Try the Materials Section of this forum.  Most of the people that know materials visit that section but often not this one.

In my country we do not have A-36 but I see it seems to be equivalent to our gr43 or 300W. Minus 20dF = minus 28dC.

I cannot refer you to a standard but can give you a few pointers in case you have to (and are indeed allowed to!) make up your own mind:

You surely need to be concerned about the ductile-to-brittle transition temperature of your steel (or, NDT - nil ductility transition).  And, I would say it also depends on the application and your load specifics (impact and fatigue loading might pose a problem).  Fracture toughness or impact energy vs temperature might be useful if you could lay your hands on it.

Using thinner sections are better from a fracture toughness point of view (not exceeding required stress levels).

I do not know the details of your project but I think there is a chance that your A-36 might just do the job. Probably more expensive but I do know that aluminium and stainless steel are better suited for cryogenic service.

Gert

Cold storage warehouses throughout the US are built with A-36 columns/beams and with high strength steel joists. these have average temperature for blast freezing of minus 40 degrees.  Simple spans and multiple columns at change of temperature are required to prevent heat transfer through the insulation of the walls.  Heated subgrade of the footings/floor are required for frost protection.

You should look again in the codes: I'm sure there is a rule for that.
Eurocode 3 for structural steel has an entire section dedicated to this problem. Variables to be considered are the test temperature of Charpy-V impact test for material,  level of stress in concerned elements and operating reference temperature: the calculated value per code is the maximum thickness allowed for different materials under those conditions.
As an example a material with 275 MPa (40ksi) minimum yield in low thickness products, when loaded close to the allowable stress, will have a maximum design thickness of 25 mm for a reference temperature of -30°C when tested for a minimum impact energy of 27J at 20°C, and a maximum thickness of 55 mm if impact tested for the same energy at -20°C.
So there is no simple answer to your question: you should definitely find the applicable code.

prex

http://www.xcalcs.com
Online tools for structural design

In the USA, A36 steel has been effectively phased out and replaced by A992 steel. Since you describe this as a new project, suggest that you carefully consider which material you specify (regardless of the cold weather requirement). Here is an article from AISC Modern Steel Construction on this changeover
http://www.aisc.org/template.cfm?template=/ContentManagement/ContentDisplay.cfm&ContentID=21429

About the cold weather requirement, no steel in the AISC manual is really suitable for use at -20F. May be best to get specific material recommendations. This may be one place to begin looking
http://www.crrel.usace.army.mil/

Here is general information on cold weather use of structural steel (near the bottom of the page)
http://www.qfam.org/techlib/stltext/StlTxt003/index.htm

www.SlideRuleEra.net

A36 steel is still the preferred grade for channels, angles, S-shapes, plates, and others.  A992 is the preferred grade for wide flanges (W-shapes).  We don't know what shape the original poster is talking about...

The AASHTO bridge code has three temperature zones:

Zone  Lowest anticipated service temperature
1     0°F and above
2     Below 0° to -30°F
3     Below -30°F to -60°F

ASTM A 709 Gr. 36 is equivalent to A 36 with CVN testing, and A 709 Gr. 50S is equivalent to A 992.  If you specify the CVN toughness requirements given in A 709 for Zone 3 or even 2, and also take a look at the AWS D1.5 Bridge Welding Code for their Zone 3 (or 2) requirements (if you're going to weld), you should be okay, or at least as okay as the bridge designers are.

Hg

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I talked with some pretty knowledgable materials guys last week and they indicated that for building structures, even those located in cold arctic environments, A36 and A992 steels would be adequate provided close attention is given to their Charpy notch-toughness tests.  The 13th edition AISC manual (see p2-53) actually refers to ASTM A709 Section S83 for determining the proper level of notch toughness (see HgTX info previous post).

For reallly realllly cold structures (e.g. vessels for holding liquid oxygen etc), steels with higher nickel content can be used - see the bottom of:

http://www.key-to-steel.com/AR/default.aspx?ID=CheckArticle&NM=85

If you go looking for A 709 S83 in a new version of A 709, it's now Table 9.  They moved the CVN requirements from supplementals into the main body of the standard.

Hg

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