World Trade Center collapse
In response to the article about the collapse of the World Trade Center (11 February, p 40) and the letter entitled “Steel’s weakness” (4 March, p 25) I can advise that we engineers can explain the collapses of the three buildings (WTC 1, WTC 2 and WTC 7) without resorting to any help from missiles or pre-placed demolition charges. In fact the lessons learned from the different but related mechanisms behind the collapses of these three buildings are of vital importance to those of us who strive to improve the performance of steel framed buildings subjected to extreme conditions involving fire.
To provide this explanation within the confines of a letter to the editor is a challenge, not made easier by the need to provide an overview of the key structural characteristics of the three buildings before explaining the principal mechanisms behind the collapse of each of them.
Each building comprised the following structural components:
鈥� a closely spaced, very stiff and strong perimeter frame on each side, which resisted the sideways loading from wind and carried its share of the vertical loading from the floors.
鈥� a set of columns in the centre of the building, called the core, carrying its share of the vertical loading from the floors and supporting building services such as lifts, stairs, water, power, etc.
鈥� spanning between the core and the perimeter frame, a stiff, strong and lightweight floor system which, in addition to supporting the contents of the building, provided support to the perimeter frame at each floor level, preventing this frame from buckling under the applied loading.
The supports connecting the floors to the perimeter frame or core were designed to transmit their share of the floor loads into the core and perimeter frame. However, these supports had little reserve to cope with overloading or deformation of the buildings under extraordinary events. They were the “Achilles heel” in each of the three buildings when subjected to the extraordinary events of 11 September 2001.
Simply put, the collapses of the three buildings were caused by the following principal chain of events.
In the North Tower (WTC 1), the plane cut through the perimeter frame and severely damaged the core between the 90th and 94th levels, destroying or severely damaging over 50 per cent of the core columns within these levels. This caused the core to drop slightly immediately on impact and to continue to drop as progressive failure continued in the core, causing the upper floor loads supported normally down through the core columns to have to be carried through the floor connections into the perimeter frame. The greatest effect was concentrated into the top few storeys, as the floors above the impact zone were to an increasing extent trying to hang off the floors above. Finally, the connections between the floors and perimeter frames at the uppermost levels failed, causing the top floors to pancake down within the confines of the perimeter frame. This caused an unzipping failure of the floors below, culminating in all floors pancaking within the perimeter frame and the total collapse of the building.
In the South Tower (WTC 2), the plane cut into the south-east corner at around the 80th level, destroying some six to 10 levels of floor in that corner and their support to the perimeter frame on the east side of the building. It also started a severe fire in the north-east corner, which attacked the remaining floor systems and their support to the perimeter frame, further destabilising the perimeter frame on the east side. Finally, the perimeter frame on the east side failed over some six levels, causing the floors above to lean sideways, break up and fall down onto the floors below, pancaking the building. Some of the floors above the impact region fell onto an adjacent hotel building, destroying it.
In the case of WTC 7, the construction of the building at the lower levels was significantly different. There the building was built over an existing electrical substation which had to remain operational during the construction of the building. To achieve this, many of the core columns at the levels immediately above ground were supported on massive transfer trusses, which transferred the loads past the substation into the ground. When the adjacent WTC 1 collapsed, the disruption and damage caused a fire in the substation which burnt for 8 hours, fed by fuel tanks supplying the electrical substation generators which were located beneath the transfer members supporting the core. These long-burning fires, probably in conjunction with damage to the insulation material on the transfer trusses and maybe the trusses themselves, finally caused these transfer trusses to sag and pull the support out from many of the core columns. The resulting failure mode was essentially the same as for WTC 1, with the floors ripping out from the perimeter frames at the top of the building initiating a complete pancake collapse of the floors within the perimeter frames, followed by the collapse of the perimeter frames.
It is a sad fact of life that building failures provide the best lessons to structural engineers on what is required to make buildings safer under extreme events. The collapses of WTC 1, WTC 2 and WTC 7 have been no exception and the lessons learned will be very valuable in improving the performance of multistorey buildings under extreme events.
Not a dale
Laura Halliday tells of Damfino Creek, supposedly deriving from a surveyor’s mishearing of “Damned if I know” (Feedback, 4 March). Here in North Yorkshire we have a Snotterdale, said to be derived from the surveyor being told “That’s not a dale” by a local.
For the record
鈥� The sidebar on the fall of the Minoans in our story about the Mediterranean sea floor (4 March, p 44) stated that Crete is the second largest island of the Mediterranean Sea. It is in reality only the fifth, after Sicily, Sardinia, Cyprus and Corsica.
鈥� The Feedback story about Martian Church Art (25 March) should have been credited to Caroline Mills, not David Mills.
