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Originally Posted by fastom
I am really, really aware of thermal expansion and use it to advantage all the time. We are still talking tiny distances, not the tower expanding out over the Bronx.
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some pictures:
http://upload.wikimedia.org/wikipedi...Arrangment.jpg
This shows a top down view of a floor. 60 feet separate the edge of the building and the elevators
http://www.tms.org/pubs/journals/JOM...Eagar/fig5.gif
This picture (Not To Scale) shows how the 2 are connected by several long struts, (picture calls them floor joist) each being approximately 60 feet long, of solid steel. The struts rest upon the angle clip, also known as a gusset plate. The structural integrity of the floor is solely based on them resting on top of that ledge. From what I have gathered gusset plates are 4 x 2 x 3/8 inches.
Ok now onto thermal expansion of these struts. 60 feet of building grade steel. Building grade steel has a thermal expansion coefficient of 1.2 x10^-5 or so says my physics book. According to
http://www.tms.org/pubs/journals/JOM...agar-0112.html the steel never heated past 750C no where near enough to melt it. A 60 foot beam heated to 750C from 20 C will expand by a little over 6 inches. If the fires were all uniform, the structure would have expanded at about the same rate, but the fires were not uniform, there were points of hotness and points of coolness, depending on the supply of oxygen and fuel, this discontinuity was the major cause of the weekend structure.
Take this scenario, 2 beams heated to 750C while a third beam in the middle is only heated to 250C. The 2 outer beams will expand by 6 inches, while the center beam will only expand by 2 inches, a difference of 4 inches. the outer beams having no where to grow will push the outer wall out by 6 inches, where as the center will expand by 2, but its outer wall is pushes out 6 inches, leaving a 4 inch gap, now since the gusset plate is only 2 inches deep, it just fell off the plate and that section of floor collapsed, all because of a 500C difference. This impact would then stress the already weekend lower floor, causing the inevitable collapse.
Now if we go backwards we can find the exact difference in temperature we need to get 2 inches of separation, it turns out it is 231.5 degrees C, in a building fire it is easy to get pockets of heat, from the flow of fuel and wind. Unfortunately in the real event took place in 3d and is much more complex than this example, but the concept is still the same, the little gusset plates and the uneven spread of the fire was the towers downfall. Furthermore, it would actually take less than 2 inches to make the gusset plates fail, as the weight is placed closer and closer to the edge of the gusset plates, the forces acting on the gusset plates increase due to leverage.
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Originally Posted by fastom
Another thing that seems to confuse some of you is flame temperature vs adjacent steel temperature. The steel ain't burning and heat is picked up from the flame but is also radiated out from the steel. Maybe an experiment can be tried if you have a stove with gas burners. Turn the stove on and heat your frying pan. Crank 'er up, no eggs to burn. Let it bake like that for an hour. Use the thermometer like ya stick in the Thanksgiving turkey to get a temperature reading. Now stick the thermometer into the burners flame and see if it's maybe any hotter.
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Doing so would not only damage my thermometers, but also my nice pots and pans. however, over the course of an hour, the pan would heat up to the maximum temperature of the flame, although it would take a while (the fires burned for a while in WTC as well) but touching the pan with a thermometer would not conduct the heat very well, so the reading from a kitchen thermometer would read much lower then the real value. as for a thermometer directly in the flame, it would read the maximum temperature of the flame quickly, due to its low mass and the high abundance of heat from the flame.
Although your example is asinine, it still serves, to show you have little concept of heat, temperature, and science. Due to the steels high specific heat capacity, it would heat up slower then most of its surroundings, meaning that they would radiate to the steel, not the other way around. Heat always flows from masses of higher temperature to lower temperature, for the most part; all of the heat would be flowing into the steel, not out of it, keeping it nice and toasty.
Temperature
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Originally Posted by http://en.wikipedia.org/wiki/Temperature
Temperature is a measure of the average energy contained in the microscopic degrees of freedom of a system. For example, in an ideal gas, the relevant degrees of freedom are translational, rotational, and vibrational motion of the individual molecules. In this case, temperature is proportional to the mean kinetic energy of the constituent atoms. But in more complicated systems, magnetic, electronic, photonic, or other exotic degrees of freedom can play a significant role in determining temperature.
Thermal motion is the reason gasses have pressure, since the particles in the gas collide with the walls of the container and exert an outward force. Although very specialized laboratory equipment is required to directly detect thermal motions, thermal collisions by atoms or molecules with small particles suspended in a fluid produces Brownian motion that can be seen with an ordinary microscope. The thermal motions of atoms are very fast and temperatures close to absolute zero are required to directly observe them. For instance, when scientists at the NIST achieved a record-setting cold temperature of 700 nK (billionths of a kelvin) in 1994, they used optical lattice laser equipment to adiabatically cool caesium atoms. They then turned off the entrapment lasers and directly measured atom velocities of 7 mm per second in order to calculate their temperature.
Molecules, such as O2, have more degrees of freedom than single atoms: they can have rotational and vibrational motions as well as translational motion. An increase in temperature will cause the average translational energy to increase. It will also cause the energy associated with vibrational and rotational modes to increase also. Thus a diatomic gas, with extra degrees of freedom like rotation and vibration, will require a higher energy input to change the temperature by a certain amount, i.e. it will have a higher heat capacity than a monatomic gas.
The process of cooling involves removing energy from a system. When there is no more energy able to be removed, the system is said to be at absolute zero, which is the point on the thermodynamic (absolute) temperature scale where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the “classic” (non-quantum mechanical) sense. By definition, absolute zero is a temperature of precisely 0 kelvin (–273.15 °C or –459.67 °F).
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Heat
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Originally Posted by http://en.wikipedia.org/wiki/Heat
Under the First Law of Thermodynamics, heat (and work) are processes that change the internal energy of a substance or object. Heat is the transfer of energy over the boundary of a system owing to a temperature gradient. Its SI unit for heat is the Joule, though the British Thermal Unit is still occasionally used in the United States.
Heat is a process quantity, as opposed to being a state quantity, and is to thermal energy as work is to mechanical energy. Heat flows between regions that are not in thermal equilibrium with each other; it spontaneously flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy, a state quantity that is related to the random motion of their atoms or molecules. When two bodies of different temperature come into thermal contact, they will exchange internal energy until the temperature is equalized; that is, until they reach thermal equilibrium. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy: heat is related to the change in internal energy and the work performed by the system. The term heat is used to describe the flow of energy, while the term internal energy is used to describe the energy itself.
In common usage the term heat denotes the warmth, or hotness, of surrounding objects and is used to mean that an object has a high temperature. The concept that warm objects "contain heat" is not uncommon, but hot is nearly always used as a relative term (an object is hot compared with its surroundings or those of the person using the term) so that high temperature is directly associated with high heat transfer.
The amount of heat that has to be transferred to or from an object when its temperature varies by one degree is called heat capacity. Heat capacity is specific to each and every object or substance. When referred to a quantity unit (such as mass or moles), the heat exchanged per degree is termed specific heat, and depends primarily on the composition and physical state (phase) of an object. Fuels generate predictable amounts of heat when burned; this heat is known as heating value and is expressed per unit of quantity. Upon changing from one phase to another, pure substances can exchange heat without their temperature suffering any change. The amount of heat exchanged during a phase change is known as latent heat and depends primarily on the substance and the initial and final phase.
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Originally Posted by fastom
Phone calls... Mom it's me, Mark Bingham!  |
for all that don’t know who mark is, he is one of the passengers on flight 93, and was alleged to have rushed the cockpit with others to bring it down, making fun of a dead guy is a new low for you fastom.