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More specific technical issues:

n     The fires – how hot did they burn?

Fire intensity

n     Kerosene burned off quickly – FEMA and NIST agree

n     Fuel = office contents

n     Black smoke means cooler, oxygen-poor fire

n     Lack of fuel in service cores

n     Core designed to prevent acting as a chimney in a fire

            

Notes:  As the FEMA report quoted below explains in detail, almost all of the JP-4 jet fuel 
(essentially highly refined kerosene) from the two planes was consumed in the first 5-10 minutes 
after impact, both in the initial fireballs and in fires on the floors near the impact points.  This means 
that the jet fuel had disappeared as a heat source long before the collapses, and cannot have been 
an important influence beyond helping to ignite the office contents, which would have had to supply 
the lion's share of the energy needed to raise the temperature of the core columns.

Modern office contents, including furniture, computers, floor and wall coverings and curtains
are not a rich source of fuel and generally incorporate fire-retardant materials.  They are 
spread out through a large volume of space as well, creating a diffuse, lower-intensity fire 
as discussed below by Eagar and Musso.  The lack of fuel was even more of a problem 
in the so-called service cores where the central support columns of the towers were located. 
These housed the elevator systems, stairways and utility shafts, but contained no significant 
amount of flammable materials.

The cores were specifically designed not to allow passage of air in the event of a fire or
other disaster, and included automatic fire shutters to close off the elevators.  This means that 
the only air available to a fire in the core would have been from broken windows on 
the periphery of the buildings.  Since the cores could not act as chimneys, the smoke and hot 
gasses from such fires would have to travel beck along the ceilings to escape, and would not 
produce a strong draft to pull fresh air inward toward the core.

The FEMA/ASCE Study (2.2.2):

"The fire is the most misunderstood part of the WTC collapse. Even today, the media report (and many scientists believe) that the steel melted. It is argued that the jet fuel burns very hot, especially with so much fuel present. This is not true.

Part of the problem is that people (including engineers) often confuse temperature and heat. While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the density (or mass). Temperature is defined as an intensive property, meaning that it does not vary with the quantity of material, while the heat is an extensive property, which does vary with the amount of material. One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the size of the fire or the length of time the fire burns, or a combination of the two, doubles. Thus, the fact that there were 90,000 L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire. The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.

In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame. A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber. Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet. This is what occurs in a jet engine, and this is the flame type that generates the most intense heat.

In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions. It does not attain the flame velocities of a jet burner. An oxyacetylene torch or a Bunsen burner is a pre-mixed flame.

In a diffuse flame, the fuel and the oxidant are not mixed before ignition, but flow together in an uncontrolled manner and combust when the fuel/oxidant ratios reach values within the flammable range. A fireplace flame is a diffuse flame burning in air, as was the WTC fire.

Diffuse flames generate the lowest heat intensities of the three flame types.

If the fuel and the oxidant start at ambient temperature, a maximum flame temperature can be defined. For carbon burning in pure oxygen, the maximum is 3,200°C; for hydrogen it is 2,750°C. Thus, for virtually any hydrocarbons, the maximum flame temperature, starting at ambient temperature and using pure oxygen, is approximately 3,000°C.

This maximum flame temperature is reduced by two-thirds if air is used rather than pure oxygen. The reason is that every molecule of oxygen releases the heat of formation of a molecule of carbon monoxide and a molecule of water. If pure oxygen is used, this heat only needs to heat two molecules (carbon monoxide and water), while with air, these two molecules must be heated plus four molecules of nitrogen. Thus, burning hydrocarbons in air produces only one-third the temperature increase as burning in pure oxygen because three times as many molecules must be heated when air is used. The maximum flame temperature increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000°C—hardly sufficient to melt steel at 1,500°C.

But it is very difficult to reach this maximum temperature with a diffuse flame. There is nothing to ensure that the fuel and air in a diffuse flame are mixed in the best ratio. Typically, diffuse flames are fuel rich, meaning that the excess fuel molecules, which are unburned, must also be heated. It is known that most diffuse fires are fuel rich because blowing on a campfire or using a blacksmith’s bellows increases the rate of combustion by adding more oxygen. This fuel-rich diffuse flame can drop the temperature by up to a factor of two again. This is why the temperatures in a residential fire are usually in the 500°C to 650°C range.   It is known that the WTC fire was a fuel-rich, diffuse flame as evidenced by the copious black smoke. Soot is generated by incompletely burned fuel; hence, the WTC fire was fuel rich—hardly surprising with 90,000 L of jet fuel available. Factors such as flame volume and quantity of soot decrease the radiative heat loss in the fire, moving the temperature closer to the maximum of 1,000°C. However, it is highly unlikely that the steel at the WTC experienced temperatures above the 750–800°C range. All reports that the steel melted at 1,500°C are using imprecise terminology at best.

Some reports suggest that the aluminum from the aircraft ignited, creating very high temperatures. While it is possible to ignite aluminum under special conditions, such conditions are not commonly attained in a hydrocarbon-based diffuse flame. In addition, the flame would be white hot, like a giant sparkler. There was no evidence of such aluminum ignition, which would have been visible even through the dense soot."

