Effect of Heat on Structural Steel 1

[1437n6]

Does anyone know a general rule of thumb to what the effect of heat is on A36 Structural Steel? What temperature does it start to carbonize, and what temperature does it begin to fail?

 

[frv]

I guess it would depend on your definition of failure. I'm fairly certain it starts losing it's strength at a relatively low temperature, but what do you consider failure? 90% of design strength or 50%?

I know that there are temperature-load carrying capacity tables published for bolts; I'm sure they'd have the same for other shapes/grades.

 

[msquared48]

As I recall, there was a lot of discussion on this subject in the engineering world regarding 911, the temperatures the steel saw, and how that affected the collapse. There may be some string in the archives in this forum regarding that.

I believe you start running into strength losses in excess of 10% above 1000 degrees farenheit, and the losses increase markedly as you progress to a total failure around 2000 degrees or less. Don't qoute me on that though - check the archives.

Mike McCann
McCann Engineering

 

[namanges]

A36 steel loses more than 10% of Fy at 500 degrees farenheit and loses more than 50% of Fy at 800 degrees.

Fy(A36) @ 400 deg. F. is about 33 ksi
Fy(A36) @ 800 deg. F. is about 16 ksi

These are the values that I have been exposed to and have used, but it would be great if someone else could verify these.


namanges

 

[TGS4]

From ASME Section II, Part D, Table Y-1 and U-1:
Temperature(°F) Yield Strength(ksi) Tensile Strength(ksi)
-20 to 100 36.0 58.0
150 33.8 58.0
200 33.0 58.0
250 32.4 58.0
300 31.8 58.0
400 30.8 58.0
500 29.3 58.0
600 27.6 58.0
650 26.7 58.0
700 25.8 58.0
750 24.9 57.3
800 24.1 53.3
850 23.4 48.5
900 22.8 43.3
950 22.1 38.0
1000 21.4 33.4

Although exposures above 800°F will result in the carbide phase converting to graphite. Also, above 700°F, time dependant effects (creep) become important.

 

[Robertmet]

Are you asking about short term or long term ? Graphitization takes time .The old rule of covering steel with 2" concrete is to insulate the steel from high temperature .World Trade Center insulation came off easier than thought, exposing steel to high temperatures.Applications where steel would be exposed long term would require a different alloy such as adding Mo to slow graphitization.

 

[Qshake]

You should also consider the effect of heat on the steel modulus. The modulus of elasticity and or Young's modulus is also dependent on temperature. Recall that both deflection and buckling equations are based on the Modulus. As a result, you could have a displacement or buckling failure prior to reaching a failure via normal or shear stress.

Some time ago I recall that steel begins to change properties at about 600 degrees F. This was about the same temperature it took to light an unlit match by touching to the steel.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[bridgebuster]

Perhaps we should ask Rosie O'Donnell, as she put it"...everyone knows fire can't melt steel."

 

[1437n6]

Thank you all for your insight. It was very useful in determining what I needed!

 

[miecz]

From an old ASME Boiler and Pressure Vessel Code, Section III, Division 1, (for Nuclear Structures, I believe) Table I-6.0, for Carbon Steel with carbon content 0.30 or less (includes A-36);

Temperature Modulus of elasticity
<70[&deg;]F 27.9 x 10⁶ psi
<200[&deg;]F 27.7 x 10⁶ psi
<300[&deg;]F 27.4 x 10⁶ psi
<400[&deg;]F 27.0 x 10⁶ psi
<500[&deg;]F 26.4 x 10⁶ psi
<600[&deg;]F 25.7 x 10⁶ psi
<700[&deg;]F 24.8 x 10⁶ psi

For Carbon Steel with carbon content above 0.30;

Temperature Modulus of elasticity
<70[&deg;]F 29.9 x 10⁶ psi
<200[&deg;]F 29.5 x 10⁶ psi
<300[&deg;]F 29.0 x 10⁶ psi
<400[&deg;]F 28.3 x 10⁶ psi
<500[&deg;]F 27.4 x 10⁶ psi
<600[&deg;]F 26.7 x 10⁶ psi
<700[&deg;]F 25.4 x 10⁶ psi

 

[Bagman2524]

There's a discussion in the steel manual about the affect of high temperature on steel. I believe it contains much of what msquared's post has.