Cold-formed and stainless steel (purlins, girts, studs and stainless members): how they carry load, where they fail, and when to choose them

The reference structure

What it is

Euro Inox (European industry guidance, not a UAE rule) gives the definition of stainless steel in EN 10088-1: a minimum chromium content of 10.5% by weight and a maximum of 1.2% carbon.⁴

How it carries load

The JRC's Eurocode background book (European teaching guidance, not a UAE rule) describes the same system. Purlins and side rails act as secondary beams that the building envelope often restrains, and because Z and C sections can be sensitive to lateral-torsional buckling, the book says it is important to take advantage of the sheeting where it is effective. The sheeting is generally assumed to carry the load component in its own plane, while the purlin resists the component normal to it; the sheeting may also hold the purlin sideways and, where its flexural stiffness is substantial, as for trapezoidal sheeting, and it is properly connected, partly restrain its twisting. Under EN 1993-1-3, where trapezoidal sheeting is connected to the purlin and the shear stiffness it provides meets a condition set by the purlin's warping, torsional and minor-axis stiffness, its span and its height, the purlin may be regarded as laterally restrained at the connection; if the sheeting is connected to the purlin at every second rib only, that shear stiffness S is replaced by 0.20 S.⁷⁸

To evaluate that restraint, EN 1993-1-3 as the book describes it treats the free flange, the one not fixed to the sheeting, as a beam on an elastic foundation; the book's figure shows a single span under uplift. A lateral spring stands for the restraint, and its stiffness combines three things: the rotational stiffness of the connection between sheet and purlin, the distortion of the purlin's cross-section, and the bending stiffness of the sheeting across the purlin's span. That is a design model of EN 1993-1-3, which this library does not hold, any more than AISI S100, so no stiffness, spring value or check is stated here; the restraint a real roof gives depends on its sheeting, its fixings and the purlin maker's tested arrangement.⁸

The book also shows that where the restraint acts matters (European guidance, not a UAE rule). For one simply supported Z purlin in pure bending, 202 mm deep with 75 mm flanges, 20 mm lips and 2.3 mm thick (after Martin and Purkiss, 2008), restraint at the junction of the web and the compression flange changes its buckling a great deal: holding that flange against sideways movement stops lateral-torsional buckling, and holding it against rotation raises the critical stresses of local and distortional buckling significantly. Applied at the junction of the web and the tension flange, only the rotational part of the restraint influences lateral-torsional buckling. Those curves belong to that section and loading only. EN 1993-1-3 represents the partial torsional restraint by a rotational spring that combines the stiffness of the sheet-to-purlin connection with the flexural stiffness of the sheeting, values the standard gives in its Section 10.1.5 or that can be found by test under its Annex A5; none is given here. The book notes that the same restraint model can also be used for other claddings, such as a sandwich panel on a purlin.⁹¹⁰

AISC 360-16 recommends the AISI North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100) for cold-formed members, except cold-formed hollow structural sections, which it designs itself.² Its approved materials include sheet to ASTM A606 and A1011,³ and for the members it does design it classifies sections for local buckling by the width-to-thickness ratios of their compression elements.¹¹ Slender-element compression members are designed on an effective area built from reduced effective widths, with the strength the lowest of flexural, torsional and flexural-torsional buckling interacting with local buckling.¹³

For stainless, Euro Inox explains (European industry guidance) that corrosion resistance comes from a passive, chromium-rich oxide film that forms naturally and re-forms after scratching where there is enough oxygen, and that in small oxygen-starved areas, such as mechanical joints, tight corners and incomplete or poorly finished welds, the surface can become active and suffer crevice or pitting attack.⁴

Where and how it fails

For light-gauge open sections, a user note in AISC 360 recommends AISI S100 for cold-formed members other than hollow sections,² and AISI S100 is not held, so no rule for the restraint a purlin needs under uplift is stated here.

