations.

Manufacture of Cold-formed Steel Roof Purlins

Erection of purlins on a distribution warehouse in Doncaster

Cold-formed steel roof purlins and side rails are manufactured from pre-galvanised steel strip, in the form of coils, which have been slit to the desired width from the parent or “wide coil” supplied from the mills.

The purlin is formed gradually by feeding the strip continuously through successive pairs of rolls, which act as male and female dies. Each pair of rolls progressively forms the strip until the finished profile is produced. For example, five to eight “stages” or “passes” will be needed to form a lipped Z purlin, depending on the thickness of the material. More complicated profiles will require 15 or more passes.

The strip is pre-pierced and cut to length in-line, prior to roll-forming.

The principal advantage of cold roll-forming, compared with other methods of fabrication, is the high production capacity achieved. Sections produced by cold roll-forming are essentially uniform in cross section and can be manufactured to very close dimensional tolerances.

In building uses, the most common and simplest of profiles are the lipped Z and C sections – both defined by a central vertical web joined to horizontal flanges at either end. Additional lips are included because unstiffened, wide, thin plates are unable to resist significant compression and the flanges become inefficient.

Z sections are usually installed with the top flange pointing up the roof slope so that the load is applied through the shear centre and serious twisting of the sections does not occur.

Cold-rolled steel purlins are manufactured from hot-dipped galvanised steel to the European standard BS EN 10346:2009, generally with a yield strength of 450N/mm² and with a Z275 coating containing 275g/m² of zinc.

Key Features of Purlin and Side Rail Systems

Lipped Z and C section purlins

Sleeves are generally used in purlin and rail systems at the rafter positions to optimise the use of the steel. Sleeves are short lengths of section placed over joints in the purlins, ensuring the system is continuous and therefore structurally more efficient than single-span, non-sleeved purlins.

Single-span sleeved systems are preferable over double-span sleeved systems to avoid the transportation of long purlin lengths of 12-16m and to facilitate easier erection and stability of the primary steel frames during construction.

The most economical purlin solution is the heavy end bay system. This is a continuous system for a roof with a minimum of five bays and rafter spacings of up to 15m. Lighter-gauge purlins are used on the inner bays, with sleeves of the same gauge material at every joint. Heavier-gauge purlins are used in the bays at either end. At the penultimate rafter, a longer sleeve is used at each joint in the same gauge material as the end bay purlins.

Heavy End Bay Sleeved System

The section sizes are optimised based on the dead, service, imposed and wind loading on the structure.

Purlins above certain lengths and loading conditions will also require sag rods. These are small rods or angles that are bolted or clipped between the purlins to restrain the bottom flange of the purlin from twisting under wind uplift conditions. On longer spans, they also provide restraint to the purlins and maintain alignment during construction and installation of the cladding.

Side Rail Systems

Coils of pre-galvanised steel strip at the Metsec purlin manufacturing facility, awaiting roll-forming

Side rails are usually designed as a continuous, two-bay, sleeved system with heavier sections used in the first two bays where the rails are subjected to larger wind suction forces. Side rail supports are used vertically between the rails to provide restraint to the inner flange of the rail and prevent rotation. The vertical loads are then usually supported by diagonal tie wires, which transfer the cladding weight to the columns.

In horizontal cladding systems the side rail supports are substituted with a vertical C section or purpose-made section at the panel joint position. These are bolted to the horizontal side rails, usually with slotted cleats so they can be aligned accurately.

Advantages of Cold-formed Steel Purlins

  • They are cost effective in comparison to heavier, hot-rolled steel angle and channel profiles
  • They have a high strength-to-weight ratio
  • Long spans of up to 15m are possible
  • Pre-galvanised sections provide longer-term durability
  • Material is delivered to site pre-punched and cut to length
  • They are lightweight and are easy to handle and construct
  • Steel is 100% recyclable.

Design Standards

Purlins being installed at the Ricoh arena in Coventry

When light-gauge, cold-formed steel roof purlins were first introduced to the UK construction industry in the fifties, all design standards were written for hot-rolled steel. These were not appropriate for light-gauge steel sections, which were susceptible to buckling, due to their relatively thin walls. Early guidance was found in the American Iron and Steel Institute specifications and this, coupled with some industry testing, was used to establish minimum strength requirements to control the buckling.

In 1975, an addendum to British standard BS449 was published for the use of cold-formed steel sections in buildings. This was superseded by BS5950 part 5 in 1987, which was specifically written for the design of cold-formed sections and introduced limit state design. Eurocode BS EN 1993-1-3 has now replaced this standard and should be used for the design of cold-formed sections.

In addition, most manufacturers carry out full-scale testing and component testing and this, together with finite element analysis, is used to determine the performance of the sections.

Design Considerations

The set out of the purlins and rails is determined from the span characteristics of the roof sheeting and side cladding under imposed and wind loads, and the required positions of restraint for the hot-rolled frame.

Roof purlin systems should be designed to take into account the following loads:

  • The dead load from the cladding
  • Service loads from lighting, sprinklers, pipework, and so on
  • Imposed loads – for example, from uniform or drifting snow on the roof – and access loads. These should be calculated from BS EN 1991-1-1 and BS EN 1991-1-3, together with the National Annex
  • Wind loads. These should be calculated to allow for the increased loads in the eaves and ridge zones, as well as other factors such as distance to sea, site orography, sheltering effects and the geometry of the building. These are calculated in accordance with BS EN 1991-1-4 and the National Annex.

For side rails, the dead load of the cladding should be considered, together with the wind pressure and suction. Once all loads have been calculated, they should be combined using the appropriate load factors from BS EN 1990 and the National Annex. The properties and capacities of the sections are determined from BS EN 1993-1-3, using effective cross sections that take account of local and distortional buckling.

CE Marking

A load of Metsec cold-formed steel purlins with CE markings

Since 1 July 2013, the Construction Products Regulation has made CE marking a legal requirement in all member states for all construction products that are permanently incorporated into building or civil engineering works and are covered by either a harmonised standard or a European Technical Assessment.

Manufacturers and suppliers are required to provide construction products with the appropriate CE mark, the declaration of performance and any other supporting information. This ensures that any specified product will be fully compliant with the regulation and that designs are carried out in accordance with the Eurocodes. The relevant harmonised standard for cold-formed steel roof purlins is BS EN 1090-1.

Metsec has been CE marking its products since February 2011 under this standard.

Design Software

In addition to load tables, manufacturers may also produce software that enables the quick and easy design of purlins and side rails. For example, Metsec’s MetSPEC EURO 1 software calculates snow to BS EN 1991-1-3 (including snow drift conditions) and wind loading to BS EN 1991-1-4, taking into account the specific site parameters and building orientation. Using the load factors and combinations from BS EN 1990, it produces fast and economic solutions for the cold-formed steel.

MetSPEC Euro
MetSPEC Euro
MetSPEC Euro

Questionnaire and Certification

To take this module read the technical article below and click through to a multiple-choice questionnaire, once taken you will receive your results and if you successfully pass you will be issued automatically with a certificate to print for your record.

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