When east is west
I think what the manager at Whitby Youth Hostel was trying to say is that at Whitby the midsummer sun sets 鈥� as well as rises 鈥� over the sea, as at that point in Yorkshire the coast faces almost due north (Feedback, 18 March). Most of the North Sea coast of Yorkshire faces east, so perhaps it is understandable that people become confused by a section facing in an entirely different direction.
Handwritten by robots
“Would you rush to the bookstore to have a book signed by a robot?” asks Gizmo (11 March, p 25). I still treasure my copy of Tim Hunkin’s book Absolutely Everything There Is To Know, which I was given for my birthday many years ago. It is signed by a robot that was constructed by the author to take the drudgery out of book-signing sessions. The result isn’t perfect, but it’s more legible than many signatures I’ve seen.
Just to be on the safe side, my friends got the book signed by the author as well 鈥� and it’s dated 1988. Do encourage Margaret Atwood to persevere with her robot.
Tracking the spin
Cathy Craig seems to have got it all wrong when she concludes that it is impossible for people to track a curving, spinning ball accurately (4 March, p 19). We can, and do. Any tennis player will verify this, as will table tennis players. Even children playing “tether ball” in their gardens do so with considerable skill.
What makes a spinning ball so hard to track over a longer distance of, say, 30 metres is the fact that the forward speed and the spin speed, not to mention the prevailing breeze, do not remain constant, causing the Magnus effect and the effect of gravitational pull to vary constantly. This is much more pronounced with a large, light ball than with the examples previously given. Try it with a balloon.
The Magnus effect works in any direction of travel, horizontally and vertically, on a spinning object. To suggest we are not hard-wired to this is wrong.
Red rain fantasies
There appears to be an increasing tendency among scientists to come up with wild explanations when asked by the press to comment on unusual, novel phenomena. A good example is provided by comments about the recent Indian red rain phenomenon (4 March, p 34).
Red rain is morphologically similar to fungal spores or algae, as I have recently been able to confirm by microscope analysis of samples. There is no evidence that I am aware of to support suggestions that red rain is dust, sand, fat globules or blood.
From Gareth Jones
So Godfrey Louis rules out all possibilities apart from an extraterrestrial source for the red rain over India. Most meteorologists would be surprised to hear that over the course of two months, particles settling out of the upper atmosphere from a meteor airburst would be confined to such a small area.
For the upper atmosphere winds to not disperse any material over a much wider area in even as little as 10 days is surely quite extraordinary. Were there no winds at all? Is it not more likely that there was some as yet unknown local terrestrial source?
Ottery St Mary, Devon, UK
The editor writes:
鈥� For an update on the red rain story, see “Red rain puzzle is still up in the air”, this issue.
From David Pollard
Analysis of Louis’s paper in arXiv, on which I presume your article about microbes from space is based, points without significant doubt to a more mundane, albeit ghastly, explanation: namely, that the particulate fallout in the red rain on Kerala in 2001 was the result of the incomplete combustion of pesticide residues.
Among various other reasons, this hypothesis is in good agreement with the prevailing weather conditions, and is somewhat simpler. It is surprising that during all this time no report of gas chromatography appears to have been published.
Oxford, UK
From Euan Fyfe
By coincidence I am reading Life as We Do Not Know It by Peter Ward. It is disappointing that only DNA is mentioned in the red rain article: the possibility of life being based on RNA seems to have been ignored again. Ward makes a good case for the existence of RNA-based life on Earth from a very early epoch, and for the possibility that it is still around. He points out that detection will be more difficult than for DNA, but nobody seems to be even trying.
Woking, Surrey, UK
Across the border
In your article on panspermia you say that the idea 鈥� that life here on Earth was seeded from space 鈥� was championed in England by William Thomson, better known as Lord Kelvin (4 March, p 54). If so this must have been during his undergraduate years at Cambridge, between 1841 and 1845. The rest of his career was at the University of Glasgow which was, and still is, in Scotland.
The word is…
Velcro is not an abbreviation, as you state in “The word”, but a portmanteau word (11 March, p 52). Lewis Carroll, who invented this term for a blend of two different words, would be delighted to see it used in your magazine.
On the middle ground
Jonathan Wolff and Kenneth Boyd are wrong to suggest an equivalence between the animal rights movement and the Pro-Test campaign (11 March, p 20).
I do not hear the Pro-Test campaigners saying that “no animal research equals no medical progress”, as Wolff and Boyd allege. What I see is simply a group arguing for precisely what the Nuffield Council report on animal testing 鈥� the same one that Wolff and Jackson helped produce 鈥� says: “We conclude that because of evolutionary continuities in the form of behavioural, anatomical, physiological, neurological, biochemical and pharmacological similarities between animals and humans there are sufficient grounds for the scientific hypothesis that, in specific cases, animals can be useful models to study particular aspects of biological processes in humans, and to examine the effects of therapeutic and other interventions.”
