CAN STEEL BEAMS BE MELTED BY JET FUEL

When someone raises the question, “can steel beams be melted by jet fuel” images of September 11 conspiracy theories come to mind. There are some very vocal (and passionate) individuals who question the events and the failure mechanism of the world trade centre in New York which fell victim to an horrendous terror attack on September 11, 2001. This article takes a look at the facts, from a Structural Engineering perspective, to answer the question… can steel beams be melted by jet fuel.

The ambient burn temperature of jet fuel at 1,030oC (1,890 oF), is not sufficient to melt steel which has an accepted melting point of 1425-1540oC (2597 – 2800 oF). Therefore steel cannot be melted by jet fuel.

That’s the short answer. Indeed, taken in isolation, this question could start to have you questioning if 9/11 was an inside job. But this question is far too simplistic to get to the real cause and failure mechanism of the world trade centre.

Perhaps a better question should be, could a fire within a structure, fuelled by an accelerant (jet fuel) cause a total collapse of a building without requiring the steel to “melt”? Lets take a closer look…

The Melting Point of Steel

The first critical piece of information to determine if steel beams can be melted by jet fuel is the temperature at which point steel begins to melt.

This is dependant on the exact type of steel and its carbon content, but generally the melting point of steel is in the order of…

Melting Point of Steel (oC) 1425 – 1540 oC
Melting Point of Steel (oF) 2597 – 2800 oF
Melting Point of Steel (source)

What is the Temperature at Which Jet Fuel Burns

The obvious second piece of information to determine if steel beams can be melted by jet fuel is the burn temperature of the jet fuel itself.

The “open air burn temperature” of jet fuel is…

Open air burn temperature of Jet Fuel (oC) 1030 oC
Open air burn temperature of Jet Fuel (oF) 1890 oF
Burn temperature of jet fuel (source)

A simple comparison between the melting point of steel and the ambient burn temperature of jet fuel indicates that jet fuel in fact cannot melt steel.

A trending meme which followed 9/11 referring to can steel beams be melted by jet fuel
A trending meme which followed 9/11 referring to “can steel beams be melted by jet fuel”

Don’t ask Can Steel Beams be Melted by Jet Fuel but Can Steel Beams be Weakened by Jet Fuel

It should be noted that, at the melting point of steel, we are dealing with a liquid. Hardly able to support any kind of load. But how does steel behave between being a solid (at room temperature) to being a liquid (at circa 1,500 oC).

When steel is heated, it retains its strength up to a temperature of around 350oC. After this point, the steels strength progressively declines until it is deemed totally “useless” at around 1,050oC. The graph below shows this in a diagrammatic sense…

Strength reduction in steel with increasing temperature to determine can steel beams be melted by jet fuel
Strength reduction in steel with increasing temperature to determine can steel beams be melted by jet fuel

Structural Engineers design buildings with “safety margins” in place. These are referred to as “safety factors” or “factors of safety”. This accounts for a margin of error which may be caused by:

  • The building occupants unknowingly over-loading the structure above its intended load capacity
  • The construction material having a defect and not achieving the intended strength
  • Minor construction tolerances causing some structural members attracting higher loads than anticipated

The approach most design codes across the world adopt in order to account for these safety margins is:

  • Increase the expected design load applied to the structure by a certain percentage
  • Reduce the expected material strength by a certain percentage.

Depending on the region, the design load is generally increased by a factor of around 1.3 to 1.4 (or 30 to 40%). In the case of structural steel, the strength of the steel material is usually decreased by a factor of 0.9 (or 90% of its expected strength).

Adding the effects of these safety factors together would give a total safety margin of around 40 to 50%. This would mean that some form of failure of a steel member within a building could start to occur when the strength of the steel is reduced to around 60% or less.

This is the reason the 0.6 strength factor is highlighted on the graph produced earlier. The corresponding temperature being a very modest 550oC. This is well below the ambient burn temperature of jet fuel.

However the fire likely did not need to reach even this temperature to commence the collapse of the world trade centre buildings. The secondary effects of fire on steel beams/columns would have played a significant role in the collapse also…

Secondary Effects on Steel Members due to Fire

When steel is heated, it expands. When a steel member (beam or column) is heated during a fire, it is rarely heated evenly through its cross-section. The differential in heat from one side of a member to another can result in warping, twisting and increased stresses being introduced to the member.

Lets take the example of a steel beam. This example steel beam is part of a floor system within a multi-storey building.

A fire has begun directly beneath the beam. The beam directly supports a concrete slab to the level above. Here is what this arrangement looks like…

Example beam subject to a fire in a multi-level building.  The proximity of the fire and the support arrangement of the beam can result in a differential heat rise across the member.
Example beam subject to a fire in a multi-level building. The proximity of the fire and the support arrangement of the beam can result in a differential heat rise across the member.

Simply due to the proximity to the fire, the lower flange of this example beam will be hotter than the top flange. In addition, the concrete slab it supports acts as an insulator for the top flange. Concrete is a very good insulating material meaning it does not transfer heat across its volume effectively. This insulating effect would further contribute to the difference in temperature between the top and bottom flanges.

Higher heat in the bottom flange results in more elongation (lengthening) due to the effects of heat when compared with the top flange.

Bending theory tells us that under regular loading conditions at room temperature, the bottom flange is experiencing tension (elongation/lengthening) while the top flange is experiencing compression (shortening). Take a look at THIS article for a better understanding on bending theory.

When a concrete slab goes into bending such as this bottom image, the bottom portion undergoes tension and the top portion undergoes compression

The introduction of heat, and more specifically the difference in heat compared with the top and bottom flanges, further increases the bending strain the beam will experience. This effect, coupled with the steels strength reduction due to the heat can contribute to a beam “failure” at more modest burn temperatures lower than 550oC.

Conclusion

It would seem that steel beams cannot be melted by jet fuel…

However, even though jet fuel does not burn at an ambient temperature exceeding that of steels melting point, this would not mean that the events of 9/11 were an “inside job”.

The ambient burn temperature of Jet Fuel is however more than hot enough to significantly decrease a steel members strength to the point of failure.

Without the presence of an accelerant such as jet fuel in a building fire, the burn temperature can still reach levels of 1,100 to 1,200oC. This is just with conventional office items as a fuel source including computers, carpets, partition walls, ceilings and paper.

In the event of a significant office building fire (with or without the presence of an accelerant) you have two hours, maximum, to evacuate before significant collapse of the building may occur.

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Quentin Suckling is our founding director.  Prior to starting Sheer Force Engineering, he spent almost 2 decades working as a practicing Structural Engineer at Tier 1 engineering consulting firms delivering multiple billions of dollars worth of projects and managing large multi-disciplinary engineering teams. View More Posts

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