*******

Quote from  G Charles Clifton - This is an early attempt by a Structural Engineer from New Zealand to explain the collapses.  Though his discussion of fire intensity is very cogent, he goes on to explain the collapses as being due to very intense localized heating of the core columns late in the course of the fires, unspecified "severe fire conditions," without addressing the obvious conflict with his previous debunking of the idea of extremely hot fires.

"How Severe Were the Effects of the Fires?

In my opinion the fires had a less important role to play in the collapse of both

towers than the damage from the initial impact. It took both to cause the

collapse, however the fire was in no way severe enough to have caused the

collapse on its own. The reasons for this opinion are as follows:

1. If the temperatures inside large regions of the building were in the

order of 700+ deg C, then these regions would have been glowing red

hot and there would have been visible signs of this from the outside.

Also there would have been visible signs of flames. If one looks at the

photos of the Cardington fire tests, the flames and glowing of the

steelwork is clearly visible even in the large enclosure test where the

maximum fire temperature was only 700 Deg C. In contrast, the

pictures of the towers after the impacts and prior to the collapses show

sign of severe burning over only relatively small regions of the tops of

the towers, even pictures taken from the air looking horizontally into the

impact region (eg Fig.9).

Photos of the First Interstate Bank fire in Los Angeles in the early

1990s? show what appears to be greater heating effects and over

larger regions than were apparent in either tower.

This does not mean that there were no regions subjected to severe

heating. It is likely that temperatures in some parts of the impact region

would have exceeded 700 deg C for some or all of the time between

impact and collapse, especially on the South side of the North tower.

However, the extent of impact damage would have been such as to

leave the residual vertical load carrying system within the core regions

of both buildings vulnerable to further weakening at temperatures lower

than 700 deg C.

In contrast, had the columns in the core and the perimeter frames

remained intact and protected ( an impossible scenario given the

magnitude of the impact) then it is expected that the building would

have remained standing, with significant floor damage, even when

subjected to fire temperatures of 1000 deg C and having suffered the

loss of the fire rated suspended ceiling to the floor slabs.

2. When fully developed fire conditions ( temperatures of over 700 deg C)

are reached within a region of a building, this results in the breaking of

glass in any external windows within that region. This continuous

breakage of glass as the fully developed fire spread through the floor of

the First Interstate Bank, for example, was the most hazardous feature

of the fire to those at ground level around the building.

In contrast, once the blast and fireball effects of the impacts had

subsided, there appeared to be little ongoing window breakage from

either tower, either as evidenced from pictures/video footage or as

reported from the ground. Significant areas of window even remained

intact within the impact region (see eg Fig.9). This is further evidence

that fully developed fire conditions did not spread much through and

beyond the initial devastated region, following the impacts.

3. If there had been severe fires burning in the core regions of the

building due to the fire load from the plane combining with the fire load

from the buildings, this would have adversely impacted on the

conditions in the stairwells below the impact region. This would have

especially been the case for the North Tower, where the core was

destroyed by the impact, leaving the regions within the core below fully

exposed to fire conditions within the impact region, such as the ingress

of burning fuel and other combustibles. However the stairwells below

the impact region on the North Tower were sufficiently clear to allow

some occupants close to the impacted floors to escape and to allow

firemen to reach at least the floors around the 70th level, as reported by

survivors from the building. In the South Tower, at least one stairwell

remained operable past the impact region after the impact. Given the

damage that must have been done by the impact to the walls

surrounding this stairwell, the resulting fire is unlikely to have been

“incredibly severe”, otherwise the few survivors from the South Tower

above the impact region would not have been able to escape.

4. When the North Tower finally collapsed, the collapse started from the

top down onto the impact region. If the fire in this region had been very

severe at the moment of collapse, then I would have expected to see a

significant burst of fire and burning debris expelled from all around the

perimeter of the impact region as it was compressed by the collapse. In

reality, the footage of the collapse does not show much flame issuing

from the impact region as it is compressed by the collapse.

5. It is reasonable to assume that the force of the impact and subsequent

fireball would have stripped the passive fire protection from most if not

all of the steel members that remained in place within the impact

region. If this is the case and the fire had been as severe as some

have stated, the buildings would not have remained standing for as

long as they did. Left unprotected, elements of any steel members

exposed to severe fire conditions would have quickly reached

temperatures close to the fire temperatures. We know this from the

large-scale real fire tests conducted in recent years, in which the

bottom flange and webs of unprotected beams and columns exposed

to the fires reach 90% or more of the fire temperature and closely

follow the fire time-temperature curve. The floor beams and exterior

columns of these towers were relatively light members and would have

heated up rapidly. Unprotected core columns would have heated up

more slowly, but even in that case would not have survived 1 ¾ hours

of severe fire exposure prior to collapse, especially if they had also

been distorted by the impact. This is because the columns would have

tried to expand with the heating and, being unable to do so by the

surrounding cold building, would have instead buckled sideways

leading to further loss of load carrying capacity. That was probably the

mode of failure of any damaged core columns following the initial

impact, however the fact that the buildings survived as long as they did

after the impact indicates to me that most of the members that survived

the initial impact were not then heated to very high temperatures."

From Collapse of the World Trade Centre Towers

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