The JRC book explains why these sections twist so readily (European guidance, not a UAE rule). They are normally thin, so their torsional stiffness is low, and many of the shapes produced by cold-forming are mono-symmetric, with the shear centre away from the centroid; a load applied off the shear centre generally twists a thin-walled beam considerably. Such beams usually need torsional restraint at intervals or continuously along their length, and Z and C purlins that are not properly braced may buckle flexural-torsionally because of their low torsional stiffness.¹⁴

AISC 360 classifies a section as slender-element where any compression element exceeds its limiting width-to-thickness ratio,¹¹ and designs slender-element compression members on a reduced effective area, the effective width of each slender element depending on its slenderness and the member’s buckling stress.¹³

The JRC book classes the buckling of cold-formed members as local, distortional, global (flexural, flexural-torsional or lateral-torsional) and shear buckling (European guidance, not a UAE rule). Local buckling of the individual flat plate elements has a relatively short wavelength; distortional buckling, by contrast, in which the fold lines move relative to each other, has a wavelength generally between those of local and global buckling. The two sectional modes can interact with each other and with global buckling, and for a given cross-section which mode appears depends on the buckling length. Edge or intermediate stiffeners can improve the load-carrying capacity and buckling strength of thin compression walls considerably.¹⁵¹⁴

The book illustrates this with a finite-strip analysis of one lipped channel in compression, after Hancock, plotting its buckling strength against the buckle half-wavelength (a worked illustration, not a design value). The local mode has its lowest buckling stress at a half-wavelength of 65 mm and the distortional mode at 280 mm; the distortional stress is slightly higher, so a long, fully braced length of that section is likely to buckle locally rather than distortionally. At long wavelengths it buckles flexural-torsionally, up to half-wavelengths of about 1800 mm, and flexurally beyond. Those lengths belong to that section only. Interaction between the sectional and global modes makes the member more sensitive to imperfections and erodes its theoretical buckling strength, and the book notes that, because imperfections are always present, some mode interaction always occurs in thin-walled members.¹⁵¹⁶

This library holds no rule for distortional buckling of cold-formed sections: AISC 360 recommends AISI S100 for those members,² and AISI S100 is not held.

For wide-flange sections and similar built-up shapes, AISC 360 checks concentrated forces for web local yielding and web local crippling, with separate expressions for a force near the member end, and calls for stiffeners or doublers where the required strength exceeds the available strength.¹⁷ Section J10 applies to those shapes only, leaving guidance for other members to its Commentary,¹⁸ so it is not a check for a cold-formed purlin web.

The JRC book gives the reasons web crippling at concentrated loads and supports can be a critical problem for cold-formed members and sheeting (European guidance, not a UAE rule): load-bearing and end-bearing stiffeners are often impractical, which is always so for continuous sheeting and decking over several supports; their webs usually have larger depth-to-thickness ratios than hot-rolled members and are often inclined; and the load reaches the web through a bend of finite radius, so it is applied eccentrically from the web. Design codes include special provisions against it; those provisions are in AISI S100 and EN 1993-1-3, neither of which this library holds.¹⁴

AISC 360 Section J3 is written for bolts and threaded parts, with bolt strength from the nominal stress and the unthreaded body area.¹⁹ Screwed connections in thin sheet are not covered by the held documents; the screw manufacturer’s data and a cold-formed design specification would govern, and neither is held.

For hot-rolled material AISC 360 checks bearing and tearout at each bolt hole,²⁰ and block shear rupture along a shear path with a perpendicular tension path.²¹ The equivalent rules for thin sheet are in the cold-formed specification, which is not held.

