Sub-Zero Structural Steel 1

[HSIII]

Can anyone advise me on how bending and shear strength are affected if a structural steel member (say, A36 steel) is exposed to a constant sub-zero condition (-49 F)? Would I follow the generic temperature formulas found in the AISC manual, or are those just to consider dramatic changes in temperature?

Links to any information would also be greatly appreciated.

 

[cetiger]

Have you tried contacting anyone with AISC? I would think they could point you in the right direction.

Shooting from the hip...I would think that A36 steel performs poorly to other grades due to the high amount of carbon content, which causes this grade of steel to be one of the most brittle of the steel alloys. I would think the colder temperature would further complicate the steel's ability to remain ductile under heavy loads/stress.

Interesting question...I'll be interested to hear if anyone has any info or experience.

 

[Guest102023]

For any structure built for more than ballast service, I would require impact testing. A36, not being normalized, doesn't have a hope at that low temperature.

I think you are looking at a High-Strength, Low Alloy (HSLA) steel. I don't have ASTM standards at hand, but the attached will give you some pointers. Most structural designers should be able to tell you immediately.

 

[racookpe1978]

You seem to be addressing the shrinkage (reduction is length) due to low temperature, not the change in structural strength/impact resistance/fundamental properties. True?

It is the latter that are significant - as mentioned above.

 

[frv]

cetiger..

Out of curiosity, why do you say A36 is brittle?

If you take a fairly common comparison for ductility, Fu/Fy, A36 is actually one of the more ductile steels around.

 

[BAretired]

The subject of low temperature steel design does not arise very often in Canada, so far as I know even though our temperatures can occasionaly dip to -40C or colder. There are some steels which are considered tough insofar as resistance to brittle fracture in cold temperatures. But, for the most part, exterior structures are built without regard to cold temperature notch toughness. This may not be true for bridges, but it is true for external cranes which are typically designed without regard to cold weather properties.

Perhaps this has been an oversight, but it does not seem to have caused any issues to date, although I do recall one very cold day when a load of rebar was offloaded from a truck...it was dumped down and shattered when it hit the ground. I didn't actually see it, but I heard about it.

A36 steel may not be recommended for use in very cold temperatures, but it was used quite successfully for years in exterior applications in Alberta where the temperature could get down to -40C.

BA

 

[hokie66]

That's a convenient temperature, BA. -40C = -40F

I have never dealt with this either, but think A36 steel would be better than most, as it is quite ductile. Reinforcing bars are more brittle.

 

[WillisV]

From discussions I've had with some materials guys, they indicated that for building structures, even those located in cold arctic type environments, A36 and A992 steels behave adequately under the typical AISC specification provided that close attention is given to their Charpy notch-toughness tests which is typically called a Charpy V-Notch or CVN for short.

The 13th edition manual discusses cold temperature effects on p2-33/p2-34. See item 7 of the lists and the following discussion. This discussion indicates that ASTM A709 “may be useful” for determining proper levels of notch toughness. As discussed in the manual, this ASTM standard is generally used for steel intended for bridges, but can be applied to buildings to determine the appropriate level of Charpy notch toughness to specify for cold conditions given that ASTM A709 is not dissimilar from standard A36 and A992 steels.

If you look in ASTM A 709 the temperature classes are divided into three zones based on the minimum expected service temperature. Zone 1 is 0deg F. Zone 2 is 0 to -30 deg F. Zone 3 is -30 to -60 degrees F.

For grade 36 and 50 material a minimum CVN of 15 ft-lbf is required for all three zones but the testing must be performed at 70 deg. F (Zone 1), 40 deg. F (Zone 2), or 10 deg. F (Zone 3).

Note that this value of a CVN is probably easily obtainable; however, if it is deemed important to double check, you will need to specify that CVN test results be provided as it is NOT typical for CVNs to be reported unless requested. See

http://www.modernsteel.com/steelinterchange_details.php?id=131

 

[IFRs]

For above-ground storage tanks (API and AWWA) steels are grouped according to their toughness and listed for minimum temperature versus thickness. Thicker steels are more subject to brittle fracture than thin sections. In my world, A36 is not a very tough steel for thicknesses over one half an inch and I use other grades.

 

[dhengr]

HSIII:
-49° F is a pretty nasty temp. for structural steel. You should be talking with AISC about this subject, and doing some independent reading; talking with your steel supplier about your requirements and steel availability; and specifying Charpy V-Notch values (toughness values, in ft-lbsf @ a temp.) for the steel you are ordering, so you know this toughness value and have a record on the mill certs. for the heats of materials you are using. Then you should be designing so as not to negate this improved toughness. What ‘generic temperature formulas’ from AISC are you talking about? And, what are you going to do with them?

Decreased temps. usually don’t have an adverse effect on the mechanical properties of structural steels, such as yield stress and tensile strength, modulus of elasticity, and fatigue strength; in fact there is generally some increase (improvement) in these properties as temps. decrease. However, decreased temps. reduce the ductility of these same steels. And, there is a temp. (transition temp.) below which a structural steel subject to tensile stresses may actually fracture with little or no plastic deformation. That is brittle fracture, sudden fracture, fracture by cleavage; as opposed to a failure by shear, which exhibits plastic deformations and yielding. Remember von Mises, we keep getting lots of questions about him and his stresses, he played around with this subject, or mode of failure.

At lower temps. tensile stresses, stress gradients, combined stresses and max. stresses, loss of ductility, and any geometric discontinuities such a notches, reentrant corners, weld undercuts and other defects, arc strikes and the like combine to cause brittle fracture. This becomes a fracture mechanics problem, not just a nominal stress problem. This brittle fracture can occur at stresses lower than those which would cause problems at normal temps; because of the lack of ductility, any discontinuities, stress gradients and strain rate become so dominant in the equation and the mode of failure.

frv’s comment, ‘common comparison for ductility, Fu/Fy,’ is reasonable for normal structural considerations since it represents the part of the stress/strain curve where normal ductility, plastic stress and strain or action occurs; usually where we are designing. But, this same type of ductility doesn’t exist (can’t be assured) at lower temps. The design approach must pay much more attention to details, fab. methods and processes, etc. The approach is similar to that taken for designs which consider fatigue, although not high cycle, many of the same detail considerations are involved. BA’s and WillisV’s comments are right on too, we haven’t generally paid much attention to this issue in normal structural design work, and it doesn’t seem to have caused much trouble. The spectacular brittle failures seem to have mostly occurred in large grain and fuel storage tanks, they just split up the side and burst open; in bridges, where low temps., poor details and cumulative damage finally added up, thus AASHTO’s and AISC’s interest; in pipe lines; and in ship hulls; and the like. But brittle fracture and cold temps. are not real often directly blamed for other relatively static structural failures. However, given what we now know, you ignore this issue at your peril, when it can cause problems.