August 2015


Welcome to ‘Stainless Solutions’ from IMOA. Each month, we will cover a different stainless steel issue with tips on design and specification, and links to technical resources. This month we are discussing specification of structural sections.
Specifying Structural Sections

Stainless steel is an ideal choice for both visible and hidden structural sections in corrosive environments because of its corrosion resistance. The superior stiffness retention of austenitic stainless steels in fire relative to carbon steel means that fire protection may not be required in non-industrial buildings with lower fire temperatures. This combination of characteristics makes it possible to use uncoated stainless steel for many aesthetic structural applications.

There are, however, other reasons for its specification.  Austenitic stainless steels in particular have much better impact resistance then carbon steel at ambient temperatures and still perform well at very low temperatures when carbon steel becomes quite brittle. These properties and its blast resistance make stainless steel an ideal choice for safety and security oriented applications. Please see IMOA’s Structural Design page for more resources.

Common stainless steel industry terminology can be confusing. Stainless steel suppliers commonly use “grade” to mean “alloy”. From a structural design perspective, “grade” means strength level and suppliers may be unaware of this terminology difference, which can cause confusion. Using specific yield and tensile strength levels and alloy names can avoid misunderstanding.

Finding Suppliers & Available Sizes

Stainless steel is available in the same product forms as carbon steel and aluminum but the range of stocked sizes is more limited because the market is much smaller. Any size and shape can be custom manufactured but that is generally more expensive and requires longer lead times. It is best to contact suppliers to determine commonly stocked sizes before starting design. This information can often be found on their websites and also for North America, via the links below:

Metals Service Center Institute Directory
SSINA Directories for Structural Service Centers and Manufacturers


Specifications & Guidelines

Always use the appropriate codes, specifications and design guides for stainless steel, including the stainless steel structural welding codes. We have a downloadable list for guidance.

Stainless Steel Specifications & Guidelines Resource

Specifications, guidelines and industry associations

An international list of widely used stainless steel product, material and welding standard specifications, structural design resources, basic rules and resource sites. Stainless steel industry association contacts are provided to assist with country specific specifications.

Download PDF (394 K, English)

Tube versus Pipe

It is not uncommon for project specifications to mistakenly use pipe or seamless tubing for structural applications. Pipe is specified based on inside diameter and intended for transporting fluids, often under pressure. It is generally not intended for structural applications.  Pressure rated tubing can also dramatically increase cost because of the added testing that must be done for certification.

Welded mechanical tubing is suitable for most common structural applications and is available in small and large diameters and light to heavy walls. Round, square and rectangular shapes are available, and are less expensive than seamless or pressure rated tubing or pipe.

Type 316 was specified for these security bollards because of deicing salt exposure.

Credit C. Houska, TMR Consulting

Selecting the Right Stainless Steel

Structural sections often have rougher finishes, such as abrasive blasted, because they are less expensive.  Rougher finishes retain more corrosive deposits and that can lead to a corrosion staining problem that might not occur with a smoother finish. IMOA’s Stainless Steel Selection System should be used for alloy selection guidance, if staining cannot be tolerated.   

If the application is in a more corrosive environment, such as sea or brackish water, please see our guidance for these environments.


Fasteners

Information on fastener specifications can be found in our Specifications & Guidelines list.  The most important rule in fastener specification, other than achieving the desired mechanical properties, is that they should be at least as corrosion resistant as the most corrosion resistant of the materials joined. If not, rapid galvanic corrosion of the fastener can occur if carbon steel fasteners are used with stainless.  Fasteners of all levels of corrosion resistance are available. Obtain assistance from a stainless steel industry association if you can not find suppliers or are unsure about the appropriateness of a stainless steel.

Avoiding Galvanic Corrosion

Galvanic corrosion can lead to rapid corrosion of any aluminum or carbon steel that is in direct contact with stainless steel. It must be considered in any critical structural application.  The Nickel Institute brochure Guidelines for Corrosion Prevention and the SSINA Galvanic Corrosion webpage provide guidance to improve understanding of the problem and suggestions for its prevention.

These Type 316 structural sections are exterior and in an environment with coastal and deicing salt exposure. They are connected to painted carbon steel. Paint is a temporary barrier, especially when parts are intended to move and abrasion will occur.  The staining in these pictures is carbon steel corrosion due to paint loss.  Direct stainless steel and carbon steel contact will lead to accelerated corrosion of the carbon steel.  An all-stainless design would have been preferable, at least in sections with moving parts.

Credit C. Houska, TMR Consulting

Stainless Steel Library

Download a free Stainless Steel Library (zip file, 390 MB) with over 280 pdfs covering building and construction applications, selection, specification, fabrication, sustainability and other common questions.

Stainless Solutions e-newsletter archive

For previous issues or to subscibe to the e-newsletter, please visit the archive page.

Continuing Education – American Institute of Architects (AIA)

IMOA is an AIA continuing education system approved provider with eight 1-hour programs that are registered for both live face-to-face and distance learning credit.

1. Stainless Steel Sustainable Design
2. Bioclimatic Design With Stainless Steel Weather Screens
3. Stainless Steel Structural Design
4. Stainless Steel Specification For Corrosive Applications
5. Deicing Salt: Stainless Steel Selection to Avoid Corrosion
6. Stainless Steel Finish Specification
7. Advanced Stainless Steel Specification and Problem Avoidance
8. Specification of Stainless Steel Finishes and Grades For Corrosive Applications

For more information or to schedule a workshop contact Catherine Houska, 412-369-0377 or email chouska@tmr-inc.com.

What is IMOA?

IMOA (International Molybdenum Association) is a non-profit industry association, which provides technical information to assist with successful specification of molybdenum-containing materials. Molybdenum is an element. When it is added to stainless steel, molybdenum increases its resistance to corrosion caused by deicing salts, coastal atmosphere and pollution.

If you have a topic suggestion for a future issue of Stainless Solutions or need additional technical advice on stainless steel specification and selection, please get in touch here.

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Disclaimer

In providing consultation or other assistance with respect to technical issues, any consultation, assistance or material is provided for the general information of the recipient and should not be used or relied upon for any specific application without first securing competent advice. IMOA and their respective employees, consultants and members (i) make no representation or warranty, express or implied, of any kind with regard to such consultation, assistance or material including no representation or warranty of  suitability for any general or specific use; (ii) assume no liability or responsibility of any kind in connection  therewith; and (iii) disclaim any and all liability for any claim that arises therefrom. 

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