IMOA (international industry guidance, not a UAE rule) explains that the galvanic corrosion rate rises with the ratio of the noble metal’s surface area to the active metal’s, gives mild steel rivets in stainless sheet as an undesirable example, and recommends avoiding or insulating dissimilar metals, choosing metals close together in the galvanic series, or making the metal likely to corrode much larger in area, and, where a couple is unavoidable, coating the noble metal near the joint rather than the active one.⁶ It warns that austenitic fasteners in the annealed condition gall readily and favours fasteners that are cold worked, of higher strength or of a grade that resists galling, and it notes that the crevice in a mechanical joint can corrode in chloride media.⁵ Euro Inox recommends a fastener grade at least equal to the adjoining stainless in moist or humid connections.²²

AISC 360 requires structural components to be designed to tolerate corrosion, or to be protected against it, wherever corrosion could impair strength or serviceability.²³ Part F requires the steelwork material specification to state steel grades and protective paint systems that are durable in the Dubai climate, specified to BS EN 12944 as a minimum with added requirements for ultraviolet degradation, with inspection and maintenance requirements and the expected coating life.²⁴

Advantages and limitations

Why this structure, and not another

Euro Inox (European industry guidance, not a UAE rule) describes austenitic grade 1.4401 (316), with its molybdenum, as appropriate for coastal or heavy industrial exposed sites and grade 1.4301 (304) for less demanding exterior environments, ferritic grades as better suited to internal or cosmetic uses, and duplex grades as increasingly specified for structural members.²²

For the experienced engineer

Hollow sections are the cold-formed members AISC 360 does design.² For hollow sections made to approved standards other than ASTM A1065 and A1085, which includes A500,³ the design wall thickness is 0.93 times the nominal thickness.²⁷

Source and applicability: Dubai Building Code 2021, Tables F.5 and F.6 (A001 PDF page 356). The Code's tabulated yield values for cold-formed hollow sections are not stated in this release, because the reading the Authority applies to them has not been confirmed.

Welding thin and stainless steel: AWS D1.1:2020 was developed for carbon and low-alloy steels 3 mm or thicker with a specified minimum yield strength of 690 MPa or less, with the Engineer to judge its suitability outside that range, and its committee points to AWS D1.3 for sheet steel up to 5 mm thick and AWS D1.6 for stainless steel;²⁸ neither is held.

Forming stainless: Euro Inox (European industry guidance, not a UAE rule) notes that austenitic grades work harden, so bending them takes about 50% more power than carbon steel of similar thickness and parts must be overbent by about 5° to allow for springback, that drills should be sharp and run at the right speed and feed to avoid bluing or work hardening, and that tools used on stainless should be kept for stainless only.²²

Heat tint and contamination (industry guidance, not UAE rules): Euro Inox explains that welding heat tint leaves a surface layer with less chromium than the bulk steel, so visible heat tint reduces the corrosion resistance of the surface, and removing all of it is good practice.²⁹ IMOA advises keeping carbon steel away during fabrication and handling, with dedicated tools and clean abrasives, and says steel shot must not be used on stainless steel, because embedded iron particles rust on the surface;³⁰ its table links cold forming and machining to stress corrosion cracking and mechanical joining to crevice and galvanic corrosion, the actual effect depending on many factors.⁶

Thermal movement: for steel members and members exposed to solar radiation, Part F takes a range equal to the maximum temperature swing as appropriate, allows a ±25 °C default only for protected steel, and leaves the Engineer to assess the thermally induced forces case by case.³¹

IMOA (international industry guidance) tabulates mean expansion coefficients from 20 to 100 °C of 16.4 × 10⁻⁶ per K for Type 304, 13.0 for duplex 2205 and 12.1 for carbon steel.³²

Which rules apply in Dubai

Part F designs structural steel to AISC 360 and the standards it references, with ASTM materials, AWS welding and the steel grades that Tables F.5 and F.6 say shall be adopted, from American or British specifications, the Engineer keeping the material specification compatible with the design basis, including detailing and ductility.³³ Other materials, such as aluminium, timber, gypsum board, glass and plastic, follow Chapters 23 to 26 of the IBC and may be used only where Sections 2, 4, 6 and 7 of Chapter 1 of the UAE Fire and Life Safety Code also permit them, and every material must be specified compatibly with the codes and designed for changes in its strength, stiffness and durability over time.²⁶ Codes are stated by number, title and edition, and where Part F and a referenced standard conflict, the most restrictive requirement is met.³⁴

What to settle before design

Scope and limits