When the first tower got hit the sprinkling system went off cooling the metal structure. When the 2nd tower got hit, the sprinkling system also went off, but at less water pressure and also lessening the water pressure in the 1st tower. The sprinkling systems were never designed to handle such a large fire in both towers at once. The second tower never received the water pressure the first tower did and fell first. Bottom line is that had not both towers been hit, the first one would not have fell in because the water pressure would have kept the first structure cooled. And that the terrorists were aware of that when they hit the 2nd tower. According to Gary North, the plane that went down in PA was headed for the Pentagon so that a similar event would occur there.

-- fred (fred@mddc.com), October 05, 2001

Answers

This is speculation, perhaps even wild. Dumb luck on the part of the terrorists, not engineering skill in planning the attack brought those towers down.

-- Joe (CactusJoe001@AOL.com), October 05, 2001.

I read that the plane that hit the 2nd tower was carring more jet fuel. The more jet fuel the greater the heat generated and spread out in a bigger area.

-- Karen (db0421@yahoo.com), October 05, 2001.

I thought it was just from the intense heat. I heard a reporter say they estimated the heat from the jet fuel fire at over 1000 degrees. Wouldn't that melt or severely weaken the steal beams?

-- Cheryl in KS (cherylmccoy@rocketmail.com), October 05, 2001.

Fred - I've been designing sprinkler systems for a long time and would like to offer the following comments:

1. A sprinkler system in a highrise building are only desiged for a fire on one floor. The impact of the plane created a fire condition on as many as eight floors.

2. High rise buildings are typically designed for a office occupancy as a hazard to protect. This being very light in combustible loading. The introduction of jet fuel would have overwhelmed the sprinkler system in the event the sprinkler system piping had survived the impact. Water alone is a very poor extinguishing agent for jet fuel.

3. The fire protection systems for each tower would have been independent of each other. Each tower would have been equiped with it's own water supply and system of multiple fire pumps and water storage tanks. There would have been multiple fire pumps in each building supplying the sprinkler/standpipe systems. The systems would have been configured into multiple vertical zones of not more than 400' per system. It takes roughly 175 PSI to lift the water in the sprinkler/standpipe systems the 400' per vertical zone.

4. An engineerning study had been done after the 1993 bombing. One of the senarios was the tower being impacted by a larger commercial air plane. The study concluded that the towers would survive the intial impact but, would altimately fail from the resulting fires. If you recall the ajacent 47 story building collapsed a number of hours after the impact of the planes even though it had not been struck by either plane. It's structure had failed as a result of the steel framing weakening from heat from fire.

Sprinkler systems work great for the application they are designed for. Typicaly only 1-3 sprinklers operate in a fire. What we saw in the WTC attack is a hazard no system could have suppressed or controled. We just don't design systems for this type of occurance.

Jeffalan

-- Jeffalan (jeff100760@aol.com), October 05, 2001.

World Trade Centre - New York - Some Engineering Aspects

http://www.civil.usyd.edu .au/wtc.htm

-- Dave (something@somewhere.com), October 06, 2001.

First, thanks to Jeff for the information and understandable explanation. It made a lot of sense even to a non-engineering type (a/k/a/ dumb social science major) like me. ;o)

Second, wasn't the second tower hit lower than the first? If it was, I'd think the absolute weight above the weakened area would be the biggest factor in which structure first collapsed. That makes sense to me, anyhow.

-- Gary in Indiana (gk6854@aol.com), October 06, 2001.

The terrorist probably did not know exactly what would happen because it has never been tested using real planes and real 110 story buildings. They (the ones that watched from the safety of their tents) were probably as surprised as we were. I have always thought that Tim McVey had no idea what he was doing. I don't think he planned for the distruction that he caused. How could he? His test was done in a field with a bucketful of nitrigon.

-- Belle (gardenbelle@terraworld.net), October 06, 2001.

Thank you for all of your posts. The post from Jeffalan seems to answer most of what I was wondering about.

-- fed (fred@mddc.com), October 07, 2001.

Fellow American Citizens: I was among the volunteer searchers in the street south of the North Tower at around midnight of Tuesday 9-11. I have looked at many photographs of the disaster since then. I am an engineer by education. (BSEE, scl) I had wondered why the terrorists struck both buildings with the planes tipped at an angle off horizontal, while banking, instead of straight on. It would have been much easier to strike the buildings in a straight-on manner, without the turning and banking they did. I noticed that the result of turning was that the planes struck the building on many floors at once, with each wing (including fuel tanks) entering the building on different floors. I concluded that this result was their intention, to distribute the fuel on multiple floors. It would make sense, in light of Jeff's comments, that the terrorists were concerned that a fire- extinguishing system might succeed in saturating/extinuishing the inferno if it were contained on one floor. By distributing the fire on multiple floors, they overcame that risk. AND, by distributing the fuel on multiple floors, tehy also heated up a longer span verticle of steel collumns. The longer a column (or pipe) of a certain cross-section is, or the longer the weakened segment of a column is, the faster it will buckle under load. Thus, the terrorists intended to bring the buildings completely down, and had performed engineering analysis to do it. This is proved by the fact that they took the trouble to turn and bank the planes at the time of impact, instead of flying into the buildings straight-on. Mark Ferran, Patent Agent, BSEE scl

-- Mark Ferran (mferran@iplawusa.com), October 11, 2001.

Thank you Mark for your explanation. It has been my opinion from the moment that it happened that they knew what they were doing and totally intended to bring them down.

-- diane (gardiacaprines@yahoo.com), October 11, 2001.

My understanding is that they also struck at a height where the building structure was less protected. That is, I heard (one source, unconfirmed) that lower floors of the building had asbestos insulation around their girders, but the planes struck where the insulation left off.

-- Don Armstrong (darmst@yahoo.com.au), October 11, 2001.