I see in the Pro-Test campaign a group of people who are prepared to stand up and publicly challenge those who won’t accept this consensus. It requires some courage for them to do so.
From Tony Maggs
I have had several online discussions on this matter, as a result of which I have been labelled at different times as an animal rights extremist and a barbaric torturer, based simply on a few words in my comments that people have interpreted as placing me at one extreme of the spectrum or the other.
In fact I fall somewhere to the animal rights side of the middle ground. I acknowledge that some animal experimentation is required, but I believe a lot of the animal experimentation that takes place is pointless.
Chelmsford, Essex, UK
From Jonathan Balcombe, Physicians Committee for Responsible Medicine
Wolff and Boyd underestimate the travails of a laboratory animal’s life when they assert that “not all research causes suffering”.
A recent review of published studies that I participated in (Contemporary Topics in Laboratory Animal Science, vol 43(6), p 42) found that lab routines (such as injections, blood collections and even handling and cage cleaning, cause pronounced and lasting stress responses. Stress hormones flood the animals’ bloodstream, heart rate accelerates and blood pressure spikes.
Another review by me to be published in Laboratory Animals concludes that the stacks of sterile “shoe-box” cages typical of rodent lab housing are also inhumane.
Washington DC, US
Herceptin hype
Ralph W. Moss is right to highlight the overeagerness among doctors and insufficient questioning by journalists that have resulted in the spread of incomplete information about Herceptin and its risks (4 March, p 22). The potential for harm to desperate patients, and the relationship between scientific consultants and the drugs about which they make positive statements, pose ethical problems that need to be resolved.
The hype about Herceptin should serve as a reminder to patients everywhere of the need to question doctors thoroughly about risks or side effects before accepting a new drug or device. Disseminating medical information brings with it a substantial responsibility. Moss’s article vindicates those physicians and patients who exercise a healthy dose of scepticism. He deserves to be applauded for confronting the lax status quo.
Teacher integrity
When I hear about the teaching of creationism in British schools, I find myself asking: where are they finding the biology teachers? Surely any biology graduate worth his or her salt should respond to the suggestion that they teach mythology in a science class with a snort of derision followed by the word “No”.
Gene changes
David James correctly points out that the European Union, in its directive 2001/18/EC, considers a genetically modified organism (GMO) to be an organism, with the exception of human beings, in which the genetic material has been altered in a way that does not occur naturally by mating and/or natural recombination (11 March, p 22). By this logic he concludes, presumably with tongue in cheek, that altering plants through gene mutation or by application of chemicals to induce chromosome doubling, or polyploidy, is unnatural too, so such plants should qualify as GMOs.
To clarify the situation: articles 2 and 3 of the directive James cites expressly exclude from the definition of GMOs those organisms derived from polyploidy induction, mutagenesis and cell fusion (including protoplast fusion), on condition that they do not use recombinant nucleic acid molecules.
This interpretation of the term GMO has been in place since the first directive on their deliberate release into the environment was published in 1990 (directive 90/220/EC).
Deep sea variety
In your article on sharks patrolling the high seas, you first describe the seas as “featureless” and then go on to mention two of the many features which they do have: faint electrical fields, and scents from other organisms (4 March, p 30).
To an oceanographer, the depths of the ocean are anything but featureless. There are changes in temperature, density, salinity and a host of more subtle chemical variables on both horizontal and vertical scales. Discharges from large rivers are clearly discernible by their chemistry hundreds or even thousands of kilometres from land. Likewise the deep outflow of Mediterranean water or eddies spinning off the Gulf Stream are identified by differences in salinity, density and temperature from surrounding water, some or all of which would be clearly detectable to many marine organisms. Then there are the ever-changing patterns of noise 鈥� from cetaceans, crustaceans, ships, waves, rain and seismic activity.
The deep ocean is only featureless to the human eye. Bring other senses into play, and it becomes a place of infinite variety.
From Carrie Davison
Writing about the team working on the idea of remote-controlled sharks, Susan Brown says: “That team is among a number of groups around the world that have gained ethical approval to develop implants that can monitor and influence the behaviour of animals, from sharks and tuna to rats and monkeys.”
I’m curious to know who determines that this research is “ethical”. Is there a particular governing body?
Toronto, Ontario, Canada
Susan Brown writes:
鈥� The work with animals was done at universities in the US, where the law requires researchers to gain approval from their university’s Animal Care and Use Committee before they begin the work. Such committees include veterinarians, representatives from the university and at least one outside community member. Every research institution that receives any money from the US government is required to have a committee of this kind.